THE UNIVERSITY 
 
 OF ILLINOIS 
 
 LIBRARY 
 
 <ll r 
 
 ILLINOIS LIBRARY 
 
 MC URBANA CHAMPAIGN 
 
 BOOKSTACKS 
 
MODELS OF BLAST FURNACES 
 FOR SMELTING IRON 
 
 BY 
 
 HENRY W. NICHOLS 
 Associate Curator of Geology 
 
 FIELD MUSEUM OF NATURAL HISTORY 
 
 CHICAGO 
 
 1922 
 
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Field Museum of Natural History 
 
 DEPARTMENT OF GEOLOGY 
 Chicago, 1922 
 
 Leaflet Number 2 
 
 Models of Blast Furnaces for 
 Smelting Iron 
 
 Three models represent the hot blast furnace of 
 today, the cold blast furnace of seventy years ago and 
 the small forge in which iron was smelted one hundred 
 and fifty years ago. 
 
 I. THE MODERN IRON BLAST FURNACE. 
 Practically all the iron in use is extracted from its ore 
 in blast furnaces, one of which is represented in mini- 
 ature by the model in the right half of the case. It is 
 called a blast furnace because its operation is main- 
 tained by a blast of air blown in from below. Its prod- 
 uct is pig or cast iron which, remelted and shaped in 
 molds, forms our cast iron objects. All steel and 
 wrought iron is refined from this pig iron. The process 
 of smelting iron ore is based on simple principles al- 
 though the practice of smelting is decidedly complex in 
 detail. A mixture of ore, limestone and fuel is fed in at 
 the top of a tall furnace (the furnace reproduced is 
 sixty-five feet high) . The fuel, coke prepared from soft 
 coal, is made to burn with great intensity by a blast 
 of hot air blown in from below. Hot gases formed by 
 the incomplete combustion of the coke attack the ore 
 which is a combination of iron and oxygen and take 
 away its oxygen. This leaves the iron behind in a 
 free state. Lower in the furnace this iron absorbs 
 carbon from contact with hot coke, melts and falls to 
 the hearth below whence it is drawn off from time to 
 time in molten condition. The earthy impurities of 
 the ore and the ash of the coal combine with the lime- 
 
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2 Field Museum of Natural History 
 
 stone and form a slag which melts and runs from the 
 furnace. The furnace runs continuously day and 
 night from the time it is blown in until it is necessary 
 to shut down for repairs. As the charge wastes away 
 below it is replenished by additions of ore, limestone 
 and fuel from above. 
 
 The model represents in some detail the furnace 
 in which the smelting is accomplished and some of its 
 accessories. At the center of the model is a circular 
 tower which houses an elevator by which materials to 
 be fed to the funiace are lifted to the top. At the left 
 of the elevator is a blast furnace and some of its ac- 
 cessories. At the right the furnace and accessories 
 are repeated in section, that is cut open to show the 
 inside. 
 
 THE FURNACE or STACK is a tall structure of 
 brick enclosing a central shaft lined with the best 
 quality of fire-brick and cased on the outside with 
 boiler iron. This furnace is sixty-five feet high and six- 
 teen feet wide at its widest part. Many furnaces are 
 larger than this. The disk-like structure which rests 
 on top of the furnace is a circular room of boiler iron 
 which encloses the CHARGING PLATFORM from 
 which the furnace is fed. The material fed to the 
 furnace is the CHARGE. The charge is a mixture in 
 carefully proportioned quantities of ore, flux and fuel. 
 THE ORE is a combination of iron and oxygen and is 
 the source of the iron which is the product of the fur- 
 nace. THE FLUX is limestone. This combines with 
 any infusible matter which may form in the furnace, 
 causes it to melt and washes it from the furnace in the 
 form of slag. It also prevents waste of the iron of 
 the ore and serves other useful purposes. THE FUEL 
 is generally coke although some few furnaces use char- 
 coal. The fuel provides the necessary heat and also 
 the reducing gases which free the iron from the ore. 
 
