410 APPLIED SCIENCE 



at a time so as to ascertain the heat in the center of the 

 mass. When the box has been exposed to the fire the desired 

 length of time, its contents are quickly dumped into cool 

 water. 



Sieves of iron netting are laid on the bottom of the tub 

 into which the case-hardening material is dumped, so that 

 the hardened articles may be conveniently taken up from the 

 water by one of the sieves. The case-hardening material 

 itself is taken out by another sieve which is of very fine 

 netting and which is placed under the first one. The material 

 is dried and used over again with the addition of a little new 

 material. 



When articles are finished before hardening, this pro- 

 cess gives a very fine color to both soft steel and wrought 

 iron. 



Case-hardening may also be effected by packing the articles 

 in soot, but this process does not give a good color. Horn 

 and hoof are also used for case-hardening. Malleable iron 

 may be case-hardened, but requires careful handling to 

 prevent its cracking and twisting out of shape. 



470. Annealing. Of the several methods of annealing 

 steel, the most common, when but a few pieces are to be 

 treated, is to heat the metal red hot and then bury it in 

 ashes, powdered charcoal, or lime. If the pieces are small, 

 it is advisable to heat a heavy piece of iron red hot and bury 

 it in the ashes or other material, leaving it until the articles 

 to be annealed are heated to the desired temperature. The 

 heavy piece is then removed and the articles are placed in 

 hot ashes. If the small pieces of steel are buried in cold 

 ashes or lime, or if the material is damp, the metal may be 

 chilled and thus rendered hard. Large pieces are not easily 



