Vehicles of Paints.

The Scientific American 39, 5.6.1858

Oil is the common vehicle employed for paints, and it is undoubtedly the best; it has, however, some defects requiring correctives. Thus drying linseed oil, which is made by boiling it with some metallic oxyd, &c., has such an affinity for oxygen as to promote chemical union with it and the coloring pigments, and thus ultimately destroys the beauty of their color. There are many delicate and beautiful colored pigments which cannot be employed with oil in paint, without suffering injury. This is the case with chrome yellow, verdigris, gamboge, and a number of the lakes. But there is a very useful corrective for this deteriorating quality of the oil, that is pure beeswax. It was the principle vehicle of the ancient painters before oil painting was invented, and some of the old paintings exhibit a freshness of color perfectly wonderful. Wax is a powerful antiseptic, has great preservative powers, and it would be well to apply it as the first coating for canvas designed for oil paintings. Wax added to painters' varnishes tends to prevent them cracking, the latter being an evil which has destroyed the beauty of many excellent works of art. It is said that the famous Titian painted on a red ground, and imbued his canvas at the back with beeswax dissolved in oil; this may account in a measure for the enduring brilliancy of his colors. It has also been asserted on the other hand that Sir Joshua Reynolds used a great deal of wax with his colors, and it is well known that their beauty has been very short lived, and his paintings have all become very faint. But it has also been denied that he used wax as a vehicle, because it is the most unalterable of unctuous bodies, and would have preserved his colors. Bleached wax is easily dissolved in loot oils, both volatile and fixed; it is not changed by exposure to the atmosphere, and is but very feebly acted upon by the strongest acids. Its appropriateness, therefore, as a vehicle for paints is self-evident.

Oils contain a considerable portion of glycerine, which is a hygroscopic fat, and prevents unprepared oils from drying. It has been found that some metallic oxyds possess the quality of combining with the glycerine in the oil, and rendering it susceptible of readily drying in the atmosphere. The oxyd of lead, sulphate of zinc, and the oxyd of manganese, boiled with oils, communicate to them great drying properties, and for this reason oils treated in this manner are called drying oils, and are in common use.

Some works written by incapable authors recommend the use of both sulphate of zinc and the acetate of lead mixed together for making drying oil. These two metallic salts when brought together produce two new compounds by double decomposition, namely, the acetate of zinc and the sulphate of lead, and the oil is restored to its original condition. The acetate of zinc should never be employed in paints, because it is a bad drier. Few painters, we suppose, are aware of the foregoing action.

Fixed oils, even those which are bleached, when exposed to the air become rancid, yellow, and acquire an acid reaction. They absorb oxygen from several pigments, but this, in a great measure, is prevented by the use of wax or a little resin, such as gum shellac. Many persons mix shellac varnish with common paint in order to render the latter less expensive, because a considerable quantity of water can be added to the varnish and combined with the paint owing to the alkaline agent employed to dissolve the gum. Thus, if we talse three ounces of the bicarbonate of soda, and place it in three pints of soft water, it will dissolve a pound of gum shellac by boiling, thus making a lac varnish. To this is usually added half a pint of alcohol and two uarts of soft water, and it is then mixed with common oil paint. For inside work in houses it will answer well, but it should never be applied to the outside of buildings, because it cannot resist atmospheric influences like paint which contains only boiled oil and a pigment. Gum shellac varnish made with the carbonate of soda does not stand the action of rain an well as shellac varnish for which alcohol has been employed as a solvent. It should, therefore, never be used for any work exposed to water or the weather.