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 five day period, while most dissolved resins 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 paint should ever be let down with only a thinner such as turpentine. This wash 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 wide spread. The practice of using oils, which imparted smoothness to a painted sur face, 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.
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 months 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 century. The better commercial grades are warmed and aged to remove the grossest particles. Raw lin seed 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 seven teenth 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°F. 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.
Poppy Oil 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 making 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.
Driers, or Siccatives
DRIERS, or siccatives, are usually metallic salts that are combined with oils or resins and then mixed into the paint and/or medium and/or varnish to accelerate the drying time by speeding the rate of oxidation and polymerization. But it is important to remember that driers diminish the life of the paint or varnish. With good judgment and experience, however, they can be used safely. The following guidelines may prove helpful.
l. Use a drier only in glazes or in thinly painted pictures.
Never use more than 3 percent concentrate to media consisting primarily of a drying oil, and 6 percent to oil-resin media combinations.
Never apply a faster-drying paint film over a wet, slower-drying paint film.
Test a drier before use on something other than your final picture.
Cobalt drier, siccative de Courtrai (primarily lead linoleate), and siccative de Haarlem (primarily dammar resin) are among the more common driers. Cobalt and dammar are the least harmful because they work primarily while the paint is in a liquid state, their action becoming progressively less as the paint film hardens. This is not the case with lead driers, which can continue to act even after the paint film hardens. Siccative de Haarlem is safer than siccative de Courtrai because of its lower lead content and proportionately larger amount of dammar resin. Similar drying effects can be obtained by mixing small amounts of faster drying colors into slower-drying colors, for example, mixing cobalt, manganese, viridian, and lead-containing colors with slower-drying colors such as phthalo blue or lamp black.
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 overthinning 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 com monly 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 liquified 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 South east 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 re-dissolve 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 (see page 264) 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 fom1. It also dries clearer and with more gloss than dammar. Many people who use mastic use it in com bination 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.
OIL-MODIFIED ALKYD RESIN
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 nonyellowing 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 overthinned, so add no more than 25 percent thinner to resin.
Was as a base for a painting medium since the first century B.C. and remained popular until about A.D. 700. Today, there has been a resurgence in the use of various waxes in painting. These include fossil waxes, paraffin waxes, and beeswax. The use of such waxes in painting is called encaustics, or encaustic painting. The term "encaustic" is derived from the Greek, meaning "burnt in," which occurs after the painting is completed. This process involves heating the surface to remove all brush or knife marks and to fuse the painting into one solid paint film. Tradition not withstanding, this final burning process is rarely used today. The best surviving examples of early encaustic are the mummy portraits from the Egyptian district of al-Fayyum from the first century B.C. A recent use of encaustic in painting is the Target and Flag paintings by Jasper Johns (c.1955). In these paintings, the pigmented beeswax was applied in translucent layers over strips of newspaper clippings.
The primary advantage of using beeswax, which must be kept warm during application, is that the surface can be worked over as soon as the beeswax pigment mixture cools to room temperature. Since the wax can be softened and dissolved in turpentine or mineral spirits, an encaustic painting can be reworked with a brush and thinner.
Fossil waxes are actually far more commonly used today than the more expensive beeswax. The great advantage to fossil waxes is that they do not require heat to make them pliable. Dorland's Wax Medium is made commercially of fossil wax, which can be mixed directly with oil paint. It is ready to use with or without heat and is relatively inexpensive. It hardens as the solvent evaporates, which takes several times longer than cooling beeswax. Beeswax, despite its great expense, the difficulty of finding it in a pure and bleached state, and its need to be heated, is still preferred for heavy impasto technique.
Wax media set up quickly, drying to the touch within thirty minutes, although final curing is quite slow. A wax-medium paint film will dry with a matte sheen. If desired, a semi-gloss appearance can be created by polishing the heated and cooled surface with a silk cloth. Surface quality has become an important consideration for many contemporary artists, who feel that surface reflection detracts from the direct experience of their work. The major disadvantage in the use of wax is the danger of melting when it is exposed to high temperatures. If you wish to retain the matte quality, remember that this can easily be lost if the surface is polished by improper handling.
