Metal Working Additional Processes 63 



method in laboratory work. The object may be difficult of 

 access or it may have attached to it some combustible material 

 not convenient to remove ; consequently one has to exercise 

 considerable ingenuity in the accomplishment of one's task. In 

 one actual case, for instance, the steel pin of an air pump pinion 

 had to be re-hardened while in position, surrounded as it was 

 by rods, barrels, and pillars. This was attained by polishing, 

 touching with a hot poker till the correct colour appeared, and 

 quenching by means of a jet of water from a wash bottle. 

 Difficulties such as this are of everyday occurrence to the 

 repairer of scientific apparatus, and they may usually be dis- 

 posed of by the exercise of a little imagination and ingenuity. 

 Any one who has estimated bromine in the presence of nitrates 

 and iodides will have little trouble with mechanical difficulties. 

 The following suggestions may be of use. 



Before tempering steel, it is necessary to " draw "the temper 

 completely by heating to a full red heat for a few minutes, and 

 subsequently cooling rapidly. This makes the metal very hard. 

 It is then heated up a second time, and tempered. The method 

 of doing this greatly depends upon the article to be tempered, 

 a chisel or heavy solid which retains heat for some time being 

 treated as already described on page 41, but a watch spring 

 obviously cannot be so treated. Such a spring would be heated, 

 cooled quickly by dropping in water, and then polished. An 

 iron plate, sufficiently large to take the whole spring, would 

 then be made red hot, held above a bucket of water, and the 

 spring dropped upon this plate. When the spring became 

 purple, it would be dropped off the plate into the water, and 

 so be tempered correctly. The success of this operation 

 depends upon the accurate contact of the spring and plate 

 over the whole surface covered by the spring. Small twist or 

 wing drills may be similarly treated. Flat springs may be 

 treated by first hardening, smearing over with tallow, attaching 

 a wire about a yard long to the spring, and putting in a fire 

 until the tallow has just burnt off completely, then withdrawing 

 sharply and swinging round in the air until cold. Penknife 

 blades may be tempered similarly. 



Various mixtures are recommended for quenching, from 



