MINERAL PAINTS. A terns applied to mineral substances which are mined, ground. and sometimes purified for use as pigments. It in clude- a variety of natural materials as well as some artificial produets. The essential character istics of mineral paints are permanence of color and sufficient adhesion when applied to a surface to prevent sealing and to keep out moisture. Aunleg the important substances included under the heading of mineral paints are ortire, sienna, and r. These are clays which owe their• color largely to limonite, although sienna and umber are colored in addition by manganese. Ochre cx•eurs 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. Pnuber and sienita are found in but small quantities in the United states, and the chief supply of them is obtained from abroad. and shah are for paint. the former being• the refuse from slate quarries. The colors obtained from them are usually red. green. blue.
yellow. and brown. Gig rib', or laarat,s as it is oalleri is used as a substitute for or an adulterant of white lead in the nianimfavture of whit,• pigments: for this purpose it must be free• from iron. and therefore its preparation for
market (en-i-ts not only in grinding. hut in sorr, cases may include treatment with sulphuric acid to remove the stains. The main supply of barite is obtained from 31issouri. 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 divi sion 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 mak ing black paint, and have been found specially desirable fo• the coating of metallic surfaces. The main supply for this purpose is obtained from Ceylon. although small quantities are mined in the United States.