Whenever a paint is thinned it should always be thinned with a medium, of which the simplest form is a combination of thinner and drying oil. The amount of thinner used should be less than 50 percent of the recipe because there will always be sufficient drying oil replacing whatever might be washed away by the thinner.
A medium is chosen not just to thin a paint, but also to alter its working characteristics, drying time, and final appearance. Resins, waxes, and driers are used as additives to basic thinner-drying oil media to make these changes. No matter what formula is used to create a medium, the principle of fat over lean should never be violated in its application. ("Fat" means a medium rich in oil and "lean" refers to a medium rich in thinner. A lean medium or paint mixture should never be applied over a fat one.)
Most of the following recipes have been offered by artists who have had success using them. Some are from previously published literature to which I have made small changes, primarily to update them for currently available materials. I would like to thank all those who laid the groundwork for these recipes by their courageous experimentation with different materials on their own work.
The following diagram describes two distinctly different media. The drying oil turpentine medium is for thinning paint mixtures and the wax-turpentine medium is for thickening paint mixtures.
DIAGRAM OF SIMPLE MEDIA
Drying Oil or Wax
(linseed oil, poppy oil, stand oil, or wax)
(turpentine, petroleum distillate)
A few drops of drier may be added to speed the drying time if wax is not used.
Drying Oil-Turpentine Medium. A drying oil-turpentine medium is the most common type of medium used. It is the simplest to make and many paint manufacturers sell it premixed. It consists of 60 percent drying oil, usually linseed oil or stand oil, and 40 percent turpentine. The percentage of turpentine should be reduced as layers of paint are built up over one another.
The disadvantage of this method is the increased drying time as the thinner is reduced and the oil is increased in the fat over lean process. Turpentine-drying oil media also have no body and tend to run down the canvas, especially in glazes. (A glaze is a paint that has been thinned greatly with a medium to disperse the pigments in the paint. This makes the paint transparent and allows the application of a thin veil of color over another.) The drying time of this type of medium is slow, from two to five days, unless mixed with driers or a fast drying color like manganese blue.
Wax-Turpentine Medium. A wax-turpentine medium may be heated and used with oil paint or mixed with dry pigment. It is ideal for impasto technique. The wax should be warmed in a double boiler reserved for just this purpose and dry pigment may be added directly. (Keep in mind that dry pigment is difficult to handle safely and can be extremely dangerous.)
1 part wax (preferably beeswax) 3 parts rectified turpentine
Warm in a double boiler and stir until the wax is dissolved. Pour into a wide-mouth container.
May be worked over in 30 minutes.
The following recipes are easy to prepare and provide greater versatility than simple media.
DIAGRAM OF INTERMEDIATE MEDIA
(If natural resin is involved use only genuine turpentine.)
Resin or Wax
(dammar solution, dammar varnish, Venice turpentine, or wax) A few drops of drier may be added.
Copal-Drying Oil Medium. Copal media are ideal for producing very hard and durable, although brittle, paint films that can be applied without softening or redissolving previous paint layers. This allows for an egg-tempera like effect where one color can be painted over another after it has dried without the two colors mixing.
1 part copal varnish
1 part genuine turpentine
1 part linseed oil (Stand oil may be substituted. The drying oil reduces the brittleness of the copal resin.)
The drying rate is approximately two hours if the copal varnish used contains driers;
if it does not contain driers, the drying time will be approximately 1½ days.
(Please note that this means workably dry, not totally dry.)
Dammar-Drying Oil Medium. The difference between this medium and that of the copal-drying oil medium is that the dammar will soften and partially re-dissolve when successive layers of paint mixed with this medium are applied. This allows the two paint films to interlock and to appear less isolated from each other. Dammar also yellows and darkens much less than copal. It is also less brittle. Dammar is not as hard a resin as copal and is therefore less durable, but not sufficiently so to cause concern. The following recipe also has better leveling properties than a copal medium.
