244 



METALS. 



sued. It consists in burying the wrought metal in 

 iron boxes containing charcoal, and heating it for 

 several days, till combination with a certain portion 

 of the carbon is effected. 



604. ANNEALING. The hardness of 



How is steel . ..... 



made soft or steel depends upon the rate at which it is 

 cooled. By heating it to redness, and 

 cooling it slowly, it is rendered as soft and malleable as 

 wrought iron. This process is called annealing. By 

 cooling it very suddenly, it becomes as hard and brittle 

 as cast iron. Steel instruments are commonly ham- 

 mered out of the soft steel, and subsequently hardened. 

 How is steel 605. TEMPERING STEEL. Steel hardened 



tempered? as above described is too hard and brittle 

 for most uses. Any portion of its original softness and 

 tenacity may be returned to it, by reheating and slow 

 cooling. To restore the whole, a red heat would be 

 required. To give back part, and make a steel so 

 tough as not to break readily, yet sufficiently hard for 

 cutting, a lower temperature is employed. This process 

 is called tempering. The sort of temper imparted de- 

 pends upon the degree of heat which has been em- 

 ployed. 



606. The proper temperature is ascertained 



How is the , . , , , 



proper heat by the color which the steel assumes when 



ascertained? heated< Toolg for cutting metal are heate d 



till they become a pale yellow ; planes and knives, to 

 a darker yellow ; chisels and hatchets, to a purplish 

 yellow ; springs, till they become purple, or blue. In 

 each case they are afterward slowly cooled. These 

 colors are owing to a film of oxide of iron, which is 



