Composition of Household Paint
Information on the composition of ordinary household paints.
Household paint is composed of two parts: a liquid, which is mainly oil in most house paints, and a powdered solid, which gives color, and body to the mixture.
The liquid part is especially termed the vehicle; the solid, the pigment. The oil is linseed oil, made from flax-seed; raw oil is the natural oil, which will dry or become hard enough to handle, if exposed to the air in a thin film, in about a week; and boiled oil is the same, to which has been added a small proportion of "drier", and will dry in twelve to twenty-four hours. Paint drier, also known as japan, or paint japan, is a compound of lead, or manganese, or both, soluble in oil; it takes up oxygen from the air, and passes it over to the oil; for it should be known that oil dries, not at all by evaporation, but by absorbing oxygen from the air and uniting chemically with it, so as to make a new material, which is not a liquid, but is a tough, leathery solid, and weighs a fifth or a quarter more than the oil from which it was made. This is technically called linoxyn. The drier is, therefore, a chemical agent, and acts toward oil somewhat as kindling-wood does to coal, only it doesn't entirely burn up, but keeps on acting for a long time. If too much of it is used the oxidation, or combustion, goes on too strongly, and the oil gets over-oxidized, or burned, and its toughness and elasticity are impaired; therefore we must be sparing in its use, for we find the slow-drying paints are the most durable. Paint drier is usually sold as a liquid, and to a gallon of oil in paint not more than five or ten per cent of drier should be added; less for outside than for inside paints. The best driers do not contain rosin, but most cheap driers do. It is not often necessary to use drier with boiled oil, as the manufacturer has already put in as much drier as is desirable. Drier should never be added to mixed paint, as the maker has exactly proportioned the ingredients to give the best results; there are cases where it is proper to add oil to a mixed paint, and sometimes turpentine, but not drier.
Turpentine is sometimes an ingredient of paint. In this connection it always means the essential oil of turpentine, a spirits of colorless liquid, lighter than water, Turpentine highly inflammable, volatile. It mixes perfectly with oil, and increases the fluidity of paint. If we mix a considerable amount of it with paint, it makes a thinner film and one which is not glossy when dry, but dull, what painters call "flat". Most paints stick better to a "flat" surface than to a glossy one, and so turpentine is often added to undercoats; also sometimes a "flat" surface is preferred, as a matter of taste, as a finish; but it is always less durable than a glossy one. Turpentine substitutes are usually benzine, a petroleum product between light naphtha and kerosene; often mixed with rosin spirits, or with wood distillates to conceal the odor. Genuine turpentine is always preferred, but the substitutes are cheaper, and for some purposes are fairly good.
Turpentine should not be put in with the pigment until after the oil; because with most pigments there is a sort of attraction between the oil and the pigment, and it is important not to prevent them from uniting as intimately as possible; and turpentine seems to do this. After the oil is all in and well mixed is the time to put in the turpentine. On the other hand, if we are to add drier to paint, it is to act only on the oil, and it is well to mix it with the oil before the latter is put with the pigment. Moreover, the amount of drier is to be proportioned to the oil, not to the finished paint. To make paint thinner and more fluid, turpentine is much more efficient than an equal volume of oil; as it is volatile it does not act as a binder or cementing material, and therefore lessens the elasticity and durability of the paint.
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This is Composition of Household Paint.
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