When iron castings are taken from the mold they present a rough surface 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 separate 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 neutralizes the acid. Otherwise the acid will continue to eat or corrode the metal.