Page:Cyclopedia of Painting-Armstrong, George D (1908).djvu/439

Rh fumes, if breathed, are highly injurious, causing ulcers on the lungs, whilst drops of the acid on the skin will act like a red-hot iron, and produce very painful sores, which are not very easily healed. Opal can also be treated as described above, and the letters colored or enamelled afterwards, or china colors may be used and the opal fired. White letters on colored glass may be obtained by using flashed glass, and treating the flashed side with acid, which is soon eaten through, leaving the plain glass underneath.

Zinc Sheeting Paints. A very durable weather-resisting paint for zinc sheets is made by mixing oxide of zinc with a fluid silicate, such as water glass and potash and soda, to which the required pigments are added. The proportion should be about three-quarters of a pound of zinc white to every pound of silicate, with or without water. This zinc silicate paint becomes insoluble in water in about twenty-four hours. It is equally useful for interior and outside work, but it must not be applied to greasy surfaces, nor to old coats of paint. New zinc, not being oxidized, should first be prepared by the application of a solution of 1 part of soda in 10 parts of water, and then be thoroughly washed with water only. A quick drying, weather-resisting paint of dark color for zinc sheets is made by mixing 5 pounds of graphite with 1 gallon of vinegar. The oxidized surface of the zinc, previously well brushed, is painted with the above, one coat giving a sufficiently dark color. New sheet zinc, however, requires two coats, and must first be oxidized by the following application, which is not strong enough to cause any deterioration of the metal: 1 part each of chloride of copper, nitrate of copper, and sal ammoniac, dissolved in 64 parts of water, and 1 part of hydrochloric acid added to the solution.