MANrKAi TUH: AND TIIAI'K. 



MANUFACTURE AND TKADE. 



sro 



the Mcond ml third being the results of in extension of the pro- 

 ecooi necessary for the production of the first. 



Pig-in*. The firrt prooeM in that of reducing the iron-atone IT :.. 

 or, as it U technically called, the mite, into a metallic state by means 

 of fusion. This operation is conducted in a ltlal-fun>arr, the form 

 and construction of which will be understood from the following Motion. 

 The interior of this furnace in the broadest part, which in called the 

 BaMn. is usually from 14 to 17 feet in diameter, iiutl this U gradually 

 decreased to about half that diameter at the top. The whole is built 

 of masonry, the lining to the furnace being composed of fire-bricks 

 carefully jointed together with fire-clay : the whole furnace is strongly 

 bound together with iron hoops or stays. The furnace is again con- 

 tracted below the boshes, and into thin lower part the melted iron falls 

 aa it is formed. The ground-plan of this lower part of tin- furnace is 

 constructed as shown in the following diagram, where the unshaded 



Fig. 1. Ground Plan of Blast Furnace. 



square in the centra represents the hearth , and in about 3 feet square, 

 The three tubes leading to this hearth (two of which are shown in the 

 vertical section), and which are called tuyere*, are used for introducing 

 the blast of air required to give the degree of intenseness to the heat 

 necessary for fusing the ore. 



The size and form of the blast furnaces have undergone much 

 change. Slight differences will be observed by comparing the section 

 in fig. 1, with that at col. 241 of the article FURNACE ; but the modern 

 alterations have been still more considerable. Half a century ago the 

 furnaces were generally about 40 feet high, 10 or 12 feet across the 

 boshes, and 34 feet diameter of tunnel-head, or cylinder, at the top ; 

 and they were blown with only one tuyere each. By degrees the 

 tunnel-head has been enlarged to 10 feet; the tuyeres have been 

 increased from one to three ; a new form has been given to the 

 lower part of the interior; and in some instances, such as that of 

 Messrs. Dixon's works at Guvan, near Glasgow, the whole furnace is 

 made cylindrical. The largest furnaces are now 60 feet high, and 

 about 40 feet square at the base. Engineers are still divided in opinion 

 concerning the best size of the throat, or contracted part between the 

 body of the furnace ami the tunnel-head ; but all agree that it should 

 be much longer than those formerly employed. 



We have now to notice the filling of the furnace with iron-ore, coke, 

 and limestone. The ore must previously have been roasted or calcined 



I 





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Fig. -..Filling HU.t Furnace. 



in a kiln, in order to drive off the water, sulphur, and arsenic, with 

 which it is more or less combined in iU native state : by this process it 

 loan one-sixth part of iU weight. One of the recipes for smeltin 



iron, in use a few yean ago, gave the proportion at 15 tons of roasted 

 iron-ore, 22} tons of coke, and about 6 tons of limestone ; but these 

 proportions vary according to the quality of the ingredient *. .u.i tho 

 routine of processes. The ingredients are supplied at oqui>i 

 charges, and must be intimately mixed together in the furnace. 

 The limestone must be bnoken into small pieces ; its use in to act as a 

 flux to the ore and promote its f usion. The mode of filling the furnace 

 varies a good deal. In W i. -. the blast furnaces are usually built on 

 the slopes of hills, so tbnt the minerals can easily be brought from the 

 mines to the top of the furnace ; but where the latter is built on the 

 level of the ground, as in most of the midland districts, the minerals 

 must be raised to the proper level by steam, water, or pneumatic 

 power. Inclined planes have generally been used ; but these are in 

 many works being superseded by a direct vertical lift. One of the 

 filling-mouths at the top of a furnace (of which there are sometimes 

 four) is shown in Jig, 3. The usual plan is to throw in the ore, coal, 

 and limestone in alternate barrowsful ; but Mr. Slate, in 1859, pro- 

 posed a new method. He makes the furnace-mouth very wide, and 

 places over it a 'bridge supported by iron girders. A cast-iron pipe 

 descends vertically from the bridge into the furnace, and is con' 

 down beneath the surface of the burning material; the greater jxirtion 

 of the fuel is fed in through this pipe; while the other materials are 

 fed in through the open mouth of the furnace ; insomuch that the furl 

 is always in the middle, and the ore and flux around it A valve lifts 

 up the cover of the central pipe ; and several scuttles or openings are 

 left to admit the other materials. How far the cast-iron pipe will 

 bear the intense heat, is a point not yet satisfactorily determined. 



