(Excerpts from ART HARDWARE: The Definitive Guide to Artists’ Materials, by Steven Saitzyk © 1987)
Resins are added to painting media primarily for three reasons. First, a mixture of drying oil and resin will offer a paint film that can be worked over within hours or, at maximum, the next day because when the solvent evaporates, which happens within several hours, the resin hardens, holding the drying oil in place until it can oxidize and solidify. Second, a resin may be used to dilute a paint for glazing without over thinning it. If a glaze is too watery, it will run uncontrollably over the painting surface. Finally, resins dry with greater clarity than drying oils and they will add brilliance to paint films.
Resins are also the primary material used for varnishing paintings. Today, the five most commonly used resins are copal, dammar, mastic, Venice turpentine, and alkyd.
Copal is a hard resin. Originally, the copal used in painting media was the fossil resin amber. Now that amber is considered a semiprecious stone and has become virtually unavailable to the artist, the copal available for artists’ use is unspecified tree-root resins. There are many grades available on the world market and the best are rarely used for making media. In addition to thickening a paint medium, adding leveling properties to the paint film, and increasing gloss, this resin also exhibits a thixotropic effect, which is particularly helpful in glazing.
Copal is a hard resin which can only be liquefied through heating with a solvent and so, when it dries, it is difficult to dissolve unless it is reheated. This is a desirable characteristic because if a protective varnish such as dammar has to be removed to clean or restore a painting, this can easily be done without great risk of dissolving underlying paint films which contain the copal resin. This insolubility at room temperature also means that a copal varnish makes an excellent isolating varnish when applying successive glazes.
The disadvantage of copal is that it darkens with age. Although the actual reason for this darkening is not known, it is speculated that it is due to changes that occur during the heating necessary to dissolve the resin. Copal is brittle and is best used conservatively with a flexible drying oil like stand oil. The difference between a “copal medium” and a “copal varnish” is that the varnish often contains driers, which add to the risk of cracking, and therefore the varnish is not best used in making a painting medium. Today, many of the commercially made copal media have no copal at all and are composed of synthetic substitutes or alkyd resins. These substitutes are safer to use as a medium, being less brittle, but they should not be used to replace copal varnish.
Dammar is collected from the fir tree genus Shorea or from Hopea trees of Southeast Asia. Dammar is a soft resin and is readily dissolved in turpentine (not in mineral spirits, because it is then partially insoluble) at room temperature and is the most popular additive to a painting medium as well as the most commonly used resin for varnishing. Dammar when used in painting media helps paint films to set up quickly so that they may be worked over within a day. It is possible to interlock paint layers by taking advantage of the two types of drying that occur with a dammar-drying oil combination. Several hours after using this combination, the dammar has hardened and the drying oil has begun to polymerize through oxidation. If, in one and a half to two days, when the drying oil is roughly half dry, a second paint layer containing the same medium is applied, the two layers will interlock. The turpentine of the medium when applied will redissolve part of the resin and soften the drying oil of the paint film underneath, and the two should lock together. This process can be used to produce beautiful and subtle effects. Dammar also adds gloss and brilliance to a painting.
Mastic is an exudate from the Mediterranean mastic shrub Pistazia lentiscus, also known as the pistachia tree. Like dammar, it is a soft resin totally soluble in turpentine and only partially soluble in mineral spirits. The use of mastic resin in painting dates back to the time of Rembrandt and the Dutch Masters and is responsible for Rembrandt’s blue backgrounds now appearing green. This is due to the yellowing of the mastic he used in the application of five or six coats of varnish. Mastic is still popular today primarily as an ingredient for media rather than as a varnish. The yellowing effect of mastic is less pronounced when it is used in a medium and many painters feel its advantages outweigh this drawback. One of the advantages is that a mastic solution dries faster than all the resin solutions-one hour in its pure form. It also dries clearer and with more gloss than dammar. Many people who use mastic use it in combination with dammar, fifty/fifty in a recipe calling for one or the other. This ratio reduces the yellowing of the mastic and helps to speed the drying of the dammar.
Genuine Venice turpentine is collected from larch trees. It is a viscous liquid resin; such resins exude from certain trees and are often referred to as balsams. Venice turpentine has been used in painting for centuries and has excellent handling and aging characteristics. It should not be confused with the thinners gum turpentine or rectified turpentine. Venice turpentine is used as an additive to thicken other media. This resin is popular because it gives body to the paint film while maintaining gloss and brilliance, and it yellows very little over time. Genuine Venice turpentine is not easily found and some companies are using a substitute resin called Canada balsam. Canada balsam has two advantages over Venice turpentine-it dries more clearly and it takes only one or two hours to dry, as opposed to three days for Venice turpentine. However, Canada balsam has a distinct disadvantage in that it is more fluid than Venice turpentine.
Alkyd resin is the category of resins that are made from mixtures of dibasic acids and polyhydric alcohols. There are many alkyd resins and each manufacturer picks a favorite and keeps it a secret. The quality and concentration of an alkyd resin or medium can vary a great deal from manufacturer to manufacturer. The selected resin is chemically combined with a non-yellowing oil, such as safflower oil, producing a workable, fast-drying medium. Most manufacturers add driers to speed up the drying time and silica to give extra body.
Alkyd resins can exhibit a thixotropic effect. Thixotropy is an unusual phenomenon where 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. This effect can be used to advantage during glazing by helping to prevent the glaze from spreading uncontrollably over the painted surface. However, this characteristic is undesirable with impasto techniques. Alkyd resins should not be over thinned, so add no more than 25 percent thinner to resin.