Metal Working Metals 31 



Brass castings, rich in zinc (up to 30 per cent), are of 

 a light colour, and are liable to contain " blow-holes," caused 

 by the vaporization of the zinc at the time of its addition, and 

 too early a " pouring " of the alloy. In making brass, the zinc, 

 being easily vaporized, is added to melted copper immediately 

 before casting takes place, and if stirred or " poled " with a 

 wooden rod, the product loses some of its hardness ; yet some 

 process of stirring should be adopted in order to secure even 

 admixture and freedom from blow-holes. 



Compromise is nowadays effected by reducing the propor- 

 tion of zinc, and producing the softer " red brass " more like 

 gunmetal, which, however, becomes too soft for the purposes to 

 which brass is applied if the proportion of zinc is lower than 

 10 per cent. 



Brass for making into sheet contains a higher proportion of 

 zinc (15 per cent.), in order that subsequent rolling may not 

 remove too much of the hardness to which the alloy owes its 

 popularity. Sheet brass may be softened for working by making 

 it nearly red hot, and quenching in cold water, the hardness 

 being restored by re-heating and allowing to cool slowly in the 

 air, or, if great hardness is required, in sand or sawdust. It will 

 be noted that the opposite effects are produced upon steel by 

 the above treatment. 



Brass is largely used in instrument making, as it does not 

 easily rust or corrode, yet is sufficiently hard and tough to stand 

 ordinary strains, and sufficiently easy to work to ensure a 

 reasonable price for the resulting product. 



Rod brass is cut with a hack saw, and sheet brass with 

 shears. Brass rod may be obtained in any size and shape of 

 section, and sheet in varying thicknesses, from ^ to J". 



Brass turns red when rubbed with an acid, as the zinc is 

 dissolved out, in preference to the copper. 



Commercial " Tin " consists of a thin sheet of iron, coated 

 on each side with tin. When the coating is thin, the iron soon 

 shows through, and rusting commences. 



The material is not in general use in laboratories, save for 

 accessories of little importance. 



It is cut with shears or snips, is very easily worked, being 



