For the Beginning… Linseed Oils – Basic Properties
Linseed Oil is extracted from the seeds of the Flax Plant, the very same plant that provides the painter with linen canvas fabric.
We have seen already that oils are fatty substances, which have in common the property of drying in presence of oxygen, i.e. they polymerise thus becoming a completely different substance after drying.
Linseed Oil is extracted in a few different ways, of which the most important are by the cold pressing method and hot pressed method. Both methods have been around for centuries, many centuries. In the cold pressed method, seeds from the plant are pressed under great pressure exuding the oil out which after some basic purification and cleansing from seed rind and other impurities, is then bottled. The hot pressing method involves the heating of the seeds which renders them softer allowing a more copious amount of oil being extracted.
Linseed Oil is treated in many ways, more than any other oil and is today the most important oil in use for oil painting. Treatment of linseed oil provides other linseed oils, of which the most important are definitely Sun-Thickened Oil, Stand Oil, Raw Cold Pressed and Refined Linseed Oil. Linseed Oil is also available as an alkyd modified. Such modifications of the oil have an important impact on the properties and drying times of each quality of oil and impart different finishes when applied to painting.
Stand Oil is processed by heating linseed oil with carbolic acid. The heating process needs to be done in a vacuum vessel, so oxygen (the chief element for the drying process) doesn’t get in the oil. The main purpose is to thicken the oil, render it out-levelling and reduce yellowing. Stand oil is the least yellowing oil of all treated linseed oils and when applied to painting a picture, it is self-levelling, eliminating brushstroke marks and producing a high gloss film which will look like enamel. It is one of the favourite oils for the production of home and studio-made oil painting mediums, a characteristic it shares with Sun-Thickened Oil. The thickness of Stand Oil is measured in Poise units, the higher the Poise, the thicker the oil. Poise is a standard method to measure viscosity levels in industry, as glass industry. Standards are that normally sold stand oil will be supplied in 40 Poise thickness, but thicker oils are also available, especially in 60 Poise. For painting mediums, 40 Poise thick is fine, but 60 Poise can also be used safely. Stand Oil is a little slow to dry, therefore it is good practice to mix it with a natural resin and turpentine in order to improve its drying rate in oil painting mediums.
Raw Cold-Pressed Oil
Cold Pressed Linseed Oil is on its own a fine ingredient for painting mediums or grinding dry pigments. We have mentioned before the process of extracting the oil from the flax seeds. Some painters prefer it to stand oil. I recommend you use only raw cold pressed for grinding dry pigments.
Refined Linseed Oil
Refined Linseed Oil is widely available and is manufactured by most art materials manufacturers. Such oils are supposed to have been freed from their mucilage matter and other impurities, thus rendering the task of doing the washing in the studio completely needless. The most popular oils are the so-called Alkali-Refined. The oil is heated up to 85 degrees Celsius in a aqueous solution of one or more alkali compounds, usually sodium hydroxide or sodium carbonate. Mucilage and impurities settle to the bottom of industrial tanks or through a spinning programme are drained off. The oil is then treated with caustic soda and finally mixed and washed with just water to remove any trace of the previous treatment. It finally is separated from water. Alkali-Refined Oils are clear, of a pale colour, slow drying but very fluid and “soft”. Some refined oils may become rancid, which is not usually the case with linseed oil. Alkali refined oils are also used for grinding oil colours. Pigment is mixed with the treated oil and then milled to disperse pigment particles equally. Colours ground in such oils have a different “feel”, less glossy and are not as “buttery” as those ground in pure linseed raw pressed oil. However such finish may be preferred by painters seeking to create modern and contemporary pictures finished with matt synthetic varnishes and waxes, etc
Any of a class of nonvolatile (non evaporating), solid or semisolid organic substances obtained directly from certain plants as exudations or prepared by polymerization of simple molecules: used in medicine and in the making of varnishes and plastics. A substance of this type obtained from certain pines; rosin.
Hard Resins: Fossil Resins
Tend to darken and crack with age. Must be boiled in oil to form a varnish; the use of boiled oil mediums has led to a considerable deterioration of the paintings executed in them and it is recommended that they be avoided by artists, though their popularity is returning in attempts to recapture the glass like layering of past paintings.
