IRON AND IRON MOLDING 375 



418. Clearing and Smoothing Castings. When iron 

 castings are taken from the mold they present a rough sur- 

 face and must be cleaned and smoothed before they can be 

 assembled into a machine. This is done in various ways, such 

 as by means of emery wheels and revolving wire brushes, by 

 rotating the casting in " tumblers" or " rattlers," by chipping 

 with pneumatic chisels, or by removing the scales by means 

 of a sand blast. In these processes a great amount of dust 

 may arise, but light polishing on emery wheels equipped with 

 good hoods and adequate exhaust ventilation gives rise to 

 but comparatively little dust. 



The scales which sometimes form on castings are removed 

 also with dilute sulphuric acid. The fumes arising in this 

 process, while the castings are draining, are very irritating 

 to the nose and throat. Small castings may be dipped into a 

 tank set into the floor. The acid is thrown over large ones 

 resting on the floor, which is so constructed as to permit the 

 excess of acid to drain back into the tank. This process is 

 termed " pickling" and the chemical action which occurs is 

 that the sulphuric or other acids partially dissolve and sep- 

 arate the scales or oxides on the surface of the metal, by 

 acting on the metal underneath the scales. Since acids act 

 on all iron, it is necessary to remove all traces of the pickling 

 by washing the casting with water and alkali, which neu- 

 tralizes the acid. Otherwise the acid will continue to eat or 

 corrode the metal. 



419. Molding. Most of the iron used in industry is 

 made into castings. Castings are made by pouring hot mol- 

 ten iron into a mold, which is a body of a certain kind Of 

 sand held in a boxlike frame called a flask. The top part of 

 the flask is called the cope, the lower part the drag (Fig. 187), 



