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SCUMMING AND
EFFLORESCENCE
2
The Richardson Lovejoy
Engineering Company
Columbus, Ohio
**
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Scumming and Efflorescence
BY
ELLIS LOVEJOY, E. M.
14
THE RICHARDSON-LOVEJOY ENGINEERING CO.,
COLUMBUS, OHIO
!
TRADES POP, COUMIET,
COLUMBUS, OHIO:
PRESS OF F. J. HEER
1907
{
W
Letty
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J
1
!
Copyrighted 1907, by The Richardson-Lovejoy
Engineering Co.
PRICE 50 CENTS
3
t
2
1
SCUMMING.
There is no question before the brickmakers and clay
workers of this country more important than that of Scumming
and Efflorescence.
It is brought up in every convention: it is discussed on
every brick yard and in every clay working establishment.
It manifests itself in every stage of the manufacture,
often appearing on the clay itself, on the ware in the drier,
in the kiln, and finally on the finished product on the yard or
after it is laid in the wall.
Scumming is the common brick yard term for the trouble,
and it is defined as a white or grayish white coating on the sur-
face of the bricks. It is sometimes called "drier white," and
"kiln white."
Efflorescence is another name for the coating, but this.
usually refers to the white, yellow, or green coating which
appears on the finished product after removal from the kiln
while stored on the yard, or often not until the ware is laid in
the wall. Months and even years may elapse before the efflo-
rescence may appear, but on products particularly subject to the
trouble, the efflorescence will begin to show after the first wet-
ting and subsequent drying out.
The Germans have begun a systematic study of the ques-
tion and have already done a great deal of work, but the greater
part of it has been confined to the wall efflorescence, Auswitter-
ung, literally weathering out.
The great trouble in systematically studying this question
in America is due to the confusion which arises from the several
sources of the trouble and its appearance in different stages of
the manufacturing processes, and on the finished ware.
We would distinguish the coating which appears on the
ware in the process of manufacture, and which is a permanent
insoluble coating, by calling it scum, and we would use the
term efflorescence for the soluble coating which appears on the
(3)
44
4
SCUMMING AND EFFLORESCENCE.
finished product in the wall, or on the stock yard, as the case
may be.
If the clayworkers of America would recognize this dis-
tinction some of the confusion which arises in discussing the
question, would be eliminated. If the manufacturer would go
farther, and learn by careful observation, whether the scumming
appears on the ware in the drying or in the burning, we would
begin to get some order out of chaos. Many manufacturers
keep accurate records of the manufacturing processes, and all
should do so, but we have never seen a record in which there
was a space for scum.
In our experience, we have seen great variations in the
amount of scum on ware from the same clay, and there must
be many yards with similar experiences. Why, then, not keep
a record of it?
The kind of clay and condition of the same, the kind of
water, state of weather, rapidity of drying, condition of ware
going into kilns, time and temperature of water-smoking, kind of
fuel, clean or dirty, wet or dry, etc., and from these records it
will be found in many cases that certain conditions give the least
scum, or none at all, and such conditions are worth working for.
It will pay to study the question and get rid of the coating,
or at least reduce it to a minimum. It may not be injurious to
many wares, such as sewer pipe, paving bricks, etc., but we
believe that the appearance of such wares may affect the sale.
Even inspectors wil hesitate to turn down bright clean ware,
but if it is dirty and discolored, their suspicions are aroused and
the ware gets a rigid inspection.
A few cities are exceptional common brick markets, in that
the bricks must be scummed to be readily salable. The idea
seems to prevail that the scummed bricks are harder burned and
in consequence more durable than the cleaner bricks.
The
builders have learned to grade the hardness of the bricks by
the appearance of the scum.
We believe that the manufacturer who succeeds in over-
coming the difficulty, and puts into these cities a clean hard
burned brick, will have a decided advantage in the market, and
that the prejudice in favor of the scummed bricks will quickly
disappear.
·
SCUMMING AND EFFLORESCENCE.
ел
"
THE WORK OF THE GERMAN SOCIETY.
The German Society of Clay, Lime and Cement Manu-
facturers issued a brochure in which a series of questions were
asked, covering every phase of the efflorescence trouble. These
questions were sent out to the clay ware manufacturers, to
architects, to builders, and to any one interested in the subject,
and from whom information might be obtained, and from all
was requested information on the question.
The replies were tabulated and elaborate tests were carried
out to determine the facts brought out in the answers.
The
Society is thus collecting valuable information, but what is more.
important, the method it is taking results in a large corps of
practical men in all parts of the country making observations.
and studying the question under the direction of such eminent.
scientists as Dr. Mäckler and E. Cramer. In this country we,
unfortunately, have no Society laboratory in which such exten-
sive work can be done, and unless we can induce the govern-
ment to take up the work in connection with the testing work
which has been so magnificently begun on the coals and which
will be extended to building materials, clay products, stone
and cement, we must rely upon our own individual efforts
and upon the work of our Ceramic Schools.
It seems to us that the German idea of a series of questions
is a good one, and may very properly be extended to cover
scumming as well as efflorescence.
As a preliminary to a more carefully prepared list, we would
propose the following questions:
I. Have you ever noticed any white, yellow, or green
coating (efflorescence) upon the clay after exposure to the
atmosphere?
2. Is the ware made from weathered clays more subject
to scum than that made from unweathered clay?
3. Are some portions of the clay bank more liable to cause
scum than other parts?
4. Is the scumming more troublesome when the water
supply is low and probably highly impregnated with salts?
5. Does the amount of water used in the manufacture
affect the degree of scumming?
6
SCUMMING AND EFFLORESCENCE.
6. Do you attribute the scumming to the clay or to the
water?
7. Does it appear in the drying or the burning?
8.
9.
If the scum appears during the drying:
a.
Is it worse under slow or quick drying?
b. Is it affected by the state of the weather?
C.
Is it affected by the source of heat in the drier,
steam heated coils, waste heat from cooling
kilns, heat from burning kilns, heat from aux-
iliary furnaces, wood, coal or gas fired? Is it
worse under open air drying?
If the scum appears in the burning:
α.
Is it affected by the degree of dryness of the ware
set in the kiln?
12.
b. Is it affected by the state of the weather?
C.
Has wet or dry fuel any influence?
d. What effect have different fuels?
e.
What effect has the draft?
f. Does the scum come in the water-smoking, in the
oxidation, or in the vitrification period?
IO.
Does the coating appear to be an exudation from the
clay body, or is it a deposit left on the bricks by the passage of
the kiln gases?
II.
Is it worse in some kilns than others of the same type?
If so in what respect do these kilns differ in time of firing; in
strength of draft; in type of furnace; in dampness under or
around the kiln?
Is it affected by the type of kiln,
draft, muffle or continuous?
-up draft, down
It seems to us that these questions with others which every
manufacturer can propose to bring out the detail of the trouble
on his yard, will enable us to examine the problem more criti-
cally, and arrive at more definite conclusions with beneficial
results in many cases.
They will lead to more careful investigation because some
of the questions cannot be answered by many manufacturers
without examination of the product, and in a number of in-
stances actual experiments must be made before the answer can
be given.
¦
SCUMMING AND EFFLORESCENCE.
7
A.
B.
It certainly will be valuable information to every manu-
facturer to know the source of the trouble on his yard, even
though he cannot overcome it at the time. At any rate, the study
of the question by each of us in reference to our own product
will enable us to compare notes with others, without the endless
confusion which always now results in any discussion.
In order to properly discuss the question involved in scum-
ming and efflorescence, we propose the following classification
based on the stage of manufacture in which the trouble appears
and also upon the origin of the scum.
}
Effloresence on the clay.
Scum.
I. Appears on clay ware in the drier.
4.
CLASSIFICATION.
5.
I.
Soluble sulphates originally in the clay.
2. Soluble sulphates in the water used in manu-
facture.
3. Soluble sulphates formed from the decomposition
of minerals in the clay by sulphurous gases
(eventually sulphuric acid) introduced in the
drier.
Soluble sulphates formed by the oxidation of the
pyrite in the clay, forming in moist air sul-
phuric acid, and the decomposition of the
minerals by such acid.
Sulphates formed by sulphur in the oil acting on
the bases in the clav.
II. Appears on the clay ware in the burning.
Same as 3 under I.
Same as 4 under I.
I.
2.
3. Coating formed from sulphuric acid evolved from
wet fuel and deposited on the ware in the cooler
parts of the kiln, and collecting the basic
constituents from the ash carried into the kiln by
the draft.
C. Efflorescence.
I. Alkali and magnesia sulphates, vanadates, or molybdates,
originally in the clay and not fully decomposed in the burning.
8
SCUMMING AND EFFLORESCENCE.
со
2. Alkali and magnesia sulphates in the water used in
manufacture and not fully decomposed in the burning.
3. Alkali and magnesia sulphates formed in the drying,
and remaining unchanged in the ware after burning.
+. Alkali and magnesia sulphates formed in the burning.
5. Alkali and magnesia sulphates, vanadates, or molybdates
formed by the disintegration through atmospheric agencies, of
the more basic silicates produced in the burning.
6. Soluble sulphates from the mortar, or sand used in
building, or formed by reaction between carbonates from the
mortar, and sulphates or sulphides in the clay ware.
Soluble sulphates or nitrates absorbed from the ground.
Soluble salts absorbed from the stone copings, caps,
quoins, etc., used in building.
9.
Soluble salts from the air.
a.
7.
8.
In the vicinity of the sea, sodium chloride and
carbonic acid in the air, or carbonates in the clay
react to form hydrous sodium carbonates.
Ammonia in the air through chemical reaction upon
sodium chloride or upon carbonates in the ware,
forming soluble nitrates.
Sodium salts acted upon by sulphuric acid from
sulphurous gases, forming soluble sulphates.
This classification might be extended to cover rare cases
involving several chemical reactions, but for all practical pur-
poses and to avoid too much complication in our discussions, the
above classification is sufficient. Even the last item under efflo-
rescence might be omitted so far as the question of scumming and
efflorescence from the clay workers standpoint are concerned.
b.
C.
EFFLORESCENCE ON CLAY.
The efflorescence on the clay may be any soluble salt con-
tained in the clay or formed by weathering. In dry weather
these salts are brought to the surface and deposited there by the
evaporation of the water. When wet weather comes, they are
taken up by the water and in part washed away with the surface
water, and in part carried back into the clay. They play quite
an important role in the weathering of clay and disintegration of
rocks. Not only do they assist in the chemical destruction of
SCUMMING AND EFFLORESCENCE.
9
the mineral bodies, but the crystalization of the salts from
saturated solutions during changes in the temperature assist in
breaking apart the rock mass, and give opportunity for the pas-
sage of the ground waters.
DRIER SCUM.
It will be noted in our classification that the first two di-
visions under the heading "Scum that appears on the ware in
the burning" are identical with the third and fourth divisions
under the heading of "Scum that appears on the ware in the
drier." We may even go further and include all the drier scum,
except, perhaps, that caused by the oil, under the heading of
the burning.
All these classes belong to the drying stages of the manu-
facture whether in the drier or in the kiln.
It is well established that the drier and related kiln scum is
chiefly sulphate of lime. In the drying process, any soluble salts
of the alkalies and magnesia will also come out with the lime,
but we believe that the percentage of these will be small, and in
any case they will be decomposed or volatilized in the burning,
leaving behind the sulphate of lime. Clays, as a rule, do not
contain appreciable percentages of the salts of the alkalies and
magnesia.
Because of their extreme solubility, they are leached out
and removed by the ground water as fast as set free through the
decomposition of the minerals containing them.
That they appear in the form of efflorescence on the burned
ware is due to the fact that the minerals containing alkalies are
decomposed in the burning, or even later, by atmospheric agen-
cies, and the salts set free to appear as efflorescence.
In the drier scum, the sulphate of lime may have been
originally in the clay as such, or it may have been formed from
lime pebbles and pyrite in the clay, or from lime in the clay and
sulphuric acid formed from the fuel gases. The related kiln
scum comes from the same source, formed in the same way, and
is in reality a drier scum.
It is impossible to draw the line sharply between the two
because they are both due to the drying process.
IO
SCUMMING AND EFFLORESCENCE.
1
>
If the clay contains sulphate of lime, we would expect it to
come out in the drier, and it becomes "drier scum." If the ware
is set in the kiln quite wet, the sulphate comes out in the kiln
instead of in the drier and becomes "kiln scum."
If the clay contains lime in any form and pyrite, we would
expect the formation of sulphate of lime, if the pyrite is given
opportunity to oxidize, and this opportunity may come in the
drier or in the kiln, and the resultant scum is "drier scum” or
"kiln scum" as it may happen to appear in either of these stages
of manufacture.
The clay may be free from pyrite and contain very little
sulphate, the lime content being in the form of a carbonate.
In the kiln, the sulphur gases will supply the necessary sul-
phuric acid to form the sulphate, and the scum is "kiln scum,"
but many driers use combustion gases, and in such cases the
"kiln scum" becomes "drier scum.”
Whether the scum appears in the drier or in the kiln, it is
connected with the drying stage, and is properly a drier scum,
and we would define it as a scum which comes during the drying
stage, and at least one of the constituent elements comes from
within the body of the ware.
This is the most common scum,
and we are safe in saying that over ninety per cent of the scum
troubles may be included in this class.
KILN SCUM.
True kiln scum is not recognized by many manufacturers.
