Linseed oil

Oil Painting

While the history of paints made from vegetable drying oils goes back to the Middle Ages, and oil paints were known to painters of the fourteenth century and earlier, they were not widely adopted for use in easel painting until the fifteenth century. By the middle of the sixteenth century, the method was in full swing in a rather well-developed form, and ever since, oil painting on canvas has remained the standard technique for artists’ easel painting.

Although all the other techniques are practised for certain advantages they have over oil painting, the latter remains standard because the majority of painters consider that its advantages outweigh its defects and that in scope and flexibility it surpasses watercolour, tempera, fresco, and pastel.

From the viewpoint of permanence, however, all these accepted and time-tested methods of painting may be considered to be of equal merit. They all possess certain inherent defects which the careful painter does his best to minimize, and each presents peculiar difficulties which he must overcome.

Materials for each must be carefully selected; oil paint, tempera, and fresco must be correctly manipulated and applied if they are not to deteriorate; and the fragility of watercolours and pastels requires that they are carefully preserved.

In the recent past, oil painting dominated the field to such an extent that from the standpoint of public acceptance, the other methods of painting were relegated to the status of minor techniques. With the development of art, and especially in modern practice with so many facets of art being appreciated in the same era, there is now not so wide a gulf between the leadership of oil painting and the secondary use of other techniques, and some of our greatly admired artists create their major works in techniques other than oil.

Benefits of Oil painting Vs Other Colours

The basic points of the oil technique’s superiority over the other accepted methods of permanent painting are:

• It’s great flexibility and ease of manipulation and the wide range of varied effects that can be produced.
• The artist’s freedom to combine transparent and opaque effects, glaze and body colour, in full range in the same painting.
• The fact that the colours do not change to any great extent on drying; the colour the artist puts down is, with very slight variation, the colour he wants.
• The dispatch with which a number of effects can be obtained by a direct, simple technique.
• The fact that large pictures may be done on light-weight, easily transportable linen canvases.
• The universal acceptance of oil painting by artists and the public, which has resulted in a universal availability of supplies, highly refined, developed, and standardized.

Its principal defects are the eventual darkening or yellowing of the oil, and the possible disintegration of the paint film by cracking, flaking off, etc. The former may be eliminated or reduced to an acceptable minimum by correct selection and use of materials, and the latter by correct handling of the technique.

PAINT

How Is Paint Made

Paint consists of finely divided pigment particles evenly dispersed in a liquid medium or vehicle; it has the property of drying to form a continuous, adherent film when applied to a surface for decorative or protective purposes.

Painting Methods & Materials

Some painting methods and materials must be more carefully or precisely applied than others. With some, the range of variation or modification is small, and standard procedure must be closely followed; others are more flexible and a considerable degree of latitude is permitted within the bounds of sound practice. The word flexible used in this rather abstract sense to describe the technique in general, as in its more literal meaning as applied to a physical property of materials, does not signify an infinite degree of elasticity; it must always be understood that there are definite limits to such a range.

Painting Surfaces

Surfaces may be coloured or decorated by applying the pigment directly; in pastel painting, the protective function may be supplied by a fixative, the application of which is separate from the decorative or colour application; and in fresco, the ground itself supplies the adhesive or binding property. However, paint, in the commonly accepted meaning of the term, usually implies a material which combines these functions—as the typical oil or tempera paint.

When a drying oil is used as a medium for painting, it performs the following four functions:

1. Executive. It allows the colours to be applied and spread out.
2. Binding. It locks the pigment particles into a film, protecting them from atmospheric or accidental mechanical forces and from being disturbed by the application of subsequent coats of paint.
3. Adhesive. It dries and acts as an adhesive, attaching the colours to the ground.
4. Optical. It has an optical effect, bringing out the depth and tone of the pigment, and giving it a quality different from that which it possessed in the dry state.

Drying Oils For Paint

A number of vegetable oils have the property of drying to form tough, adhesive films either by themselves or when assisted by the action of added ingredients. These oils do not “dry up” in the ordinary sense of the evaporation of a volatile ingredient, but they dry by oxidation or absorption of oxygen from the air.

The drying process is accompanied by a series of other complex chemical reactions, and the dried oil film is a new substance which differs in physical and chemical properties from the original liquid oil; it is dry. The solid material which cannot be brought back to its original state by any means.

The increase in weight or volume of the oil through the absorption of oxygen is compensated for to a variable degree by the loss caused by the passing off in the gaseous form of certain by-products of the reaction. These changes may be measured in the laboratory and from such figures, we gain considerable knowledge of the properties of our oils.

