LINSEED OIL over the years has proved itself relatively the surest of all oils used. Therefore it alone will be treated here. The much prized nut and poppy oils have the fatal drawback of occasionally softening without apparent reason, or even of never hardening.
Linseed oil is extracted from the seed of flax. For painting and medical purposes it is pressed out cold, for although more oil can be obtained by pressing it under heat this darkens the color and alters it in other ways. Care must be taken that the seeds are not mixed with those of any other plant or wood, as the oil will then not harden. The raw oil contains a watery slime which has t.o be removed. In old painters' manuals many recipes, some very strange ones, are given for the extraction and purification of linseed oil. Painters had then themselves to press and purify the oil. With today's science and technology it is, of course, hardly a part of the painter's art to extract oil from flax.
Opinions still vary on whether the oil is best if used when quite fresh or after it has matured. It is certain that newly pressed oil oxidizes more slowly than an oil which is six or nine months old. Rancid oil cannot be used; it is easily recognized by its smell and taste.
The oxidation of oils can be very much influenced. It can be accelerated by pre-oxidation, for instance, by blowing oxygen into it. This produces linseed varnish, which is used, instead of untreated linseed oil, exclusively for commercial paints.
For the artist's use "stand oil" is made in the following way: linseed oil is exposed in an open container to the action of air and sun in contact with lead oxide which always forms on the surface of lead plates or rods. This thickens the oil, because the hardening process has already begun, and it gives a very shiny, fat effect when applied. It hardens quicker than untreated oil.
A number of pigments have the same effect, especially the cobalt and lead compounds. They act as catalysts, accelerating the rate of the reaction, in this case the absorption of oxygen by the linseed oil. If a p.igment is said to be a "good drier" this always refers to its effect on oils, which harden by the absorption of oxygen.
Some pigments delay oxidation; lamp black is one of the worst of these. It may contain remains of non hardening oils, and, in any case, like all dark pigments it requires much more binder to make it adhere than do light pigments. All these matters are related to the formation of cracks while the paint is drying, what we have called "early crack formation"; if quick drying oil colors are painted over those which harden slowly, particularly if these have not completed this process, the upper surface will soon break. Thus, in oil painting the rule is always to use slow hardening colors over quickly oxidizing ones. If the composition demands otherwise, then traces of "quick dryers" are added to the slowdrying colors and painted over only when the under layer has become quite hard.
The old masters managed in this way to lay thick layers of white lead over dark and slow hardening colors without causing any cracking, except the inevitable late crack formation. If you examine old pictures carefully you will often see that the surface is not uniformly covered with splits and cracks, but that there are large cracks in some places and none at all in others. The reason for this is the variation in thickness of the paint and the different behavior of the pigments.
Linseed oil, like all painting oils, bleaches in the light and yellows in the dark. For this reason it is pointless to use bleached oil; it yellows and discolors all the more on the canvas. A thick layer of oil which reduces the penetration of light through to the bottom, yellows more, and more permanently, than a thin one.
It is also a great mistake to think that oil colors cannot absorb moisture. This can be demonstrated by spreading linseed oil on a glass plate and leaving it to harden for as long as a month or two. If the plate is then laid in water the oil film can be seen to swell and ultimately to come away from the plate entirely. A similar experiment with casein would have a contrary result: the casein would not be affected by the water. Transitory moisture does not damage oil paintings, and this may be why the layman imagines that an oil painting on strong canvas is the most durable and solid of techniques. He needs to revise this conception. The fact that the great masters succeeded in painting pictures which have withstood the test of several centuries is a tribute to their care and attention, but it does not make the technique in general any more certain.
Oil painting is quite useless on a wall. The film of oil seals the pores of the plaster, which then starts to rot, bubble up, and finally crumble away. It is amazing that there are still house painters who recommend washable oil plinths in buildings. A tragic example of the dan'gers of oil painting on plaster. is the famous Last Supper of da Vinci. Always experimenting with new methods, leonardo painted over his picture in oil colors. Its deterioration continues 'relentlessly, defying the most highly developed techniques of conservation and restoration.
Although pure oil colors are still produced in great quantities, many good paint factories now have reduced the content of fatty oils in paints and produce the so-called "resin" oil colors. They contain a large, unfortunately unspecified, percentage of resin and some wax, which is also added to pure oil colors. Wax both protects resin varnishes from perishing and prevents pigments from separating from the oil in the tubes or forming a sort of jelly which is impossible to paint with. Resin oil paints harden more quickly than pure oil colors. Both evaporated solvents for resins and wax reduce the amount of binder and accelerate the absorption of oxygen by the oil.
In addition to bought oil colors, which are nowadays always resin oil paints, a suitable resin oil color can be obtained by carefully warming together 100 cc. of linseed oil with 5 gr. wax melted at about 158 degrees F., and adding, when it has cooled and been placed in a jar, 100 cc. mastic essence (of a proportion 1 :2). Shut the jar immediately and shake it so that both substances are well mixed together.
The pigments are kneaded with this binder into mounds of paste and left to rest at intervals to test the amount of binder required by the different pigments. These mixed pigments can be thinned with turpentine, just as watercolors can be thinned with water, without affecting their adhering to the ground. After the turpentine spirit has evaporated, 100 parts of linseed oil, about 30 of mastic, and 5 of wax will be found in the hardened paint.
Our list of organic binders is now at an end, for all other oils, lacquers, varnishes, and such like, though they may be usable, are no better than those already listed and tend to muddle the painter and involve him in technical difficulties.
To avoid the difficulties of oily binders it is better to use emulsions instead. They provide the advantages both of oily and watery substances, while the disadvantages are rendered harmless by the emulsion texture. A simple emulsion is obtained by combining an egg yolk with glue (tragacanth or gum arabic) and linseed oil, to which an essence of resin is added later. The method is the same as for making mayonnaise.
The emulsifying properties of an egg yolk are, of course, limited; it will function for about 75 cc. of gum solution and 150 cc. of oil, depending on the size of the yolk. It is best not to try to use it to the saturation point.
Before making any emulsion it is wise to measure out the various constituents carefully, as the bit-by-bit method makes it difficult to remember exactly how much has been used. An emulsion is most easily made by mixing it in a water bath at about 75 to 85 degrees F., and then stirring it until it has cooled to room temperature to prevent its separating.
An emulsion can be made even without an emulsifier - for instance, with gum arabic and essence of resin (and with linseed oil). It is a sleight of hand possessed by chemists as part of their professional equipment. A successful result is announced by a sort of clicking sound in the mixture as it is stirred. The substance making the "honeycomb" formation should always be double in quantity to that which is being mixed in and, when necessary, thinned only with the substance which combines with it. Otherwise too much of the enclosed material would break the cell-like structure of the "honeycomb," and the mixture would curdle.
If you make your tempera colors yourself it is sufficient to make a paste only of a selection of the pigments which you intend to use. If a pigment is. to be used only for a nuance in a mixture it can be taken from the watercolor box or the collection of oil colors, according to whether the tempera reacts better to an addition of water or of oil and turpentine. But, as we have said, this addition of other types of paint can be used only for mixtures, not for putting on by itself! This method saves much trouble and rightly used does not affect the technical quality of the tempera painting.
Once a layer of tempera color has hardened completely it is less sensitive to the solvent of the following layers than glue or oil colors necessarily are, but it does remain sensitive to some extent.
The only paint which for practical purposes is completely insoluble to the following coats is a resin oil emulsified with casein - a relatively easy emulsion to make without an emulsifier.