In our classification it is the third division under the head-
ing of "Scum that appears on the ware in the burning." It is not
a salt leached out from the interior of the ware and left on the
surface by evaporation, but on the contrary, it is a coating de-
posited on the surface of the ware, and does not necessarily
derive any of its elements from the ware. We may find it not
only on the surface of the ware, but also upon the kiln walls,
the kiln castings exposed to the kiln gases, and the sand facing
of sand moulded bricks.
It is a trouble belonging entirely to the burning, and may
occur even after the ware in the kiln is fairly dry, and the danger
of the ordinary scum is passed.
SCUMMING AND EFFLORESCENCE.
II
"
FORMATION OF CLAY.
To get at the source of our trouble we must go back to the
origin of the clay. Clay as we generally apply the term is a
mixture of kaolinite, and other hydrous silicates, sand, lime,
and magnesia compounds, feldspars, mica, iron in several forms,
hornblende, and many other silicates, resulting from rock de-
composition.
It is derived from the older rock formations, -- granite,
gneiss, etc., as well as from the volcanic rocks, diorite, dolerite,
diabase, etc., through their disintegration and decomposition by
atmospheric agencies. Kaolinite comes from the decomposition
of the silicate of alumina minerals of which feldspar is a noted
example, and usually taken to describe the process of formation
of kaolinite.
Feldspar is a double silicate of alumina and the bases, lime,
soda, potash, lithia, etc. Under atmospheric agencies, this is
broken up, and hydrous silicate of alumina, or kaolinite is one of
the resultant minerals. The bases are set free and are taken up
by the ground waters, combining with silica to form other sili-
cates, with carbonic acid to form carbonates, with sulphuric acid
to form sulphates, etc. Residual clays,
Residual clays, those which have not
been removed from the places where they were formed, will
contain besides the clay base, all the products of rock decomposi-
tion, including mica, sand, undecomposed feldspar and other
silicates, and hydrous silicates formed during the process of dis-
integration.
The sorting and sifting of these residual clays begins with
their removal from the place where they were formed to lower
levels by river or glacial action. The materials are deposited,
taken up again and re-deposited, sorted by swift, slow, and
quiescent water. They are covered to great depths by other
sedimentary deposits, lime, coal, etc., subjected to great pressure
and heat.
Perhaps brought to the surface again by some movement of
the earth's crust, and again cut through by water action, and
again sorted and deposited. In the deposited beds other forces,
both chemical and mechanical, are at work. The animal and
vegetable matter which has come in during some stage, is under-
going decomposition, and the gases given off are acting on the
12
SCUMMING AND EFFLORESCENCE.
minerals in the bed, converting the iron into pyrite for in-
stance. Concretionary force is collecting certain elements into
boulder forms, carbonate of iron and lime, boulder flint clay,
pyrite concretions, etc.
A
The ground waters carrying sulphur, lime, magnesia,
alkalies, various gases, in fact any thing soluble, are circulating
through the beds of clay, giving up their soluble salts in one.
place, and taking them away in another. The works of nature
are wonderful, and all the wonders of it are represented in our
clay beds.
We can readily see how the great laboratory of nature with
its vast and varied forces and unlimited time, can arrange for
us the great variety of materials which we call clay, and we can
also understand how our troubles are stored up in it, and scum-
ming is not the least of these.
SULPHUR.
Sulphur in some mineral forms is a very common element in
the earth's crust. Geologists estimate that .11% of the earth's
crust is sulphur. Carbon which includes the great beds of peat
and the vast coal basins is credited with .14%, but a slight in-
crease over the sulphur. Lime, which is very properly a rock
forming mineral, is estimated at 3.43%. Vanadium, that very
rare element which colors the efflorescence on walls green, has,
.02% to its credit.
Except some beds of gypsum and barite, sulphur cannot be
considered a rock forming element.
In vein formations, it is a prominent element in the metalli-
ferous deposits, from which we get copper, lead, silver, tin,
nickel, antimony and other metals, but the total volume of the
valuable minerals make up but a small part of the vein formation,
and the vein itself is but a crack, fissure or fault in the earth's
crust filled with the metalliferous deposit and gangue. The
average percentage of copper in several large mining industries
in the north-west is about one and one half per cent. This is a
vein formation in which the copper sulphide is concentrated.
In small percentages we find sulphur in every rock forma-
tion, archaic, metamorphic, volcanic, and sedimentary deposits.
of every age.
SCUMMING AND EFFLORESCENCE.
13
In the older rocks we may find it as pyrite, or other sulphide
minerals, in crystal form distributed through the formations; in
clays and shales, and sandstones, it may be as pyrite in crystal
form, or in concretionary grains or masses, or in sulphate com-
position; in coals as lenticular masses or concretions of pyrite.
We venture the statement that if one hundred samples of
the earth's crust be taken at random, that at least ninety-nine
of them will show upon analysis the presence of sulphur.
That we have been so slow in arriving at conclusions in
regard to the cause of the scum on our ware, is largely due to
our chemical work. Not one clay analysis in one hundred shows
any sulphur, yet ninety-nine of them should do so. The chemist
in making a clay analysis first dries the clay, then heats it before
a blast lamp and drives off all volatile matter, which he reports
as "Combined water," or "Loss in ignition," and in this is in-
cluded much of the sulphur, and the carbon which is also one of
the clay-worker's troubles.
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
{
The non-volatile sulphates are broken up in the course
of the analysis, and only the bases, calcium, magnesium, and
alkalies, are determined, and these are reported as oxides.
In iron ore 'analyses, in coal analyses, and in limestone
analyses for furnace use, it would be criminal not to report the
sulphur and phosphorus, for upon these may depend the fitness.
of the material for manufacture.
We have no fault to find with the chemist's analyses except
that they are not complete, and when he comes to realize that
sulphur, and carbon too, are sources of trouble to the clay-worker,
he will include them in his report.
The universal distribution of sulphur in some form in the
older rock masses, from which our clays are derived, accounts
for its presence in all sedimentary deposits. With the breaking
up of the clay forming rocks, the sulphide minerals are included
and find their way into the clay mass. Sulphur springs and under-
ground sulphur waters are very common, and these are adding
their quota of sulphur to the clay beds and other formations.
Animal and vegetable life take up minute portions of sul-
phur, and their remains serve to distribute the element broad-
cast.
We shall not attempt to discuss the many changes which the
sulphur may pass through before it appears as the acid element in
14
SCUMMING AND EFFLORESCENCE.
our clay ware scum-first coming from depths as sulphurous gas,
being converted into sulphates, reduced to sulphides, back and
forth, ever changing with the conditions, and in every change
playing a part in rock decomposition and formation.
It is not strange that our clay beds should contain it in some
form, the strangeness is that any beds can be free from it.
In the New Jersey Geological report for 1904, twenty-eight
out of thirty one clays from different districts, contained soluble
salts, ranging from .06% to 1.44%, the average being .37%.
A German chemist gives analyses of twenty one clays, only
two of which did not contain soluble salts. The chances are that
these soluble salts were sulphates, and the percentages do not by
any means show the total amount of sulphur which the clays
contained.
An Ohio shale used largely in the manufacture of bricks,
contains two per cent of sulphur, one hundred and twenty or
more pounds in every thousand bricks.
SOLUBLE SALTS IN CLAY BEDS.
One of the most interesting sulphur minerals in lithological
problems is pyrite, sulphide of iron, consisting of one part
iron and two parts sulphur. Upon exposure to atmospheric
agencies the pyrite in the older rocks is oxidized to iron oxide
and sulphuric acid. The acid assists in breaking down and de-
composing the rock masses, and at the same time takes up the
bases to form sulphates.
The iron would ordinarily assume the most stable form, the
red oxide, or hematite ore.
Under cover in connection with the reducing action of de-
caying animal and vegetable matter, the iron is reduced to the,
lower oxide, and the chemical composition of other minerals is
likewise changed. Sulphur which was taken up by the animal
and vegetable matter during its growth is set free in the decaying
process, takes up the iron to form pyrite, thus coming back to the
original form found in the other rocks.
When again exposed to the atmosphere by any movement
of the earth's crust, or by erosion, or both, the pyrite is oxidized
and again we have iron oxide and sulphuric acid.
With the shifting and sorting through mechanical action, we
SCUMMING AND EFFLORESCENCE.
15
have also going on the above briefly outlined chemical changes
and many others as well, and the process has been repeated many
times, perhaps, and is still going on.
In this way we have formed calcium sulphate (gypsum),
sodium sulphate (Glauber salts), magnesium sulphate (Epsom
salts), iron sulphate (copperas), and many other salts and min-
erals which appear in our clay beds.
The sulphates above mentioned are soluble in water, and
the water we use in the manufacture of our clay ware may con-
tain them as well as the clay.
Investigators of scumming agree that the coating is chiefly
sulphate of lime, and that the efflorescence that appears on the.
material after it is laid in the wall, is, in the majority of cases,
sulphate of magnesia and the alkalies. The yellow and green
efflorescence which appears on
on buff burning wares contains
vanadium and molybdenum salts.
It is very important that we learn to distinguish the sources
of the scumming.
If the clays show efflorescence before being made up into
ware, we may be certain that they contain sulphates, and our
treatment to overcome the trouble must begin with the clay.
If the scum appears on the ware in the drying process,
regardless of the method of drying, we may look for the source
either in the clay or the water, or both, and be reasonably certain
that the sulphates were already formed.
If waste heat drying with sulphur gases alone produces the
scum in drying, we may conclude that the clay contains only the
basic element of the scum, and not the acid element in active
form.
If the dried product is clean, and the scum appears only
in the burning, we may say that the clay contains little or no
sulphate, and that the water is free from sulphur, but the clays
contain lime and perhaps pyrite.
If the water smoking is done with wood and no scum ap-
pears, we are certain that the sulphur end of the trouble is not
from the clay nor from the water, but comes from the products
of combustion.
Finally, if the scum is confined largely to the draft spaces
in the kiln, and occurs intermittently with wet fuel, or in damp
16
SCUMMING AND EFFLORESCENCE.
kilns, or during wet weather, we may consider that the trouble
is purely connected with the burning, and that the clay is safe.
The observations are not so simple as would appear because
some clays will scum at every stage of the process of manufac-
ture, due to all the causes, but in spite of the many contradictions
which such clays will bring out, we hope to get some value and
order from combined observations on the part of the manufac-
turers.
EFFLORESCENCE AND SCUM ON THE CLAY.
We have frequently noticed in bedded clays which contain
pyrite, particularly the No. 2 fire clays which come under coal
veins, a green or yellow efflorescence on the surface of the clay
after exposure to the atmosphere for some time. This is ex-
actly similar to the wall efflorescence, and consists of an alkali
or magnesia sulphate, molybdate, or vanadate, or all three of
these.
Gard
When the clays are freshly mined, the sulphur is in the form:
of pyrite. Oxidation converts it into iron oxide and sulphuric
acid, and the acid acts upon the bases in the clay, perhaps even de-
composing some of the minerals containing the bases, and taking
up the bases to form the sulphates. Sulphuric acid is a very
powerful reagent, and plays an important role in the decompo-
sition of rocks. In the laboratory, we use it to decompose clays,
in order to determine the free and combined silica. Free silica,
or sand is not acted upon by sulphuric acid, while all the other
minerals are more or less dissolved by it.
The pyrite concretions in the clay frequently carry rare-
metals, and it is from this source that we get the molybdenum
and vanadium which gives the green and yellow colors to the
efflorescence.
That the efflorescence on the clay bears some relation to
the efflorescence on the wall is evidenced by the fact that such
clays would effloresce when burned and laid in the wall, while
the same clay when freshly mined and made into ware, would
be free from wall efflorescence. In one factory the solution of
the wall efflorescence was accomplished by abandoning the use
of weathered and crop clay and using only freshly mined clay
from deep under the hill.
On the same clay in the outcrop, we have found crystals of
1
SCUMMING AND EFFLORESCENCE.
17
gypsum in large quantities mixed with the clay. As the oxida-
tion of the pyrite proceeded in depth, the circulating waters
brought the soluble sulphates to the outcrop, and with the evapo-
ration of the water, the crystals of gypsum were deposited in
the pore spaces of the clay.
A simple and easy test can be made of the clays and water
used in manufacture, and every manufacturer should familiarize
himself with it.
Get from the druggist, or from some laboratory a small
quantity of barium chloride and dissolve one part in ten parts-
of distilled water, and keep for the tests. To test the clay, boil
some of it, in distilled water for several hours with frequent
stirring.
Allow it to settle and decant or filter some of the clear
solution. Heat the latter and add a few drops of the barium
chloride solution. If there, are any sulphates present, a white
precipitate will form, making the water cloudy. The addition
of a little muriatic acid to make the solution acid will dissolve
any white precipitate except the barium sulphate, and if the
solution remains cloudy, sulphates are undoubtedly present in
the clay.
The water can be tested in the same way, by heating a little
and adding the barium chloride solution, and acidulating with
muriatic acid.
If the clay or water contains sulphates in any considerable
amount, we may expect scumming in the drying process, espe-
cially if the clay contains lime.
The sulphates of the alkalies, if they come to the surface,
may be volatilized and driven off in the burning, and the sul-
phate of magnesia may be decomposed.
These salts give trouble as efflorescence but not as scum.
The sulphate of lime does not volatilize, in any appreciable-
degree, and it cannot be decomposed except at a high heat under
reducing conditions, and any of it which comes to the surface.
of the ware in the drying process will remain as a coating after
the ware is burned. It must not be assumed because the clay
contains soluble sulphates, that these sulphates will come to the
surface in the drying. Such is not the case.
