(Excerpts from ART HARDWARE: The Definitive Guide to Artists’ Materials, by Steven Saitzyk © 1987 revised 1998)
Drying oils are fatty oils of vegetable matter that can react chemically with the oxygen in the air eventually to solidify and become dry to the touch. Nondrying oils are mineral oils and vegetable oils, such as peanut oil and cottonseed oil, that resemble animal fats and, because they do not oxidize naturally and harden, are unsuitable as a binder for paint.
Resins are either natural or synthetic organic chemicals that are solids or viscous (thick) liquids. They are used to make media for painting (they are often too brittle when used alone) to alter the working characteristic of the paint film. Resins, in a liquid form, differ from drying oils because they solidify by the,evaporation of their solvent rather than through oxidation. Drying oils thicken and harden into a paint film over a two-to fiveday period, while most dissolved te~ins thicken and harden within hours.
Paint that is taken directly from the, tube usually needs to be “let down,” or thinned to a workable consistency. No artists’ paint should ever be let down with only a thinner such as turpentine. This washes away the drying oil that coats each particle of pigment and protects a pigment from interacting chemically with other pigments. The drying oil also holds the pigments together as a paint film; therefore, the less you have, the weaker the paint film. Most of the cost in making an oil paint is in the meticulous care taken in coating each particle of pigment.
As early as A.D. 1100, Theophilus, a German monk, wrote about the use of a drying oil as a medium for painting. The slow drying rate of the oils prevented their immediate acceptance, but after it was discovered that the addition of zinc and lead to the oil reduced the drying time, painting with oil became widespread. The practice of using oils, which imparted smoothness to a painted surface, was introduced around 1390 in Italy and the Netherlands. With the beginning of the age of industrialization (from the eighteenth through the nineteenth centuries), knowledge of earlier materials and methods fell into a dark age. This was not because of deliberate secrecy, but because of disuse and the demise of the apprenticeship system, in which a student worked directly with a master to learn the craft. Much vital information about specific drying times of oil media and their use in causing paint films to interlock for durability has been lost.
A drying oil is both the binder and the vehicle for the pigments that are used in oil painting. Each particle of pigment must be thoroughly coated with oil to protect it from reacting chemically with other pigment particles, to allow proper dispersion of pigment particles for luminosity, and to provide a workable and durable paint film. It is important to understand that drying oils do not dry through evaporation, but through oxidation, which is a chemical reaction with the oxygen in the air. Painters often ask, “Why must I wait six munths to a year to varnish paintings when they seem dry to the touch?” The answer is that if oxidation is not complete below as well as on the surface of the paint film and the surface is sealed off from its supply of oxygen, the still-wet paint is trapped underneath and proper drying is prevented.
Linseed oil is made from flax seeds, which contain 30 to 40 percent oil. It dries to the touch quickly, between three and ten days, but it takes years before it dries completely. It tends to yellow with age because of the linolenic acid, which is one of its binding agents (the other is linoleic acid). A refinement process is commonly used during the manufacture of linseed oil to remove particulate matter and mucilage and to bleach out some of the yellow color.
Raw Commercial Linseed Oil is extracted from flax seeds today by crushing and steaming procedures developed in the nineteenth ci!ntury. The better commercial grades are warmed and aged to remove the grossest particles. Raw linseed oil is the least desirable for use in any artists’ paints or even commercial paint products because it contains the greatest amount of mucilage and impurities. It is perfectly acceptable for finishing raw wood furniture.
Refined Linseed Oil is most commonly made by steaming the crushed flax seeds and chemically bleaching the oil. The bleaching has only a temporary effect and the oil usually reverts to its original yellow color. There are many grades of linseed oil; the artist grade is often further treated with alkali to improve clarity and color, producing a pure oil that is pale, clear, and thin. Paint is produced by grinding raw pigment in the oil. Additional refined linseed oil is often added to prepared paint to thin the paint, and to add gloss and transparency. It is also used to slow the drying time of the paint film. If the oil is used sparingly, the paint film dries in approximately three days. Drying can take ten or more days if the oil is used more generously.
Cold-Pressed Linseed Oil is the best of the linseed oils. It is produced from the first pressing of ripe linseeds. It is the least efficient method of producing large amounts of linseed oil, but it gives rise to the purest form. Because of its high cost, it is rarely used in making paints. It is expensive because it is produced solely for artists’ use by the least efficient method. This oil resists embrittlement, has excellent flow characteristics, and adds gloss and transparency to a paint. It is used to thin paint and it allows brush strokes to level out of a paint film. Cold-pressed linseed oil is a slower drying oil than refined linseed oil.
Stand Oil was widely used in the Dutch school of painting during the seventeenth century. This oil is so called because of the old practice of letting the oil stand for long periods of time to allow the impurities to settle out. Today, stand oil is linseed oil that has been heated without air at a temperature between 525 and 575°P. This polymerizes the oil, making it viscous and thereby excellent for glazing and leveling brush strokes. Because it is a fatter oil, it is not recommended for underpainting, but rather for the top paint layers. It makes a paint film that is not only tougher, but also yellows less than regular linseed oil. The drying time for stand oil is slower than that of linseed oil.
Sun-Thickened Linseed Oil is made by placing partially covered linseed oil in the sun. This bleaches, slightly polymerizes, and partially oxidizes the oil, producing an oil that has characteristics somewhere between refined linseed oil and stand oil. It dries a little faster than both (between two and nine days, depending on the thickness applied). It dries more quickly than refined linseed oil because the oxidation process has already begun. This could, however, result in a less durable paint film.
Extracted from poppy seeds, poppy oil is slower drying-usually five days-than linseed oil. (Some companies, including Winsor & Newton, add cobalt driers to accelerate the drying time.) This oil, which is less yellow in appearance than other oils because it does not contain linolenic acid (one of the two binding agents in oil paint films, and the one that yellows more), is used in making or mixing the pale oil colors. But because it does not contain linolenic acid, there is a greater risk of cracking with paint films composed primarily of poppy oil. Consequently, it should not be the primary ingredient in a painting medium recipe.
SAFFLOWER OIL AND SUNFLOWER OIL
Safflower oil and sunflower oil have come into use only recently. They are presently used by some manufacturers as substitutes for linseed oil when Illaking some of the paler colors. They are also used in the manufacture of alkyd resins. Although at this time it appears that both sunflower and safflower oil can be safely used in the manufacture of some colors, they are not recommended for use in media and are not expected to replace linseed oil.