Drying Oils and Mediums

Drying Oils | Historical Preparation
Use of Drying Modifiers
Modern Preparation | Mediums | Application

In Preparing Resins for Use as Varnish

DRYING OILS: any of a group of oily, organic liquids that, when applied as a thin coating, absorb atmospheric oxygen and polymerize, forming a tough, elastic layer.

Linseed Oil: the yellow oil obtained by pressing the seed of flax. It is known to dry from the top down after forming a skin, and therefore takes many months to dry through completely after becoming dry to the touch. This oil contains Linoleic Acid, an unsaturated fatty acid occurring as a glyceride (an ester). Early painters neutralized this acid by the addition of lime or other siccative additives so that it would not deteriorate the painting surface. Linseed also contains Linolenic acid which attributes to its yellowing but also to its desireable flexibility. Linseed oil was one of the very first drying oils to be used in varnishing and has become the predominant oil for paint vehicles and mediums through the ages due to its superior balance of qualities. The mass production of alkili refined linseed oil for industrial use has also made it the cost effective favorite for artistic use as well. However, the best form of the oil for the artist is cold-pressed. (Old Holland oil paints use cold-pressed linseed oil)
Walnut Oil: an oil early favored by oil painters for its drying properties, since it dries quickly and evenly throughout. Slightly aged nuts produce higher quality (purer) oil. In earlier centuries difficulties in extraction and storage impaired its usefulness as a medium, making it less popular than linseed oil. Its primary use has been as a vehicle for grinding paints; its tendency to yellow less with age made it a preferred oil for mixing whites and blues which were corrupted by yellowing oil such as linseed oil. However, modern technology has made this oil easier to obtain and to preserve, making it useful and affordable for all oil colors. (M. Graham & Co. produces a full line of walnut oil paints)
Poppy Oil: derived from the seed of the poppy flower (Papaver somniferum), "it is a fine drying oil when treated." Its light color makes it ideal for grinding whites and blues and many paint manufacturers us it so. However, poppyseed oil will yellow some, so that its superiority over linseed oil in this respect is not so great when compared with its weaknesses. The absence of linolenic acid in poppy oil is what adds to its lighter color, but also to its brittleness. It dries very slowly and forms films that are soft and spongy and have a greater tendency to crack when compared to those formed by linseed oil. It is therefore not to be used in complex or layered painting techniques and is best used for alla prima, or direct (one coat) painting. The reduced flexibility of the paint film means that a white ground in pure poppyseed should never be used in underpainting and, following the rule of more flexible over less flexible, cannot be painted over linseed ground whites in top layers. Although, added in small percentage (15 - 25%) to linseed-oil-ground paint, it can reduce stringing and add a buttery consistency to some pigments (such as ultramarine). One author states that, used in this manner, "poppyseed oil might preferably be substituted for the essential oils (clove or spike lavender) when a slow-drying oil color is desired."

Other oils listed by early painters were Sesamine oil and the Egyptian oil Cicinum. Painters have also worked with the oil expressed from the seed of the Safflower plant, a thistle like composite herb, Carthamus tinctorius, native to the Old World, having large, orange-red flower heads. It is said to be unsound when layered with linseed oil, though some modern manufacturers use it to grind their whites. It generally has the same properties and problems as Poppyseed oil.

Yellowing
"Eibner and others have pointed out that the matter of yellowing concerns only clear oil films and the use of oils in techniques other than normal oil painting. In practical usage, correctly executed normal oil paintings do not turn yellow. The surface of protective coatings, dirt, and other removable or remediable conditions may cause browning or yellowing, but when the well-painted surviving examples of old oil paintings are cleaned of such extraneous matter, they are seldom if ever found to have suffered from yellowing of linseed, poppy, or walnut oil. It is only when the rules for correct procedures are disregarded, when inferior oils, poorly compounded oil colors, or faulty methods of application are employed that failures occur from this cause. The proper dispersion of the pigments will prevent settling in the film or the formation of a layer of clear oil on top; and holding the oil content to a minimum and using the best kind of drying oil for the purpose will further safeguard the paint layer from yellowing or cracking." --Mayer

I received an email from an artist who argued with the point that oil paint should be ground with as little oil as possible. The historical reason is this: excess oil will separate from the paint and, depending on the absorbancy of the ground, come to the surface. Once there, the layer of oil will increase the appearance of yellowing with age. As stated by De Mayerne: "Colours die when the oil floating on the surface dries and forms a skin which turns dark in the air."

