64. On hardening and tempering Steel 
is reduced to its foft ftate. Oneach of thefe ‘operations t 
fhall offer a few remarks, proved by long experience; and,. 
firft, on the choice of fteel for fuch purpofes as require the 
beft that can be procured for making cutting inftruments,. 
fuch as gravers, punches, turning tools, chifels, &e. &c. to 
be employed in turning or cutting tempered fteel, and fub- 
ftances that are too. had to be cut by tools made of ordinary 
fteel: for thefe purpofes caft fteel is undoubtedly the beft ; 
but even this fart differs in quality. 
_ The general mode of choofing fuch as is moft fuitable for 
the above purpofes is to break a bar, and obferve its. frac- 
ture, and ta feleét the clofeft grained ; but this mode is not 
always certain, owing to the difference made in the fraéture 
by the fteel being hammered under a greater or lefs degree 
of heat, fteel being much improved by being hammered 
under a low heat, and even when cold; and when over- 
heated, being quite {poiled for the above purpofe. It is ow- 
ing to this circumftance that the beft forts of caft ftecl are 
incapable of being welded as above mentioned. Another 
method is, to harden with as lowa heat as poffible a piece 
of fteel, and then to breakit, and obferve its fra¢ture : but this 
is not wholly to be depended upon; for fome fteel breaks 
with a very clofe grain, and yet is not of a good quality. 
But the fureft method is to have one end of a bar drawn out, 
into a {mall rod under a low heat, an obfcure red for in- 
fiance, or but little above; then heat it as before, and fud- 
denly plunge it into pure cold water: if it proves hard, and 
requires a great force to break it, it is good, let its fraéture. 
be what it may*; and I have always found that the {peci- 
mens that hardened with the loweft heat, and when in that 
ftate required the greateft force to break-them, proved the 
beft fteel. Having thus feleCted fteel fit for the required ufe, 
* This circumftance deferves particular attention, as workmen, in ge- 
neral, rejeét fuch fteel as breaks with a coarfe fraCture, even though it 
be of fuch a quality as to require a great force to break it, after bemg 
hardened, 
and, 