Linseed Oil

Linseed oil is pressed from the seeds of the flax plant which is grown in all temperate or cold climates. The seed from each flax-growing region has its own characteristics and is rated in quality accordingly. The impurity which is principally responsible for variations in quality is foreign or weed seed. This is true of any commercial vegetable oil. Sometimes foreign seeds are added deliberately.

The seed is crushed and the oil is extracted from it by pressing it in special machinery, usually with the aid of steam. The use of steam is necessary to secure the most economical results, but the quality of the oil thus produced is very definitely inferior to that extracted by cold pressing, especially from the artist’s viewpoint. The hot pressing extracts a larger percentage of substances from the seed; and despite any subsequent refining to which the oil is subjected, the oil’s resistance to embrittlement on ageing, as compared to cold-pressed oil, is definitely reduced.

Row Linseed Oil

The usual procedure is to warm the crude oil slightly and allow it to stand in tanks for some time (up to two years), during which period a considerable amount of solid mucilaginous sediment or feet settles out and falls to the bottom of the tank.

Oil from some seeds will soon purify itself to a great degree in this manner, but some varieties will throw down feet indefinitely. After such partial purification, the steam-pressed oil is fairly clear but of a rather dark yellowish-brown colour, sometimes with a pronounced greenish tinge, and it is sold as raw linseed oil. It is not used in this form in artistic painting or to any great extent in the better interior commercial paints and varnishes, as it is inferior in nearly every respect to oil that has been further refined, with the possible exception of its durability under extreme weather conditions when employed in outside house paint.

Cold-pressed oil properly aged and filtered has a medium or pale golden colour, and is used for pigment-grinding purposes without further refinement. Its paint films retain their flexibility better or embrittle less rapidly than do those made with steam-pressed oils.

Refined Linseed Oil

For most paint purposes steam-pressed linseed oil requires further refining, and the usual commercial treatment is to mix it with sulphuric acid and water, which removes the bulk of the undesirable impurities and improves its colour. This procedure is carried out on a large scale with special equipment, and the best grades are subsequently purified of all traces of water and acid. Many variations of this method are in use, employing a number of bleaching agents and other chemicals. The refined paint-grinding oils range in colour from pale straw to golden or golden amber.

Varnish Linseed Oil

The refined oils discussed above meet the requirements for materials used in ordinary painting or paint grinding. Another class of refined oils is produced commercially for use in clear varnishes, etc. The first requirement for a varnish oil is that it should be free from “break.”

When linseed oil is heated rapidly to 500 degrees F. and a flocculent mass, or cloud of particles, forms in the oil, it is said to break; oil which remains clear is acceptable as non-break oil. This break contains a large percentage of phosphorus and is not to be confused with the feet which the oil precipitates spontaneously as it ages.

Another specification is that varnish oil should have a low acid number whereas, for maximum wettability and dispersion of pigments, paint-grinding oil should have a relatively high acid number. The common method of refining varnish oils is to use alkali instead of acid in the treatment, but some of the best grades are also processed by mechanical methods.

Despite the fact that such oils are held to be less desirable than the paint-grinding oils for use in the production of industrial paints, artists’ colour manufacturers will usually select this type because of the all-important attribute of colour stability. This is the type most likely to be on sale in bottles in the artists’ supply shops; in the absence of cold-pressed oil, alkali-refined oil is the best for all-around use.

Stand Oil

When linseed oil is heated to 525-575 degrees F. and held at that temperature for a number of hours, an internal change takes place, and the physical and mechanical properties of the resulting product, stand oil, are not the same as those of raw oil.

The change is a molecular one, polymerization; nothing is added to the oil and nothing is lost. Stand oil is a heavy, viscous material of about the consistency of honey; it may be thinned to a painting consistency by mixing it with several parts of turpentine, the mixture is paler in colour than other linseed oils.

Owing to its viscosity and its low acid value, it is not so suitable as a vehicle with which to grind pigments, but when diluted with a thinner it is one of the most useful ingredients of glazing or painting mediums, as an addition to oil paints or tempera emulsions and as an ingredient in varnishes. Stand oil turns very much less yellow with age than raw oil does, and when it is diluted or mixed with other ingredients to a usable consistency, the resulting medium is practically non-yellowing.

