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 UNIVERSITY OF .UINO.S LIBRARY AT URBANA-CHAMPA.GN 
 
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 JAN Je 1976 
 
 SEP I 8 
 MAY 3 
 
UNIVERSITY OF ILLINOIS BULLETIN 
 
 Vol. VII. MARCH 21, 1910. No. 29 
 
 [Entered February 14, 1902, at Urbana, Illinois, as second-class matter under 
 Act of Congress of July 16, 1894.] 
 
 BULLETIN No. 14 
 DEPARTAIENT OF CERAMICS 
 
 A. V. BLEININGER, Director 
 
 OPALESCENCE AND THE FUNCTION OF 
 BORIC ACID IN THE GLAZE 
 
 BY 
 
 R. T. STULL AND B. S. RADCLIFFE 
 
 URBANA, ILL. 
 
 1909-1910 
 
 PUBLISHED FORTNIGHTLY BY THE UNIVERSITY 
 
[Rei'Rinted from Transactions of American Ceramic Society, Vol XII. 
 Paper read at Pittsburgh Meeting, February, 1910.] 
 
 OPALESCENCE AND THE FUNCTION OF B.O. 
 IN THE GLAZE. 
 
 BY 
 
 R. T. Stull and B. S. Radcliffe^ Urbana, 111. 
 
 In the study of whiteware glazes, at the University 
 of Illinois, the class in glaze making t-onstnieted a group 
 of glazes which showed all stages from clear colorless 
 through opalescence to opaque white. 
 
 The group consisted of six series having nine mem- 
 liers, each covering the following field : 
 
 .3 K„0 \ r L'.l to 6.1 SiO, 
 
 .4 CaO i 
 
 ,1 PbO ' I to 1.0 1J,(), 
 
 The glazes were ap]»lied to biscuit wall tile, set on 
 edge in flat tile saggers and burned to cone 3. Examina- 
 tion of the trials seemed to indicate that opalescence was 
 not a function of the quantity of boric oxide, since glazes 
 having small molecular quantities appeared as opalescent 
 or opaque as those having larger quantities. 
 
 There also seemed to be a dividing line between the 
 clear and the opalescent glazes. Although the glazes had 
 run and beaded along the edge of the tile, the thickness of 
 the glazes against the white background of the tile was not 
 sufficient to determine accurately the dividing line between 
 the clear and the opalescent fields. 
 
 In order to determine more accurately this line, a 
 series of glazes was inserted between what appeared to be 
 the clear and opalescent fields, and another series added to 
 the left of the group, thus covering the following limits: 
 
 .3 K„0 ^ (1.76 to 6.1 SiO. 
 
 .2 Na,0 ( 45 AlO -' 
 
 .4 CaO I -^^ -"^ i 
 
 .1 PbO .' ' to 1.0 B.,0, 
 
2 OPALESCENCE AND FUNCTION OF B0O3 IN GLAZE. 
 
 Each circle oii the diagram (Chart I) represents a 
 glaze. In the lower left hand corner is low boric oxide and 
 low silica. In the lower right hand corner is low boric 
 oxide and high silica. At the npper left corner is high 
 boric oxide and low silica. In the npjier right corner is 
 hio'li boric oxide and hioh silica. 
 
 T-fx-Ars'=5. AM CEK &OC. 
 
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 In the series running horizontally from left to right, 
 boric oxide is constant and silica increases, while in the 
 series running vertically from bottom to top silica is con- 
 stant and boric oxide increases. 
 
 In order to detect faint opalescence it is necessary to 
 examine thick laj-ers of the glazes. For this purpose tiles 
 from stoneware clay were made having conical depressions 
 one-half inch in diameter and one- fourth inch deep (Chart 
 IV). These depressions Avere filled with the glazes, the 
 wall tiles were dipped and the trials burned to cone 3. 
 
OPALESCENCE AND FUNCTION OF BoOg IN GLAZE. 3 
 
 Examination of tlie trials showed opalescence strongly 
 in Series 2 running- vertically. Glazes having low boric 
 oxide seem to show as strong opalescence as those having 
 higher quantities. The members in vertical Series li/. do 
 not show the slightest trace of opalescence. To the right 
 of the dividing line, opalescence increases, while glazes 
 in the field to the left are perfectly clear. 
 
