chemical composition. Pigments of this class suitable for making paint may range from 98 per cent. down to as low as 10 per cent. of ferric oxide, their value as pigments depending less upon the amount of iron present than upon their physical properties, and the composition of the material which is not oxide of iron.

Roasted iron ores often yield excellent pigments, the quality depending upon the character of the ore, the construction of the furnace, the uniformity of the heat, and the care and attention given to the milling and grinding.

Natural ores or mineral earths seldom yield pigments that are good for making metal coatings, for the reason that they contain more or less water, and are apt to contain sulphur or other unstable elements, one exception  to  this  being   the  rich  hematite