IKON 



951 



1226 



furnace from the tuyeres upwards is cased with wrought-iron plates, those of the lower 



or conical part being inch thick, while those of the barrel vary from ^ths of an inch 



below to f ths of an inch at the top. The 



interior of the furnace is lined throughout 



with fire-brick lumps 5 inches thick, and 



of dimensions varying with the internal 



diameter, no two courses being alike. The 



backing between the inner lining and the 



shell is of ordinary fire-brick. Up to a 



short distance above the tuyeres every fire 



lump is chisel-dressed on both beds and 



joints, and the same is also the case with 



the hearth lumps, which consist of two 



courses set on an edge and breaking joint ; 



the lower course being 18 inches deep and 



the upper one 3 feet. The following are 



the principal dimensions of the furnace : 



Diameter of the hearth, 8 feet; depth at 



tuyeres, 3 feet 6 inches ; diameter at the 



bosh, 28 feet ; diameter of the bell-opening, 



13 feet ; total height from the hearth to the 



platform, 83 feet ; the cubical capacity is 



30,085 cubic feet. There are four tuyeres, 



each with a nozzle 6 inches in diameter, 



and the dam opening is 2 feet wide. The 



pig-beds are necessarily of large dimensions, 



being capable of holding 1,200 moulds for 



each furnace. The slag-boxes are eight in 



number, and large enough to contain 



upwards of 3 tons of slag each. 



Calcination of the Ironstone. This is 

 effected either in kilns, or in the open air ; 

 the object being to separate carbonic acid, 

 water, sulphur, and other substances vola- 

 tile at a red heat. The operation is per- 

 formed most effectually, and probably at the 

 smallest cost, in kilns. The interior shape 

 of the calcining-kilns differs in different 

 works, but they may all be reduced to that 

 of the common lime-kiln. A coal-fire is first lighted at the bottom of the kiln, and 

 the ironstone is placed over and around, until the floor is covered with red-hot ore ; 

 a fresh layer of ironstone, with about 5 per cent, of coal, is then laid on, to the depth 

 of 8 or 9 inches ; and when this is red hot, a second layer is added, and so on gradu- 

 ally till the kiln is filled. By the time this is done, the lowermost layer is cold and 

 fit to draw ; so that the working of the kiln is a continuous operation. When the ore 

 is calcined in the open air, a heap mingled with small coal (if necessary) is piled up 

 over a stratum of larger pieces of coal, the heap being 5 or 6 feet high, by 15 or 20 

 feet broad. The fire is applied at the windward end, and, after it has burnt a certain 

 way, the heap is prolonged at the other extremity, as far as the nature of the ground, 

 or the convenience of work, requires. From the impossibility of regulating the 

 draught, and from exposure to the weather, the calcination of ore cannot be so well 

 performed in the open air as in kilns ; and as to the relative cost of the two methods, 

 Mr. Truran calculates that the quantity of coal per ton of ore is, in the kiln, one 

 hundred-weight of small, and in the open air, two hundred-weights of small, and a 

 half hundred- weight of large ; and that while the cost of filling the kiln is barely a 

 penny per ton, that of stacking the heaps on the open-air plan, and watching them 

 during the period they are under fire, amounts to fourpence per ton. Against this 

 must, however, be placed the cost of erecting the kiln, which according to the same 

 authority amounts, for a kiln of a capacity equal to 70 tons of argillaceous ore, which 

 will calcine 146 tons weekly, to 160J. The ironstone loses by calcining from 25 to 30 

 per cent, of its weight ; it has undergone a remarkable change by the operation ; in 

 the raw state, it is a grey or light brown stony-looking substance, not attracted by 

 the magnet ; after calcination it has a dry feel, adheres strongly to the tongue, is 

 cracked in all directions, is of a light reddish colour throughout, and acts power- 

 fully on the magnet. It should be carried to the furnace as soon as possible, or if 

 kept should be carefully protected from the rain. 



The calcining kilns, fig. 1227, were erected at Middlesbrough, from the designs of 