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 Some very interesting points witli respect to crazing 
 are observed. In the A series, A-Vo is a basic matt and 
 crazed. A-9, which is high in silica, is badly devitrified 
 and crazed. A-8 is slightly devitrified and slightly crazed. 
 
 In the case of these two extremes of crazing the addi- 
 tion of boric oxide has had diametrically opposite effects. 
 Boric oxide added to A-i ^ has inci-eased crazing, the craz- 
 ing increasing with increase in boric oxide. In A-8 and 
 the addition of boric oxide has overcome crazing and also 
 devitrification. 
 
4 OPALESCENCE AND FUNCTION OP B^O, IN GLAZE. 
 
 In his article^ on "The Function of Boron in the Glaze 
 Formula/' Professor Binns says: -'It was noticed that 
 from its position in the periodic curve and from its valence, 
 boron might be presumed to act like alumina, functionino' 
 as a base with strong acids and as an acid with strong 
 bases." 
 
 He also points out, that in a glaze having the formula 
 RO, .2 AUOo, l.GSiOo, whose oxygen ratio is one to two, 
 if .3 BoOo be added it is necessary to raise tlie silica to two 
 and one-half according to practice, and that if BgOo be 
 considered as base the oxygen ratio then becomes one to 
 two. 
 
 Professor Binns further states: " * * *, the opinion 
 is further expressed that BoO;, undoubtedly is basic when 
 melted with silica or silicates, and that it may be used to 
 replace alumina partially or entirely, with the exception 
 of its exercise of the property of fusibility." 
 
 Dr. Zimmer says:^ "While I admit that all the facts 
 brougth out seem to indicate that boron will act as a base. 
 there are suridy conditions under which it will act as an 
 acid." 
 
 If we arrange the oxides of the elements in groups 
 according to the periodic system, we find, as a rule, that 
 the bases of low molecular weight produce best opacity 
 and bases of high molecular weight least opacity. With 
 respect to acids, we find the opposite, i. e., acids of high 
 molecular weight produce best opacity and acids of low 
 molecular weight least opacity. 
 
 According to Seger's rules for crazing, bases of low 
 molecular weight produce least crazing and bases of high 
 molecular weight produce most crazing. Whether the op- 
 posite is true of acids has not been definitely shown. 
 
 By calculating the oxygen ratios of the glazes includ- 
 ing boric oxide, lines of equal oxj^gen ratios are obtained 
 running vertically across the diagram, as shown in Chart I. 
 
 ' Vol. X, Trans. A. C. S., p. 159. 
 2 Ibid, p. 168. 
 
opalf:scence and function op B2O3 in glaze. 5 
 
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6 OPALESCENCE AND FUNCTION OF B^Og IN GLAZE. 
 
 CoiisideriDg boric oxide as aii acid, lines of equal oxy- 
 gen ratios run diagonally downward from left to right 
 (Chart II). Calculating oxygen ratios in which boric 
 oxide is assumed to be a base, gives diagonal lines running 
 upward from left to right which appear to radiate from a 
 common center, (Chart III). 
 
 In Chart I it is observed that the dividing line be- 
 tween the clear and opalescent glazes runs parallel to and 
 between the oxygen ratio lines of 1 to 2 and 1 to 2.2. Since 
 opalescence is shown so strongly on the total oxygen ratio 
 line of 1 to 2.2, it is reasonable to suppose that the true 
 dividing line between clear and opalescent glazes lies on or 
 very near to the total oxygen ratio line of 1 to 2. 
 
 In consulting Charts II and III, it is evident that the 
 lines of equal oxj^gen ratios in which boric oxide is calcu- 
 lated either as an acid or a base, do not bear any definite 
 relation to the dividing line between the clear and opales- 
 cent glazes. 
 
 Boric oxide fuses at a low temj)erature, forming a thin 
 water-like fluid. In this condition it is one of the strongest 
 igneous solvents at our command, and there are few inor- 
 ganic oxides that it is not capable of dissolving. Silica 
 added to it is dissolved and evidently combined, since 
 silica is strongly acid in character in igneous fusion, the 
 combination is probably some silicate of boron which pro- 
 duces a milky precipitate. 
 
