(Excerpts from ART HARDWARE: The Definitive Guide to Artists’ Materials, by Steven Saitzyk © 1987 revised 1998)
The nature of oil paint is such that particles of pigment are coated and suspended in a liquid, which, in turn, will bind those particles together when exposed to air. That liquid may be a drying oil, a resin‑oil blend, an oil‑modified alkyd resin, one of the new water mixable drying oils. Oil paint in a stick form, primarily wax and pigment, will be discussed in the chapter on drawing materials.
These oils are used in grinding pigment to form an oil paint, as well as to make media for oil painting. It is the linolenic and linoleic acids present in a drying oil such as linseed oil that, when in contact with air, oxidize to form a film that binds the particles of pigments together, and then becomes insoluble in water or thinners.
Most manufacturers today substitute a refined poppy oil or safflower oil for linseed oil in preparing certain pale oil colors and whites. These drying oils have little or no linolenic acid, a binder that gives a yellow tint to oil paint. The only binder in these oils is the linoleic acid, which is not only less yellow, but also yellows less over time. However, linoleic acid by itself is a weaker binder and should not be used as a replacement for linseed oil in all cases.
Using a “pure oil” system of making paint is the least costly method and, unless driers and wax stabilizers are added to certain colors, there will be wide variations in consistency and drying times between many colors. This type of paint is referred to as “fat” paint because of the high oil content. The fatter the paint, the more yellowing and the greater the possibility for future cracking. Some manufacturers have attempted to reduce the “fat” either by an elaborate grinding system, such as that used by the Holbein Company, and / or by aging the paint before packaging between three to six months, now customary by most manufacturers of the highest quality paints.
The closest one can come to finding the characteristics most desired by the Old Masters in a commercial oil paint today is in the Mussini line of oil paints by the Schmincke Company. They have developed a resin‑oil binder that is stable in a tube. Mussini oil paints are named in honor of Cesare Mussini, a nineteenth‑century painter and teacher who was one of the few remaining repositories of some of the Old Masters’ paint‑making recipes, and who passed this knowledge on to the colormen of Schmincke. Mussini oil paint is made with such drying oils as cold‑pressed and refined linseed oils, poppy oil, and sunflower oil, which are blended with natural resins like damar and mastic, as well as with polycyclohexanone, a modem non-yellowing damar-like resin.
This elaborate formulation allows for four important effects: more even drying of paint films, which reduces the possibilities of future cracking; greater clarity and less yellowing because of the lower oil content; narrowing the range of drying times between different pigments; and smooth and even consistency with little or no separation of oil and pigments.
The mixing of a medium containing such ingredients as mastic and damar into a paint after it has been ground with only a drying oil, will certainly improve the paint’s performance. However, it is not the same as when the pigment is ground into such ingredients to start with, where less oil and more resin can be used. When the ingredients are added later to a high‑oil‑content paint, the original oil in the tube is only diluted, not replaced, and the increase in performance is not as dramatic.
Oil‑Modified Alkyd Resin.
Alkyd resin is the category of resins that are made from mixtures of dibasic acids and polyhydric alcohols. One particular alkyd resin (kept a trade secret) is chemically combined with a non-yellowing oil, such as safflower oil, to produce a workable, fast‑drying binder for oil paints. Driers are often included to speed drying even further and silica is occasionally added to give extra body.
Alkyd resins often exhibit a thixotropic effect, found in many resin-oil paint mediums. Thixotropy is a phenomenon in which a gel, or paste, suddenly loses its plasticity when disturbed or moved mechanically, resulting in a liquid. The opposite may also occur where a liquid, left undisturbed, forms into a gel. There is a certain advantage to this thixotropic property in the glazing technique, for a glaze will resist running down the canvas after it has been applied and left undisturbed. However, the tendency to liquefy rapidly would be a disadvantage in the impasto technique.
Alkyd resins, originally developed for use in making industrial paints, have been applied to make a variety of popular fast‑drying media for artists’ paints. Where alkyd media have been greeted with substantial success the opposite it true for alkyd oil paints. Today, no manufacturer makes an artist grade alkyd oil color. Some twenty years ago Winsor & Newton attempted it and found few artists would make the commitment to abandon their old paints in favor of a different system of painting. Most opted to just use alkyd mediums with their current paints even though it meant sacrificing some the advantages of a pure alkyd system. Griffin Alkyd Colours, with a limited range of colors in a quality less than artist grade and more than student grade, has replaced Winsor & Newton’s initial attempt.
Intermixing nonalkyd paint with alkyd paint, and nonalkyd medium with alkyd medium is perfectly safe, but it slows the drying time of the alkyd. Alkyd paints and alkyd media should not be over-thinned. No more than 25 percent by volume of thinner to paint or medium is recommended; more than this will break down the consistency and quality of the paint or medium. To avoid any problems with adhesion, consistent use of alkyd is necessary. If you are going to use an alkyd paint or oil paint mixed with alkyd media, you must not switch back and forth between an alkyd and nonalkyd in the same painting.
The greatest area of expansion and experimentation in several decades is in the area of water mixable oils. Its promise is to remove or reduce the need for hazardous solvents such as turpentine and mineral spirits. At this time there are three major brands, Grumbacher’s Max Grumbacher, Winsor & Newton’s Artisan Oil Colours, and Holbein’s Duo Aqua Oil. Max Grumbacher and Artisan use a chemical detergent that allows up water to be mixed into their paint up to 25% by volume. For the best results with these paints, use as little water as you can. As you increase the amount of water the gummier the feel of the paint and the tackier the resulting paint film is and the longer it takes to finally harden. Adding any water tends to make the paint foam up a bit when you mix it and it retains the additional volumes as it dries. This can be seen as either a benefit or disadvantage depending on your painting style. Either brand can be mixed with standard media but it reduces the amount of water that it can hold. Winsor & Newton offers several water mixable media to be used with their paints to maintain the working characteristics. Max Grumbacher and Artisan are not manufactured to the same standards as their respective Artists Oil Colors.
The Duo Aqua Oil is a modified linseed oil without a detergent. It could be seen as the opposite of the water mixable oils, because it is a water soluble oil paint that is oil mixable up to 30% thinner or oil media. A opposed to Max Grumbacher and Artisan, this paint can be let down with water only and thinned to a wash, and most startling, mixed seamlessly with acrylics. If you add too much oil it quickly loses its water mixability. Although the feel of the paint is not gummy and it does not foam up like other water mixable paints, it is different from conventional oils in that there is a slight drag on the brush as you spread out the undiluted color. When mixed with water alone, some colors form tiny clumps that require additional mixing to dissolve. However, these are ultimately small issues compared to the possibility of partially, if not totally, eliminating hazardous solvents. The greatest disappointment with this line of paint is the complete lack of traditional pigments such a cadmiums, cobalts, and other metallic salts. It therefore cannot be classed with other artists grade paints. I suspect the reasoning is to market this line of paint as a non-toxic or less hazardous oil paint for children, schools, and other institutions.