The second point made above by Mayer is that one should use pure, high quality oils. The problems discussed by artists of the middle ages in regard to linseed oil yellowing is due to impure oil. Painters from the cooler, cloudier north had an especially hard time, since linseed oil is best purified in the sun. Some artists proposed mixing whites and blues (most affected by yellowed oil) with lighter oils (walnut or poppyseed) or with gum water. They were also strategic in the layering of paints to isolate colors to limit discoloration. A gum water binder in paint demanded that it be used in the underpainting and varnished before oil color was overlayed or, if the gum binder was to be used over oils, the surface was rubbed with onion or garlic juice so that it would adhere. Anthony van Dycke used this method.

The chemical purification and industrial production of refined linseed oils in todays marketplace make all such practices unnecessary.

The 17th century Spanish painter, Franscisco Pacheco, used linseed oil in his whites and blues with great success. But his oil had been purified and bleached in the Spanish sun for 15 days. The resulting oil was lighter and did not yellow so badly.

Purified and bleached oil will still yellow slightly and should be kept to a minimum in the binder. "It is for this reason," said French art theoretician Andre Felibien, "that those who wish their paintings to maintain their [colours'] freshness, should use as little oil as possible and keep their colours as firm as possible." Modern tube colors may have excess oil, especially in cheaper grades, to prevent them from drying out and caking in the tube. You can remove excess oil by laying out your colors on a paper towel before placing them onto your palette.

For more information see Margriet van Eikema Hommes article entitled "Painters' Methods to Prevent Color Changes Described in Sixteenth to Early Eighteenth Century Sources on Oil Painting Techniques" in the book Looking Through Paintings: The study of Painting Techniques and Materials in Support of Art Historical Research. © 1998 - ISBN 90 6801 575 3 and 1 873132 56 5

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HISTORICAL PREPARATIONS OF OIL
FOR USE IN PAINTING

Oils should be cleansed and purified of their mucilage to prevent discoloration with age. "By removing the fermentable particles which the oil contained, its affinity for oxygen has been reduced; and a larger duration, a longer resistance to the atmosphere, is secured for it." -- Tripier-Deveaux

The early painters had numerous recipes for preparing oil. The actions taken were meant both to purify the liquid and to make it more drying, especially for use as a semi-resinous varnish, which quality it achieved upon thickening. Cennini writes of "How to prepare good and perfect oil, by baking it in the sun -- Oil may be prepared in another mode: it is thus more fit for colouring, nevertheless the fire is indispensable in preparing oil for mordants. Put linseed oil in a bronze or copper vessel, and in July or August keep it in the sun; and, if you leave it so exposed till it be reduced one half, it will be perfect for colouring [that is, colourless in itself]. In Florence I have found it to be of the best possible quality." Some instructions tell of laying the oil out in a shallow pan with a cover raised above it to keep out dust, while letting air enter. The oil is stirred occasionally as well. It is said that purified oil can be made "clear as water."

(Note: The length of time used in this preparation polymerizes the oil into a heavy resinous varnish, not the liquid that we use today for fluid painting. The modern version of this process is known as Sun-Thickened Oil which is thick, but is not left to oxidize as long. The bleaching of the sun is not permanent, and the oil will return to a more yellow appearance when dry. See Modern Preparation below)

Another method used by early painters to cleanse oil of its mucilage, was to wash it repeatedly with water (and sometimes pure white sand), letting it separate, and draining off the old water (and sand) with the mucilage. However, any water molecules left in the oil cause it problems later, making this a somewhat unsatisfactory method for home studio use. Of course, unless you're pressing your own linseed oil from seed, you don't need to worry about it today. All commercial oil has been carefully cleansed.