Even the very
soluble sulphates of the alkalies, and magnesia, may go through.
4
18
SCUMMING AND EFFLORESCENCE.
the buring unchanged and without coming to the surface. We
have evidence of this in the tests of burned ware which show
the presence of such soluble sulphates, some of which, perhaps
the greater part, was formed in the burning, but if soluble sul-
phates that are formed in the burning, may go through without
passing into silicate compounds, so also may the salts originally
in the clay. The conditions for scumming must be favorable.
If the clays contain sulphates, and enough water is added
to take them up in any appreciable quantity, and the drying
is slow and at low heat so that the evaporation will be from
the surface, and not below it, and the ware is not too porous,
there is great danger of scumming in the drier.
THE USE OF BARIUM.
The use of barium is the cheapest and most feasible method
of avoiding the scumming due to the soluble sulphates in the
clay, or water. Barium carbonate is the salt ordinarily used
for this purpose because of its cheapness and safety in use. It
may often fail to do the work, and the reason is simply due to
the fact that not enough of it can get into solution to precipitate
the sulphates. In order to have the chemical reaction of the
barium on the sulphates, solution of both is necessary.
Barium carbonate requires 14,000 parts of water to dissolve
one part, while lime sulphate only requires about 400 parts.
Without going into the question of atomic weights in order
to speak accurately, we may say that only enough barium car-
bonate can go into solution to precipitate one-thirty-fifth of the
lime sulphate that may be in solution, and from one-five-thous-
andth to one-ten-thousandth of the amount of magnesia or alka-
line sulphate that may be present. As soon as that one-thirty-
fifth has acted, the solution may take up another one-thirty-fifth,
and so on until all the lime sulphate has been acted upon and
rendered harmless.
J
The conditions are not often favorable for this repeated
action of solution and precipitation, and when not, the barium
carbonate fails. In such cases a more soluble salt of barium
must be used, - barium chloride, or barium hydrate. Barium
chloride is soluble in three or less parts of water, one hundred
and fifty times as soluble as lime sulphate, and its use will pre-
SCUMMING AND EFFLORESCENCE.
19.
cipitate all the sulphates which may be in solution in the water
in the ware.
The danger in its use is that it in itself will cause scum-
ming, if any of it remains in the clay, and it must only be used
in such quantities that all of it will be used up in the chemical
reactions.
-d
This question was discussed in a paper read before the
American Ceramic Society, which appears in Vol. VIII of the
Transactions.
SCUM ON THE WARE IN THE DRIER.
In that paper it is suggested that where barium carbonate
is not sufficient, that some barium chloride be used to precipitate
the greater part of the sulphates, leaving a small quantity to
be precipitated by the barium carbonate. The latter can be safely.
added in excess, and in consequence there can always be some
of it present to complete the work of the barium chloride.
We have already touched upon this subject in the preceding
pages of this paper, and in taking it up individually, we will
repeat, in many instances, what has already been said.
The soluble sulphates in the clay which are taken up by
the water used in making up the ware, or which may be in the
water itself, are brought to the surface during the drying pro-
cess and because of it, and with the evaporation of the water,
are left upon the surface as a white, or yellowish white coating,
which we call scum.
Sometimes the surface is only flecked with the coating, and
at other times the entire surface is covered with it and the color
of the ware hidden. No brick is uniformly coated, and no two
bricks are equally coated, and no two kilns produce it in the
same degree, nor the same kiln in different burns, and the effect
of it in the pile or wall is that of a lot of old bricks which have
been taken out of a wall and the mortar cleaned off, They are
unfit for face bricks on account of the appearance, and a de-
cided advance will be made in the manufacture and use of com-
mon bricks when we can overcome the trouble.
The quantity of water used in the manufacture has a marked
effect on the degree of scumming, and also upon the effectiveness.
of the methods of overcoming the trouble. In order to get a satur--
20
SCUMMING AND EFFLORESCENCE.
►
ated solution of sulphate of lime, four hundred parts of water
are necessary, for each part of the sulphate. If the clay is pressed
dry or nearly dry, there can be no solution of the sulphate, and
without solution, none of it can come to the surface. The greater
the amount of water we use in the manufacture of the ware, the
greater the amount of sulphate which can pass into solution, and
subsequently be brought to the surface.
Dry or semi-dry pressed bricks are less troubled with scum-
ming:
First, because very little water enters into the make-up of
the brick.
Second, because the bricks are dried in the kiln where they
are faced, and the ends set close together, and the drying is
largely through the sides of the bricks, where any scum that
might appear would not be harmful.
Third, dry pressed bricks are more porous near the surface
than mud bricks, and the evaporation may proceed at sufficient
depth below the surface to hide any scum.
While the quantity of water used may tend to increase the
scumming, yet it at the same time gives better opportunity for
the solution of the barium. The barium may and should be
added to the water before mixing the latter with the clay. The
water thus may become fully saturated with barium, ready to
act upon any sulphates which the water may take up when it is
mixed with the clay.
The chances are against getting a full saturated solution
of sulphates in the short time which the water has to act.
The advantages are in favor of the use of larger quantities
of water, when the corrective barium is used, and against it
when no barium is used. In practice we have found that tile
and bricks made from the same clay and at the same time, the
one being quite soft and the other quite stiff, would have scum
on the latter and the other would be clean. It was simply a
result of greater quantity of water in the former, giving the
barium better opportunity to conteract the sulphates which the
water could take up from the clay. If the clay contains only
a limited amount of sulphate, far below the amount required to
saturate the water even in stiff mud, then there can be no ques-
tion in regard to the advantages of running the ware softer, in
connection with the barium treatment, as a preventative of scum.
SCUMMING AND EFFLORESCENCE.
21
Whether little or much water is used, it will take up all the
sulphate, the total quantity being the same in either case, —
but the larger quantity of water will take up more barium if
the latter is added in excess, and at the same time give better
opportunity for the latter to act.
The rapidity of drying plays an important part in scumming.
We were in one case enabled to overcome the trouble by chang-.
ing the drying from a slow steam floor to a quick hot air system.
In many plants, especially those using shale or upland clay, the
clay goes into the mill comparatively dry, and any sulphates,
or the greater part of them, are in the solid form. We have
noted the solubility of lime sulphate, and also the fact that it
may be present in the clay in the crystalline form. It not only
takes time to dissolve these crystals, but any water surrounding
the crystals, or particle of sulphate, must be removed when
saturated in order to give unsaturated water a chance at the
crystal. In a clay mass there is very little movement of the
water, - the stiffer the brick the less the circulation of the water.
Without circulation the diffusion of the saturated portion of the
water will be very slow, but in time it will permeate the entire
clay mass, and during this time the water in touch with the
crystals of sulphate will be taking up more of the sulphate, which
in turn will be diffused.
Quick drying will put a stop to the diffusion and also the
continued taking up of more of the sulphate.
Time has also another effect. We have only considered the
question from the standpoint of sulphates in the clay. If the
clay contains pyrite, the pyrite will begin to oxidize as soon
as it is exposed to the air, forming in the presence of moisture,
sulphuric acid, which attacks any lime, or other bases to form
sulphates.
Naturally the longer the exposure of the pyrite to such con-
ditions the greater the amount of sulphate formed and taken up
by the water.
We have mentioned the fact that soft mud bricks will have
a greater tendency to scum than stiff mud, and also that they
can be more easily treated with barium to offset the scumming.
We have instances called to our attention in which soft mud
products would not scum while stiff mud products made of the
same clay at the same time would scum. This would seem to
22
SCUMMING AND EFFLORESCENCE.
contradict our statements, but we must consider the character
of the ware.
If both are run out through an auger machine or through a
repress, giving each equal density, except so far as the quantity
of water made a difference, the soft mud product would show
the greater amount of scum. If, however, the soft mud product
was a so-called slop product, and the stiff mud an auger or
repressed product, the soft mud may not scum. It is simply a
question of density.
The soft mud brick will have large voids and open pores
and the scum will be deposited in these pores and voids and not
show on the surface. The evaporation actually takes place be-
low the surface, and wherever it occurs the sulphates will be
deposited.
FINGER MARKS ON CLAY WARE.
In view of the above, the explanation of finger marks on
clay ware may be made. It seems to us that it is a question of
density. Stiff mud bricks as they come from the machine have
a dense face. Wherever this face is touched by the fingers, the
surface is more or less broken and the flow of the solutions will
naturally be to these open places. If we examine a finger mark
under a powerful magnifying glass, the rough broken surface.
becomes apparent. The serrated edges stand out and the surface
has the appearance of having been scratched over by a file.
The reverse is the case on a wire cut surface. Here the
entire surface is torn by the cutting wires, with the cracks reach-
ing into the bricks. The evaporation takes place from the bottom
of these cracks. Press these rough surfaces down with the
fingers, and we get a surface smooth enough to show the scum,
but not dense enough to prevent it from coming to the surface.
In dry pressed products the effect would be somewhat similar
to that on the rough wire cut surface. The density, perhaps,
would not be increased, but there would be more or less crum-
bling of the clay and the fine dust would be forced into the pores
of the brick, filling them to such a degree that the salts would
come to the surface instead of being deposited below the surface.
This statement has been questioned, and very properly, per-
haps.
Our experience has been with hard clays, and the pressed
SCUMMING AND EFFLORESCENCE.
23
body was not very dense. We can readily see how a fine, highly
plastic clay when hard pressed in a dry press machine, may be
so dense that the effect of the finger is to scratch the surface,
and allow a better passage for the solutions, just as in the stiff
mud product.
In either case the effect of the finger is to increase the capil-
larity at the point where the brick is touched, and the solutions
flow to this point.
SOURCE OF HEAT IN THE DRIER.
We cannot see that the source of heat in the drier, whether
from steam pipes, or waste heat, can effect the tendency to scum-
ming, provided the rate of drying is the same in each case.
However, with waste heat there is always more or less dan-
ger of sulphur gases coming from the cooling kilns through
leakage of dampers in the waste heat system. Beds of coals in
the ash pits will continue to give off sulphurous fumes until
practically cold.
It must also be remembered that the clay contains pyrite,
and in many cases this is not completely roasted out in the burn-
ing, and from this source we will get sulphurous acid gas, and
introduce it into the drier. The clays may not have contained
sufficient sulphate to cause scumming, but they may contain car-
bonate of lime and magnesia. The carbonate of lime is practic-
ally insoluble, one part in 28,000 parts of water, and cannot
be the cause of scumming.
But introduce sulphurous acid gas, which becomes sulphuric
acid in the presence of moisture, and the carbonate of lime will
be quickly decomposed and sulphate of lime formed.
For this reason we cannot use the products of combustion
in our drying. The length of our drying tunnels may have some
effect on the scumming. In a long tunnel, the air may become.
saturated before the end of the tunnel is reached, and beyond
the saturating point there will be a deposit of moisture, dew,
or sweat, on the ware.
The bricks may actually gain in weight for a while instead
of losing, and we have simply increased the water content, with-
out any material change in the density or porosity of the brick.
In continuous kilns, in which there were no advanced heating
24
SCUMMING AND EFFLORESCENCE.
1
flues we have tested the advanced drying, and in some instances
found the bricks had gained as much as a pound in weight due
to the moisture taken up by the bricks from the saturated kilm
gases.
CAN THE OIL USED ON THE CUTTER AND REPRESS ROLLS HAVE
ANY EFFECT?
Several times our attention has been called to scum due to
the oil. Some of our crude oils, particularly those which are of
limestone origin, such as the Lima, Ohio, oils, contain sulphur.
Kerosene will take up some water which has great affinity
for sulphuric acid, or rather vice versa, and the effect would be
to coat the bricks with a solution containing sulphuric acid, which
will act on the lime in the clay to form the sulphate.
We have before us two bricks, a red and a buff. The
red scums badly under any circumstances, while the buff is clean.
Wherever the oil has touched the red brick the scum is
markedly worse. The buff brick using identically the same oil
is free from scum. We must conclude that the oil in itself can-
not cause the scum, but in connection with lime in the clay, it
can have some effect. The oil used was common black oil mixed
with brick oil, a grade of kerosene unfit for illuminating pur-
poses.
Another illustration was brought to our attention, and the
difficulty in this case was overcome by using pure lard oil mixed
with pure kerosene, showing that pure oil will not cause scum-
ming, but crude oils may.
Crude oils may contain dirt, lime salts, etc., simply as im-
purities in suspension and with the drying and burning, the
residue will be left on the surface of the bricks.
In our own experience we have found that fairly pure kero-
sene mixed with so-called "castor" oil, gave no trouble.
answer no.
We have been asked if the addition of barium carbonate to
the oil will prevent the scumming due to it, and to this we must
The addition of barium carbonate to the oil will
simply increase the amount of sediment which the oil contains,
and this sediment will be left on the surface of the bricks after
the oil is driven off.
In the manufacture of dry pressed bricks, we have used oil
to prevent scum coming to the surface of ornamental bricks.
SCUMMING AND EFFLORESCENCE.
25
1
In this case the oil simply filled up the pores of the bricks
forcing the drying to the sides, and with it the scum.
Many manufacturers have used oil to remove the scum from
burned dry pressed bricks, and it seems to be effective if the
scum is not too heavy. We do not think there could be any ap-
preciable solvent action, but the oil may have mechanically loos-
ened the bond between the sulphate and the brick and allowed
some of the former to be washed away. Whatever the explana-
tion may be, the fact remains that oil will cause the disappearance
of light scum on dry pressed bricks.