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Models of Blast Furnaces 3 
 
 The charging platform is connected with the top 
 of the elevator tower by a bridge. Over this bridge the 
 charge of ore, coke and limestone is wheeled in bar- 
 rows or small cars. The top or THROAT of the fur- 
 nace opens at the center of the charging floor. This 
 opening is ordinarily closed by the BELL, a conical 
 valve opening do^vnwards. The barrows or cars which 
 bring the charge are dumped on top of the bell. The 
 bell is then momentarily depressed and the charge 
 falls into the furnace. When the bell is opened the 
 furnace fumes come through to the charging floor so 
 a short smokestack is provided by which they may 
 escape. The bell is closed nearly all the time and the 
 products of combustion cannot escape through the top. 
 They then pass through an opening in the furnace wall 
 immediately below the bell to the DOWN-COMER, a 
 large brick-lined pipe which passes downward out- 
 side the furnace. Through the down-comer the gases 
 pass to underground conduits which conduct them to 
 the places where they are used. THE GAS BLOW- 
 OFF is a smokestack connected with the down-comer 
 and closed at the top by a valve. By opening this valve 
 any surplus gas may be allowed to escape. The gases 
 from the furnace carry with them dust from the ore 
 and fuel in the furnace. Much of this dust settles in 
 the DUST-CATCHER, a chamber connected with the 
 down-comer. From time to time the dust is drawn off 
 through a gate in the bottom of the dust-catcher, and 
 after suitable treatment, fed back to the furnace. The 
 shaft of the furnace gradually widens downward for 
 more than half its length to the place where it is widest 
 which is called the BOSH. From the bosh it nar- 
 rows gradually to the hearth at the bottom. This 
 tapering part from the bosh down to the hearth is 
 called the BOSHES. In this area the heat is so in- 
 tense that the fire-brick walls would either melt or be 
 
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4 Field Museum OF Natural History 
 
 eaten away by slag if they were not protected. Pro- 
 tection is afforded by water cooling. This is provided 
 by water which constantly runs through a number of 
 metal pipes imbedded in the masonry of the boshes. 
 
 About five feet above the base of the furnace and 
 immediately below the boshes are six TUYERES. 
 These are metal nozzles through which a blast of hot 
 air continuously enters the furnace. The tuyeres 
 draw their supply of hot air from the BUSTLE PIPE, 
 a large brick-lined pipe which encircles the furnace 
 slightly above the tuyeres. Below the tuyeres is the 
 CRUCIBLE or HEARTH which is that part of the 
 furnace in which the molten iron accumulates until 
 there is sufficient to tap. The furnace is kept filled 
 with a carefully proportioned mixture of ore, coke and 
 limestone to a point near the top called the STOCK 
 LINE. The blast of hot air which enters through 
 the tuyeres burns the coke and in that part of the 
 furnace an intense heat is generated. The tall column 
 of charge which lies between this zone of combustion 
 and the stock line above rapidly absorbs heat from the 
 rising gases, so that the charge which is at an intense 
 white heat near the tuyeres is below a red heat at the 
 top. 
 
 The section of the furnace shows the charge white 
 hot below, passing through the several stages of yellow, 
 orange and red heats to black at the top. When coke 
 or coal is burned in an ordinary stove combustion is 
 fairly complete and the principal product is an in- 
 combustible gas called carbon di-oxide. When a very 
 deep bed of fuel is burned, the combustion is not com- 
 plete and the product is a gas, carbon mon-oxide, which 
 can itself be burned. As the charge of the furnace 
 forms a bed over fifty feet deep the gas from burning 
 the coke of the charge in the hot blast of air at the 
 tuyeres is this combustible gas, carbon mon-oxide. 
 
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Models of Blast Furnaces 5 
 
 This coming- in contact with hot iron ore abstracts its 
 oxygen and frees the iron. This iron is set free in a 
 comparatively pure form which melts with difficulty. 
 Even in the intense heat of the hotter part of the fur- 
 nace it would only become pasty. In contact with the 
 hot coke each particle of iron absorbs a small quantity 
 of carbon which gives it the composition of cast iron 
 which melts readily at furnace temperatures. The 
 iron then melts and drops to the bottom of the fur- 
 nace where it collects in a molten slate in the cruci- 
 ble. When a sufficient quantity of iron has collected 
 it is withdrawn through the TAP HOLE. This is a 
 small round hole at the base of the furnace connect- 
 ing with a shallow groove or channel in the floor in 
 front. This tap hole is closed by a stopping of fire 
 clay or other refractory material. When the time 
 comes to tap the iron, the clay stopping is broken 
 out and the iron allowed to run into the channel which 
 may conduct it to a pig bed, not shown in the model, 
 where it is cast in sand molds and takes the form of 
 rough blocks called PIGS. When a steel works is 
 connected with the furnace plant, the molten iron as 
 tapped is conveyed directly to the mills for conver- 
 sion to steel. 
 
