You need only three things to paint a picture—the paint, something to apply it to, and something to apply it with. These can all be purchased at commercial art stores; generally the modern artist elects to buy his materials prepared and ready for use. There are occasional advantages, however, in making at least some parts of your own equipment. In any case it is advisable to understand both the special properties and the limitations of the many materials available. This will help you decide which to select for a given job and how best to use them.
The use of oil as a painting medium has been traced back to the 11th century. It has become increasingly popular among painters and has undergone many changes in handling and use. Paint consists of a dry powder or pigment, ground in an appropriate liquid medium. The pigment gives body and color while the liquid carries the pigment in suspension and on drying acts as a binder. In oil painting the liquid used is called a drying oil. Linseed oil is the common variety. As it dries it absorbs oxygen to the extent of 13 percent of its weight and is converted into a solid. This process of oxidation is much less rapid than the drying, by evaporation, of moisture in water-color painting. An oil painting may be dry to the touch in 24 hours but the oxidation continues for several years. Driers are often added to linseed oil to increase its rate of hardening. These substances, manganese or cobalt, are dangerous to the permanence of the painting unless used sparingly.
The pigment which is ground in the oil may be of organic or inorganic origin. Pigment composed of different colored earths and minerals are inorganic ; those in dyes extracted from the plants are organic. Such colors are vermilion, yellow ochre, terre verte, malachite green, azurite blue, ultramarine, and Venetian red are found in the earth and need only grinding to be used in painting. They are extremely permanent. The chemist has added such inorganic and permanent pigments as the cadmium reds, oranges and yellows, oxide of chromium greens, artificial ultra-marine, zinc, white, cobalt blues and yellows, and zinc yellow. Some painters object to the use of chemically manufactured pigments, believing them to lack permanence. Modern chemistry, however, is able to produce pigments of the same quality and purity characteristic of the natural products.
c. In the past, painters ground their own colors, and for those who still prefer to do so a simple method is outlined in chapter 25. But today most artists buy their colors already ground and in tubes. Often the ingredients are completely noted on the labels.
What does the oil paint need as a support ? The most generally accepted material is a canvas of linen, hemp, cotton or jute, stretched on a framework. Canvas may be obtained in various weights, from a smooth closely-woven material to one of coarse, heavy grain. Cotton canvas tends to stretch after painting. Linen is more acceptable and quite durable. Hemp and jute, coarse and of heavy ground, are suitable for large paintings. Oil paintings have been made on panels of wood, sheets of metal, and slabs of stone, but size and weight render them generally impractical. At the present time some of the light, strong wall boards and plastics are coming more and more into favor for painting in oil.
The canvas or board must be prepared for painting. The acid in linseed oil rots linen, and the porous quality of the material requires a dressing of some insulating material. It is usually a thin coating of size (gelatin, glue, or casein). The linen may be further prepared by being primed with a filler. This does two things: It hides the weave and gives it greater body. The priming is generally made of size and a filler of finely ground chalk, gypsum or zinc white. One of the oldest forms is gesso, a mixture of whiting and size, which yields a dead white, absorbent ground and can be smoothed with abrasives to an ivory-like finish. Some painters feel that priming increases the danger of cracking in a picture, but gesso grounds in Renaissance paintings have stood the test of time remarkably well. Prepared canvas, already sized and primed, can be bought at art supply stores either by the yard for mounting on stretchers, or glued to cardboard panels. Such canvas has usually been treated with an oil or chalk ground and a plasticizer to insure pliability. They are ready for the painter's use without further preparation.
The technique of applying paint to canvas is personal to the painter. He has brushes of various sizes, shapes, and degrees of stiffness and the palette knife. These all affect the appearance of the paint as it is applied to the canvas. A stiff bristle brush will leave a ridged mark that is clearly discernible and quite different from the stroke of a soft sable brush. Paint smoothed on the canvas with a palette knife retains the broad mark of the blade, but this method is not widely used and is generally limited to certain parts of the picture.
Different effects may also be obtained by varying the consistency of the paint and the depth to which it is built up on the canvas. This is known as impasto and may run from translucent washes of color radically diluted with turpentine and oil, to heavy layers of unthinned paint used as it comes from the tube.
While "direct" painting is perhaps the commonest method in use today, the technique of "indirect" painting (a combination of under-painting, glazing, and scumbling used with success by the great Venetian masters) is still favored by many of our modern artists. A glaze is a thin transparent film of color superimposed on another color to modify the tone or enhance its effect. The undertone appears through the glaze which is often darker than the color upon which it is laid. Scumbling, on the other hand, is achieved by going over the work with a stiff brush containing very little paint, the tone of which is opaque and often produced by a mixture with white. The brush is drawn loosely over the previous painting, which must be dry and firm.
There are many variations in the use of glazing and scumbling. The commonest method is to start with an underpainting in various shades of gray or of a single color (monochrome). This gives the entire modeling of the forms, their lights and shadows. Over this the colors of the finished picture are glazed in thin transparent washes of paint diluted with an oil or varnish medium. Scumblings of white are then applied, picking out the highlights, which in turn are reglazed with less color. Great richness and depth of tone may be obtained in this way.
When a painting is sufficiently dry, about a year after it is completed, it is varnished. If you varnish too soon you are apt to produce cracking in the paint film. Varnish protects the surface of the painting from dirt, humidity, and injurious gases in the atmosphere. It must be colorless, transparent, and easily removable, since all varnishes darken in time.