Other names: berenice, verenice, vernix, glessum (glas or glassa). a pale yellow, sometimes reddish or brownish, fossil resin, translucent, brittle and capable of gaining a negative electrical charge by friction. It comes chiefly from the resins of pine trees that grew in northern Europe. These resins were gummy materials mixed with oils in the trees. When the oils became oxidized, hard resins were left. These pine trees were buried underground or underwater, and the resins slowly changed into irregularly shaped lumps of amber. The largest supply of amber lies in the Baltic Sea area. It comes from a species of pine tree that is now extinct. Some experts consider this amber the only true amber. Central America has important deposits of amber from other sources. Most amber is mined from a claylike soil called blue earth and was thought by the early Greeks to be a mineral.
A hard, lustrous resin obtained from various tropical trees, it has the same recommendations as amber and nearly the same defects. The best Copal is said by Eastlake to come from South-Africa and were imported into Europe through India. The Hard Copal resins today come from Sierra Leone or Zanzibar. The general argument against copal, as with other resins, is its reported tendency to darken and crack. However, artist/investigators of this medium such as Melville Holmes argue that good quality copal mixed as no more than 25% of the medium with a drying oil (linseed or nut) produces a good quality paint film with no proven and documented signs of extreme yellowing or cracking (as opposed to normal craquelure). Mixed in small quantities with oil it promotes the flow of the paint and gives a painting lustre and brilliance, but this resin is too brittle to be used as a varnish thinned only with Turpentine, and in its hard (semi-fossil) form is insoluble when dry. While this insolubility makes the resin an improper choice for a final varnish, it is precisely this feature which makes it desireable as an additive in medium. The old “No.1 Water-white transparent” congo copal was best used, but is no longer in production since the Belgians were driven from the Congo in the 1960s, making the procurement of measured quality copal difficult to come by. The term copal refers to a number of resins, from different botanical origins and chemical constitutions, from both living (soft resin) and hard or semi hard (fossilized) sources. Therefore, copal mediums purchased ready-made may contain unknown and variable qualities of resin, making them susceptible to the darkening and cracking of which they have often been accused.
Soft Resins: Recent Resins
Extruded from live plants. Dissolved in cold diluent. Resoluble.
Milan resin tapped from live trees can be dissolved in alcohol. Many varieties exist of differing grades. Unlike its fossil counterpart, soft resin is resoluble, therefore causing the same concerns as any resoluble resin when used as an ingredient in painting medium.
Other names: red varnish, vernix, common glas. Resin from the African arbor vitae, known as Thuja articulata, similar to the Juniper and often referred to by that name. It is of lower quality than amber or copal for making varnish.
Also known as white varnish. An aromatic, astringent resin obtained from a small anarcadiaceous evergreen tree native to the Mediterranean region. Believed to have been the resin that replaced Amber in the European’s thick medium, known as Megilpe, which was mixed with boiled linseed and litharge (huile grasse). It darkens and is more brittle than dammar which has replaced it for creating essential oil varnishes as well as in mediums.
Resin of Turpentine:
Also known as glorie, white resin, colophone, or concrete turpentine. Is said to be clear and not to yellow over time. This resin in only available as dark rosin for wood varnish today, and not recommended for fine art. see oleoresin below.
A soft resin derived largely from dipterocarpaceous trees of southern Asia and used for making colorless varnish. Any of various similar resins from trees of other families. Due to its colorless nature and the fact that it does not harden and crack as badly as the above listed resins, it has become the preferred resin for oil painting in the 20th century. It is mixed with oil of Turpentine to create an essential oil varnish. And when the dried varnish becomes soiled on the surface of the painting over time, it can be removed again with the same essential oil of Turpentine. However, dammar becomes less soluble with age so strong solvents may be needed for its removal when aged. Modern writer Melville Holmes states that “Mastic and Dammar have been studied in considerable detail by conservation scientists because of their use as picture varnishes and their relatively rapid darkening is well documented.” I have not found statements claiming such darkening takes place when dammar is incorporated into an oil medium.
A mixture of an essential oil and a resin, found in nature.
Any of various oleoresins derived from coniferous trees, esp. the longleaf pine, and yielding a volatile (one that readily evaporates) oil [essential oil of Turpentine] and a resin [concrete Turpentine or rosin] when distilled. Rosin is considered an adulterant in modern painting mediums.
BALSAMS: A turbid, viscid (sticky) , water-insoluble liquid resin. Any of certain transparent turpentines.
