Some Simple tests to determine best Kaolin for Paint Industry:
1. Kaolin is a hydrated silicate of alumina and hence it should be insoluble in water, alkali or dilute acid solutions. However long boiling with strong sulphuric acid should form alumina sulphate in solution and a precipitate of silica.
2. To test the fineness, spread the China Clay mixed with turpentine on a strip of dry glass. This test by permitting the turpentine to evaporate will also serve to test whiteness or absence of discoloration too.
3. To test opaqueness, Mix Clay and ultramarine blue powder with as many drops of oil as necessary for rubbing on a slab of marble or glass. If the clay is deeply colored with blue then it is less opaque as it does not resist color, else it is more opaque. This test will also show how much oil is required by china clay for rub-out.
4. Check the Grit content on 400 mesh. It should be zero percent to give a uniform texture to the paint.