PYRITE IN CLAY.
We can readily understand how sulphates existing as such
in the clay or in the water used in manufacture may cause scum-
ming.
Where, however, the clay and the water are free from sul--
phates, there will be no scumming unless the minerals in the
clay can be converted into sulphates. We have noted the com-
mon occurrence of pyrite in bedded clays. Under oxidizing.
conditions, pyrite is decomposed and the iron and sulphur con-
verted into oxides.
The sulphur tri-oxide in the presence of water becomes sul-
phuric acid. Sulphuric acid is a powerful agent and decomposes
many minerals. Besides pyrite, clays contain lime. It may be
safely said that no clays are free from it, while many of them
run high in lime carbonate, such as clay marls, and all common
clays contain appreciable percentages. The feldspars are also
common in clays and they contain lime and alkali bases.
The sulphuric acid will deompose the lime carbonates, feld-
spars, etc., taking up the lime or other bases to form sulphates,
and in this way we may get sulphates formed from the ingre-
dients in the clay, where the clay originally contained none.
However, there can be no great degree of oxidation if the
drying is done rapidly, and at the same time the heat in the drier
is not sufficient to roast out the sulphur in the pyrite and set it
free to form sulphuric acid and subsequent sulphates. We think
there is little danger on this score in any clay ware drier.
The pyrite, therefore, if not allowed to oxidize cannot be the
cause of scumming in the drier, but we may look for trouble
from it in the kiln where the temperatures are higher.
26
SCUMMING AND EFFLORESCENCE.
f
A temperature of 750 degrees F. is required to start the
sulphur from pyrite by volatilization. Sulphuric acid is volatil-
ized at 680 degrees F., and no moisture could remain in the ware
above this temperature, and without moisture there can be no
scumming.
Such temperatures are not obtained in the drier, but in the
kiln the top courses are often heated above 750 degrees, while
the bottom courses may contain sufficient moisture to cause scum-
ming.
In rotary driers, or any driers, in which clay is dried before
being made into ware, we may look for trouble, because here we .
may have combustion gases in connection with moisture, or at
the hot end we may have heat enough to start the sulphur from
the pyrite in the clay.
WASTE HEAT GASES.
All fossil fuels, many oils and gases, contain sulphur com-
pounds, and in the burning these are set free and oxidized in
the presence of moisture to sulphuric acid. All clays con-
tain sulphur compounds, and during the burning these are par-
tially roasted out, but in many cases not fully so. The introduc-
tion of waste heat, even though taken from a cooling kiln, means
the possible introduction of sulphurous acid gas and the forma-
tion of sulphuric acid, in the drier. If we take the products of
combustion into the drier, the presence of sulphur gases is very
apparent. If we take the heat from a cooling kiln, while the
ashes in the furnace are still hot, we are certain to get sulphur
gases. The sulphuric acid thus introduced into the drier will
be deposited on the bricks and remain there long after the mois-
ture is driven off except such moisture as may be taken up by
the acid. Capillarity will take the acid into the pores of the
bricks, and sulphates will be formed by the acid acting on the
lime in the clay, which will be brought to the surface and de-
posited as scum. Sulphuric acid does not volatilize until a tem-
perature of over 600 degrees F. is attained. and it may cause
scumming even after the bricks are dry and free from moisture,
except the hygroscopic water which remains in the clay until
quite high temperatures are reached. The acid has great affinity
for water, and will take it up and hold it at temperatures above.
the boiling point, and in fact the last remnant of water in the
1
I
SCUMMING AND EFFLORESCENCE.
27
acid is not driven off until the volatilization point of the acid itself
is reached.
Here we have the drier scum coming from identically the
same source. The cure in the drier is not difficult. If the ware
can be dried without scum, and only scums when waste heat is
used, it will be necessary to either do away with waste heat, or
get rid of the sulphur gases. Naturally the first step would be
to remove any hot ashes from the furnaces, and to allow the
kiln to stand a few hours until the sulphur gases have passed
off, or become diluted below the stage where they would be
harmful.
Heating stoves are being designed to make use of products
of combustion, somewhat on the principle of the old-time furnace
hot blast stoves, and their use will be extended as the advantages
of an abundance of heat in gases free from sulphur become ap-
preciated.
SCUMMING CAUSED BY THE ADDITION OF GROG.
We have had our attention called to a case of scumming due
to the addition of grog made from ground bricks. The use of
such material to counteract shrinkage and prevent checking dur-
ing the drying process is quite common but the appearance of
scum from its use is quite rare. Dr. Mäckler, in an exhaustive
series of analyses of burned bricks, shows the presence of appre-
ciable amounts of soluble salts. As a matter of fact the soluble
salts in the burned bricks may exceed in amount that in the un-
burned clay.
}
If the clay contained lime or other carbonates, these are con-
verted into the more soluble oxides, and if the heat is not carried
high enough to pass these into silicate formations they will re-
main in the clay. They are much more likely, however, to be-
come sulphates, which will remain unchanged at much higher
temperatures than the oxides. Thus an insoluble carbonate in
the clay becomes a soluble sulphate in the burned brick.
Feldspar, a common mineral in clay, is insoluble in water,
but it may be decomposed by heat, or even after the burning,
it may be decomposed in a measure by solutions of sulphates,
and thus add its quota of soluble salts to the burned bricks, which
the raw clay did not contain.
I
28
SCUMMING AND EFFLORESCENCE.
The filling around most factories is largely the ashes from
the furnaces, which contain notable percentages of sulphates.
Brick bats dumped on these and allowed to stand exposed
to the weather, until wanted for grog, will take up these sul-
phates from the ground. Thus the grog may contain appreciable
amounts of soluble salts, and its use in the clay will cause scum-
ming, while the clay itself, and the clay from which the grog
was made would not scum.
WHY IS SCUM INSOLUBLE?
The question may have occurred to the reader as it often
has to the writer, why, if scum is a soluble salt, deposited from
solution, which goes through the burning unchanged, it becomes
insoluble. It is established that scum is made up of sulphates,
chiefly lime. The alkali sulphates are volatilized in the burning
and driven off, but the sulphate of lime remains unchanged. It
may be broken up at high temperature under reducing condi-
tions, and the lime content pass into silicate formation, but under
ordinary common brick kiln conditions and temperatures, it re-
mains largely as sulphate, and if so it should be soluble and
removable by washing, which is not the case.
•
H. Günther in discussing this question stated that the sul-
phate becomes insoluble through the destructive influence of the
burning, which is undoubtedly true in vitrified paving bricks,
and clinkers, where we have more or less reducing action in the
burning and higher temperatures than in burning common ware.
Even in burning common ware, some of the sulphate prob--
ably becomes reduced and broken up during the burning, and the
lime combines with the silica in the clay to form an insoluble
silicate.
Such a silicate on the face of the brick will remain, and we
believe the scum will prove to be, in part, a silicate.
In the analyses by Dr. Mäckler, our attention is called to
the presence of soluble silica and alumina in every analysis.
These analyses were of burned bricks and the soluble silica is
attributed to the effect of the fine grinding for the analyses.
and instances of the effect of such fine grinding on rocks is cited.
Silica is more or less soluble in hot alkaline solutions, and
we would have such solutions in the clay during the drying
process.
1
SCUMMING AND EFFLORESCENCE.
20
The presence of sulphuric acid would materially increase
the amount of soluble silica, since many silicates are decomposed
by sulphuric acid, and the silica set free is soluble until dried
and burned. Thus we may have going out with the water in
the drying not only soluble sulphates but also soluble silica and
alumina, which would collect upon the surface of the bricks.
During the burning the soluble silica will become insoluble,
and some of it will enter into silicate combination with the clay
materials on the one side, and with lime from any reduced sul-
phate on the other, perhaps aiding in the reduction of the sul-
phate, and will serve as a matrix holding fast the particles of
unaffected lime sulphate, perhaps coating them, and firmly attach-
ing them to the face of the brick. The coating then becomes a
mixture of sulphate, silica, and silicates, in which the latter are
insoluble, and in smaller percentages, but sufficient to hold the
larger amount of the soluble sulphates, and prevent their being
washed away.
-
HOW TO PREVENT THE SCUM IN THE DRYER.
In the preceding pages the method of prevention of scum
in the drier is expressed or intimated. If the cause is wholly or
in part due to slow drying, to sulphur in the gases, to impure
oil, to excess of water, to impure water, the cure is obvious.
If the trouble is with the water regardless of quantity, it
can be overcome with barium carbonate. It is safe to say that
water properly treated with barium carbonate will be freed from
all soluble sulphates and cannot give any scumming trouble.
While one part of barium carbonate requires fourteen thous-
and parts of water for solution, and lime sulphate only requires.
four hundred parts of water, and can in consequence be far in
excess of the barium carbonate in solution, yet as soon as the
one-fourteen-thousandth part of barium is used up, the water
can take up another one-fourteen-thousandth part, and continue
to do so, as long as any sulphate remains in solution. It is only
necessary to have a tank with a stirrer. Into this introduce the
powdered or precipitated barium carbonate and keep it stirred up.
To overcome the sulphates in the clay itself is quite a differ-
ent problem. If the clay contains very little sulphate compounds,
the introduction of barium carbonate with the water, may be
30
SCUMMING AND EFFLORESCENCE.
and often is sufficient to overcome the difficulty. If, however,
the clay contains larger quantities of sulphate than can be over-
come in this simple way, it becomes necessary to resort to the
use of more soluble salts of barium. The quantity of water and
time are important factors in overcoming scum with barium car-
bonate, and as we decrease the quantity of water, and shorten
the time of its operation, and the opportunity to get at the salts,
we must resort ever more and more to the use of the more soluble
salts of barium, the chloride, hydrate, bi-carbonate and oxide.
We cannot hope to overcome scumming in clay prepared direct
from the clay pit for dry or semi-dry pressed ware with barium
carbonate, because we cannot get enough of the latter into solu-
tion to precipitate the sulphates which may be in solution. Stiff
mud products will be limited in the successful use of barium car-
bonate, and it is useless to add heavier and heavier doses of the
carbonate, when the proper amount fails to bring results.
Soft mud products will lend themselves more readily to the
use of barium carbonate in overcoming scum, because of the
greater amount of water, and the better opportunity of the barium
to act.
There are many clays which can only be corrected by the
slumming process. If it is practical to first treat the clays in a
blunger with a large excess of water, to which barium carbonate
has been added, the trouble with the sulphates will be entirely
overcome.
KILN SCUMMING.
We must recognize two distinct kiln scums. One which is
quite common and closely related to the drier scum, and the other,
a coating of rarer occurrence, which is in no way related to the
drier scum, and could hardly be produced in it. In the drier and
related kiln scum, one or both of the elements making up the
scum is inherent in the clay or water used, while in the true
kiln scum, neither element comes from the clay.
THE COMMON KILN SCUM.
If the bricks go into the kiln only half dry, we may expect
to find upon such ware the scum which would ordinarily come
out in the drier. This we have already discussed under the drier
scum and need not repeat it. There are, however, many clays
1
SCUMMING AND EFFLORESCENCE.
31
which come from the drier clean, clays which contain car-
bonate of lime instead of sulphate. The carbonates are prac-
tically insoluble and cannot cause scumming, since they cannot
be in solution in any appreciable quantity. Such a clay can be
dried without showing any scum. We consider a clay dry when
it looks and feels dry, but it is impossible to expel all the moisture
from a clay in any ordinary drying process.
The water smoking stage in the kiln is the expulsion of the
hygroscopic water, and this is followed by the expulsion of the
chemically combined water in the clay. The former will require
a temperature of several hundred degrees while the latter does
not start until a temperature of eight or nine hundred degrees F.
is reached. In the burning with fossil fuel, we are producing
sulphurous gas, which is converted into sulphuric acid, and this
acts on the lime in the clay producing sulphates, which are de-
posited on the surface as scum. The clay itself may contain the
sulphur, in pyrites, which has not previously had a chance to
oxidize, and in the burning, before all the water is expelled, the
sulphur may begin to pass off and act like the sulphur from the
fuel.
We had the same thing taking place in the drier when com-
bustion gases were introduced, but probably the sulphur in the
clay as pyrite would not be started off at drier temperatures,
and this at least may be called kiln scum. We would call atten-
tion to the fact that the bottom of a kiln is often more badly
scummed than the top, due to the fact that the bricks are colder,
and the sulphuric acid formed in the upper part of the kiln
will be deposited on the bricks in the lower part of the kiln, and
remain there until the temperature is sufficient to volatilize the
acid.
We cannot reach this scum with a barium treatment, simply
because there is so little water, that practically none of the barium
will go into solution. If it were practical to convert all the car-
bonates in the clay into sulphates, then precipitate the sulphates
with barium, there would be nothing upon which the sulphurous.
gases could act, and no scumming could result.
This would be putting the clays in condition to scum, then
correcting them with barium. Such a process is not to be rec-
ommended. The most practical solution is the heating up of the
bricks above the volatilizing point of sulphuric acid (680 deg. F.)
32
SCUMMING AND EFFLORESCENCE.
.
with wood, or other non-sulphurous fuel. The dry sulphurous
gas formed above that temperature cannot give trouble because
no solution can be had, and without solution, no scum can come
to the surface.
The sulphur from the pyrite will not come off as rapidly
nor as completely under oxidizing kiln conditions as under re-
ducing conditions, and the water smoking should be done under
oxidizing conditions, with large excess of air, and strong
draft.
