Drying Oils and Mediums
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)
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.
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
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."
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:
"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.
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.
Use of Retarders
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.
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.
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 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.
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.
History, Definitions, and Techniques | Drying Oils and Mediums
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