 The blast furnace runs continuously and all that 
 enters must escape in a gaseous or liquid state as there 
 is no provision for removing solids. Earthy matters 
 not easily fusible are present in the ores and in the 
 ash of the fuel. Limestone from the furnace charge 
 combines with these and under the influence of heat 
 forms a fusible waste product, the SLAG or CINDER. 
 This floats on the molten iron in the crucible and runs 
 off through the SLAG EYE. This is a small opening 
 a little below the tuyeres and to the right which a man 
 with an iron rod is cleaning. The blast of air is pro- 
 vided by BLOWING ENGINES which do not appear 
 
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6 Field Museum of Natural History 
 
 in the model. These are large air-compressors driven 
 either by gas engines or by steam engines which take 
 their steam from gas-fired boilers. These engines or 
 boilers use the furnace gases which are drawn from the 
 top of the furnace through the down-comer and led 
 to the engine room by underground conduits. Other 
 conduits lead the compressed air for the blast from 
 the blowing engines to the STOVES where it is heated. 
 The stoves are the tall structures which appear at the 
 outer edges of the model. They are built of fire-brick 
 and cased with boiler iron. They are so designed that 
 the inside consists of numbers of narrow fire-brick 
 passages. Each furnace is provided with two or more 
 stoves although only one appears in the model. A pipe 
 connects the stove with gas ducts which are fed from 
 the down-comer of the furnace. Gas is burned in the 
 stove until all the fire-brick passageways become hot. 
 Then the gas is shut off and air from the blowing en- 
 gine enters at the top. This air becomes heated in the 
 passage and emerges below into underground ducts 
 which conduct it to the bustle pipe of the furnace. 
 Stoves are intermittent in operation, one supplies air 
 to the furnace while another is being heated. The out- 
 put of the furnace is pig iron, slag and gas. The gas 
 is consumed in the stoves and in operating the blowing 
 engines although there is sometimes a surplus which 
 may be used for other purposes. The slag is mostly 
 a waste product although some is used in the manu- 
 facture of Portland cement and in other ways. The 
 principal product is pig iron. This iron contains small 
 quantities of various impurities, which have an im- 
 portant effect in its properties. From long contact 
 with hot coke in the furnace it has absorbed a quantity 
 of carbon which has made it fusible and suited for 
 the manufacture of cast iron articles. Most pig iron 
 is refined to steel. Refining removes impurities and 
 
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Models of Blast Furnaces 7 
 
 makes the iron stronger, less brittle, and more diffi- 
 cult to melt. When practically all the carbon is re- 
 moved the product is wrought iron or mild steel 
 according to the method of refining employed. When 
 the refined product contains certain definite amounts 
 of carbon it is tool steel or other high grade steel. 
 The output of a blast furnace is large. Some furnaces 
 have yielded over 900 tons of iron per day. The fur- 
 nace represented by the model yields 300 tons daily. 
 
 Earlier Types of Furnace 
 
 Two models in the left half of the case represent 
 two obsolete types of iron smelting furnaces. One 
 was in use one hundred and fifty years ago, the other 
 sixty years ago and even later. They illustrate the 
 immense advance in the art of iron smelting during 
 the past two hundred years and call attention to the 
 relatively recent introduction of the modern furnace 
 with its large capacity and moderate cost of operation. 
 Before the time of the Catalian forge, the earliest of 
 those shown, more primitive devices were employed 
 and the progress from this forge to the modern furnace 
 was marked by a number of furnace types other than 
 those represented by these models. 
 
 II. THE COLD BLAST IRON SMELTING 
 
 FURNACE 
 
 The central model of the three represents a cold 
 blast iron smelting furnace modelled on the same scale 
 as the hot blast furnace to the right. It is called a cold 
 blast furnace because the blast of air which keeps it in 
 operation is not heated. This furnace was the im- 
 mediate predecessor of the hot blast furnace of today 
 which has completely superseded it. It was in general 
 use in this country at the time of the civil war and for 
 
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8 Field Museum of Natural History 
 
 some years after. It operated on the same principle 
 as the modern furnace, but as it did not have the hot 
 blast, did not utilize the furnace gases and lacked other 
 refinements, its efficiency and capacity were low. Its 
 most noticeable features were the massive construction, 
 the large flame at the top and the primitive character 
 of the equipment. 
 