One of the most striking characteristics of stand oil is its levelling property, that is, its tendency to dry to a smooth, enamel-like film, free from brush marks, and its ability to impart this tendency to paints and mediums when it is added to them. For satisfactory results, stand oil can be made only by large-scale industrial methods from selected varnish-type oil.

High-quality pale stand oils are now made by heating the oil in special equipment, under high vacuum or in an atmosphere of carbon dioxide. Formerly, stand oil was made in open kettles and heated for as long as eighteen hours or more; the resulting products were dark, not uniform, and were partially oxidized. The modern light-coloured polymerized linseed oils have great colour stability and, unlike the oils refined by other processes, are not so liable to revert to darker colours.

The product specifically called stand oil is made to conform to a standard range of viscosity. Some polymerized oils are made in lighter grades and some in very heavy consistencies, so heavy that they can scarcely be poured. Lithograph varnishes are very similar to stand oil, but in general, it is not a good practice to substitute them for the latter because they are usually made by variations of the high-temperature process which are designed to impart special properties for use in the preparation of printing inks. Even when thinned with turpentine to the same consistencies as those in which the true stand oils are used, these varieties have working properties considerably different from those of stand oil.

So far as the painter is concerned, there does not seem to be any precise or critical viscosity to which stand oils must conform; there is some range of viscosity among the stand oils sold as such to artists, but all of them appear to give characteristic results. The heavier ones obviously require the addition of a little more turpentine and perhaps a bit more varnish in the preparation of painting mediums; some carefully controlled laboratory work would be required if the exact limits for a good stand oil were to be determined. Mixed and diluted to desirable working consistencies, the end products show little differences.

Blown Oil, Bodied Oil, and Boiled Oil

Linseed and other drying oils may also be thickened or bodied by an entirely different change, oxidation or combining with oxygen. This is the same process by which the oil dries when exposed to the air, and it results in a product far inferior to stand oil, although resembling it in superficial properties. Oxidized oil is produced by blowing air through the oil; heavy, viscous blown oils are prepared commercially in large quantities. They should not be confused with stand oil. Boiled oil is a misnomer; the oil commonly sold under this name is not boiled but heated with driers until very slightly thickened. A good deal of the boiled oil of commerce is raw oil to which liquid driers have been added; such oil is derisively called bunghole boiled oil to distinguish it from heat-treated or kettle-treated oil. Neither should be used for permanent painting.

Sun-Refined or Sun-Bleached Oil

An older process of refining artists’ oils, which dates from the fourteenth century or earlier and which usually produces a more rapidly drying product, consists in shaking up the oil with about an equal amount of water, sometimes salt water, and exposing it, in glass jars or trays, to outdoor sunlight for a few weeks. The vessels are loosely covered in such a manner as to exclude soot and dust but admit air, and the oil and water must be thoroughly mixed every day for the first week.

It is not possible to name any definite length of time for the exposure of the oil, since the time required varies according to the purpose for which the oil is intended, and the consistency and color desired, the actinic power of the sun in the particular locality and season, the type of oil used, and the size of the container. The gelatinous or albuminous matter is removed by filtering the oil through coarse filter-paper or fine cloths; if a little clean sand is put into the jar at the start, it will help the settling of such impurities. At the end of the treatment, the oil is most easily separated from the water by the use of a separatory funnel .

The action of the sun and air is threefold; it partly oxidizes, partly polymerizes, and effectively bleaches the oil. Although this oil has been used with good results by past generations of painters, there is considerable opinion that the oxidation part of the process robs it of a good deal of its life, and that it will eventually behave in the same manner as a blown oil or one in which oxidation is begun by the addition of driers; certainly its superficial properties resemble those of a blown oil.

If the free access of air is permitted, the oil will thicken to a considerable extent; this action may be retarded by using a narrow-mouthed jug or otherwise limiting the amount of air. Allowing the oil to bleach to the palest possible shade, as mentioned on pages 129-30, is not always recommended, as there is an optimum degree of permanent bleaching to which any given sample of a vegetable oil may be carried; beyond that point, it may, upon ageing, revert to a deeper colour, a common fault in some commercial bleached oils. The usual correctly sun-refined linseed oil is a light golden or pale amber hue, rather than an extremely pale straw colour.

In common with other treatments which increase the viscosity of oil, sun-thickening decreases its wetting power, pigment dispersion, acid number, and tree brushing quality, but increases its speed of drying and its levelling 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.

Sun refining is the only oil treatment which can be carried on successfully by home methods; as noted in connection with the cooking of varnishes, no heat-treating can be well done by other than large-scale industrial methods, and only the most antiquated recipes call for home boiling of oils.

