Paint is a mixture of a finely-divided solid substance with a liquid which, when spread on a solid surface with a brush or otherwise, will adhere and in a short time form - by evaporation, or more commonly by oxidation - a somewhat hard and tough film. The finely divided solid is called the pigment; the liquid part, the vehicle. The most common vehicle is linseed oil. This is an oil obtained by pressure (or extraction by solvents) from flaxseed.

When spread out in a film and exposed to the air, linseed oil is converted into a tough, leathery, elastic substance called linoxin, insoluble in water and all common solvents. This change is brought about by absorption and chemical union of the oxygen of the air, whereby the weight of the oil is increased about one-fifth or one-sixth. It is therefore a mistake to suppose that oil paint gets dry as whitewash does, by the evaporation of the liquid. Instead of that, it gets heavier. There are some other vegetable oils which have this property in some degree, but none which are used for paints to any considerable extent; some are used a little for artists' colors.

Linseed oil should stand at least a month or two before using. It should then be perfectly free from sediment or cloudiness; if it is not so, this is a sign that the oil has not been properly aged, and such oil is not fit for making paints. In this natural state, it is called raw oil; and the price of linseed oil as commonly quoted refers to raw oil. Boiled oil is this raw oil which has been heated, usually to 450° or 500° F., with the addition of a small amount of oxide of lead or oxide of manganese, or a mixture of the two (occasionally some other lead or manganese compounds are used). Boiled oil is darker (browner) in color than raw oil. but differs from it chiefly in that it dries five to ten times as rapidly. A thin film of raw oil on a glass or metal surface will dry at ordinary temperatures in five or six days, so as to feel no longer greasy; but boiled oil will do the same in a day or half a day. Oil dries best in warm, dry weather and out of doors.

The pigment is mixed with the oil by stirring the two together. This is usually done by power, in a vessel called a paint mixer. The mixture should then be run through a paint mill; some paint mills are of steel, but the best have a pair of mill-stones, between which the paint is ground and most thoroughly mixed. Paints mixed in this manner are much better than those which are mixed only by stirring.

Besides oil and pigment, paint sometimes contains a volatile thinner, the most important thinners being turpentine and benzine. Turpentine is a well-known essential oil, volatile, boiling at about 320° F., but evaporating at ordinary temperatures when exposed to the air. Benzine is a mineral oil, lighter than kerosene and heavier than gasoline; the kind used in paint and varnish is called "62-degree benzine," its specific gravity being 62° on the Baume scale for liquids lighter than water. Linseed oil weighs 7.7 lbs. per gallon; turpentine, 7.2 lbs.; and 62° benzine, 6.1 lbs. But linseed oil is sold by the oil makers and dealers on the basis of 7.5 lbs. per gallon.

A dryer, in some form, is an essential ingredient of oil paint. A dryer is a compound of lead or manganese (generally both), soluble in oil, and is usually sold, under the name of paint dryer or paint japan, as a solution of such material in a mixture of oil, turpentine, and benzine. It is usually of such strength that an addition of from 5 to 10 per cent of it to a raw-oil paint will make it dry in from six to twelve hours sufficiently to be carefully handled. Paints are not dry enough to use, until they have stood four times as long as this; and they continue to harden for months. The strongest drying japans are dark in color; but such are more injurious to the durability of the paint than those which are paler, especially if the latter do not contain rosin. The buyer should always ask for a guarantee that the dryer is free from rosin, if great durability in the paint is needed. Not more than 10 per cent of any dryer or japan should ever be used in any paint. Slowly drying paints are more durable than quick ones.

In house painting, the white pigments are the most important, because they are the base of all light-colored paints. The most important white pigment is white lead. This is sold either as a dry powder, or more commonly as paste white lead, which is made of 90 lbs. dry white lead and 10 lbs. linseed oil. This can be thinned with boiled oil to make a white paint. White lead is a very heavy pigment; and with a given quantity of oil, more of it can be mixed than of any other pigment, except red lead. It has great opacity, or covering power. It is discolored by gases containing sulphur. becoming brown or black; and unless exposed to fairly strong light, it becomes yellowish even in pure air. It is better it' it has been mixed with the oil for some time -a year or more.

White zinc is a somewhat purer white than white lead; not so opaque. Three coats of lead are reckoned equal to five coats of zinc.

It becomes harder than lead, but is somewhat liable to peel off; while lead, after exposure to the air for a long time, becomes dry and powdery on its surface, and chalks.

A mixture of two parts of lead and one of zinc is much liked.

Zinc-lead, however, is the name of an entirely different pigment, made by furnacing ores containing about equal parts of lead and zinc, in which the lead is present as a sulphate. This pigment is free from the liability to turn brown if exposed to sulphur gases; it is said to be not quite so pure a white as the preceding. It is a comparatively new pigment, but is coming rapidly into use, being somewhat cheaper than the others. Lithopone is another white pigment of considerable merit.