Obtained from the larch Larix Europea or Larix decidua, this is a viscous yellow liquid from which resin acids cannot be crystallized and in this respect it differs from common turpentine. It is soluble in alcohol, ether, acetone and turpentine but only partly soluble in petroleum hydrocarbons. When purified, it no longer shows its natural tendency to darken a painting and produce cracks, but is very cohesive mixed with a fixed oil. It should be used sparingly since it is resoluble, but a little is all that is usually needed to create the desired effect of glassy sheen. It was the most common varnish found in early recipe books and had been commonly used in Europe at least from the time of Rubens.
The French called it Thérébenthine de Venise and said that it flowed without an incision from conifers (meleses, pins, sapins). They said it was a golden liquid, clear as water which thickens with age turning lemon yellow. However, in the 17th century, common pine turpentine (rosin) was often sold as Venetian turpentine. “Large quantities were sold with water on the top to mask the (inferior) reddish liquid below.” True Venetian Turpentine is only from the larch tree.
From the white fir abies pectinata, this is paler than Venice turpentine and very difficult to obtain. Seventeenth-century compilers of recipes were unanimous in preferring Strasbourg turpentine over Venice turpentine due to its better color and odor. But both dry into desirable film when mixed with drying oils, etc.; compared with the customary oils and varnishes, they are acceptably permanent, nonyellowing, and durable, and tend to impart more flexibility and life to the films than do most resins.
***When Venice or Strasbourg turpentine is mixed with stand oil, the resulting varnish is superior for artists’ mediums to the cooked oil-resin varnish group; and when liquid driers are added, the resulting product is superior to those varnishes into which driers have been cooked.
Said to resemble Strasbourg turpentine, Canada balsam is relatively pure and valuable for its transparency and its high refractive index. It is usually more expensive than Strasbourg and has been less tested.
Of variable composition and viscosity, Copaiba balsam was found to be impossible to remove completely and to display excessive darkening and shrinkage. It is now avoided in picture making.
Any of a class of volatile oils obtained from plants and possessing the odor and other characteristic properties of the plant: used chiefly in the manufacture of perfumes, flavors, and pharmaceuticals.
Examples in Painting:
Oil of Turpentine:
Colorless, flammable, and volatile, having a penetrating odor and a pungent, bitter taste. Used in essential oil varnishes where it gives a matt finish, and for thinning paints and cleaning brushes. The overuse of spirit of turpentine in thinning oil paint will make them appear flat and dull and, reducing the binding medium, can cause flaking due to lack of adhesion.
Spike Lavender Oil:
The oil of the Lavender plant, Lavendula latifolia, it can be used in the same ways as oil of Turpentine, but dries more slowly. Has a “pungent odour” stronger than turpentine. Can be used to retard the drying of oils but “it is generally felt that they do the paint films no good,” according to one writer. He may, however, have been using the oil of the female plant, known as lavenda vera, which has a stronger odour and is used in perfumes. For painting one should use the less aromatic oil of the male lavendula spica.
Any of various liquids composed only of hydrogen and carbon that boil below 450 degrees F. which are distilled from other products. Unlike the essential oils listed above, this diluent is distilled from a mineral rather than a plant. The danger is in its low flash point, between 20 and50 degrees F, making turpentine the preferred solvent in modern painting.
Naptha (or Naphtha) can be made when coal tar [the pitch used by early shipbuilders to waterproof their vessels], a sticky substance made from soft coal, is distilled. It was the first volatile oil used to dissolve resins for varnish and considered the purest and most unchanging diluent.
In todays terminology solvent naphtha refers to a rather impure by-product of coal tar distillation belonging to the benzol group. It is a good solvent for coal tar and some asphalts, but is a poor general solvent for paint and varnish materials. It is sometimes useful for this reason, as it has little effect upon the oils and resins in an oil painting and may be used to wash off superficial dirt, wax, etc., without the film’s being affected.
V.M.&P. Naphtha (Varnish Makers and Painters Naphtha) is a modern variety of petroleum distillate intermediate between mineral spirits and gasoline. With a lower flash point, a higher rate of evaporation, and usually a more disagreeable pungent odor it is not so well suited to paint purposes as is mineral spirits.
