IRON. 



143 



in various places, and allowed to burn till the whole 

 surface is In combustion ; when it is covered with the 

 ashes of a former fire, and left to go out. The coke 

 is made harder and more pure, when the cooling of 

 the heap is quickened by throwing on cold water. A 

 slight knowledge of chemistry is sufficient to show, 

 that much of the coal must be converted into ashes 

 before combustion can be carried a sufficient length 

 to coke the heap, and the more economical process 

 of coking in a close oven, or furnace, is now becom- 

 ing more general. The ovens are of a hemispherical 

 form, about ten feet wide at the base, and two feet 

 at the aperture, the wall being of brick, eighteen 

 inches in thickness. There is a door-way in the side, 

 for the purpose of taking out the coke, and the 

 opening at the top is for charging the oven with 

 fresh coal. Small refuse coal is used. The oven being 

 filled up to the springing of the arch, and the heat of 

 the oven from the former coking being adequate to set 

 the coal on fire, the door-way is filled up with loose 

 bricks, and the air, rushing through the crevices, sup- 

 ports the combustion until the whole charge is lighted 

 up, when the door way is plastered up, excepting the 

 top row of bricks, and in twelve hours after covered 

 entirely. The chimney remains open until the flame be 

 extinguished, when it is closed, and the whole allowed 

 to remain for twelve hours more, after which the coke 

 is withdrawn from the doorway. The coke thus 

 formed is of a grey colour, metallic lustre, and very 

 hard ; but when it is required to be of a nature more 

 resembling charcoal, the coking is prepared in a 

 place similar to a baker's oven, the door of which is 

 kept constantly open, and the coals frequently stirred. 

 Coke made in this way is black in colour, porous, 

 and very light more inflammable than the first de- 

 scription, but not capable of affording such intense 

 heat, nor so durable in the smelting furnace. 



The construction of the smelting furnace, will be 

 understood, from the subjoined section. 



Fig. 1. 



The interior of the furnace, is a cavity, formed by the 

 frusta of two cones joined at the base, and terminated 

 in cylinders both at top and bottom, as will be seen 

 at G, in the figure. The wall d d of this cavity, con- 

 sists of the best fire brick, well cemented together, 

 the thickness of the wall being generally fourteen 

 inches. At a distance of about six inches behind 

 this wall, a wall or casing of brick is built all round 

 the former, and of a thickness of fourteen inches. 

 The space between these two walls is filled up with 

 river sand, crammed in compactly. Sand being but 

 an indifferent conductor of heat prevents the casing 

 b b from being much affected by the fire of the fur- 

 nace. The whole is enclosed by the outer wall, A 

 A, of ashlar stone, or brick. This wall is built 

 very strong and thick; the interior is of course made 

 circular to envelope the casing b t, but the exterior 

 face of the wall, is made to terminate in four faces, 

 tapering to the top, so that the outward appearance 



of the furnace, is a truncated quadrangular pyramid. 



The inside of the furnace, G, is made to terminate in 

 a cylindrical chimney, and at the bottom, in a deep 

 quadrangular pit H. Such is the construction of 

 the furnaces erected in this country till of late, the 

 whole building being made for substantiality as thick 

 as possible. But the strong heat of the furnace, 

 frequently so expanded the material, as to burst the 

 mason work, and the modern furnaces are all 

 constructed of comparatively thin walls above A A, 

 nor is there any space left in them for the intro- 

 duction of sand between the two interior walls of 

 l;rick work. At the top of the chimney, there are 

 formed two or more doors by which the workmen in- 

 troduce the ore, coke arid flux, and above this there 

 is a semicircular wall E erected for the purpose of 

 preventing the flame from blowing upon the work- 

 men while they are feeding the furnace. The mate- 

 rials are drawn up on a mound of earth at the back 

 of the furnace, either by machinery or by animal 

 strength, and being set fire to at the bottom, are 

 allowed to burn, the combustion being afterwards 

 accelerated by a blast from a blowing machine. (See 

 blow ing machine}. The ends of the pipes from the 

 blowing machine enter nearly at the bottom of the 

 furnace, as may be seen by inspection of figure 4. 

 These blast pipes, the nozles of which are technically 

 called tweers, are two in number, and enter the 

 furnace opposite to each other, and a little above that 

 point where the melted metal rests. The ore, coke 

 and flux, in the body of the furnace, are acted upon 

 by the heat, just as they would be in a close vessel, 

 the oxygen of the ore combining with the carbon or 

 the coke, and forming carbonic acid, or rather 

 carbonic oxide. On the ore parting with its oxy- 

 gen, the carbon combines with the metal, and the 

 mass being reduced, falls down to a lower part of the 

 furnace, and in this way, makes room for more 

 to come down to the hotter part, and in its turn be 

 smelted, and the liquid metal to fall down to the 

 bottom of the part H, called the hearth. It may 

 be observed, that of late it is not usual to construct 

 the hearth as deep in proportion as it is shown in 

 the foregoing section. 



There is an opening in the wall at the bottom of the 

 hearth, at the mouth of which a stone is placed, called 

 the dam stone, beyond which an opening is made in 

 the side of the outer wall, in order to run off the liquid 

 metal when it rises so high as to canse the scoria to 

 flow over the dam. The opening in the outer wall 

 is closed by a lute of sand mixed with clay, during 

 the process of smelting; but when there has been a 

 sufficient quantity of metal formed, the lute is remov- 

 ed, and the iron allowed to run off into a channel, 

 made in a kind of sand. From this channel, called 

 the sow, numerous side branches are led, called pigs, 

 and as the melted metal flows along the sow, it is 

 checked frequently by the workmen introducing a 

 piece of wood which causes it to flow into the side 

 channels, and thus the masses of iron called pig 

 iron are formed. 



The height of the smelting furnace is sometimes not 

 less than sixty feet, but the usual height of the fur- 

 naces in this country, is about forty-five or fifty feet. 

 The proportions of the parts may be guessed at, on 

 inspecting figures 1. and 2. in this article. It may 

 be stated in addition to the description already given, 

 :hat there are numerous small openings through the 

 sides of the walls, to permit the escape of the va- 

 pours and gases formed during the process, and to 

 snsure durability, the whole of the mason work is 

 aound with bars of iron. Dr Ure states, that a 

 furnace of ordinary dimensions, will make about 

 :hree and a half tons of cast iron, these furnaces being 

 tapped once in twelve hours. For the production 