The strong draft carries the moisture out of the kiln, and
reduces the danger which comes from soaking the bricks in an
atmosphere laden with moisture and acid. We believe that
if the bricks are thoroughly dry before going into the kiln, and
the early stages of the water smoking be done with wood, that
there will be no danger of scumming in the water smoking of
pyritiferous clays.
Continuous kilns are particularly subject to scumming in
the water smoking because the waste heat gases are used for
the water smoking, and these are taken from hot compartments
to cooler ones, and as the heat decreases, the amount of moisture
is increasing, until, if the gases are not drawn off soon enough,
the dew point is reached, and moisture laden with sulphuric
acid will be deposited on the bricks. The presence of sulphuric
acid very materially raises the dew point, and moisture may be
deposited while the gases are quite hot. Continuous kilns should
be constructed with advanced heating flues to take the hot air
from cooling compartments and introduce it into drying cham-
bers ahead of the fires, until such compartments have been heated
up several hundred degrees, when the products of combustion
may be safely admitted.
Scum which has been formed from kiln conditions as above
explained is not readily reached by any chemical treatment of the
clay. The presence of barium carbonate in the clay will tend to
prevent in some measure the kiln scumming during the water
smoking, in that the sulphuric acid has greater affinity for the
barium than for the lime, and will act upon it first so long as
any remains and leave the lime carbonate untouched, but there
is such an excess of sulphurous acid gas to form sulphuric acid,
that we cannot hope to add enough barium carbonate to counter-
act all of it.
SCUMMING AND EFFLORESCENCE.
33
KILN COATING.
We now come to the phenomenon which is much rarer, and
which is a true kiln scum in no way influenced by the character
of the clay.
We cannot tell how many cases of reported kiln scum may
be the ordinary kind which comes in the water smoking stage of
the burning, and how many may be of this rarer kind, which
comes in the later stages of the burning.
We have in several instances seen a kiln scum which was
unquestionably a coating deposited on the bricks, and not from
the bricks, and our observations have been confirmed by others.
It differs from the water smoking scum, in that it has less
of the nebular appearance of the latter, and is more like a coating,
such as antimony will make when volatilized.
It appears most strongly in the draft spaces, and that it is
not an efflorescence from the bricks is evidenced by the fact that
we have found it as a fringe around the perforations in the fire
brick kiln floor. It is always more marked in the lower part of
the kiln (down draft). We have seen it deposited on the sand
used in making sand mould bricks, and in such cases it can be
brushed off with the sand.
We have experienced it more with wet dirty fuel high in
sulphur, and believe it comes entirely from that source.
The sulphates in greater or less degree are volatile, and to
this extent any sulphates in the fuel, or formed during the burn-
ing, will be carried over and deposited upon the bricks. The
more volatile salts will be revolatilized as the heat advances in
the kiln and will be driven off entirely before the burning is
finished.
It is possible that we may get a little kiln scum in this way,
but the greater part of the scum of this character probably comes
from the sulphuric acid deposited on the bricks, and the lime.
from the ash mechanically carried over and acted upon by the
sulphuric acid to form lime sulphate. We have shown how sul-
phuric acid is formed from the gases, and also that it may be
deposited on the bricks at any temperature below 680 degress F.
If the bottom of a kiln is wet and cold and heats up slowly,
the difference in temperature between the top and bottom of
34
SCUMMING AND EFFLORESCENCE.
the kiln may be quite marked. The sulphurous gas, and moisture
from wet fuel, will form sulphuric acid which will collect upon
the surface of the ware in the colder parts of the kiln. At the
same time, and especially during the stirring of the fires more
or less ash will be carried over, and will be collected by the sul-
phuric acid to be converted into sulphates, etc., and be burned
into the bricks and remain as a permanent coating.
J
Our first experience with it was in a continuous kiln which
was not underdrained and was in consequence very wet. The
draft carried at that time was very strong and a great deal of
ash was carried over. The conditions were favorable for the
formation of this kiln scum, and we had abundance of it. The
same clays burned in down draft kilns were not coated.
In another instance, we got it in down draft kilns, but only
during bad weather when the fuel was wet and dirty.
Such kiln scumming is not common, and can be readily over-
come by having properly constructed kilns, and using only clean
dry coal.
EFFECT OF SCUMMING ON CLAY MARLS.
While not properly scumming, yet the discolorations which
come on limey clays, are sufficiently closely related to warrant a
brief description.
Clays containing high percentages of lime, notwithstanding
they may contain sufficient iron to burn a red color, usually burn
to a greenish yellow color, of which the Milwaukee brick is a
notable example.
Under fire the lime carbonate is decomposed, the carbonic
acid driven off, leaving the caustic lime. This combines with
the iron and with silica to form a lime iron silicate, which is light.
colored. Now in the presence of sulphuric acid formed from the
fuel gases and the moisture, the lime is converted into lime sul-
phate.
The sulphate of lime is not readily decomposed by heat,
especially under oxidizing kiln conditions, and this mineral does
not readily combine with the iron, and in consequence the latter
is left free to color the bricks red. Kiln conditions are constantly
varying, and the bricks and parts of bricks are differently
affected, and the result is a lot of bricks irregular in color,
!
SCUMMING AND EFFLORESCENCE.
35
streaked with red, dirty red, yellow, etc. In appearance they
are worse than a kiln scummed red brick, and are unsalable.
The sulphate of lime is decomposed in a reducing atmos-
phere, and the sulphur driven off. The procedure to get clean
buff colors would therefore be to alternate the fires from oxidiz-
ing to reducing conditions in order to keep down the amount of
sulphate of lime, and give the caustic lime opportunity to com-
bine with the iron and pass into silicate formation.
RECAPITULATION.
Soluble sulphates in the clay may be decomposed and rend-
ered harmless by the use of barium carbonate, or other salts of
barium, and the water used in manufacture may be purified in
the same way.
Where the sulphates in the clay are not excessive, the car-
bonate of barium will remove them, or rather decompose them,
but where they are excessive, and probably not all in solution,
the barium carbonate will only remove those in solution, and only
to the extent of its opportunity to go into solution itself, and
act on the sulphates.
Increasing the quantity of water gives the barium better op-
portunity to go into solution, and if the sulphates are not cor-
respondingly increased, the barium carbonate may suffice to cor-
rect the evil. Where the salts are excessive and more than suffi-
cient to saturate the water, we cannot expect barium carbonate
to succeed, and it is of no use to increase the amount of the
barium carbonate. Such clays can only be corrected by a slum-
ming process with barium carbonate, which will bring all the
sulphates into solution and give the barium oportunity to pre-
cipitate them. Where such slumming is not practicable, the so-
lution of the difficulty must depend upon the use of more soluble
salts of barium, as the hydrate, chloride, oxide, or bi-carbonate.
Where the scumming is due to sulphur gases introduced into
the drier, the simplest method of overcoming the difficulty would
be to eliminate these gases. The waste heat from burning kilns
may be used to heat up properly constructed stoves, through
which pure air may be drawn and heated before being taken into
the drier.
In the drying process long, slow drying has greater tendency.
36
SCUMMING AND EFFLORESCENCE.
to produce scumming, and all ware should be dried as quickly
as possible.
When the scumming is due to the pyrite in the clay, the sul-
phur can be roasted out by careful burning and the sulphates
be either prevented or destroyed by reducing action. The danger
from pyrite is largely past when the water smoking is finished
and if the kiln be properly constructed, practically all the water
smoke should be off before the sulphur in the pyrite is started.
The kiln scum due to wet, dirty fuel can be prevented by
the use of better fuel and keeping it dry, and by having the kiln
properly drained and dry. We, in America, do not appreciate
the value of underdrainage and insulation of the kiln walls and
bottom from ground moisture, and when we come to realize the
loss on this account, we will build better, and get better results
with less fuel.
A German has a patented method of getting rid of scum-
ming, which consists of coating the bricks as they come from the
machine with a prepared mixture, something similar to dextrine,
with perhaps some barium in it, carbolic acid or some other anti-
septic perhaps sometimes some coloring matter. The evaporation
during the drying takes place through this coating, and any salts.
brought to the surface, come into this coating, and any deposits.
due to the kiln or drier gases likewise adhere to this coating.
During the burning this coating burns off, or to such extent that
it can readily be removed, and with it the soluble salts are re-
moved. The method, so far as we know has not been used in
America, but it has been used in Germany successfully on badly.
scumming clays.
EFFLORESCENCE ON THE WARE AFTER IT IS BURNED.
The efflorescence which appears as a white, yellow or green
coating on bricks or other burned clay products after they are
burned and removed from the kiln and exposed to atmospheric
agencies, has been the subject of much discussion. In this coun-
try no systematic work has been done toward investigating this
trouble, but the Germans have taken up the question, as previously
mentioned, and have done a great deal of work. The work is
far from complete and the conclusions reached are not satisfac-
tory. They began the work with the publication of a series of
SCUMMING AND EFFLORESCENCE.
37
it w
questions, which were sent out to interested parties to get their
opinions. The questions all relate to efflorescence, and it will
not be out of place to give a translation of them here.
I.
What does one understand by efflorescence?
2.
Of what does the efflorescence consist?
How does it originate?
4. When does the efflorescence ordinarily show itself?
5. What is made accountable for the efflorescence?
6. What material as a rule appears as the basis of the efflor-
escence?
7. How is it to be ascertained what building material is re-
sponsible for the efflorescence?
8. Does clay ware which contains no soluble salts show it?
9. What remedy will render harmless the injurious influ-
ence of the soluble salts in the clay?
IO.
How can the water used in the manufacture be of in-
Aluence?
II.
What influence has the degree of burning?
I2.
What influence can the place of storing have?
13. What influence can the mortar or lime have upon the
origin of the efflorescing solutions?
14. In what manner is the origin of the salt in lime brought
about in the burning of the lime?
15. When is the sand used in the mortar responsible for
the efflorescence?
16. Can treatment of the mortar affect the occurrence of
the efflorescence?
17. Is the efflorescence harmful and in what way does it
exert its influence?
18. How is it to be explained that in the spring many build-
ings are coated with white efflorescence which soon disappears?
19. How can the efflorescence appearing on the walls be
removed?
20.
How can the efflorescence be prevented in the construc-
tion of a building?
The method of determining whether a block of clay ware
will effloresce is simple, and with some modification upon the
German method will give satisfactory results. A glass bottle
or flask is filled with distilled water, a piece of filter paper placed
over the open end and the bottle inverted and placed on the
38
SCUMMING AND EFFLORESCENCE.
piece of clay ware to be tested. As the water is absorbed by
the brick, air is allowed to enter the bottle. The water enters
the clay body and takes up any soluble salts which are brought
to the surface as the water evaporates. In the wall, however, the
evaporation is only from one face, and in making tests, if we
coat the test piece with parafine, except one face, so that all the
evaporation will be from that face, we will get wall conditions.
Mortars, sand and such materials can be tested by putting them
in the bottle, filling the bottle with distilled water, and testing.
upon a piece of clay ware which has been previously tested for
soluble salts, and found to show no efflorescence.
We shall not attempt to review the German work in detail.
Full discussions of it are to be found in the Ton-Industrie
Zeitung, and in the papers by Dr. Seger, E. Cramer, Dr. Mäckler,
Dr. Günther and others.
The conclusions reached are far from satisfactory, and
much work remains to be done. The general opinion of the
members of the Society for the Clay, Lime, and Cement In-
dustries, was that hard burning would prevent the efflorescence.
This question was made the subject of exhaustive research by
Dr. Mäckler, but unfortunately the work was confined to clays
from near Berlin, most of which do not effloresce. It is to be
regretted that clays from other districts were not included in the
tests, especially the coal measure clays, in order to make com-
parison and arrive at more confirmed conclusions.
Of seventeen clays tested, only three showed any efflo-
rescence. Of these one efflorescenced in soft, medium and hard
burned samples. Of the other two, only the soft burned samples
effloresced. One foreign clay was tested and it effloresced badly.
Only three of the hard burned samples tested showed less than
10% of water absorption. All of the others ranged from 10%
in the hard burned samples to 29% absorption in the soft burned.
The conclusions drawn from these tests are as follows:
I. The percentage of porosity, (absorption) is no measure
of efflorescence.
2. The percentage of soluble salts is no proof of the ten-
dency to effloresce. Both porosity and soluble salts are essential.
conditions for efflorescence, but the samples showing the efflores-
cence were not the most porous, nor did they contain the highest
percentage of soluble salts, nor vice versa.
SCUMMING AND EFFLORESCENCE.
39
}
3. Sulphate of lime, gypsum, — is not the most trouble-
some salt in efflorescence, though it is found in all the samples
tested, and in one case cited by E. Cramer, the efflorescence
consisted of sulphate of lime entirely.
4. The more soluble salts, magnesium sulphate, and the
sulphates of the alkalies, are the salts which are largely the
cause of the efflorescence.
5. That in hard burning, the sulphates are decomposed
and taken up into silicate formations, and become thereby harm-
less.
It must be noted that in every analysis of the burned
samples, soluble salts were found, silica, alumina, iron, sul-
phate of lime, sulphate of magnesium, and sulphate of the
alkalies.
The writer had the opportunity to study this question a
number of years ago, and arrived at the conclusion that hard
burning was the best method of overcoming the trouble, and
where the salts in the clay were present as sulphates, the burn-
ing must be carried to an extreme limit, to be effective. An
analysis showed the efflorescence to be largely sulphate of the
alkalies and these sulphates came from the clay. The mortar,
sand, and backing up brick had very little influence. We must
admit that there may be cases of efflorescence directly traceable
to the mortar, sand or backing up bricks, but we think they are
comparatively rare. A notable exception, however, must be
made of many of the sandstones used in connection with bricks
or other clay ware in building.
