Chemistry
The metallic salt or binder used must be inert and insoluble in the vehicle, and it must be white or very neutral. It must have low tinting strength so that the dye itself determines which wavelengths are absorbed and reflected by the resulting precipitate. In ancient times chalk, white clay, and crushed bones were used, being sources of calcium carbonate and calcium phosphate. The salts that are commonly used today include barium sulfate, calcium sulfate, aluminium hydroxide, and aluminium oxide (alumina), all of which can be produced cheaply from inexpensive mineral ores.
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