The ancients often boiled the oil, alone or in mixture with other substances, skimming off the mucilage as the cooking progressed. Additives were often used to speed up oxidation and increase the drying speed of the oil (see driers below). Litharge (red lead) is a heavy, earthy poisonous solid used as an additive in preparing oils to make them more drying (siccative). Other additives included calcined bone, white copperas, white lead, umber, or pumice stone. However, it was found that slowly boiling oil makes it increasingly thick and drying without the use of metallic oxides or other drying agents. Sometimes small amounts of hot water were added during boiling. All these additives oxygenate the oil which thickens it. It is not recommended that drying oils be boiled today for three reasons:

First, the action of sun and air are adequate to prepare a very good quality oil for painting.

Second, boiling oil was a method of preparing it into a thick semi-resinous varnish, and not as a fluid substance for detailed painting.

And Third, because the action of boiling these oils can lead to serious fire unless the proper methods are precisely followed using the needed tools and containers. To avoid this last problem, the oil was sometimes cooked by sitting the pot in hot sand or a hot water bath. Any one wishing to prepare their own oil as a varnish should read Eastlake's book carefully and follow the instructions of the ancients. But conservation and testing have shown cooked oils to be much less archival than other methods, and modern boiled and blown oils are never used in the production of artists paints today. Their use is mostly limited to industrial purposes.

USE OF DRYING MODIFIERS

"In order to accelerate the drying compounds of drying or semidrying oils, reactive materials are added, which have the power of starting, accelerating, or forcing the absorption of oxygen by the paint film, or of overcoming conditions that inhibit drying."

In Germany, a species of metallic sulfate (vitriol) known as copperas was ground to a fine powder and mixed with oil. 2 oz of calcined white copperas added to 1 lb. of linseed oil will speed drying, even of lakes, to 2-3 hours.
Modern texts list Vitriol as zinc sulfate. Driers added to colors, especially while cooking the oil, can be progressive, continuing the drying action long after the paint film has dried through, thereby weakening it.

Today driers are generally discouraged in thick paint films beyond the small amounts which are added cold by paint manufacturers to reduce the range of dry times in their paint line. Driers added to thick paint create unequal drying of the paint film since linseed oil dries from the top down.
However, all the thin glazes of the early masters seem to contain metallic driers. In this manner they can be used, because the thin film can dry through completely in a short time, allowing earlier varnishing of the finished work. The best (least harmful) modern drier is said to be cobalt linoleate. It is less progressive than others and darkens less with age, but should be added in as small amount as possible to get the desired effect.
Generally, if a more quick drying paint film is desired, one may wish to use sun-thickened linseed oil in glazes and to use quick drying pigments (containing lead, manganese, or cobalt) in underpainting.

See Discussion with National Gallery Conservator

Use of Retarders
When one wishes to extend the wet time of oil paints in order to carry over or extend one's work time through a long day or into the next, certain slow drying essential oils may be added. The slowest drying of these is said to be Oil of Cloves, followed by Oils of Lavender and Rosemary. It is not known if these interfere with the proper formation of paint films.

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Modern Preparation:
Linseed oil
is extracted from the seed by pressure. Modern oils are usually hot-pressed with the aid of steam, but cold-pressed oils are also available and some claim these are superior, especially for grinding pigments into paint. Cold pressed oil has greater wetting capacity due to its acid content and, though it is more yellow to start, is said not to change as much over time. It also has the benefit of being more flexible upon aging than hot-pressed oils.
After being pressed, oils are further processed to remove impurities. This can be done naturally, by allowing impurities to settle, by washing, or by chemical means such as acid or alkali refining. Acid-refined Oils that are bleached to a straw color revert, upon aging, to a dark or deep amber shade so that they have no benefit over cold-pressed oil. Alkali-refined oil with an added wetting agent is generally used in the production of modern oil colors because it is the clearest oil commercially produced. However, these oils create a suede effect when brushed out and become more brittle than cold-pressed upon aging. All U.S. made oil colors ground in linseed use alkali-refined oil. The only foreign company that I know of producing a line of cold-pressed oil colors is Old Holland Classic Oil Colors.

