Method of coating inorganic pigments (ultramarine and bronze powder) with dense amorphous silica

A process for preparing a stable inorganic pigment which comprises adding an aqueous solution of sodium silicate having a concentration of sodium silicate of from 1 to 10 wt % as SiO.sub.2 and an aqueous solution of one or more compounds simultaneously to an aqueous slurry of pigment having a pigment concentration of about 50 to 300 g/l to react the two solutions and thereby form and deposit a continuous non-porous coating of amorphous silica on the surface of said pigment, wherein said reaction is carried out at a temperature of at least about 60.degree. C. to 100.degree. C. for 1 to 6 hours such that the pH of the slurry is controlled constant at a predetermined pH within the range of from 7 to 11, said compounds having a solubility of at least about 0.01 g/100 g water at 20.degree. C., providing a pH of at least about 1.0 in a 3.0 wt % aqueous solution and being selected from the group consisting of aliphatic carboxylic acids, oxycarboxylic acids, an ester or aldehyde of said above acids, a sodium salt of hydrogen sulfate or hydrogen carbonate, and ammonium salt of a halogen acid or a sulfurous oxyacid and ethylene carbonate, wherein said pigment is selected from the group consisting of ultramarine and bronze powder.