The pigment, domestic or imported, is classed as hydrated silicate of alumina and should therefore be insoluble in water, alkali or dilute acid solutions. It is decomposed, however, by long boiling with strong sulphuric acid, forming alumina sulphate in solution and a precipitate of silica. The finer the grade the more greasy the feel between the fingers, while domestic kaolin always feels more rough, though it may be perfectly free from grit. To test for fineness, spread it mixed with turpentine on a strip of dry glass, treating a selected standard similarly. This test by permitting the turps to evaporate will also serve to test whiteness or absence of discoloration. Some china clays or kaolins are more opaque than others, and when the pigment is to be used as an extender for white paint, the most opaque should be selected, while when used for reducing color that with less opacity is best, as it does not absorb so much of the color, which is usually much higher in cost than the clay. A simple test will determine this. Weigh out one drachm of each clay to be compared, also for each sample of clay 3 Troy grains of ultramarine blue and on a slab of marble or glass, mix the clay and blue with as many drops of oil as is necessary to make a rub-out, comparing all on a strip of glass, side by side, the clay that is colored most deeply by the blue being the least opaque, because it does not resist color as much as that which is more opaque. If this test is carried out accurately it will also show which of the clays requires most oil by noting the consistency of the rub-out.
China clay is not used to the extent it deserves in paint making, because of its great oil absorption and on account of its becoming rather transparent when ground in oil. It should not really be classed as an adulterant for the reason that it does not pay to use it as such, as there is more oil required to mix and grind it than is the case in grinding some of the pigments that are really adulterated. The average specific gravity of china clay or kaolin is 2.25, and a gallon of bolted or pulverized dry clay packed will not weigh over 6 1/2 to 6 3/4 pounds. It requires 30 pounds, or nearly 4 gallons of linseed oil to mix 70 pounds dry china clay to a stiff paste, while 55 pounds of oil and 45 pounds of clay will be about the right consistency to be spread with a brush. Wherever whiting is barred out as an extender for heavy pigments for the reason that the presence of carbonate of lime makes the paint subject to disintegration from contact with sulphur gases, or that the alkalinity of whiting affects the color, as is the case with Chinese or Prussian blues, china clay or kaolin will be the best pigment to replace it. It is really a better suspender for heavy pigments than the ordinary grades of whiting and only its higher cost, and the fact that it is so great an oil absorber, are against its more extended use. Bolted English china clay has also been offered to the trade under the name of English kalsomine for use in tints with colors that are not alkali proof, such as blues, greens and reds. Many liquid fillers for soft woods, or in fact, most of them contain china clay as the only pigment, while it has also been used in part as pigment for paste hardwood fillers along with some other white mineral substances or with starch. It is not so long since, that it was the custom to prepare the lower priced shade cloth by running muslin through a size prepared by cooking equal parts by weight of cheap starch and china clay in water to a paste, coloring same with aniline colors, and running the muslin so filled or painted over three heated cylindrical rollers of a calender, thus obtaining a fine and fairly well wearing finish on these low-priced shades.