The best choices of oil to be used as a vehicle in grinding pigment into paint possess free fatty acids that help in wetting and dispersion of the pigment. For this reason Cold Pressed Linseed Oil is considered ideal. Many companies use Alkali Refined Linseed Oil that has had a wetting agent added because of its lighter color and commercial availability.

Linseed, in its low acid forms (stand oil and sun-thickened), is of greater durability than Poppyseed oil or Walnut oil and is recommended as the only drying oil for mediums. Although M.Graham and Co., makers of walnut oil paints and medium, argue this point for contemporary production methods.

Methods:
Polymerized Oil (stand oil):
oil that has been heated in the absence of air so that it undergoes a molecular change which polymerizes it, making it more viscous. It is best used in mediums where it imparts excellent flow and leveling properties to oil paint when thinned with turpentine. It dries more slowly than an oxidized oil, is more flexible, and is practically non-yellowing.
Sun-bleached and Sun-thickened Oil: these oils have undergone polymerization with oxidation, so they dry faster than stand oils. This is the form of the oils used by European artists since the time of the Greeks. It is prepared by mixing it with water and exposing it to the sunlight in loosely covered flat trays. The oil and water are mixed thoroughly every day for a week and then left for some weeks until the required viscosity is reached. The oil is then filtered and separated from the water when it is ready for use. The bleaching action of the sun merely bleaches out the fugitive plant coloring material and does not affect the yellowing which occurs with age. Some believe that any pre-oxidation of oils robs them of a good deal of their life and that sun-thickened oil will eventually behave like a boiled oil or one in which driers have been added to begin the oxidation process. Sun-thickening decreases its wetting power, pigment dispersion, acid number, and free brushing quality, but increases its speed of drying and its leveling and protective qualities. It is therefore more suitable for clear varnish, or glaze and painting medium purposes, than as a vehicle in which to grind pigments.
Boiled and Blown Oils: The use of boiled oils has led to a considerable deterioration of the paintings executed in them and they should be avoided by artists. They are sometimes still found as drying oils but, nowadays, liquid driers are often added to the oil without heating. Boiled oils are becoming popular in mediums with mastic which are used in an attempt to reproduce the affects of the old masters. Caution and research in using these mastic/ boiled oil mediums (such as Maroger's Medium) is advised. Many writers discourage the use of any such medium in permanent painting. Blown oils have air blown through them during heating. This oxidizes the oil to give a heavy product similar in appearance to stand oil but inferior in quality, tending to be considerably less flexible.

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MEDIUMS

In the Middle Ages through the Renaissance colors were reported to be mixed with an oleoresinous varnish medium. The resins amber, copal, sandarac, mastic and/or the white resin of turpentine were mixed with linseed oil. The resin was said to add firmness while the oil added toughness and flexibility. However, some conservators who have taken core samples of existing museum oil paintings report that they have found no resins in the paint film, only drying oil - usually linseed - into which the pigment was ground. This suggests that the reported use of such mediums was 1.) used for other varnishing and commercial coloring purposes rather than for fine art painting 2.) discussed more in theory than applied in practice (perhaps due to its deleterious affects) or 3.) the paintings using such mediums have not survived to us.

Nonetheless, the historical records report the following methods of production.
A dark varnish could be made by boiling 1 lb. of sandarac or mastic in 3 lbs. of linseed oil. While a light colored varnish was prepared by boiling 1 lb. of gloriat (concrete turpentine) with 2 lbs. of linseed oil. A few drops of either of these could be added to oil ground color to use as a medium, but the dark varnish should only be used in mixture with dark colors, whereas the light varnish should be used with light colors, highlights, white, and blues (which would have their color spoiled by the yellow-brown color of the dark varnish). Another recipe for a vehicle for color is listed as: 4 oz. of amber added to 1 lb. of linseed oil, then add 1 lb. of concrete turpentine resin. These mediums can all be thinned with naptha, spirit of turpentine, or spike oil of lavender. However, be cautioned that these volatile oils evaporate quickly, speeding the drying of the varnish once immixed and dulling their gloss.
See Techniques of Past Masters.