 The massive pyramidal structure near the center 
 of the model is the furnace. It is built of limestone 
 and encloses a small central shaft similar in outline to 
 that of a modern furnace. It is built at the foot of a 
 cliff so that the charge may be fed at the top without 
 hoisting. The top of the furnace is connected with the 
 top of the cliff by a bridge over which the charge of 
 ore, charcoal and limestone is wheeled in barrows. The 
 top is enclosed in an iron hood which is provided with 
 a smokestack so that the charging platform may be 
 sufficiently free from gases to permit men to work 
 there. The top is open and the gases generated in the 
 furnace escape and burning continuously form a large 
 flame visible for miles. Iron and slag form as in the 
 modern furnace and are drawn off in a similar manner 
 although the provisions for this are more complex than 
 they are in modern furnaces. At the right of the model 
 is the wooden blowing engine called the TUBS. At 
 the extreme right is the water w^heel which drives the 
 blovving engine. This engine consists of two wooden 
 upright cylinders, the tubs, provided with wooden 
 pistons and appropriate valves. The tubs deliver their 
 air to a third tub above which acts as a receiver. From 
 this receiver an underground conduit takes the air 
 direct to the tuyeres which feed it to the furnace. The 
 output is low. Such a furnace as the model represents 
 might yield thirty tons of pig iron a day. 
 
 III. THE CATALIAN FORGE. The model at 
 the left represents the Catalian forge which was in 
 
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Models of Blast Furnaces 9 
 
 general use in Europe during the eighteenth century 
 and in this country at the time of the Revolution and 
 for some years after. Forges of the same general 
 type have been used among peoples of the more primi- 
 tive cultures in relatively recent times and they may 
 have persisted in places to the present day. The forge 
 is small, not much larger than an ordinary blacksmith's 
 forge. The model is on a scale of one inch to the foot. 
 At the left is the large hammer which finishes the iron. 
 In the center is the hearth in which the ore is reduced 
 and at the right is the trompe in which air is com- 
 pressed to provide the air blast which urges the fire 
 on the hearth. 
 
 THE TROMPE which provides the air blast for 
 the hearth is a simple device by which falling water 
 is made to trap air and deliver it under pressure. It 
 consists of an upright hollow log fed with water from 
 a flume above and connected below with an air tight 
 tank. A valve at the top controlled through a lever 
 and hanging cord admits and shuts off the water. 
 Near the top of the log are a number of perforations. 
 Water falling freely draws air in through these per- 
 forations and delivers it to the tank below under a 
 pressure dependent on the height of the log. The 
 water escapes through a gate near the bottom of the 
 tank and the air passes through a pipe at the top to 
 a conduit which conducts it to the hearth. 
 
 THE HEARTH which appears in the center of 
 the model is much like a blacksmith's hearth with a 
 blast from the tromp instead of from bellows. On 
 this hearth a charcoal fire is built and urged to in- 
 tensity by the air blast which enters through a nozzle 
 called the TUYERE which passes through the rear 
 wall. This fire is fed by a mixture of iron ore and 
 charcoal. Under the influence of heat the ore, which 
 is a combination of iron and oxygen, gives up its oxygen 
 
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10 Field Museum of Natural History 
 
 to the charcoal and metallic iron appears in the form 
 of small granules. The heat of the forge is not suffi- 
 cient to melt the iron but does cause it to become pasty. 
 These pasty grains of iron settle to the bottom where 
 they cohere and form a spongy mass of iron called the 
 LOUPE. The pores of this iron sponge are filled with 
 CINDER, a liquid or pasty mass in which the im- 
 purities of the ore and the ash of the charcoal are 
 combined with unreduced oxides of iron from the ore. 
 After the hearth has been in operation for some six 
 hours the- loupe has grown to the proper size for the 
 next treatment. The smelting is then interrupted and 
 the loupe removed. A new fire is built and smelting 
 is resumed. 
 
 THE HAMMER. This large hammer shown at 
 the left of the model is driven by a water wheel con- 
 cealed by the rear wall of the building. The shaft 
 of the water wheel passes through the wall and is 
 armed with teeth or cams which act on the haft of the 
 hammer lifting it and letting it fall. The hot loupe is 
 placed under this hammer, which squeezes out the 
 cinder and compacts the iron into a solid BLOOM 
 which is merchantable. This forge can produce either 
 wrought iron or steel as the details of the operation 
 are varied. Cast iron never forms except by accident. 
 The output of the forge is of excellent quality but the 
 quantity is very small say three hundred pounds of 
 iron each six hours. 
 
 Henry W. Nichols. 
 
 These models are exhibited on the second floor of the 
 Museum in an alcove in Frederick J. V. Skiflf Hall. 
 
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