70 ANNUAL OF SCIENTIFIC DISCOVERY. 



Hence, also, the necessity of moving about in the water even articles of a 

 pound or two in weight, to remove them away from the steam as it is gen- 

 erated upon their surfaces, and thus promote more rapid cooling. 



" It is a good plan to harden hammer-faces, where there is a tuh and 

 water-tap conveniently near, by plunging the red-hot hammer, held with the 

 face upwards, into the water, so that a stream from the tap may fall upon its 

 face. The face of hammers and anvils is ground after being hardened, but 

 should never be tempered." Orr's Circle of the Sciences. 



HARDENING IRON AND STEEL. 



A correspondent of the London Engineer Journal gives the following in- 

 teresting memoranda, the result of his experience on the hardening of iron 

 and steel : 



" Hardening steel is a very peculiar operation, and is one of the greatest 

 contingencies in the manufacture of articles into which it is transformed. 



O 



Under the most careful management, I have seen very expensive articles in 

 tools and cutlery rendered perfectly useless through the seeming caprice of 

 the two elements, fire and water ; if such articles had been rubbed in prus- 

 siate of potash, which gives the metal a sort of liquid case, I think cracking 

 in the water, so common an occurrence with superior articles, would be pre- 

 vented, particularly if the water used were soft, and by the infusion of a 

 little hot water rendered lukewarm. In hardening iron the very opposite 

 course should be pursued ; have the water cold as possible, the harder the 

 better; a little quick lime in it would also be an improvement, and if the 

 iron to be hardened be heated nearly to a white heat, rubbed with or rolled 

 with pulverized prussiate of potash, a steel surface is sure to be obtained. 

 The use of prussiate of potash might be a great improvement to the tools 

 used by miners. Their picks and spades would wear longer if hardened 

 with it in the manner I have described. It must be remembered that it is 

 only the surface of the iron which is affected, and the hardening will not 

 penetrate more extensively than the thickness of ordinary tin plates ; but the 

 resistance is so superior to that of iron unhardened, that it would be a great 

 saving in the cost of working-tools. There is another advantage ; it would 

 not render the iron brittle, consequently there would not be an increase in 

 breakage, which is of considerable importance to the owners of extensive 

 workings." 



From another soursc also, we obtain the following : When a piece of 

 steel is crooked, or when some portions of it are thicker than others, or 

 the whole is very thick, the ordinary process of hardening in water is im- 

 practicable, as the piece would crack or twist. For this reason, when ex- 

 actness is required in a piece of machinery, it is made of soft steel, though 

 hardened metal would be preferable in most cases. The following process 

 does away with the difficulties just mentioned. Let a vessel be half filled 

 with clear water, half with oil; when the heated piece is blood-red, dip it in 

 the upper layer of oil, and as soon as this ceases to boil let it go to the bot- 

 tom into the water. This plan gives steel the proper degree of hardness for 



