Cast SteeL 
431 
j of the fire, and drop the pieces into the water in such 
manner as will allow them to come as little as possible in 
contact with the air. This method answers two good 
purposes at once, causing the heat to be more equally ap- 
plied, and preventing the contact of the air, and of course 
any scaling ; and when the work has been polished and 
well defended from the air, it comes out nearly as clean as 
it was before. When the greatest possible hardness is 
required, it may be obtained by using quicksilver instead 
of water ; but this can only be employed for small arti- 
cles. For some purposes steel is required to have a su- 
perior degree of hardness given to its surface, such as in 
the case of files, &c. This is obtained by using a coarse 
powder made of leather slightly burned, hair or horn, 
either in raspings or in powder ; this is mixed with a 
little common salt, and the files when just red hot, are 
thrust into a heap of this powder, some of which adhering 
to their surface is carried into the fire with them, and gives 
them a case hardening: the salt fluxes upon their surface, 
and defends them from the air while passing from the 
fire into the trough of water, into which they are plunged 
to harden. The workmen say, the longer this water is 
used for this purpose the better. 
We are now come to the last process called tempering, 
for one method of which see Mr. Collier’s paper ; but 
that method cannot be conveniently applied in all cases, 
and has several disadvantages, some of which I shall 
mention. First, each piece must be made bright that the 
change of colour may be better seen, and must be heated 
singly or nearly so ; and pieces of irregular figure can- 
not be made to receive an equal degree of heat in all their 
parts, so that some will be softer than others. These 
circumstances would retard the manufactory of many ar- 
ticles very much, and prevent their being afforded at the 
present prices, such as the springs of gun locks, door 
