118 Professor Brande on the Manufacture 



though not Very easily explained upon mechanical or chemical 

 principles. We know by experience, that it is a property of 

 this highly valuable substance, to become excessively hard, if 

 heated and suddenly cooled ; if, therefore, we heat a bar of 

 soft malleable and ductile steel red hot, and then suddenly 

 quench it in a large quantity of cold water, it not only becomes 

 hard, but fragile and brittle. But as a die is a mass of steel 

 of considerable dimensions, this hardening is an operation 

 attended by many and peculiar difficulties, more especially as 

 we have at the same time to attend to the careful preservation 

 of the engraving. This is effected by covering the engraved 

 face of the die with a protecting paste, composed of fixed oil 

 of any kind, thickened with powdered charcoal : some persons 

 add pipe-clay, others use a pulp of garlic, but pure lamp-black 

 and linseed oil answer the purpose perfectly. This is thinly 

 spread upon the work of the die, which, if requisite, may be 

 further defended by an iron ring; the die is then placed with its 

 face downwards in a crucible, and completely surrounded by 

 powdered charcoal. It is heated to a proper temperature, that 

 is, about cherry red, and ii^hat state is taken out with proper 

 tongs, and plunged into a cistern of cold water, of such 

 dimensions as not to become materially increased in tempera- 

 ture ; here it is rapidly moved about, until all noise cease, and 

 then left in the water till quite cool. In this process it should 

 produce a bubbling and hissing noise ; if it pipes and sings, we 

 may generally apprehend a crack or fissure. 



I have found no process answer better than the above simple 

 and common mode of hardening dies, though I have given 

 others repeated and fair trials. It has been proposed to keep up 

 currents and eddies of cold water in the hardening cistern, by 

 means of delivery-pipes, coming from a height ; and to subject 

 the hot die, with its face uppermost, to a sudden and copious 

 current of water, let upon it from a large pipe, supplied from 

 a high cistern ; but these means have not in any way 

 proved more successful, either in saving the die, or in giving it 

 any good qualities. It will be recollected, from the form of the 

 die, that it is necessarily only, as it were, case-hardened, the 

 hardest strata being outside, and the softer ones within, which 

 envelope a core, something in the manner of the successive 