Many of our sandstones are impregnated with salts. They
were laid down from seas from which were also deposited beds
of limestone, of gypsum, of common salt, etc., and the sandstones
are reservoirs of the water supply, which contains many mineral
salts.
Our oils and natural gas come largely from sandstone beds.
The gas will be in the higher levels, the oils at a lower level,
and the so called salt water, below the oil. The Berea grit in
Ohio is an excellent building stone. Below cover, it is in many
places the gas rock, at others, the oil rock as at Macksburg, O.
At other places it is the salt (sodium chloride) rock and has
been pumped for many years for this mineral, notably at
Pomeroy, Ohio. When exposed on the out crop, the water runs
40
SCUMMING AND EFFLORESCENCE.
away and evaporates, leaving behind considerable soluble salts.
In the early days it was customary to quarry the rock some time.
before it was required for the building, giving the salts time to
weather out to the surface, and when the stone was dressed for
use, the salts were removed with the surface rock dressed off,
but now we use the rock direct from the quarry, and the salts
go into the building with the stone. The writer has examined
a great many buildings, and in the majority of cases, a dirty scum
is found under the sandstone trimmings. Part of this is due to
the dirt which has collected on top of the stone and washed down,
but much of it comes from the salts in the stone which have
leached out into the bricks below, and come to the surface of
them as efflorescence. We have seen so many buildings ruined
in this way, that it is a matter of surprise to us that architects
have taken no steps to overcome it. The use of stone looks well
in the architect's plans, but if the owners could look beyond the
plans and see the stone in the building after a year or two, much
of the stone work would be cut out. We have recommended for
a number of years the use of a thin sheet of copper under all
stone courses, with the edge carried out slightly beyond the
mortar joint and turned down to form a drip. This will effec-
tually put a stop to the salts finding their way into the bricks
below, and with a suitable drip cut in the stone where it projects,
to carry away any washings from the top of the stone, our build-
ings would be much improved in appearance.
Efflorescence comes from several sources, and a full dis-
cussion of it, would lead us away from the causes in which clay
workers are interested, and much of it would be of no value
except as a technical study. We will therefore confine ourselves
to the more common causes.
1
WHAT IS EFFLORESCENCE?
Efflorescence is a white, green, or yellow coating which ap-
pears on the ware after it is removed from the kiln and stored
on the yard, or laid in the wall. It consists usually of soluble
sulphates of magnesium and the alkalies. It also contains sul-
phate of lime in varying percentages, and there are exceptional
cases in which it is made up entirely of sulphate of lime.
We are little concerned with the hydrous sodium carbonates
1
SCUMMING AND EFFLORESCENCE.
41
3
which may be formed by the action of the sea air, nor with the
nitrates which may be formed from ammonia in the air, ab-
sorbed from the ground, or caused by bacterial action.
The question may be asked why is it that sulphate of lime
is the prominent salt in scumming and of little consequence in
efflorescence.
Sulphate of lime is not as readily soluble as the other sul-
phates, and efflorescence and scumming is a question of solu-
bilities and volatilization. If the clay contains these sulphates,
or the conditions are present for their formation, the water in
the clay will contain them; the water used in manufacture may
also contain them; the quantity of water used in making a mud
brick is greater than a well burned brick can contain; slow dry-
ing in a hot atmosphere gives excellent opportunity to fully satu-
rate all the water in the clay. In consequence there will be much
more of the soluble salts brought to the surface of a green mud
brick than can come to the surface of a well burned brick, simply
А
· because there is a greater amount of the solution and it bids fair
to be fully saturated. This means that proportionately more sul-
phate of lime comes out. During the burning the more volatile
salts are driven off leaving the lime sulphate as scum. In the
burned bricks, the salts are in a dry state, partly combined into
weak silicates, perhaps, or included within a fused matrix. The
amount of water which the brick can take up is limited, and in
view of their combination and enclosing insoluble films, the salts
are not readily available for solution. These conditions are not
favorable for a large amount of salts to pass into solution at any
one opportunity, and naturally the more soluble salts will pre-
dominate, and as there is no volatilization, the salt remains on
the surface as efflorescence until washed away.
EFFECT OF SULPHATES IN THE CLAY.
We have called attention to the fact that sulphate may go
through the burning unchanged. Under reducing conditions and
hard burning, the quantity may be reduced, and in many cases.
rendered harmless. Under oxidizing conditions, in the presence
of sulphur gases, the amount of sulphates in the clay may be
materially increased and a clay containing none at all may come
out from the burning with appreciable percentages. The sul-
42
SCUMMING AND EFFLORESCENCE.
phate of lime is subject to decomposition more than the sulphates
of the alkalies, and this together with its lesser solubility may
account for the fact that it is not an important feature in efflor-
escence.
The sulphates of the alkalies are not readily decomposed
by heat but may be volatilized. That some decomposition takes.
place, in connection with the chemical reaction going on within
the clay body during the burning, or bonding period, must be
admitted, yet we believe that microscopical examination of the
burned clays will show in the fused or semi-fused matrix, in-
cluded particles of the alkali sulphates, and these need only the
proper conditions in the wall, to come to the surface.
Moreover, it seems to us reasonable that when sulphates
are decomposed in the burning, and their bases passed into
silicate combinations, that such silicates are frequently not of
weather resisting character, and sooner or later under the action
of atmospheric agencies, will be broken down and their bases
set free.
We have ample illustration of the destructive effect of the
atmosphere in the soft burned bricks which disintegrate in the
wall.
Such bricks may come from the kiln in perfect form, and
only the sorter, who has learned by experience, can tell when the
brick is too soft for the market. No sooner is the brick exposed
to the weather, than the destructive influences of the atmosphere.
are at work breaking up the silicates that form the bond, just
as the same agency broke up the silicates in the original rock to
form the clay. A soft brick may resist a week, a month, a year,
or many years, but eventually it will go to pieces and in doing
so will set free the bases which blossom out as efflorescence.
EFFECT OF SULPHATES ON THE BOND IN THE CLAY.
The bond in burned clay ware, which makes the ware inde-
structible is brought about by the decomposition of some of the
minerals in the clay and the combination of their bases with
silica to form silicates. Some of these minerals decompose at low
heats and the fusible silicates formed surround the undecomposed
minerals, binding them together. As the heat advances, other
minerals are decomposed, additional silicates are formed, thus
SCUMMING AND EFFLORESCENCE.
43
increasing the percentage of fusible material to serve as bonding
material. These silicates are constantly readjusting themselves
as the heat advances and as more bases are brought into use.
The minerals in the clay decompose at different heats.
Magnesium carbonate begins to decompose at 509 degrees
F. and its decomposition is complete at 950 degrees. Calcium
carbonate does not form basic carbonates and its decomposi-
tion takes place at 1537 degrees F. This sudden decomposition
of calcium carbonates explains why limey clays are not safe.
. If there is sufficient fluxing material in the clay to bond the
body at a temperature safely below the decomposition of the
limestone, the clay can be burned successfully, but if the tem-
perature is advanced to the temperature at which the limestone
decomposes, it means the sudden introduction of the oxide of
calcium, which is a strong flux, and the mass becomes viscous
in consequence of the amount of silicate thus formed which is
fusible at this temperature.
We have shown the presence of sulphates in the clay, and
also that they may be formed during the burning.
Calcium sulphate can only be decomposed at high tem-
peratures.
This will occur during the latter part of the burning under
increased chemical reaction, perhaps because of some changes
in the firing, and the calcium, or other bases from the sulphates
decomposed, enter into the silicate bodies already formed. This
means a further readjustment with the absorption of additional
silica and alumina, otherwise unstable silicates are formed, which
given time and exposure, will decompose.
It must be remembered that the sulphates may not be uni-.
formly distributed through the clay mass, but are often found as
crystals and particles here and there, and when these decompose,
the silicates in the immediate vicinity are affected by the excess
of flux.
Efflorescence comes not only from a gradual leaching out
of the soluble sulphates in the clay which have gone through the
burning, but also from the breaking down of the more imperfect.
silicates, and the combination of the bases with sulphuric acid.
from the atmosphere or from any remaining pyrite in the burned
ware. In this way we may explain why bricks may be exposed for
44
SCUMMING AND EFFLORESCENCE.
1
a long time without any sign of efflorescence, and then suddenly
blosom out with it.
PYRITE IN THE CLAY.
We have shown how pyrite oxidizes to form sulphates, and
when so oxidized ceases to exist. When not oxidized it is broken
up in the burning, the sulphur being roasted out.
It is doubtful if any ordinary burning will remove all the
sulphur, particularly under oxidizing conditions, and we may
have many wares coming from the kilns containing pyrite, or
rather a lower sulphide of sulphur. Under atmospheric action,
this is decomposed and oxidized into ferric oxides and sulphuric
acid, and the latter will have great effect in breaking down the
silicate bodies, and will take up the bases to form sulphates.
The sulphurous gas in the atmosphere of our smoky cities
may play the same role as the sulphur from the pyrite, and we
may have efflorescence, even though the ware contains no sul-
phates, nor pyrite, but in which the bond was made up of weak
silicates.
POROSITY.
The porosity plays an important part in efflorescence.
If a brick is vitrified so that it cannot take up any water,
no sulphates can come to the surface. On the other hand, the
brick may be so porous that the sulphates will be deposited below
the surface. The German investigations show that neither the
ware that contained the most soluble sulphate, nor that which
took up the most water, was necessarily the one which showed
efflorescence.
The almost universal presence of sulphates or sulphate
forming conditions in the ware may lead one to the belief that
all wares must effloresce sooner or later, but when we con-
sider the fact that there must be sufficient sulphate present, or
formed, to make a visible coating, that sufficient water must
enter to take up the sulphate and bring it to the surface, that
there must be suitable porosity to get the full effect of the salts
in solution, and that the absorption requirement of the ware
must be satisfied before any salts can come to the surface, we
understand why efflorescence is not as universal as analyses of
clay wares would lead us to expect. Let one of these condi-
SCUMMING AND EFFLORESCENCE.
45
tions fail and the wares will not effloresce, or perhaps only here.
and there a piece, as we often see single bricks in a wall.
ABSORPTION.
A study of absorption gives us some very interesting facts
in regard to efflorescence. If we take a piece of burned clay
ware, and place it in a vessel in the bottom of which is a salt
solution, let us say a colored solution, the first result will
be a dissociation of the water and the salt solution, and com-
paratively pure water will be taken up by the ware.
After a
time the colored solution will begin to be taken up. A certain
per cent of the solution must be absorbed by the ware, be-
fore the salt solution can advance. Some experiments along this
line gave us the following data: Under like conditions a strong
solution will advance faster than a weak solution; with bodies
of different porosities, the salt solution will advance faster in the
more porous body; hot solutions will advance faster than cold.
1
An interesting experiment is to take the piece of ware from
the salt solution after the latter has risen, say one half inch, and
place it in clear water. The salt solution will continue to ad-
vance, and we have a band of clean water at the top which was
dissociated from the salt solution, then a band of the colored
salt solution, and below a band of clean water.
We have not the same conditions in our walls that we had
in the above experiments. In the latter, we were dealing with a
dry clay ware body, and a central solution. In walls, we have the
solution distributed through the clay ware. They collect in the
cavities in the ware, and are strongest in proximity to any
soluble, or decomposing, minerals which supply the soluble salts.
Thus far, at least, the conditions are similar.
The first result in the drying out process will be some dissocia-
tion of the water, which will be drawn to the surface and evapo-
rated. This will be followed by the salt solutions drawn from
the cavities in the ware, into the capillary tubes or channels.
and these salt solutions can only appear on the surface after the
absorption requirement of the capillary tubes have been satisfied.
So long as the cavities in the ware contain any solutions,
just so long will the salt solution continue to advance to the
46
SCUMMING AND EFFLORESCENCE.
surface. When the cavities are drained, the advance ceases and
the salts lining the capillary tubes remain in the ware.
We are of the opinion that further drying is due to a pro-
gresive vaporization of the water from the solution beginning
at the surface and gradually extending to the center of the clay
body. It is impossible to get clay or clay ware absolutely dry
at normal temperatures. There always remains some moisture
in the ware. As we increase the temperature, we get off more
and more of the moisture, but only at the boiling point, or
slightly above, can we drive off all the moisture, and at this
temperature, it is certainly true that the moisture is driven off
by vaporization within the clay body, progressing to the center
with the temperature.
Our last experiment with the piece of clay ware and the
colored salt solution, in which the ware was removed from the
salt solution and placed in clean water, after some of the salt
solution had been absorbed, illustrate nicely the conditions we
sometimes get in walls, and explains many ugly blotches on the
wall due to excessive efflorescence.
If, in any manner, water can get into the back of a brick,
and continue to come, as the evaporation proceeds from the sur-
face of the brick, it will carry all the salt solution in the brick to
the surface, and leave it there as efflorescence. Not only this,
but any salts which may come in with the water, will be carried
to the surface. Porous sandstones are often used for copings,
sills, etc., and these are good reservoirs of water.
The water soaks down through the brick wall, and passes
off by exaporation from the surface of the wall, carrying to the
surface any soluble salts in the wall. Even with pure water
there might be efflorescence by carrying forward the salts in the
clay, which otherwise would be insufficient to satisfy the absorp-
tion requirement of the clay body, but as we have shown, sand-
stones often contain soluble salts, which serve to intensify the
evil.