 Opalescence in glasses containing boric oxide is not 
 caused by crj'stallization, nor by that peculiar form of 
 segregation which sometimes precedes it. If it were, then 
 sudden cooling would leave the mass in an amorphous con- 
 dition, thus giving a clear glass. A fusion of silica and 
 boric oxide when suddenly cooled will show strong milki- 
 ness. Fritts containing boric oxide frequently show opal- 
 escence though cooled suddenly by quenching in water. 
 
 It is trus that a glaze cooled slowly will show stronger 
 opalescence than the same glaze cooled quickly, but it must 
 be remembered that precipitates require time to form. As 
 
OPALESCENCE AND FUNCTION OF B0O3 IN GLAZE. 7 
 
 aij example, the comi:)lete precipitation of magnesia by 
 sodium ammonium phosphate recjiiires several hours. 
 
 CONSIDERATIONS ON CRAZING. 
 
 Boric oxide when added to A-Yo (which is basic in 
 character), undoul)tedl3^ plays the pai-t of an acid. In- 
 creasing boric acid has increased crazing. The more boric 
 acid present, the greater is the degree of crazing. 
 
 In the case of A-8 and 9, which are highly acid in 
 character, it is more than probable that the addition of 
 boric oxide has combined as a base, and in so doing has 
 overcome both devitrification and crazing. 
 
 In comparing the dividing line between crazing and 
 non-crazing glazes, with lines of total oxygen ratios on the 
 three different charts, we find that this line does not con- 
 form to any oxj'gen ratio lines, whether boric oxide be ex- 
 cluded from the calculations, or included either as acid or 
 as base. 
 
 It has been noted that crazing has increased with in- 
 crease in boric oxide in glazes whose previous total oxj^gen 
 ratios are low. It has also been stated that this crazing is 
 due to the presence of boric oxide as an acid, and as such, 
 whether free or combined, its tendency is to reduce crazing. 
 If boric oxide combines as a base, its influence should be 
 that of overcoming crazing, since it is a base of low mole- 
 cular weight (in accordance with Seger's rale). 
 
 If boric oxide when combined as a base acts like alum- 
 ina and can replace alumina, then, any increase in boric 
 oxide would decrease crazing the same as alumina, in ac- 
 cordance with the statement of Purely and Fox.^ 
 
 Boric oxide present as acid and boric oxide present as 
 base influence crazing in opposite directions, which might 
 be expected since the properties of bases and acids are 
 opposite in character. In glazes which craze from excess 
 boric acid, an addition of silica has a double influence in 
 
 » Fritted Glazes Trans. A. C. S., Vol. IX. p. ]78. 
 
8 OPALESCENCE AND FUNCTION OP B2O3 IN GLAZE. 
 
 preventiug crazing. By converting boric acid to boric base 
 it decreases crazing acid and increases non-crazing base. 
 
 The reason tliat tlie line between crazing and non- 
 crazing glazes does not conform to anj^ oxygen ratio lines 
 is because both free crazing boric acid and combined non- 
 crazing boric base may both be present, and it takes a 
 definite amount of the one to overcome the influence of the 
 other. With reference to the dividing line between devitri- 
 fied and crazed glazes and non-devitrified and non-crazed 
 glazes, Chart III shows that the line coincides in direction 
 with the oxygen ratio lines when boric oxide is considered 
 as a base. But when referred to Charts I and II, it is shown 
 that the line does not conform to the oxygen ratio lines in 
 either of these two cases. 
 
 Although there are no absolute chemical proofs at 
 hand, the following theories are advanced from the fore- 
 going evidence : 
 
 First, boric oxide when added to a glaze whose pre- 
 ^ ions oxygen ratio is less than 1 to 2 is present as an acid, 
 combined, tending to produce a chemical equilibrium. Any 
 excess is present in acid, uncombined, in solid solution. 
 When present in the acid form its influence is that of pro- 
 ducing clear glazes and crazing. 
 
