Where castings are to be worked, either in the lathe or planer, to dimensions only a little less than those when rough, they should be pickled. This consists in washing them with a solution of sulphuric acid and water. The castings may be either submerged in or swabbed with the solution. The effect of pickling is to cause the scale to drop off in flakes, leaving the metal bare, unprotected, and rusty. The casting should then be washed with a sal soda solution. A good pickling solution for this work is to use 1 part of commercial sulphuric acid in 10 parts of water.

Cold Chisels

It is well to use a coarser grade of steel for cold chisels than for lathe or planer tools. A coarse-grained metal is preferable because the continual hammering in use and redressing will gradually modify the granular structure until it is microscopic in texture. In dressing, it should never be heated above a cherry red, and the temper should be drawn well down so that the soft metal backs up the edge. A capacity to receive a multitude of grindings is not what is wanted. The tool must be able to endure the severe service for which it is intended. It must cut into a distorted mass of

J metal, where every blow gives it a shock tending to form a new arrangement of its particles. It never receives the steady pressure of the lathe tool; hence its powers of endurance must be greater.