IRON AND IRON MOLDING 381 



especially in heavy work. Loam work varies greatly in the 

 degree of skill required. Some classes of loam molds permit 

 the employment of inexperienced workmen, while others 

 demand extraordinary experience, skill, and good judgment 

 in their production. 



While the finished products of the three types of molding 

 differ, as outlined, in shape, size, and use, the method is 

 practically the same in all cases. 



424. Brass Foundry. Brass castings are made by heating 

 copper and zinc in pots in a furnace, and pouring the alloy 

 into a mold. The principal differences between molds for 

 brass and molds for iron are that brasswork molds are 

 made from finer and cleaner sands; that in brass molding a 

 greater allowance must be made for contraction; and that 

 different facings, parting sands, and finishings are used. 

 Very nearly the same blackening mixtures are used in brass 

 molding as in iron molding, and the methods of drying 

 and venting are practically the same in both classes of work. 

 The method of cleaning brass castings is the same as that of 

 cleaning iron castings. 



425. Properties of Cast Iron. Cast iron has certain ad- 

 vantages and disadvantages as a material. It is easy to give 

 it any desired form by molding. It resists oxidation (rust) 

 better than either wrought iron or steel. Its compressive 

 (crushing) strength is very high, but its tensile (stretching) 

 strength is comparatively low. It cannot be riveted or 

 welded by forging. It is brittle, breaking off without giving 

 much warning, and stretching but little before giving way. 

 It is liable to have hidden and small surface defects and air 

 bubbles, which make its strength uncertain. 