DAMMAR-DRYING OIL MEDUIM
1 part dammar varnish (Dammar heavy gum solution may be substituted for extra thick body.)
1 part stand oil
to 5 parts rectified turpentine
(This proportion may also be varied for desired fat or lean quality.)
15 drops of cobalt drier may be added to every 8 ounces of medium to speed drying.
The drying time is between 2 and 3 days (without the drier).
Stand Oil-Venice Turpentine Medium. The combination of stand oil and Venice turpentine will give a thick, resinous medium with ename-llike leveling proper ties. Glazes with this medium will be exceptionally clear and brilliant.
STAND OIL- VENICE TURPENTINE MEDIUM
1 part stand oil
3 parts Venice turpentine 0 to 3 parts turpentine
(Vary turpentine according to fat over lean principles.)
It is necessary to warm the stand oil and Venice turpentine first to make mixing them easier.
Paint films using the formula can often be worked over in 1 to 2 days.
Greater brilliance as well as a shorter time between application of paint films can be had by adding some dammar heavy gum solution (this is a concentrated form, which can be bought or made).
The following media are significantly more difficult to prepare, yet are among the most versatile.
DIAGRAM OF COMPLEX MEDIA
(If natural resins are involved use only genuine turpentine.)
(i.e., dammar varnish)
(mastic solution, Venice turpentine, or wax) A few drops of drier may be used.
Dammar-Oil-Venice Turpentine Medium. This is ideal as an all-purpose painting medium. If you use less turpentine, it is excellent for glazing. Both the dammar and Venice turpentine allow for the interlocking of successive layers of paint.
DAMAR-OIL-VENICE TURPENTINE MEDIUM
9 parts dammar varnish (to add gloss, increase brilliance, and speed drying)
4 parts stand oil (to aid leveling and to give body)
2 parts Venice turpentine (This is a resin and its purpose is
to thicken the medium and add brilliance.)
4 to 9 parts rectified turpentine (To comply with fat over lean, the amount of turpentine should
be reduced to 4 parts in successive layers. Do not substitute another thinner.)
Without the addition of a drier, this medium will often be ready to work over in 24 hours. A small amount of drier can speed this up.
Linseed Oil-Mastic Media. The combination of linseed oil and mastic, called megilp, was popular in the eighteenth and nineteenth centuries as a painting medium, although it is now blamed for many of the disastrous effects seen in paintings of this period. At first, the mastic resin was thought responsible for the cracking, blistering, and browning that occurred. It is now thought, however, that these undesirable affects were caused primarily by the extended heating of the mastic resin called for in the recipe and by the excessive use of driers in combination with impure turpentine. Megilp was so popular because it appeared to dry, in some cases, in only fifteen minutes, and in several hours in heavier applications. It reportedly had a wide range of handling properties and was excellent for impasto and glazing. There has been a resurgence of interest in linseed oil-mastic combinations, and various "improved" formulas have evolved. The resurgence started in the 1930s; however, fifty years is not enough time to make an unequivocable recommendation one way or the other. Time is the only known test for such media as these. There are many variations of megilp recipes, none of which I feel sufficiently confident about to offer at this time.
Flemish medium is an example of an all-purpose painting medium made with mastic and oil that has been handed down through time. It has excellent handling characteristics, and the mastic creates a brilliantly clear paint film. The following recipe is used today with excellent results. The ingredients are similar to those used to make the infamous megilp; however, there are several important differences regarding heat, turpentine, and the use of driers-the mastic is not added until the end of heating, only rectified turpentine is used, and the amount of drier is small.
20 parts linseed oil (cold-pressed preferable) 14 parts rectified turpentine
7 parts mastic tears, or crystals
1 part litharge (lead monoxide-This is highly poisonous and is not sold in art supply stores;
it must be obtained through chemical supply houses. If pale drying oil, which is made with litharge,
is used instead of linseed oil, then this ingredient is unnecessary.)
Mix the poisonous litharge in a small amount of oil, using a palette knife, until a paste is formed. (This will prevent the litharge from settling to the bottom of the cooking pot when it is later added.) The remaining oil should be heated outdoors or with proper ventilation over a low flame in a covered enamel cooking pan. The pan should hold twice the volume of the ingredients. An asbestos mat placed between the flame and the pan will reduce the possibility of scorching.