The same applies to moisture absorbed from the ground and
leached out through a wall surface. Pipes in the wall, which
may leak, or upon which the moisture in the air may condense
and be taken up by the brick and carried to the wall face and
with it any salts the wall may contain, are oftentimes the cause
of the trouble.
16
SCUMMING AND EFFLORESCENCE.
47
Chimneys, which are often the most beautiful feature of a
structure, are particularly subject to efflorescence.
They are frequently capped with stone, which adds its quota
to the trouble. When cold, the rain gets on both sides of the
wall, and there is very little evaporation from the inside.
In drying out, the moisture within assists in carrying the
salts to the outside, thereby increasing the natural tendency of
the brick to effloresce. When in use, the sulphuric acid from the
combustion gases is constantly eating its way into the wall, break-
ing down the mineral structure, and setting free the soluble salts
to be taken up by the water, when opportunity presents itself.
These problems are not for the clay worker, but for the
architect and builder, and many of them are so simple that there
is no excuse for the disfiguration of the walls built of clay ware.
Capstones should be made impervious, or be laid in au im-
pervious bed. Walls should be insulated from the ground
moisture.
All copings, sills, and projections of whatever material.
should be provided with proper drips. Chimneys should be lined
with fireproofing, and not be built with a single piece stuck out
of the top for appearance sake, as we have often seen. The
lining should be kept away from the wall and stayed by pro-
jecting bricks, but even when carelessly built, it is our exper-
ience that flue lined chimneys are not subject to much efflores-
cence. Wall pipes should not come in contact with the wall,
and the drip or leak from them should not fall upon the wall.
We do not see any difficulty in such construction, and builders.
should insist upon such specifications by the architect and such
construction by the contractor.
EFFECT OF HARD BURNING.
The value of hard burning to prevent efflorescence is too
evident to need description. The harder burned the bricks are,
the better the bond and the more permannt the silicates forming
the bond. The sulphates are decomposed or sealed up. The sul-
phur in the pyrite is roasted out more completely, and the density
of the body is increased. The density prevents the access of the
atmospheric water, and delays in proportion the decay of the
silicates, and the taking up of the sulphates.
48
SCUMMING AND EFFLORESCENCE.
1
BACKING UP BRICKS, MORTAR, SAND, ETC., USED IN BUILDING.
We hear a great deal about the effect of the mortar, lime,
sand, and backing up bricks in producing efflorescence, but we
think the effect has been greatly over-estimated. Years ago it
was customary to lay all the efflorescence to these materials.
When some manufacturers succeeded in making a brick
which they would guarantee not to effloresce, while their competi-
tor's bricks laid in the same mortar would effloresce, there was a
change in opinion in regard to the effect of the mortar, etc.
Theoretically these may be the cause of efflorescence, and
occasionally are, in practice, but the occasions are very rare.
In our own experience, with a brick made from a clay from
which we might get efflorescence, and could get it if desired,
we do not recall a dozen instances in as many years in which
efflorescence came out, yet these bricks were shipped all over the
country and laid up in all kinds of mortar, sand, and with all
kinds of backing up materials.
Caustic lime is soluble, and with a freshly made mortar, and
with wet bricks, there may be more or less diffusion of lime into
the bricks. The chances are that this lime will be converted into
carbonate and remain in the body of the brick, but it is possible.
that some of it may find its way to the surface, and there become
a carbonate and remain as a coating on the face of the bricks.
When the building is cleaned down with acid, this coating
wil be removed, and there will be little danger of its return.
Greater danger comes from the sulphates in the limestone,
or formed during the burning of the lime, and especially cements,
many of which contain as high as 3% of calcium sulphate. Such
sulphates might find their way into the bricks by diffusion, and
later come out as efflorescence. Sand likewise may contain
soluble salts, and these may find their way into the bricks. Back-
ing up bricks, which are often soft bricks, and if exposed would
effloresce badly, might under favorable conditions pass their sul-
phates into the face bricks, but we are inclined to think that in
the majority of cases the backing up brick will dry out faster
from the inside, than from the outside, and the tendency would
be to take the solutions away from the face bricks rather than
toward them. Whatever the case may be in regard to the drying
out of the wall, the fact remains that we have found that the
+
SCUMMING AND EFFLORESCENCE.
49
backing up bricks do not affect face bricks which do not them-
selves effloresce.
HAS EFFLORESCENCE ANY INJURIOUS EFFECT ON THE WALL?
The salts which appear as efflorescence undoubtedly have
influence in breaking down the structure of the ware.
Chemically, they increase the solvent action of the water,
and mechanicaly they tend to disrupt the bond, through crystalli-
zation of the salts from saturated solutions. Calcium sulphate
is more soluble in cold than in hot water, and any increase in
the temperature will cause crystallization of some of the salt.
The sulphates of magnesium and of the alkalies are more
soluble in hot water and any lowering of the temperature causes
crystallization. Silica and silicates are more soluble in alkaline
solutions than in pure water, and also more soluble in hot solu-
tions than in cold ones.
Geologists use these facts in discussing the disintegration.
of rocks, and the discusion applies equally well to clay ware,
but geologists have ages for the accomplishment of the work,
while we need consider the question only from the standpoint of
years of time. If our ware is well burned, efflorescence will not
destroy it, but if it is soft burned, it will go to pieces and efflor-
escence is only one of the factors in its disintegration.
EFFECT OF STORING BRICKS ON THE YARD.
On acount of the ashes which are frequently used in filling
around a brick yard, and the storing of piles of coal, the ground
is often saturated with sulphates. If bricks are piled direct upon
such ground they will absorb these sulphates, which will come
out as efflorescence. This is very common, and may be seen on
almost any brick yard. As a preventive, the bricks were piled
upon strips of wood leaving an air space under each bench of
brick, and in such piling, we had no further trouble.
HOW CAN EFFLORESCENCE BE OVERCOME?
If the clay contains pyrite and no sulphates, much of the
sulphur may be roasted out by hard burning, and the danger of
50
SCUMMING AND EFFLORESCENCE..
efflorescence greatly reduced. In our practice, we rejected all
crop clays, and discontinued weathering. The hard clays from
under cover were crushed, ground, screened, steamed, reground,
and screened, and put into the kiln as quickly as possible, and
hard burned, but not in this case vitrified in any sense of the
word. The result was a brick which would not effloresce. Many
plants are using No. 2 fire clay, and such plants can drop out the
crop clay, and use only the unoxidized mine clays, putting them
through a rapid disintegrating process to take the place of
weathering.
SULPHATES IN THE CLAY.
Plants using surface clays, perhaps shales, and such bedded
clays as lie near the surface like many of the Tertiary clays, have
not the advantage of dropping out the clays containing sulphate,
and in consequence will have greater difficulty in overcoming the
trouble.
If the clays are not heavily loaded with sulphates, hard
burning will remove the trouble in a large measure. If the pro-
ducts will allow a reducing kiln condition, the decomposition of
the sulphates will be hastened, and opportunity given for the
bases to pass into silicates. The hard burning at the same time
decreases the porosity and after a certain stage is reached, any
decrease in the porosity decreases the tendency to effloresce.
When bricks are vitrified and practically non-absorbent, the
danger of efflorescence is eliminated.
We do not know that any attempt at chemical or mechanical
treatment has been made to get rid of efflorescence.
Many products will not stand much expense in the way of
such treatment, but we believe much of the trouble could be re-
moved by preliminary treatment and the cost need not be ex-
cessive.
A
Washing the clays by slumming or blunger process need not
be too expensive for many products,- terra cotta, roofing tile,
etc., and in addition to the removal of much of the sulphates,
such treatment would put the clay in fine condition for working.
The difficulty in this treatment would be to get water free
from sulphates with which to do the washing. In this work the
use of barium carbonate would be of benefit. If we convert
all the sulphates into carbonates, with the resultant insoluble
SCUMMING AND EFFLORESCENCE.
51
barium sulphate, there can in the first place be no scumming, and
if the watersmoking is done with wood, there can be no sul-
phates formed.
The carbonates would be broken up by the burning and go
into silicate formations, and could not give serious trouble in
efflorescence. If we can get the fluxes into shape to pass readily
into silicate form during the burning, and properly adjust them-
selves, we will have a relatively permanent bond, and the de-
structive influence of the atmosphere will be so slow as not to
set free enough of the bases to form at any one time sufficient
sulphate to make a visible coating.
Barium is not decomposed at ordinary kiln temperatures to
any great extent and will go through the burning unchanged.
Recent experiments with barium carbonate show that the
temperature at which the carbonic acid is driven off is about
2,600 degrees F. Any burning at or above this temperature will
convert the carbonate into oxide, in which form it is an active
flux, and combines with silica. Any barium carbonate which
might go through the burning, would be taken up to the limit
of its solubility by the water entering the ware in the wall, and
to this extent, the soluble salts which cause efflorescence would be
rendered harmless, but the effect would be small on acount of
the limited amount of barium required to saturate the solution.
We do not believe that an extreme case of efflorescence can
be overcome in this way. Hard burning, permanent silicates,
and low porosity, and preliminary treatment of the clays, must
be the corrective in such cases.
PRELIMINARY TREATMENT OF THE CLAYS.
We have discussed this in several places in the fore-going
pages, and will not repeat it, except to say that we believe much
can be done by the slumming process at comparatively little.
cost.
Pyrite can be removed by strong electro magnets, but this
involves fine grinding, and the cost of getting rid of the pyrite
in this way would be excessive.
Pyrite may also be removed by fine grinding followed by
air separation, but this method is also expensive, and not ap-
plicable to the wares in which efflorescence is a troublesome
factor.
5
*
THE USE OF BARIUM COMPOUNDS IN
PREVENTING SCUM.
BY ELLIS LOVEJOY, COLUMBUS, OHIO.
(Transactions American Ceramic Society, Vol. VIII.)
Barium as a preventative of "scum" has been used many
years, but there is so little literature on the subject that many do
not understand its use, how it acts on the salts in the clay, why
it frequently fails, especially beyond a certain limit, and some
doubts exist as to its efficacy. Theoretically barium is a perfect
preventative of "scum," but in practice it often fails.
The purpose of this paper is to call attention to lines of in-
vestigation which the writer believes would give valuable results.
in the use of barium to overcome efflorescence, nor are the lines
suggested by any means a complete list.
Seger in his paper, "Concerning the Addition of Barium
Compounds to Clay," says: "With a content of 0.1% of calcium
sulphate in clay there will be required 0.127%. of barium car-
bonate," or "0.170% of crystallized barium chloride." In the
following paragraph, the solubilities of some of the salts in-
volved in the question of scumming are given :
Calcium sulphate, CaSO,, 2H,O, is soluble in 386 to 451
parts of water according to the temperature.
Ferrous sulphate, FeSO4, 7HO, is soluble in 1.64 to .27
parts of water.
Magnesium sulphate, MgSO4, 7H2O, in 3.17 to 1.25 parts.
Potassium sulphate, K,SO,, in 10.31 to 3.82 parts.
Sodium sulphate, Na,SO,, 10H₂O, in 4.34 to 0.32 parts.
2
In practice, we do not have to contend with any such quan-
tities as these solubilities represent, except, perhaps, in the case
of calcium sulphate. That the water which we use in our clay
work may be fully saturated with calcium sulphate when it enters
the clay, or become so before leaving it in the dry-house, cannot
be denied. Calcium sulphate is the chief source of our trouble,
and we shall confine our discussion to it.
(52)
(
SCUMMING AND EFFLORESCENCE.
53-
24
1
In passing, however, we wish to say that it would be
valuable to determine what effect, if any, solutions of these other
salts would have upon the solubility of calcium sulphate in con-
centrated solution, and upon its corrective, barium carbonate.
It is possible that the soluble salts resulting from the reaction
of these with barium carbonate may affect the solubility of the
latter, and explain its sometimes erratic behavior in the preven-
tion of "scum."
EFFLORESCENCES CLASSIFIED.
There are three distinct lines of efflorescence, and we must
confine this paper to one of them.
The efflorescence which appears on the material after it is
laid in the wall is a problem in itself, and whether any additions.
of chemical compounds to the clay can prevent it, students of
chemistry must determine for us.
Kiln white, a much rarer phenomenon, concerning which we
have little data, seems to be outside of any possible effect of
chemicals in the clay.
Dry-house white, the common "scum," which appears on the
material in the drying stage and continues unchanged through
the burning, is usually calcium sulphate, and to it we will con-
fine our attention.
DRY-HOUSE WHITE.
If we assume that a dry pressed brick contains eight ounces
of water, and that sixteen bricks can be made from one hundred
pounds of clay, then we have in one hundred pounds of clay
prepared for this process, 128 ounces of water.
This, since calcium sulphate is soluble, in round numbers,
in 400 parts of water, will dissolve 128/400 ounces of calcium
sulphate, or 0.32 ounces.
The equation for the precipitation of calcium sulphate by
barium carbonate is as follows:
BaCO3 + CaSO,, 2H,O= BaSO, + CaCO3 + 2H₂O.
By weight, 172 parts of calcium sulphate require 197 parts
of barium carbonate, or one part of calcium sulphate requires.
1.15 parts of barium carbonate. The amount of 0.32 ounces of
calcium sulphate, which may completely saturate the water in
one hundred pounds of dry press clay, will require 0.32 X 1.15
1
54
SCUMMING AND EFFLORESCENCE.
