Report on the Art of making fine Cutlery. tig 



throughout. This difficulty forme* a very confiderable impediment to my fucccfs in a 

 courfe of delicate fteel work, in which I was engaged about feven years ago j but after 

 various unfuccefsful experiments, I fucceeded in removing it by the ufe of a bath of melted 

 lead, which for very juftifiable reafons has been kept a fecret till now. Pure lead, that i$ 

 to fay, lead containing little or no tin, is ignited to a moderate rednefs, and then well 

 ftirred. Into this the piece is plunged for a few feconds ; that is to fay, until when 

 brought near the furface that part does not appear lefs luminous than the reft. The piece 

 is then fpeedily ftirred about in the bath, fuddenly drawn out, and plunged into a large 

 mafs of water. In this manner a plate of fteel may be hardened fo as to be perfedlly brittle, 

 and yet continue fo found as to ring like a bell ; an efFe£l which I never could produce in 

 any other way. Mr. Stodart has lately made trial of this method, and confiders it to be a 

 great acquifition to the art, as in fa£l I found it. 



The letting down, or tempering of hard fteel, is confidered as abfolutely neceflary for 

 the produdion of a fine and durable edge. It has been ufual to do this by heating the 

 hardened fteel, till its bright furface exhibits fome known colour by oxidation. The firft 

 colour is a very faint ftraw colour, becoming deeper and deeper by increafe of heat, to a 

 fine deep golden . yellow, which changes irregularly to purple, then to an uniform blue, 

 fucceeded by white and feveral fucceflive faint repetitions of thefe feries. It is well known, 

 that the hardeft ftate of tempered inftruments, fuch as razors and furgeon's inftruments, 

 is indicated by this ftraw colour ; that a deeper colour is required for leather cutter's knives, 

 and other tools that require the edge to be turned on one fide ; that the blue which indi- 

 cates a good temper for fprings, is almoft too foft for any cutting inftrument except faws, 

 and fuch tools as are fliarpened with a file ; and that the lower ftates of hardnefs are not 

 at all adapted to this ufe. But it is of confiderable importance, that the letting down or 

 tempering, as well as the hardening, fliould be efi'eiSed by heat equally applied, and that 

 the temperatures, efpecially at the lower heats, where greater hardnefs is to be left, fliould 

 be more precifely afcertained than can be done by the different ftiades of oxidation. Mr. 

 Hartley firft pra6lifed the method of immerfing hard fteel in heated oil, or the fufible com- 

 pound of lead five parts, tin three, and bifmuth eight. The temperature of either of thefe 

 fluids may be afcertained in the ufual manner, when it does not exceed the point at which 

 mercury boils : and by this contrivance' the fame advantages are obtained in lowering the 

 temperature of an whole inftrument, or any number of them at once, as have already been 

 ftated in favour of my method of hardening. Oil is preferable to the fufible mixture for 

 feveral reafons. It is cheaper \ it admits of the work being feen during the immerfion by 

 reafon of its tranfparency ; and there is no occafion for any contrivance to prevent the 

 Work from floating. 



I requefted Mr. Stodart to favour me with an account of the temperatures at which the 



feveral colours make their appearance upon hardened fteel ; in compliance with which he 



made a feries of experiments upon furgeons needles hardened, highly poliihed, and ex- 



VoL. IV. — June 1800, S pofed 