Are a petroleum distillate intermediate between Kerosene and Gasolene (petrol) with properties similar to those of turpentine when used as a paint thinner. It has several advantages: it leaves no sticky, gummy (resin) residue upon evaporation, it does not deteriorate with age, its price is a small fraction of that of gum turpentine, and it is less likely to affect persons prone to allergic reactions. It can replace turpentine in most studio uses except in dissolving dammar.
A preparation for finishing or coating wood, cloth, or other materials, consisting of resinous matter, as copal or lac, dissolved in an oil, alcohol, or other volatile liquid. In contemporary use, the sap of certain trees used for the same purpose.
When used in this way, oleoresinous refers to a resin dissolved by heating into a drying oil such as linseed or walnut oil to create a thick liquid varnish.
Amber and Copal varnishes turn very dark and are red to begin with. Sandarac has the same quality, with all eventually drying and cracking to the point of disintegration, though Sandarac is worse than either Amber or Copal. They are all thick, dark, and slow drying. Florentine and Sienese schools used a green underpainting in their skin tones which, when varnished with one of these red varnishes, was neutralized, giving a proper effect to the skin color.
Mastic and Concrete Turpentine (colophone) are clearer in color, but still tend to crack over time.
From Berenice (amber), the constellation of Berenice’s (golden) hair. Nero referred to the golden tresses of his empress, Poppoea and Pliny observed that, because of Poppoea, amber-colored hair became fashionable. A varnish of the middle ages made with Amber (or Sandarac) heated and mixed with drying oil. As defined by Cardanus: “The juice which flows from the Juniper (Thuja) is called vernix. — From dry vernix and linseed oil, liquid vernix is made: this is calculated to resist all effects of the atmosphere, and therefore is applied to pictures.”
Early Recipe for a thick Vernice Liquida: 3 parts linseed, 1 pt. Sandarac or Amber, and sometimes (the white resin) concrete Turpentine was added in 2 or 3 parts. The concrete Turpentine aids in the liquefaction of the Sandarac making the varnish lighter and adds gloss. The recipe with Sandarac is called common vernice liquida, while that with amber is vernice liquida e gentile. With the glossy turpentine (rosin) added, it was especially prized for varnishing cross bows and furniture.
A resin or balsam dissolved in a volatile oil. Not til the close of the 15th or beginning of the 16th century did painters use an essential oil varnish.
Examples of Varnish:
Essential oil and balsam, sometimes with a resin added. Another recipe used clear (silver) fir Turpentine (resin) mixed with an equal part naptha. It was applied warm and is a thin, but glossy varnish.
The Flemish sometimes used “the clearest Venice Turpentine boiled with an equal quantity of essential oil of Turpentine.” It protects from the effects of air and moisture and can be painted over. Vandyck used 2 parts Turpentine to 1 part Venice Turpentine boiled as before and applied cool. Do not allow the essential oil to evaporate or the Venice will not dry well and becomes too thick. This was considered ordinary painters varnish. A retouch varnish used between layers to “oil out the colours” was made of “essential oil of Turpentine, Spike Oil, or (naptha) Petroleum, with (white resin of) Turpentine itself, dries at last and prevents the varnish from cracking. Note: very little is necessary; the tenth or twelfth part.” (ratio of essential oil to resin = 10:1 or 12:1) -De Mayerne
This thin resinous film does not yellow, “undergoes no alteration… leaves a comparatively fresh surface which takes the colour easily; and, having scarcely any body, does not affect the superadded tints.” Best used as a retouch varnish between layers to oil out the color.
Varnish sold by Pomet in 17th century France:
1.) Spike oil, Venetian Turpentine, and Sandarac melted together to produce a siccative varnish.
2.) Venetian varnish or White varnish –
spirits of Turpentine, Venetian Turpentine and Mastic melted together.
3.) Spirit varnish of Sandarac, Amber, Gum Elemi and Mastic with Alcohol.
4.) Common varnish – resin of Turpentine (rosin/colophone) melted with Spirit of Turpentine
Some of the earliest recipes for varnish were for Venetian varnish, used to varnish and paint engravings.
De La Fontaine’s recipe for Venetian Varnish, which was used to coat paintings, mixed 1 oz. of Venetian Turpentine with 1/2 oz. of spirits of Turpentine and boiled in a water bath until the balsam melted. Allow to cool – but warm slightly to increase flow before applying. Other recipes from the period are similar, though some incorporate mastic.
A mixture of mastic ground with linseed oil could be used to grind colours which would be more resistant to the air.
