Oxide of zinc, or zinc white, is durable in water or oil; it dissolves in hydrochloric acid; it does not blacken in the presence of sulphuretted hydrogen; and it is not injurious to the men who make it, or to the painters who use it; but on the other hand, it does not combine with oil well, and is wanting in body and covering power, and is difficult to work. It is easily acted upon by the carbonic acid in rain water, which dissolves the oxide, and it therefore is unfit for outside work. The acids contained in unseasoned wood also have a great effect upon it. When pure and used for inside work, it retains its color well, and will stand washing for many years without losing any of its freshness. When dry it becomes very hard, and will take a fine polish. This paint is suitable for any place that is subjected to vapors containing sulphur, or in places where foul air is emanated from decaying animal matter. In such positions, of course, zinc paints should not be mixed with “patent” or other driers which contain lead. The best driers to employ with it are sulphate of manganese and sulphate of zinc. This white is recommended as being preferable to white lead for painting on a dark ground. The reason of this is that the soap formed by the combination of the lead and oil in lead paints is semi-transparent, and the dark ground shows through it. The want of density, however, in zinc paints, is a great drawback to their use, 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 give as good satisfaction as the zinc whites made in Belgium.
Vehicles.—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 in 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. Its qualities when pure are excellent, and it may be considered the best of all oils for use in paint, putty, and other similar substances. It oxidizes and becomes thick upon exposure to the air. This property is very much increased by adding other substances to it and boiling them together. It is superior in drying powers, tenacity, and body to any other fixed oil. The best oil comes from the Black Sea and the Baltic; that from the East Indian seed is inferior, as the seed is less carefully cleaned, and contains too much stearine. Raw linseed oil is clear and light in color, works smoothly, and is used for internal works, for delicate tints, and for grinding up colors. Boiled oil is much thicker, darker, and more apt to clog. It is used for outside work, as its greater body and rapidity in drying make it a quicker and more efficient protection.
Volatile Oils are generally obtained by distillation, and have an odor resembling that of the plant from which they are obtained. They are, as a rule, colorless at first, but upon exposure to air and light they become darker, thicker, and eventually are converted into a kind of resin. Spirits of turpentine is the best variety of this class for mixing with ordinary paints. Naptha and benzine are sometimes used instead of turpentine, but not often, and their use is not recommended when the latter can be obtained. Good spirits of turpentine is lighter in weight and more inflammable than bad. It is colorless and has a pleasant pungent smell, whereas the smell of inferior qualities is disagreeable. It is used as a solvent for resins and other substances in making varnishes; also in paint to make it work more smoothly. It is useful also in flatting coats, but will not stand exposure to the weather.
Driers.—Driers are substances added to paint in order to cause the oils to thicken and solidify more rapidly. The action of these substances is not thoroughly understood. Chevreuil has shown that the drying of linseed oil is caused by the absorption of oxygen; and there can be no doubt that for the most part driers act as carriers of oxygen to the oil, a very small quantity producing considerable effects.
The best driers are those which contain a large proportion of oxygen, such as litharge, acetate of lead, red lead, sulphate of zinc, verdigris, etc. They are sometimes used to improve the drying qualities of the oil with which the paint is mixed, or they may themselves be ground up with a small quantity of oil, and added to the paint just before it is used.
Litharge or oxide of lead is the drier most commonly used, and is produced in extracting lead from its ores. It can be produced on a small scale by scraping off the dross which forms on molten lead exposed to a current of air. Massicot is a superior kind of litharge, being produced by heating lead to an extent insufficient to fuse the oxide. Sugar of lead, or, as it is more frequently called, acetate of lead, ground in oil, and copperas and white vitriol (sulphate of zinc), are also used as driers, especially for light tints. Oxide of manganese is quicker in its effects, but is of a very dark color, and seldom used except for deep tints. Japanners’ gold size and verdigris (acetate of copper) are also much used for dark colors. Care must be taken not to apply too much of the size, or it will make the paint brittle. Red lead (oxide of lead) is often used as a drier when its color will not interfere with the tint required. It is not so rapid in its action as litharge or massicot. Sulphate of manganese is the best drier for zinc white, about 6 or 8 ounces only being used for 100 lbs., of ground zinc white paint. The manganese should be mixed with a small quantity of the paint first, and then added to the bulk. If great care be not taken in mixing the drier the work will be spotted. Sulphate of zinc is also a good drier for zinc paint.
Patent driers contain oxidizing agents, such as litharge or acetate of lead ground and mixed in oil, and therefore in a convenient form for immediate use. There is great danger, however, in using such driers, unless they are of the best quality from a reliable maker. Some of the inferior descriptions depend for their drying qualities upon lime.
The following points should be observed in using driers:—
1st. Not to use them unnecessarily with pigments which dry well in oil color.
2d. Not to employ them in excess, which would only retard the drying.