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)
Thinner (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
(damar solution, damar 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.
A TYPICAL RECIPE FOR A COPAL-OIL MEDIUM
1 part copal varnish (see 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 1 1/2 days.
(Please note that this means workably dry, not totally dry.)
Damar-Drying Oil Medium. The difference between this medium and that of the copal-drying oil medium is that the damar will soften and partially redissolve 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. Damar also yellows and darkens much less than copal. It is also less brittle. Damar 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.
DAMAR-DRYING OIL MEDIUM
1 part damar varnish (Damar heavy gum solution may be substituted for extra thick body.)
1 part stand oil
1 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 enamel-like leveling properties. 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 damar 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., damar varnish)
(mastic solution, Venice turpentine, or wax)
A few drops of drier may be used.
Damar-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 damar and Venice turpentine allow for the interlocking of successive layers of paint.
DAMAR-OIL-VENICE TURPENTINE MEDIUM
9 parts damar 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 purposeis 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. Care should be taken to reduce the possibility of scorching.
When the mixture is warm, approximately 320°F, or 160°C, add the litharge.
Gradually increase the temperature, over 2 1/2 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 widemouth 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
10 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.
Care should be taken to avoid 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 2 1/2 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 widemouth 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, Zec by Grumbacher, and Res-n-gel by the Weber Company, are some examples of primarily oil-moditied-alkyd resins. The pastelike 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 readymade 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.