Mr. J. Napier on Copper Smelting. 347 



These brief descriptions will enable us now to consider the 

 nature of the materials best fitted for forming the different parts 

 of the furnace ; and although modifications and improvements 

 have been proposed in the construction of some of these furnaces^ 

 which will be referred to afterwards^ they do not affect the pre- 

 sent inquiry'. 



The hearth or chamber of the furnace may be looked upon as 

 a large crucible, in which the ore or other matters undergoing 

 the process of manufacture are melted; it will therefore be 

 obvious that the matters forming this crucible must be capable 

 of withstanding a higher heat before melting than the svib- 

 stances put into it. Lime, silica, alumina, &c. alone, v/ill stand 

 any degree of heat before fusing ; but a mixture of these at a 

 high heat would combine and form a fusible substance ; hence 

 the relation of the materials forming the lining of the furnace 

 with the matters to be heated in it has also to be considered. 

 Thus, were Me to line a furnace with pure silica, which no fur- 

 nace heat would touch, and heat in that furnace lime, oxide of 

 iron, oxide of copper, &c., the silica and these oxides would soon 

 fuse and form glass ; so that great care is required to prevent this 

 combination of the bricks with the substances undergoing fusion. 

 Thus soiiie kinds of brick will answer for one sort of furnace 

 and not for another ; one kind may stand well in the fire-place, 

 and not be suited for the hearth, and vice versa. Some kinds 

 will stand in the melted matter that will not stand on the roof 

 exposed to the fumes ; bricks with much alumma in them will 

 not stand exposed to melted copper. All kinds of brick or 

 clay give way rapidly round the sides of the furnace at a line 

 corresponding to the surface of the fused metal. It need hardly 

 be stated that these remarks are equally applicable to the clay 

 or other matters used for making the bricks ; for good bi'icks 

 are of little avail if there be not equally good clay to bind them 

 together. 



Another property requisite in the bricks and clay used for lining 

 furnaces is, that they must not be liable to crack, cither by the 

 intense heat or by a sudden current of cold air passing over them 

 when hot, which in furnaces cannot be altogether avoided. A 

 crack taking plac • in the bottom or sides of a furnace is a most 

 serious affair; neither should the bricks be porous. In making 

 crucibles, their liability to crack is lessened by mixing with the 

 clay some sand, grovmd fire-brick, and gra])hite. Hessian cru- 

 cibles, the best of all clay crucibles, are made by mixing the clay 

 with half its weight of sand. Black-lead crucibles are made by 

 mixing graphite or plumbago with the clay ; but when too much 

 of these materials is used, the crucible becomes porous, and 

 allows the matters fused in them to pass through. The fol- 