 Second, boric oxide when introduced into a glaze 
 whose previous oxygen ratio is 1 to 2, is present as an un- 
 combined acid in solid solution, imparting clear and craz- 
 ing tendencies to glazes. 
 
 Third, boric oxide when introduced into a glaze with 
 a previous total oxygen ratio greater than 1 to 2 tends to 
 produce chemical equilibrium by combining as a base, thus 
 producing opalescence and non-crazing tendencies. Any 
 excess boric oxide is present as an uncombined acid in solid 
 solution. 
 
 Fourth, opalescence is caused by the precipitation of a 
 silicate in which boric oxide is evidently basic. The in- 
 tensity of opalescence does not depend upon the quantity 
 of boric oxide present but upon the quantity combined as 
 a silicate. 
 
OPALESCENCE AND FUNCTION OF BgOg IN GLAZE. 9 
 
 It is interesting- to note that, although the line divid- 
 ing crazing and non-crazing glazes does not conform to any 
 (jxygen ratio lines, it conforms nearest to the lines of equal 
 oxygen ratio ^^•hen boric oxide is assumed to be base. The 
 oxygen ratio line of 1 to 2, when boric oxide is considered a 
 base, passes through the middle of the field of best glazes 
 found in this group. 
 
 DISCUSSION. 
 
 Mr. Parinelee: I have this suggestion that I wish 
 some of those interested in the matter of opalescence would 
 determine A\hether that is opalescence or fluorescence. 
 Opalescence, as I understand it, is due or at least com 
 monly attri])uted to internal fractures, while fluorescence 
 is a different thing. It would be a comparatively simple 
 thing to take a sample of glaze showing this description of 
 opalescence and determine which it is. I think it Avouhl 
 be a nmtter of some interest. 
 
 Mr. ^'^TiiJI: The trials just came out of tlse kiln ;i 
 little while before the convention. The intention was to 
 have thin sections made of them and examine them micro- 
 scopically. We used the term opalescence because that is 
 the term most generally understood, and because it ap- 
 pears like opalescence, — it has the color. We know that 
 some precipitates are opalescent in appearance. The pre- 
 cipitates of aluminum hvdroxide and silicic acid are sug 
 gestively opalescent. Kichards, in his work on the determ- 
 ination of the atomic weight of silver, jjroduced strong 
 opalescence in the silver chloride which passed through 
 the filter. The presence of minute solid particles in sus- 
 pension may give the reflection of light nearly as well as 
 cracks or flaws will do. 
 
 }fr. ^Shaic : I have had some experience with borax 
 but I do not believe I can add anything. I have been won- 
 dering what is the relation between opalesc(»nce and opac- 
 ity. Substances that produce opacity if fused hard enough 
 will produce opalescence. I have produced white enamel 
 
10 OPALESCENCE AND FUNCTION OF B0O3 IN GLAZE. 
 
 and very often had it "burned." The opac-ity is all burned 
 out of it, and tlie glaze after that time very often shows a 
 great amount of opalescence. 
 
 Mr. Binns: Would you define opalescence as a par- 
 tial opacity? 
 
 Mr. ^haw : That is exactly what I think. 
 
 Mr. ^tull : I agree with that, that opalescence is a 
 ''dilute" state of opacity. Here is F9, (exhibits trial), the 
 one highest in BoOg, and highest in silica. It shows opaque 
 white. 
 
 Mr. Bin us : I would like to ask Mr. Stull whether he 
 has any explanation of the connection between devitrifica- 
 tion and crazing. We have generally considered the two 
 things were influenced by opposite conditions. I would be 
 interested to know what suggestions he has to make on 
 that p'oint. 
 