When the mixture is warm, approximately 320°F, or 160°C, add the litharge. Gradually increase the temperature, over 2½ hours, with the last 30 minutes at exactly 482°F, or 250°C. Stir occasionally with a wooden spoon.
Allow the medium to cool to 392°F, or 200°C, by placing the pan on a stack of old newspapers and then stirring and fanning. Now add the mastic in small amounts, stirring constantly. The medium will foam up as you add the mastic; if stirring is constant the foam will rapidly dissipate.
Next, very slowly add the cold turpentine. The medium will again foam up; this will dissipate if stirred and fanned.
Pour the warm medium, approximately 284°F, or 140°C, into a wide mouth container and allow it to cool before covering.
Beeswax and Oil. This combination was used by Rubens because of its extreme versatility. It could be effective in impasto or as a glaze, but the disadvantage is that it is a dark medium and tends to discolor all colors. The following is a traditional recipe which has been updated.
BEESWAX AND OIL MEDIUM
IO parts linseed oil (cold-pressed preferable) 2 parts beeswax
1/8 part litharge (lead monoxide--This is highly poisonous and must be obtained from a chemical supply house.
However, this ingredient can be omitted if pale drying oil is substituted for the linseed oil.)
Mix the poisonous litharge in a small amount of oil, using a palette knife, until a paste is formed. (This will prevent the litharge from settling to the bottom of the cooking pan when added later.) The remaining oil should be heated outdoors or with proper ventilation, over a low flame in a covered enamel cooking pan. The pan should hold twice the volume of the ingredients. An asbestos mat placed between the flame and the pan will reduce the possibility of scorching. When the mixture is warm, approximately 320°F, or 160°C, add the litharge and broken pieces of beeswax. Gradually increase the temperature over 2½ hours, with the last 30 minutes at exactly 482°F, or 250°C. Stir occasionally with a wooden spoon.
When the mixture appears black with brown fumes rising, it is ready. Allow the medium to cool; do not refrigerate or place the hot pan on a cold surface (a stack of old newspapers works well). Pour the cooled medium, approximately 284°F, or 140°C, into a wide mouth container and allow it to cool before covering.
COMMERCIALLY PREPARED MEDIA
Commercially prepared media have a narrow range. They are primarily simple media, although occasionally intermediate and complex media can be found. Among the most commonly offered commercial painting media is a mixture of stand oil and turpentine, which usually carries a brand name. That such media as these continue to sell, and sell in quantity, testifies to either incredible laziness or a profound lack of understanding of these materials. With the exception of these media, many of the other painting media offered by manufacturers can be a justifiable convenience. In fact, most other commercial media consist either of all or part of some version of an oil-modified alkyd resin, which would be impossible to prepare at home. The use of alkyd as well as other synthetic resins has become so widespread that they have all but replaced many of the traditional ingredients used in Commercially prepared media.
Prepared media such as Liquin by Winsor & Newton, Zee by Grumbacher, and Res-n-gel by the Weber Company, are some examples of primarily oil-modified-alkyd resins. The paste-like quality found in some of these media is often due to the addition of silica. Grumbacher's Gel is designed to give body to a paint mixture necessarily speeding the drying time. Liquin behaves similarly to a copal-drying oil medium and, in fact, similar formulations have been successfully offered by other manufacturers as a substitute for this medium. LeFranc & Bourgeois offers a ready-made Flemish Medium, based upon mastic and thickened oil with oxides, which has the consistency of a gel and which is sold in tubes. It also produces a medium it calls Venetian Medium, which is a ready made wax-oil medium prepared similarly to its Flemish Medium, except that a hard wax is used rather than mastic. This medium could be a viable alternative to preparing the beeswax and oil medium previously described.
The only serious caution I would add about thinning prepared media that contain alkyd resins is that these resins are sensitive to overthinning and can break down chemically. I recommend that you use no more than 25 percent thinner to prepared media. So far, over the past ten years, these new prepared media have demonstrated great flexibility and durability whether applied thickly or thinly.
