96 "On hardening and tempering Steel. 
of hardnefs given to its furface, fuch as in the cafe of files, 
&c. This is obtained by ufing a coarfe powder made of lea- 
ther flightly burned, hair or forte either in rafpings or in 
powder: this is mixed with a little common falt, and the 
files, when juft red hot, are thruft into a heap of this powder, 
fome of which adhering to their furface is carried into the 
fire with them, and gives them a cafe hardening: the falt 
fluxes upon their furface, and defends them from the air 
while paffing from the fire into the trough of water, into 
which they are plunged to harden. The workmen fay, the 
Jonger this water is ufed for this purpofe the better. 
We are now come to the laft procefs called tempering, 
for one method of which fee Mr. Collier’s paper; but that 
“method cannot be conveniently applied in all cafes, and has 
feveral difadvantages, fome of which I fhall mention. “Firft, 
each piece muft be made bright that the change of colour 
may be better feen, and muft be heated fingly or nearly fo ; 
apd pieces of irrerular figure cannot be made to receive an 
equal degree of heat in all their parts, fo that fome will be 
fofter than others. Thefe circumftances would retard the 
manufactory of many articles very much, and prevent their 
being afforded at the prefent prices, fuch as the fprings of 
gun locks, door locks, various articles in clock and watch 
work, &e. &e. The neceflity of making them bright 
enough to mark the change of colour is obviated by fmear- 
ing them with oil or tallow, which helps to apply the heat 
more uniformly, and marks the temper as well as by obfery- 
ing the colour, or nearly fo; or by putting the things to be 
tempered into a proper veffel, and adding fo much oil or 
tallow as will cover them, and then holding them over the 
fire or the flame of a lamp until a fufficient heat is given. 
By this means the moft irregular pieces may be uniformly 
heated, and, great numbers maybe done at ong time, and 
with great certainty: thus are clock and watch pinions, 
watch verges, balances, &e, tempered; fometimes, many 
"dozens at once; and no more time is neceffary for the 
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