0.368 ounce of barium carbonate to satisfy the requirements
for complete decomposition of the calcium sulphate. Barium
carbonate requires 14,137 parts of water for solution. One hun-
dred and twenty-eight ounces of water will therefore take up
128/14137 ounces of barium carbonate, or 0.009 ounce. If
0.368 ounce of barium carbonate are required to precipitate 0.32
ounces of calcium sulphate, then 0.009 ounce of barium carbonate
will precipitate 0.008 ounce of calcium sulphate. Thus, 0.008
ounce is 21% of the amount of calcium sulphate which may be in
solution in one hundred pounds of clay prepared for dry pressed
bricks, and this is the initial value of a solution of barium car-
bonate.
The result of the initial decomposition is the formation of
barium sulphate and calcium carbonate. Barium sulphate is
soluble in 400,000 parts of water, and calcium carbonate in
28,000 parts, so that all the barium sulphate and practically ali
of the calcium carbonate formed, is precipitated. With this pre-
cipitation, more barium carbonate may pass into solution, fol-
lowed by the reaction and precipitation until one or the other of
the compounds is exhausted. Theoretically, therefore, we need
only add enough barium carbonate to precipitate the salts which
cause the "scum," and this seems to be the prevailing opinion.
in practice.
If we were dealing with these compounds suspended in
water, we might expect the solution and reaction to continue to
the end, but in a block of dry pressed clay, we have very little
circulation of the water, and cannot get the results which might
be obtained from a free circulation. There is room for con-
siderable divergence of opinion in regard to the possible value
of an excess of barium carbonate in the water used in dry pressed
bricks, but the writer holds that there is little value in any excess
beyond the initial saturated solution, after, of course, the pre-
cipitation of any calcium sulphate in the water prior to its ad-
dition to the clay. The water, with barium carbonate in solution
and suspension, upon being added to the clay is taken up hygro-
scopically by the particles of clay and becomes a part of the
hygroscopic water already in the clay, and the barium precipi-
tates there, having reacted with calcium sulphate up to the limit
of the barium in solution.
The barium carbonate in suspension cannot follow the water
•
SCUMMING AND EFFLORESCENCE.
55
into the particles of clay, and is left as a coating on their surface.
The water is not a circulating medium. It is temporarily fixed
in the clay particles, softening them and developing plasticity,
and when the drying stage begins, it passes off to the surface,
not, however, in all directions, as it entered, and in its passage.
taking up the barium carbonate deposited, but along certain
lines ever towards the point drying fastest. That there will be
some solution and precipitation during this stage must be ad-
mitted, but it will be quite limited. Whatever barium carbonate
may line the channels through which the water is escaping, may
pass into solution and accomplish its purpose, but the great bulk
of the barium remains as a coating on the particles of clay.
The water during its stay in the particles of clay will take
up the calcium sulphate in them to the point of saturation, so
that we haye a strong solution of calcium sulphate escaping from
the clay to the surface of the mass with very little opportunity
of bringing it in touch with sufficient barium carbonate to pre-
cipitate the sulphate. The conditions existing in a clay body are
not favorable to the solution of such a difficultly soluble compound
as barium carbonate nor to the reactions which bring about the
precipitation, but they are favorable to the solution of the calcium
sulphate which finds its way to the surface without much loss in
passage. A common practice is to add the barium in the
powdered form, in the dry pan or other grinding machinery,
and the little we may get in solution is quickly used up by the
impurities in the water, or by the sulphates in the clay, more
readily soluble than the calcium sulphate.
If the assumption that only the initial precipitation is effec-
tive can be maintained, then the addition of barium carbonate
to the dry pressed ware is of no practical value. Our efforts
should be directed towards increasing the initial precipitation by
getting stronger solutions of the barium.
A stiff mud brick contains two pounds of water, more or
less four times as much as a dry pressed brick. This will dis-
solve 1.28 ounces of calcium sulphate in one hundred pounds of
clay, which will require 1.47 ounces of barium carbonate to pre-
cipitate.
It will take up 0.036 ounce of barium carbonate which in
turn will precipitate 0.032 ounce of calcium sulphate.
The extremely low percentage, 0.02 per cent. of calcium
56
SCUMMING AND EFFLORESCENCE.
sulphate which may be in solution in dry pressed ware explains
why manufacturers of such products are less troubled with
"scum" than the manufacturer of stiff mud ware. Moreover,
the porous face of the dry pressed ware will tend to hide the
deposits of calcium sulphate which may come to the surface in
the drying stage. Increase the quantity of calcium sulphate four
or five times, and deposit it on a dense face of ware, and we
readily see why a compound which gives little trouble in the one
process, becomes a serious problem in the other.
In one factory using both processes it was necessary to treat
all the stiff mud products, but no barium was used in the dry.
press work. While "scumming" is of much rarer occurrence in
the latter, yet when it does occur it is much more difficult to
overcome.
The writer's explanation of the ineffectiveness of barium
carbonate in dry pressed work is based upon some observations
in stiff mud work. In the manufacture of stiff mud bricks, we
first ran the product quite soft, treating it, of course, with barium
carbonate in the wet pan, and had very little trouble with "scum."
With the introduction of a repress, we made the bricks stiff
enough to be repressed as they came from the separating belt,
and they quickly assumed a leather hard condition, showing the
absorption of the water by the particles of clay. These bricks
came from the dry house and kilns badly coated with "scum."
We doubled and quadrupled the quantity of barium without
effect. We cut out portions of the clay which we thought made
the trouble. We put in tanks in which the barium was stirred
up with the water before being mized with the clay, all to no
purpose.
During this time roofing tile and hollow blocks made from
the same clay were generally free from "scum," particularly the
roofing tile, but of course they were run much softer than the
bricks, so much so, in fact, that the water could actually be
squeezed out of the tile clots under the press.
As a possible explanation, it might be said, that in case the
content of calcium sulphate in the clay was insufficient to satu-
rate the water, the increase in the quantity of water would not
increase the quantity of calcium sulphate, but would simply di-
lute the solution, while at the same time the barium carbonate
would be increasing in quantity, maintaining its saturated con-
•
SCUMMING AND EFFLORESCENCE.
57
dition from the barium carbonate in suspension. In the
stiff mud we may not have had enough barium in solution to
precipitate all of the calcium sulphate, while in the soft mud.
process the precipitation was complete. This argument reads
better than it looks in figures.
Two pounds of water to each brick means 0.036 ounce of
barium carbonate in solution in one hundred pounds of clay,
which will precipitate 0.032 ounce of calcium sulphate. This
would be only 0.02%, a mere trace in commercial analysis.
Such a minute quantity will not give any trouble by "scumming."
In the series from dry pressed products, through leather-
hard stiff-mud, stiff-mud, soft-mud to slumming clay, we pass
from hygroscopic water, through a mass saturated with water
in which there would be more or less circulation of the water,
to water with clay in suspension. In the latter extreme we would
have perfect opportunity for the continued solution and preci-
pitation until the barium carbonate or calcium sulphate was ex-
hausted, but in just the degree that we decrease the circulation
of the water, we reduce the efficiency of the barium carbonate,
until in the dry pressed ware, and perhaps in the leather-hard
stiff mud, the efficiency of the barium carbonate is limited to the
initial solution, or 2.5 per cent. of the possible contents of calcium
sulphate in solution. We repeat, therefore, that our aim should
be to increase the initial value of the barium solution.
USE OF BARIUM CHLORIDE.
The use of barium chloride, BaCl, 2H,O, which is soluble
in 3 to 1.66 parts of water, is hardly to be recommended. While
it will precipitate all the sulphur and thus remove all the solu-
ble sulphates, yet any excess remains in solution and is drawn
to the surface in the drying and there deposited to appear after
the bricks are burned. Seger's advice not to use it in excess
is to be heeded. Moreover, the resulting calcium chloride is also
readily soluble and it, too, would find its way to the surface to
affect the color. Its use would be safer in connection with
barium carbonate. We could thus use less chloride than would
be required for complete decomposition of the calcium sulphate,
but yet enough of it to remove the bulk of the trouble, all of
which would be used up in the initial reaction, together with any
58
SCUMMING AND EFFLORESCENCE.
carbonate in solution, and for the decomposition of the remaining
calcium sulphate, we would have to rely upon the secondary re-
action of the carbonate, which in dry press and leather hard,
stiff clay would be of little value for reasons already pointed out,
but even the little value thus obtained might remove the final
trace of trouble. In any case we would have the soluble calcium
chloride to contend with. It is very desirable that the problem
be solved without the use of barium chloride.
USE OF ACID CARBONATE OF BARIUM.
Fresenius says that barium carbonate is soluble in water
containing free carbonic acid, forming the acid carbonate.
Other authorities confirm Fresenius' statement, and in one
instance give the solubility as one part in eight hundred and
thirty parts of water. To pass carbon dioxide through tanks
containing barium carbonate in suspension would be a
paratively simple matter.
com-
It is not within the province of this paper to go into the cost
of producing the carbon dioxide gas, but we do not think the
problem would be a difficult one nor the cost prohibitive. If
the acid carbonate method with its more or less valuable by-
products proved too expensive, we could fall back on the pro-
ducts of combustion of wood or oil, first using the heat value
in some of our work, and then the chimney products in the above
duty. The gas must be practically free from sulphur. The re-
action of the acid carbonate with the calcium sulphate would be
as follows:
BaH₂ (CO3)2 + CaSO, BaSO4 + CaH, (CO3)2.
If the acid barium carbonate has a solubility of 1 in 830,
we have raised the initial value of the solution from 2.5% to
50%, which would eliminate the trouble from a great many clays.
We may have an excess of the acid barium carbonate in
solution to give trouble, but its instability is a safeguard in this re-
spect, and further, what has been said in regard to using barium
carbonate with barium chloride applies equally well to solutions of
the carbonate and acid carbonate. The compound in solution
is the acid carbonate and that in suspension the carbonate.
By keeping the former near to but below the quantity
=
Į
SCUMMING AND EFFLORESCENCE.
59
actually required, and relying upon the carbonate to decompose
the remaining traces of calcium sulphate, we can have no trouble
with any excess of barium solution.
The acid carbonate of calcium formed is a soluble salt, and
like calcium chloride, remains in solution, but we are dealing
with a compound a great deal more insoluble than calcium
chloride, or even than calcium sulphate, and decrease the danger
of efflorescence in proportion to the decrease in solubility.
The solubility of the acid carbonate of calcium is given as
I in 1200. Furthermore, the acid carbonate of both barium and
calcium can exist only in solution, and the chances are strongly
in favor of their being unstable and passing back into the carbo-
nate form and precipitating long before the drying process is
complete.
An investigation of these acid carbonates is greatly to be
desired.
USE OF BARIUM HYDRATE.
Burning barium carbonate with charcoal will reduce it to
the oxide, which is quite soluble, according to one authority in
1.5 to 70 parts of water. The hydrate thus formed is soluble in
20 to 0 parts of water, being soluble in its own water crystal-
lization, at 75 degrees, nominally its melting point.
Barium carbonate does not readily give up its carbon
dioxide, and it must not be inferred from the above bald state-
ment that we can convert barium carbonate into barium oxide
as readily and simply as we convert calcium and magnesium
carbonate into "lime," but the simple process of mixing barium.
carbonate with charcoal and burning, especially under forced
draft, will give a partial decomposition sufficient, at least, for
our purpose.
The process would be crude and the stuff would have to be
freshly made, but if by its use we could introduce at once suf-
ficient barium in solution to precipitate all of the sulphur and at
the same time not leave dangerous by-products, we have solved
the difficulty of any clay, and our trouble with "scum" would
be over. The use of barium hydrate would result in the for-
mation of calcium hydrate, and this in the burning would pass
into the oxide. Calcium hydrate is soluble in 550 to 1,000 parts.
of water, and the oxide in 750 to 1,500 parts, in either case a
1
60
SCUMMING AND EFFLORESCENCE.
decided gain over the solubility of calcium sulphate. Our
greatest danger would be from any excess of the very soluble
barium hydrate. If it can be kept below the point of excess, we
would have little trouble with "scumming."
The tendency of both the barium and calcium hydrates to
absorb carbonic acid from the atmosphere and pass into insoluble
carbonates is in their favor. Add to this the tendency of calcium.
hydrate, and perhaps of barium hydrate, to combine with the
silica of the clay, insures, in the opinion of the writer, freedom
from any "scumming" because of their presence in the clay.
In conclusion the writer wishes to repeat the remark made
in the beginning of the paper, that there is a valuable field for
investigation in the use of barium. Many factories now making
common bricks could put their product on the market for face
work if the "scum" could be overcome, and factories making
face bricks would reduce the loss from this cause. Nor is the
trouble confined to brick manufacturers; it is only more ap-
parent in their case. What effect the presence of "scum" may
have on salt glazing and upon slip and enamel work is purely
problematical.
In all this work the following caution must be observed:
Barium in any soluble form is an active poison, and must be
handled with care. The sulphate being insoluble has no effect
except in so far as it may be made soluble by the digestive
juices; the carbonate being difficultly soluble is safe to use;
the chloride and hydrate being readily soluble should be re-
spected and men who are handling them should be careful to
wash their hands before eating their lunches or handling their
tobacco, or anything taken into the mouth. Naturally, the anti-
dote would be some soluble sulphate.
PA 191
35
}

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UNIVERSITY OF MICHIGAN
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Gaylord Bros.
Makers
Syracuse, N. Y.
PAT. JAN. 21, 1908
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THE PYERATORS,
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