 Mr. HtuU: Most generally crazing is conceded to be 
 a function of the co-efficient of expansion and contraction, 
 but personally I think there is another factor which we do 
 not consider, and that is crystalline tension. We know 
 that crystals form from gases, from fluids and from 
 solids. As a time-worn illustration, the iron beams of rail- 
 road bridges frequently become so crystalline from the jar 
 of the trains that they become weak and break. We have 
 plenty of evidence of crystallization going on in solids. A 
 substance in crystallizing usualh^ does one of two things, 
 \\ either increases in volume or decreases in volume. In 
 either case if crystallization should continue down into the 
 solid glaze it would produce a tension strong enough to 
 cause crazing. In experimenting with crystalline glazes 
 of the type which crystallize in assicular groupings, I have 
 failed to make one crystalline glaze yet which did not 
 craze. In these crystals which radiate from centers, it is 
 interesting to note that the craze marks did not run in 
 irregular zigzag lines, but ran approximately in broken 
 concentric rings around the crystalline center in this man- 
 ner. (Indicating on the board as shown herewith.) If it 
 
OPALESCENCE AND FUNCTION OF B0O3 IN GLAZE. 11 
 
 is not due to crystalline tensions, then what is it clue to? 
 Crystalline glazes with oxygen ratios all the way from one 
 to one up to one to five Avill craze if they are crystallized. 
 In practice I tried to overcome it by slow cooling from 
 vanishing red heat down. The slower the cooling the worse 
 the crazing appeared. 
 
 FlssLCUtar Grouizin^ 
 Showinq Craze Mccr/Cs 
 ff vi2Tiinq Concentrcca//y. 
 
 Mr. Binns: I would like in that connection to make 
 another suggestion. We commonly theorize or philosophize 
 this way : Silica in a body, being crystalline, has a high 
 €0-efficieiit. In the case of excess of silica in the glaze and 
 consequent devitrification, it seems to me quite possible 
 that the silica has returned to its crystalline form, and 
 thus regained its high co-efficient. I have no doubt that 
 Mu. StulTs theory of crystalline tension strains has some- 
 thing to do with it. At the same time I think the fact that 
 the silica has returned to its crystalline form is also an 
 important factor. I have frequently noted in that connec- 
 tion that a short fired glaze almost invariably crazes sim- 
 ply because the silica has not been thoroughly taken into 
 the solution, and has thus not reached the amorphous form 
 with the low co-efficient. 
 
 And now the question of boron, in which I am natur- 
 ally very much interested. I would like to say just a word 
 on that. This is the first time it has been alluded to since 
 I brought it up two years ago. It is quite gratifying to 
 find that at least a portion of my contention is substan- 
 tiated. Dr. Singer showed, in his paper, that it was pos- 
 sible to produce compounds in which boron sesquloxide, or 
 boric oxide, functions as a base. 
 
12 OPALKSCENCE AND FUNCTION OF BoOs IN GLAZE. 
 
 Ever since the paper was presented two years ago T 
 have worked upon the theory, which I then enunciated, 
 that in a normal glaze under normal conditions boric oxide 
 functions as alumina. And my students in the blackboard 
 work have said to me, why do you put boric oxide under 
 the alumina when all the books do the other thing and put 
 it under the silica? I tell them if they live long enough 
 they will find that all the books will have it under alumina. 
 
 Mr. Stull has brought up certain exceptional condi- 
 tions, as for instance, in a liighh^ basic glaze, where it is 
 quite probable that a portion of the boric acid or all of it 
 may function acidically. And by the Avay, I wish we had 
 some better term than acid and base to express this action. 
 
 For many years I groped in the dark with regard to 
 fritted glazes, and I believe a good many are doing so still. 
 But when we once established that the bi-silicate ratio was 
 the best thing for a clear glaze under proper conditions, 
 there did not seem to be any philosophy or working reason 
 why fritted glazes should not come under the same rule, in 
 other words, why boric oxide should introduce an entirely 
 new set of laws. But by the simple arrangement of placing 
 boric oxide with the sesquioxides the whole thing is solved. 
 Fritted glazes now obey the same law. They are normally 
 bi-silicate, or closely approximating it. I have no more 
 difficulty now in laying down fritted glazes than I used to 
 have in raw glazes. But until this point was reached there 
 was no law which could be used as a guide. 
 
 We are very glad to have this word from Mr. Stull 
 not only on the boric question, but on that rather obscure 
 and interesting phenomenon of opalescence wliich has oc- 
 cupied us today. He has also demonstrated his work in a 
 Acry interesting manner. His enclosing the panel of glaze 
 in the cell is quite a new idea and one well worthy of con- 
 sideration, when we come to deal with the consistency of 
 the glaze matter. 
 
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