Contemporary painters generally avoid these resins which crack and darken readily. The preferred natural resin for modern use is damar mixed with stand oil and spirit of turpentine to create a medium for painting. Mineral spirits (including the low odor solvents like turpenoid) will not dissolve damar. Because damar can be redissolved by turpentine, it should be used in no more than equal amounts with oil, and preferrably less.

A quick drying medium consists of 2 parts sun-thickened linseed oil to 1 part dammar diluted with 6 parts rectified turpentine.

A superior, but slower drying medium consists of 3 parts stand oil with 1 part damar. Dilute with turpentine to desired consistency, decreasing the amount of turpentine in each successive layer following the "fat-over-lean" principle.

A third medium, one recommended by Ralph Mayer as a good universal medium and one which I use, mixes 1 part stand oil with 1 part damar thinned with 5 parts turpentine. The turpentine can be increased to as much as 7 parts for more fluid underpainting and reduced proportionally in each successive layer as above. The increased damar cuts the viscosity of the stand oil, speeds drying (since it dries by evaporation), and adds toughness to the film, while mixing with stand oil maintains the paint film's flexibility. When stirred into manufactured tube colors that have been ground in linseed oil, the percent of damar is cut to an acceptable level to resist the action of solvents in varnishing, overpainting, or cleaning. A few drops of cobalt drier may be added for faster drying glazes if you desire. However, I have never found the need for it.

For a glossier glaze medium mix 2 parts sun-thickened or stand oil with 2 parts damar and add 1 part Venice turpentine and dilute with spirit of turpentine.

Other Venice turpentine mediums include the following:
For a slow drying medium good for soft blending mix equal parts of stand oil, Venice turp., and spirit of turpentine.

For layering, mix 4 parts Venice turpentine with 1 part stand oil and dilute with spirit of turpentine. For following layers increase the amount of drying oil and dilute with turpentine as needed.

Rubens used a medium similar to the following in at least some of his work: 3 parts Venice turpentine, 2 parts sun-thickened oil, and 1 part damar. For the glossy look of the Flemish painters, the use of Venice turpentine is advisable in place of resins that have a greater tendency to darken and crack (amber, mastic, copal, etc.)

Linseed oil (stand oil or sun thickened) can be used, thinned with turpentine as needed, as a medium by itself.

See Discussion with National Gallery Conservator

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APPLICATION

Superimposed paint layers must be as, and preferably slightly more, flexible than the layer beneath. To achieve this, each successive layer of paint film must contain an increasing amount of fatty oil. This is known as painting Fat-Over-Lean. This truth is applicable not only to the medium used, but to the amount of oil necessary as a binder for a given pigment. Therefore artists should be aware of the type and amount of oil that is used in producing a given pigment. For more information, see Pigments Present.

Dangers:
If an excessively fat layer is applied over a dry, lean layer, the oil in the top layer will shrivel and wrinkle.
If a layer of paint is applied with less fatty oil than the preceding layer (lean-over-fat) then the surface of the painting will dry more quickly and will be less flexible than the layers still drying (and moving) beneath it, causing it to crack. Reed Kay explains, "linseed oil oxidizes as it dries. It unites with oxygen from the atmosphere, becoming heavier in the process. Furthermore, it moves as it dries, expanding and contracting its bulk considerably. Since the film dries from the top (where the air is) toward the bottom, it may be dry or tacky on the surface while it is still oxidizing and swelling below the surface. If a film of "leaner" paint containing less oil is placed over such a half-dry underpainting, the lean film may become thoroughly solid and dry before the fat film has completely gone through its drying process. In such a case the movements of the lower film may cause the dry upper film to crack and fracture, in much the same way that heaving ground may cause a concrete sidewalk to crack." Linseed oil, once it reaches its peak weight gain during drying, declines in weight at a slower rate than poppyseed oil which is why it is better suited to layering techniques.

See Discussion with National Gallery Conservator

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History, Definitions, and Techniques | Drying Oils and Mediums
Resins and Varnishes | Pigments Past | Pigments Present | Pigment Chemistry
Supports for Painting | Grounds on Canvas | Techniques of Past Masters
Discussion with National Gallery Conservator

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