Supposing the fire to be lighted and the minerals introduced, the 

 next matter is the important one of the blat. The heat that would 

 be produced in any furnace' by merely setting fire to the fuel which is 

 thrown into it would be altogether insufficient for the fusion of the 

 ore, if its intensity were not promoted by the forcing in of a current 

 or blast of air. For this purpose it is necessary to use a strong 

 mechanical force. Water-wheels, where they can be had, are suitable 

 agents ; but there are not many places where a sufficiently copious 

 and regular supply of water at all seasons can be commanded, and 

 the success of an iron-work would be destroyed by the failure of the 

 blast in any degree for even a short time. Steam-engines are now, 

 therefore, almost universally preferred. This power is applied to the 

 working of a blowing cylinder, which may be many times the area 

 of the cylinder of the steam-engine. If the blast thus produced 

 were passed immediately from the blowing cylinder through the 

 tuyeres to the furnace, the effect would be intermitting and irregular, 

 ceasing at the end of each stroke of the steam-piston. To remedy 

 this inconvenience the blast is carried into an intermediate chamber of 

 a spherical or cylindrical shape, called a regulator ; and as the air is in 

 a state of condensation when admitted, its effort to expand itself again 

 to its natural volume causes the continuous and regular supply to the 

 furnace which is necessary. The air thus forced into the furnace keeps 

 the heat at a high degree of intenseness. Until about thirty years ago, 

 the air thus supplied was uniformly at the temperature of the atmos- 

 phere from which it was immediately taken ; and the effect v. 

 only to produce a stream of cold air, but also to supply a quant 

 moisture which is prejudicial to the smelting process. Atmospheric 

 air always contains moisture in some degree or other, but holds a 

 larger proportion in hot than in cold weather, for a very obvious reason, 

 and this causes the furnaces not to work so well in summer as in 

 winter. By the previous drying and heating of the air these incon- 

 veniences are remedied, the consumption of fuel is lessened, and the 

 absence of moisture is said to have a beneficial effect upon the quality 

 of the iron produced. This improvement was the invention of Mr. 

 Neilson, of the Clyde iron-works, and was made the subject of a 

 jMMit. in 1829. The air, before it is forced into the furnace, is heated 

 in cast-iron vessels to 300 Fahr., or more, and is thus more nearly 

 than when at its natural temperature in a condition to support i-mn. 

 bustion. 



The precise value of the hot-blast has been a subject of very animated 

 controversy. That it has contributed greatly to the advance of the 

 manufacture is beyond all question ; but some persons appear to carry 

 this estimate to too high a degree. Just before Mr. Neilson introduced 

 his method at the Clyde Works, it was customary in that establishment 

 to use eight tons of coal for making one ton of iron; in tl. 

 following year the quantity was reduced to five tons and a <p 

 with an addition of eight cwt for heating the air before using. In 

 1881, Mr. Dixon, of Calder Works, found that if the temperature of 

 the blast were raised far above 300 Fahr., raw coal might be used 

 instead of coke in the blast furnace a most important discovery, r.i- 

 it rendered unnecessary the cost of time and money in coking the coal 

 for making certain descriptions of iron. From that time it became 

 usual to raise the blast to 600* Fahr., about sufficient to melt lead or 

 cine. In 1833, it was asserted that three tons of coal sufficed, including 

 that for heating the air, for smelting one ton of iron ; and that the 

 same quantity of air would blow four furnaces as had previously 

 used for three. The hot-blast has from the first been more favoured 

 in Scotland than in Wales or in Staffordshire; nmrtheless it i in- 

 creasing everywhere in use. Mr. Truran takes exception to some of 

 Mr. Mushet's statements concerning the enormous saving effected by 

 using the hot-blast. He contends that only a part of the economy 



