(Excerpts from ART HARDWARE: The Definitive Guide to Artists’ Materials, by Steven Saitzyk © 1987)
Varnish is resin dissolved in turpentine or in a mixture of turpentine and a drying oil such as linseed oil. There are several resins available with which to make a varnish, and the selection of a resin is guided by use.
Picture Varnish is used as a final coating for a painting to protect the picture and unify the appearance of the surface. It can, for example, make the surface appear either matte or shiny, or protect the paint film from chemical reactions with the atmosphere and from mechanical abrasion.
Retouch Varnish is used to give a full and wet look to the surface of the unfinished painting before work is resumed. This prevents the eye from being fooled by the unevenness of the light reflected from parts of the painting's surface.
Mixing Varnish is used as an additive to the painting medium to accelerate the drying time, to add gloss, and to give body to a glaze.
Isolating Varnish is a resin that is insoluble in turpentine or mineral spirits. When this varnish is applied to a paint film, it will protect it from being affected by the turpentine or mineral spirits in the next layer of paint to be applied.
Types of Varnishes
The following are descriptions of various types of varnishes and their uses. Commercially prepared varnishes are readily available, or varnish can be made from recipes given in this section.
Dammar Varnish is the most popular of all varnishes and is used as a retouch, a mixing, and a picture varnish. It does not bloom (develop a chalky appearance) and it yellows less than most natural resins. Dammar varnish can be readily purchased already prepared or you can prepare it according to the following recipe.
1 part crushed dammar resin
wrapped in muslin
Leave wrapped resin to soak in the rectified turpentine for 1 or.2 days, or until the resin has dissolved.
Filter if necessary.
Dries in approximately 1 hour.
Mastic Varnish is clearer than dammar and can be applied more easily. However, it does tend to bloom (develop a chalky surface) in humid climates, as well as yellow more than dammar. Mastic and mastic varnishes are much more expensive and harder to come by than dammar. Today, mastic is usually used as a mixing varnish and rarely as a picture varnish. A mastic solution (concentrate) for painting as well as a varnish can be prepared from the following recipe.
1 part mastic tears
The preparation of mastic solution is sensitive to impurities and heat. The mastic tears should not be crushed to speed the process. The tears should be suspended in a gauze bag or nylon stocking and allowed to dissolve without use of heat. This takes 1 or 2 days.
Mastic has a more stable shelf life as a solution (heavy gum solution is best) than as a varnish. Therefore, you may wish to consider diluting a mastic solution to make the varnish. This may be done by adding 1 part rectified turpentine to 3 parts mastic solution.
Mastic varnish will dry in 1 hour.
Copal Varnish and copal medium are becoming increasingly difficult to find because amber has become semiprecious and rare. It is being replaced by such synthetics as alkyds or unspecified tree-root resins.
Copal varnish makes an excellent isolating varnish, when used in moderation, and a very hard-surface final varnish. However, copal varnish is more often produced as a convenient source of liquid copal for use in media than as a final protective varnish. Since it has been found that copal varnish darkens and often cracks with age, even without the driers that are commonly added, most recipes call for the use of stand oil or sun-thickened oil to reduce the risk of cracking. The following is a recipe for copal-oil varnish.
parts dry copal resin
The oil should be heated to approximately 482°F, or 250°C, for 45 minutes to 1 hour. A sign of readiness is the darkening of the oil. Heat copal resin separately until it melts (between 356° and 644°F, or 180°C and 340°C ). Then slowly add the hot oil, stirring with a wooden spoon. Allow the medium to cool, then store in wide-mouth bottles.
Shellac is a resin gathered from the lac insect, Laccifer lacca. The best shellac is called "true orange shellac," and it is not dyed to look orange. Shellac is insoluble in turpentine, but is soluble in alcohol. It dries to a hard, tough, flexible film when applied to something other than the surface of a painting. On paintings it tends to crack and to darken with age. It can be useful as a sizing or an isolating varnish between paint layers (especially egg tempera). It is also a good, cheap fixative for charcoal and other drawings. It dries in about thirty minutes.
Synthetic Varnishes are, in most cases, composed of ketone or acrylic resins dissolved in mineral spirits. The advantages are that they dry rapidly, are crystal clear (unless a wax is added to create a matte finish), and are non-yellowing. The disadvantage is that, with the exception of polycyclohexanone, they can be used only as a picture varnish.
Polycyclohexanone is a synthetic dammar like resin, developed recently in Germany, that can be used alone or in conjunction with dammar for media or varnish. It is prepared in the same way as dammar. The advantage is that it is optically clearer than, and yellows less than, dammar. It also forms a harder and more durable paint film. However, it is slightly more brittle than dammar and, consequently, is often used with other, more flexible resins. The polycyclohexanone adds clarity, and resins like dammar add flexibility.
