The New International Encyclopædia/Mineral Paints

MINERAL PAINTS. A term applied to mineral substances which are mined, ground, and sometimes purified for use as pigments. It includes a variety of natural materials as well as some artificial products. The essential characteristics of mineral paints are permanence of color and sufficient adhesion when applied to a surface to prevent scaling and to keep out moisture. Among the important substances included under the heading of mineral paints are ochre, sienna, and umber. These are clays which owe their color largely to limonite, although sienna and umber are colored in addition by manganese. Ochre occurs at a number of localities in the United States, the larger supply being obtained from Pennsylvania and Georgia. It is usually ground, washed to remove sand, and screened before shipment. Umber and sienna are found in but small quantities in the United States, and the chief supply of them is obtained from abroad. Slate and shale are ground for paint, the former being the refuse from slate quarries. The colors obtained from them are usually red, green, blue, yellow, and brown. Barite, or barytes as it is called commercially, is used as a substitute for or an adulterant of white lead in the manufacture of white pigments; for this purpose it must be free from iron, and therefore its preparation for market consists not only in grinding, but in some cases may include treatment with sulphuric acid to remove the stains. The main supply of barite is obtained from Missouri, North Carolina, Virginia, and Tennessee. It is cheaper than white lead, and does not turn yellow on exposure to the air. Metallic paint is a term applied to certain materials obtained by the grinding of hematite ores. The color of the paint is often changed or improved by previous roasting; the iron ore most frequently employed is that known as the Clinton ore, occurring in the Clinton division of the Silurian system of rocks. Metallic paint is mined in several States, notably New York, Pennsylvania, Wisconsin, and Tennessee; it is frequently employed for coloring mortars. Graphite and graphitic shale are used for making black paint, and have been found specially desirable for the coating of metallic surfaces. The main supply for this purpose is obtained from Ceylon, although small quantities are mined in the United States.

Artificial Mineral Paints. Under this heading are included Venetian red and Indian red, which are pigments obtained by roasting iron sulphate or copperas; white lead, produced by the action of acetic acid on metallic lead; red lead, formed by roasting of litharge; litharge, the reddish, partially roasted protoxide of lead; orange mineral, formed by the oxidation of white lead on the hearth of a reverberatory furnace; and zinc white, or zinc oxide, which is produced by the roasting of zinc ores.

The production of mineral paints in 1901 amounted to 232,347 short tons, valued at $18,417,068, while the imports were valued at $667,094. Consult: Jones, Testing and Valuation of Raw Materials Used in Paint and Color Manufacture (London, 1900). For statistics of production, consult Mineral Resources, issued by United States Geological Survey (Washington, annually).