Le Blond de la Tour complained that all varnishes of his day yellowed.
A retouch varnish can be made by mixing liquid Dammar varnish with Rectified Turpentine in a ratio of 1:2. While a final varnish can be made by mixing the same ingredients in a ratio of 2:1 or as much as 4:1. Brush on with a flat 2″ wide hogs hair bristle brush. This is the preferred artists’ picture varnish for the reasons listed above, under Resins: Dammar.
Other modern varnishes used primarily for woodworking and industrial purposes are made using Lac, a resinous substance deposited on the twigs of various trees in southern Asia by the female of the Lac insect. Shellac is lac that has been purified and formed into thin sheets, used for making varnish, or a varnish made by dissolving this material in alcohol or a similar solvent.
Shellac tends to turn dark and to crack with age and is therefore not to be used as a retouch or final varnish. But White or Orange grade Shellac can be thinned with alcohol to a watery consistency to be applied as an isolating varnish, or thin size, over a preliminary oil wash underpainting. The alcohol used should be ethyl, or grain, alcohol. Pure Grain Alcohol is 94% pure alcohol and 6% water, while Anhydrous or Absolute Ethyl Alcohol is 100% alcohol. Either will work to mix an extremely thin solution of shellac that can be used as a sizing for porous surfaces and as an isolating layer between films of paint in tempera painting. Or it can be applied just thickly enough over a wash drawing or underpainting in oils to seal the image, make the surface less absorbent and give it tooth for the adhesion of the body painting over it. Since it is not soluble in turpentine or mineral spirits it can protect a paint layer from further solvents in the overpainting. The drawback is that alcohol (shellac’s solvent) can have a destructive effect on oil paint layers and, after the alcohol has evaporated, any layer of shellac must be completely covered in pigmented layers which may be contrary to the desired effect in the painting. Perhaps a safer and more flexible method is to seal an ink drawing on gesso with a glue size or to add a thin coat of damar retouch varnish between layers on canvas.
A protective coating consisting of a resin, cellulose ester, or both, dissolved in a volatile solvent, sometimes with pigment added. Any of various resinous varnishes, esp. one obtained from a Japanese tree, Rhus verniciflua, used to produce a highly polished, lustrous surface on wood or the like.
A compound produced by the reaction between an acid and an alcohol with the elimination of a molecule of water, as ethyl acetate or dimethyl sulfate.
An inert carbohydrate, the chief constituent of the cell walls of plants, wood, cotton, hemp, paper, etc.
Pure Methacrylate, or acrylic, solution in mineral spirits has been accepted by museum and technical specialists since the early 1930s. It is colorless and dries to a dull satiny finish, often preferred to the higher gloss of damar. This is best used on light colored, or “blonde”, works of art since dark pictures may be made to look grayish by matt varnishes. Acrylic’s great flexibility may allow it to be applied to paintings that have not cured if a work must be shipped out of the studio prematurely.
A pure solution of Polycyclohexanone (keytone) in mineral spirits may be better suited for dark images as it dries very closely to the gloss of damar and has been used since the 1950s.
These are best used as final varnishes on oil paintings since they can be easily removed with mineral spirits. One disadvantage of the synthetic varnishes is their tendency to form films that are softer, with tendencies to attract dirt and dust to a greater extent than damar, thus potentially requiring more frequent cleaning. And their softer films may not protect the painting surface from abuse and scratches as well as the harder damar.
Instructions for Varnishing a Painting
Make sure your painting is dry to the touch, and no areas are still wet as the paint might run. Paintings can be varnished within 2-3 weeks after completing the painting, but if you wait longer it is recommended you wait 6 months.
Use a soft, natural bristle brush (Flat, preferably 1.5 or 2 inches).
Dip the brush into the jar of varnish, and press against the rim to remove excess varnish. Brush the varnish onto the painting, spreading the varnish as you go to create a thin layer. Do not scrub very hard, just enough to spread the varnish around. The varnish will start to set up and get tacky quickly, so don’t go over areas many times. When necessary, dip brush into jar again and continue brushing onto painting until fully covered with an even thin layer.
Paintings usually require anywhere between 1-3 coats. Make sure varnish is again dry to the touch and not tacky before applying another coat. Leaving a painting in the sun will usually dry the varnish in 1 day. Depending on climate, you may have to wait a few days to apply another coat.
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