Soluvar is a synthetic varnish composed of acrylic resin produced by Binney & Smith Company, which is available in both matte and gloss. It is used as a final, protective picture varnish on both oil paintings and acrylic paintings. It is receiving greater recognition because it can easily be removed if the picture needs to be cleaned or restored.
Matte Varnishes are made by adding to a varnish a flattening agent, which is usually a wax (such as beeswax or a fossil wax) or a wax-like substance such as aluminum stearate. Winsor & Newton's wax varnish and Dorland's Wax Medium are thick and are applied by rubbing the medium onto the surface of the painting and polishing it with a soft brush or silk cloth when dry. Liquid matte varnishes are made by suspending a wax in a varnish. The application of this kind of varnish is facilitated by warming the varnish until the cloudiness disappears and then applying it with either a soft brush or a warmed airbrush.
The advantage of a matte varnish is that there is no surface glare to interfere with viewing artwork. This has become an important consideration for many contemporary abstract artists. A slight sheen can give the illusion of a secondary color where there is none. This is less of a consideration in figurative artwork where there are usually many variations in color and shape, which help to camouflage the shiny spots on the surface.
The disadvantages of matte varnish are that too much can leave a milky or cloudy appearance on the surface, which is also easily damaged by rubbing.
Application of Varnishes
When applying varnish, the first consideration is whether the painting is really dry. Although a painting may feel dry to the touch within days or weeks, the layers below the surface may not be thoroughly dry. A paint film dries by reacting with the oxygen in the air. If a painting is varnished before this reaction is completed in the paint layers below the surface, these paint layers are sealed off from their source of oxygen and cannot complete their drying process. The painting may remain soft and sticky for a considerable length of time and, with improper drying, the paint film may not bond properly to other film layers. Another problem caused by premature varnishing is that the solvent of the varnish may penetrate the paint layers that are not completely dry, thus softening them and affecting the appearance as well as the stability of the paint films.
Most paintings of average thickness and painted with a lean medium will be ready for varnishing between six months and a year after completion. Unless driers were used throughout the painting, one year is usually the safest choice when in doubt. If the paint is thick, one year will not be long enough. Never heat or place a painting in the sun to accelerate the drying process. Because drying of oil paint is a chemical reaction with oxygen, rather than evaporation, rushing the process can cause wrinkling and other horrors. It is best to store the painting where there is light, ventilation, warmth, low humidity, and loving care.
Paintings that must be displayed before they are thoroughly dry can be shown either unvarnished or coated with a retouch varnish, which will even the surface appearance and will provide some protection. It will also slow the ultimate drying time, but will not prevent proper oxidation.
During the lengthy drying process, the surface of the painting may collect dust or dirt, which must be removed before varnishing. Any cleaning must not involve the use of water because the water can penetrate the paint layers, thus reaching the ground and causing it to swell. This will weaken the bond between the ground and the paint and can result in serious cracking. The best way to remove dust is first with a feather duster or a pigeon wing. Then take a loaf of fresh bread and pull out the center, squeeze it into a ball, and roll this over the surface of the painting. If there are slight grease stains, they may be removed by blotting with mineral spirits. If there are problems beyond those described here, professional advice is preferable to experimentation.
When the painting is dry, has a clean surface, and is in a dust-free, dry, warm environment, the varnish can be applied. The two basic methods of application are spraying and brushing. Spray varnishing can be successful when applied to a surface that has a minimum of texture, but a spray cannot cover textural irregularities as well as a brush. If a spray is held too close to the surface, the application will be too heavy and may run or pool. If the spray is too far from the surface, some of the particles of spray may partially dry en route to the surface and give it a frosted or powdered look. If you begin to spray off the surface and then move evenly onto the surface, pooling can be avoided on the surface because the areas where you start, stop, or change direction will be outside the painted area. Two thin coats are superior to one thick coat.
For textured and irregular surfaces, brush application of varnish is best. The varnish can be worked into areas that are not easily accessible with a spray. For a heavily textured surface, a hog or bristle brush is necessary to force the varnish into difficult areas. Ox hair is excellent for smoother surfaces.
After the application of the varnish, the painting should be laid flat to dry for one or two days. The surface should be protected from falling particles and dust. This may be accomplished by laying a board over some books or strips of wood placed on opposite sides of the painting. This will bridge the painting and will keep the protective covering a few inches off the surface.
(Excerpts from ART HARDWARE: The Definitive Guide to Artists’ Materials, by Steven Saitzyk © 1987)
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