Scientific American 9, 28.2.1863
Linseed oil is undoubtedly the best vehicle for paints that are to be exposed to the weather. It absorbs oxygen and becomes solid and waterproof, and yet it always possesses some elasticity which prevents it from cracking. Oils contain a considerable portion of glycerine, which is hygroscopic fat. It has been found that some metallic oxides possess the quality of combining with glycerine in the oil, and rendering it susceptible of readily drying in the atmosphere. The oxide of lead, sulphate of zinc, and the oxide 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 recommend the use of both sulphate of zinc and the acetate of lead mixed together for ming 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. The drying linseed oil has such an an affinity for oxygen as to promote chemical union with it and the coloring pigment, and thus destroy the beauty of the color. There are many delicate pigments which cannot be employed with oil in paint, without suffering injury. This is the case with chrome yellow, verdigris, gambogem and a number of the lakes. But wax is a very useful corrective for this deteriorating quality of the oil. Wax is a powerful antiseptic, and has great preservative powers. Added to painters' varnishes it tends to prevent them cracking - an evil which has destoyed the beauty of many excellent works of art. It is said that Titian painted on a red ground, and imbued his canvas at the back with beeswax dissolved in oil. Bleached wax is easily dissolved in hot 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. 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. Thus, if we take three ounces of the bi-carbonate 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 quarts of soft water, and it is then mixed with common oil paint. For inside work in houses it may answer, but it should never be applied to the outside of buildings, because it cannot resist atmospheric influences like paint which contains only oil and a pigment. Gum shellac varnish made with the carbonate of soda does not stand the action of rain so well as varnish for which alcohol has been employed as a solvent. It should, therefore, never be used for nay work exposed to the weather. In Cosmos it is stated that M. Oudry, of Auteuil, France, has found that benzine and coal oil are the best vehicles for paints of metallic basis (lead, zinc, &c.), as they dry rapidly and have no smell after the first twenty-four hours.
Ei kommentteja :
Lähetä kommentti