Nothing is more necessary to the successful use of polish ing powder than equality in the grain. Fine dust clogs the action of coarse grinding powders, and prevents them from cutting with rapidity the object to be ground; coarse particles mixed with fine polishing powder scratch the article to be polished, and render grinding and polishing necessary again. To secure fineness and uniformity no process equals that of elutriation, which is thus performed: Suppose it were desired to separate the ordinary flour of emery into three different degrees of fineness. Take three vessels (such as tin pails or glass jars) and mix the emery with a large quantity of water - say a quart of water to l 1/2 oz. of emery. Stir the mixture until the emery is thoroughly diffused through the liquid, and allow to stand five minutes. By this time all the heavier particles will have settled, and on pouring the fluid into a second jar only the finer portion will be carried over. So continue to wash the first residuum until nearly all the particles have subsided at the end of five minutes, and the water is left comparatively clear. You will now have the coarse portion, No. 1, by itself.
So, from the sediment collected from the washings of No. 1, you may collect a portion, No. 2, having a second degree of coarseness. The last and finest will be obtained by letting the final wasliings stand ten or fifteen minutes, pouring off the liquid and allowing it to settle.
The principal polishing powders are chalk or whiting, crocus or rouge, emery, oilstone powder, and putty or tutty, which latter consists chiefly of oxide of tin. Other powders, such as tripoli, bath-brick, sand, etc., are rarely used for the finer kinds of work. Emery is so well known that it does not need description.
Chalk is a native carbonate of lime, consisting of the remains of minute creatures known as for-aminifera, and when simply scraped or crushed under a hammer or runner, it is sometimes used for polishing such side substances as bone, ivory, etc. As it contains particles of silica of varying size, it cuts freely, but is apt to scratch. To remove the gritty particles, the chalk is ground, and the finer parts separated by washing. It then becomes whiting, which is generally sold in lumps. Whiting has very poor cutting qualities, and it is therefore used chiefly as plate powder for cleaning gold, silver, glass, etc., and for absorbing grease from metals which have been polished by other means.
This is a manufactured article, prepared by adding a solution of carbonate of soda to a solution of chloride of calcium (both cheap salts), so long as a precipitate is thrown down. The solutions should be carefully filtered through paper before being mixed, and dust should be rigorously excluded. The white powder which falls down is carbonate of lime, or chalk, and when carefully washed and dried, it forms a most excellent polishing powder for the softer metals. The particles are almost impalpable, but seem to be crystalline, for they polish quickly and smoothly, though they seem to wear away the material so little that its form or sharpness is not injured to any perceptible degree.