Diluents, or Solvents
Solvents are used in painting primarily to thin the paint or medium, and should then evaporate from the paint film without leaving a trace. The next most important function of thinners such as turpentine is to clean tools and brushes. Keep these two principles in mind as you read the following descriptions of the various solvents.
Many thinners and solvents can be hazardous to health. They are not only inhaled and ingested, but are absorbed directly through the skin.
Gum Turpentine is made from distilling the resinous gum from pine trees. Gum turpentine usually contains a small amount of sticky residue, which can be imparted to a painting if this kind of turpentine is used in large quantities. It may remain in the layers of paint, inhibiting proper drying and, in time, causing discoloration. Unless cost is a serious consideration, I do not recommend using gum turpentine with artist-grade paints and media. It is, however, perfectly acceptable for cleaning tools and brushes.
Rectified, or Artist-Grade, Turpentine is double distilled to remove the last bit of residue from the pine-tree gum. This thinner is ideal for oil paints and media because it does the job and then evaporates from the paint film without a trace. Artists often buy the finest paints and media and then use the cheapest thinners. This is like buying a Rolls Royce and putting kerosene in the gas tank. There are areas where compromises can be made, but they should be made with common sense.
Venice Turpentine is a resin used as a thickening agent in the preparation of media. Petroleum Distillate, Mineral Spirits, and White Spirits are byproducts from the manufacture and refinement of petroleum. Although they are all basically the same type of product, the actual chemical composition may differ vastly from brand to brand. This is of little concern when purchasing a product made for artists' paint such as a turpentine substitute. There can be a serious problem, however, if this solvent is used with house paints or if house paint mineral spirit thinners are used with artists' materials. Always test a solvent with a particular paint no matter what the label says.
The advantage of using petroleum distillates that are produced for artistic use is that they are less expensive than turpentine, have a longer shelf life, leave no sticky residue, and according to government standards, are less hazardous than turpentine. They are also recommended for painters who are allergic to turpentine. There are, however, two drawbacks. The first is that dammar and mastic are partially insoluble in these solvents. The other is that they have a decidedly oily smell, which some people may find offensive.
Odorless Paint Thinners are essentially the same as petroleum distillate or mineral spirits except that the unpleasant odor has been removed or masked. All are fine turpentine substitutes if you avoid using them with dammar or mastic.
Remember, however, that the paint thinner that is sold in hardware stores is not the same, and may contain other solvents that can destroy an artist-grade paint, ruin brushes, substantially increase health risks, or cause other dangerous or unpredictable results. One artist reports that she was using a paint thinner purchased at a hardware store, when all of a sudden the hairs were falling out of her brushes in clumps. She had been using hardware-store paint thinner without serious problems until this incident. After investigation, it was discovered that many of the solvents packaged for hardware stores under the name "paint thinner" may be changed by the manufacturer without notice. These changes may be perfectly satisfactory for hardware use, but in this case the last batch of paint thinner the artist purchased was, unfortunately, particularly good at dissolving the glues used to hold her brushes together.
Oil Spike of Lavender is one of the oldest known turpentine substitutes. It is the essential oil from the lavender plant and possesses a most wonderful scent. It evaporates more slowly than turpentine, is nonflammable, and is primarily used as a turpentine substitute by those who are allergic to turpentine. Since it is slower drying than turpentine, it is occasionally used to slow the drying time of oil paint. Its disadvantages are its extremely high cost and limited availability. Currently, the only sources of this product are Winsor & Newton, Sennelier, and Schmincke.
Varnish is a 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 reaction 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
4 parts rectified turpentine
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.
(from which the varnish is made)
1 part mastic tears (The better mastic comes in the form of tears, or little round balls.)
3 parts rectified turpentine (The best turpentine should be used, especially if the varnish will be used in painting.)
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.
2 parts dry copal resin
3 parts stand oil or sun-thickened oil
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° 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 nonyellowing. 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 enroute 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.