It is prepared by distilling metallic zinc in retorts, under a current of air; the metal is volatilised, and white oxide is condensed. It is filled into canvas bags, and pressed to increase its density.
Zinc white is durable in water and oil; it dissolves in hydrochloric acid; it does not blacken in the presence of sulphuretted hydrogen (the sulphide of zinc being white); and it is not injurious to the men who make it, or to the painters who use it.
On the other hand, it does not combine so well with oil, and is wanting in body and covering power, and is difficult to work (see p. 421).
"The want of density is a great drawback to the use of zinc white, and the purest zinc oxide is not always the best for paint on account of its low specific gravity; and in this respect the American zinc whites, which are frequently very pure, do not generally compete with the zinc white supplied by the Vieille Montagne Company, as made in Belgium." 1
Uses, etc. - Oxide of zinc is the basis of zinc paint. It has considerable advantages in certain positions, as mentioned at p. 421.
Oxy-Sulphide of Zinc is used as the basis of Griffith's patent white paint. It is stated by Dr. Phipson to be prepared by precipitating chloride or sulphide of zinc by means of a soluble sulphate - of sodium, barium, or calcium. The precipitate is dried; and levigated, while hot, in cold water.
Oxide of Iron is produced from a brown haematite ore found at Torbay in Devonshire, and at other places. It forms the basis of a large class of paints of some importance (see p. 425).
The ore is roasted, separated from impurities, and then ground. Tints, varying from yellowish brown to black, may be obtained by altering the temperature and other conditions under which it is roasted
Oxide of iron is also produced as a bye product in the manufacture of aniline dyes.1
Oils are divided into two classes - Fixed oils and volatile oils.
Fixed Oils are extracted by pressure from vegetable substances; they are of a fatty nature, do not evaporate on drying, and will bear a temperature short of 500° Fahr. without decomposing. They are subdivided into
Drying Oils, which become thick upon exposure to air. Of these, linseed oil is most commonly used as an ingredient for paint; nut oil and poppy oil are also used (see p. 411).
Volatile or Essential Oils are generally obtained by distillation, and have an odour resembling that of the plant from which they are obtained. They are, as a rule, colourless at first, but upon exposure to air and light they become darker, thicker, and eventually are converted into a kind of resin.
Oil of turpentine, commonly called spirits of turpentine, is the only variety of this class that is much used for ordinary paint.
Mineral Turpentine or Petroleum Oil is often used as a cheap vehicle instead of ordinary turpentine.
Coal Naphtha is one of the products of the distillation of coal tar. It is purified in a mill with sulphuric acid; the sediment and water drawn off, the pure washed spirit remains.
Petroleum, a mineral oil, comes from America in casks. It is then distilled, and from it oils of various density are obtained, and used for burning in lamps, etc.
Benzoline is one of the products obtained from petroleum, and is much used as a solvent for bituminous paints. Paints mixed with benzoline or the heavier oils from petroleum do not set nearly so well, nor do they dry with so much cohesion as those in which naphtha is the solvent, but benzoline is much cheaper, and is therefore often sold as naphtha, and used instead of it.