POPPY AND WALNUT OILS

Oils pressed from the seeds of the poppy and from mature, rather stale kernels of the common or English walnut have been known and used as drying oils from the earliest recorded times down to the present day. The history of walnut oil is coeval with that of linseed, and that of poppy oil nearly so, but it is interesting to note that these two oils have always occupied a position inferior to that of linseed oil in popularity among painters.

Characteristics of poppy oil

Poppy oil is a naturally colourless to straw-coloured oil, with none of the characteristic golden or amber colour of linseed. Whites and pale colours ground in it present a somewhat clearer and more brilliant appearance than when they are ground in linseed oil, and the dried, clear poppy oil film has less tendency than the linseed oil film to turn yellowish. However, because it will turn yellow to some extent under the same test conditions, and because in pale colours even slight changes are apparent, its superiority over linseed oil in this respect is not so great as is popularly supposed.

Manufacturers of oil colours are prone to use it for whites in place of linseed oil because it gives their product a more brilliant appearance as it comes from the tube, because the pastes can be stored in bulk or in finished packages more satisfactorily, and because a buttery consistency is more easily produced.

The serious defects of poppy oil are the frequency with which its paint films will crack upon ageing and its slow drying rate. It owes its property of yellowing less than linseed oil to the smaller percentage of linolenic acid it contains, but it is just this difference in the composition that causes it to form weaker films. Compared with linseed oil, it dries very much more slowly, its film tends to be softer, spongier, and more likely to crack, particularly when painted in successive coats and especially when the undercoats contain reactive pigments or when the top coat contains very finely divided pigments.

In all- around paint qualities, poppy oil is a fair artists’ material but distinctly inferior to linseed, and as one which requires more careful and precise observance of the laws governing the correct technique of oil painting.

Perhaps the best use to which poppyseed oil can be put is as a modi- fying ingredient in linseed oil colors. Poppy oil colors have frequently been recommended for use in direct, simple or alia prima painting, but not for use in mixed techniques or those which call for complex or multiple layers of underpainting or overpainting.

Walnut oil is similarly inferior to linseed oil in all-around paint qualities; some investigators rate it above, and some below, poppy oil. In the past its cost has precluded its wide use for industrial paints; therefore, little modern scientific research has been done on it. It dries more rapidly than poppy oil, being nearly the equal of linseed in this respect.

The finest grades of both walnut and poppy oil are cold-pressed; they may be further refined, bleached, or bodied by the processes used with linseed oil; however, sun- or heat-thickened poppy or walnut oil is not ordinarily in use, because both of these oils are employed primarily as paint-grinding vehicles, where a normal oil consistency is required. Linseed oil is always the choice for clear painting mediums because of its greater durability.

Poppy oil has very little odour or taste and is used in France as an edible or salad oil; walnut oil will grow rancid on storage and develop a strong odour, as is common with other nut oils; its properties as a painting medium are believed to be thereby impaired. Fine grades of cold-pressed poppy and walnut oils are made in the United States, the former being pressed from imported seeds.

At present the average American manufacturer of oil colors uses linseed oil as a basis, and mixes in or substitutes poppy oil, according to his judgment and experience with the various pigments. Since the nineteenth century poppy oil has been used to some extent in hand-ground colors, but it is not ordinarily employed as the sole vehicle in artists’ colors.

Controversies over the merits of poppy and linseed oil have by no means come to an end, especially as applied to various specific techniques, and some of the foregoing opinions are disputed by other writers.

So far is known, pure linseed, poppy, and walnut oils may be mixed with one another in any proportion without any special ill effects. A tube of poppy oil colour can be immediately identified by smelling it; linseed oil has a definite characteristic odour while the poppy oil colour is completely odorless.

OTHER DRYING OILS

A number of Other oils of vegetable origin have a drying or semi-drying properties, and some are used in the paint and varnish industry. For the most part, they are interior to linseed oil and are employed as cheap substitutes for it.

Bean Oil

Bean Oil is a widely used industrial substitute for linseed oil, but is distinctly inferior to it. It may be bleached to a very pale color, but it always requires driers. Processed with alkyd resins it has long been an ingredient of some of the best industrial non-yellowing varnishes and white enamels.

Perilla Oil

Perilla Oil, obtained from crop-grown plants in Manchuria, Japan, and India, has been used in industrial varnishes and enamels to impart hardness and toughness. It is not cheap and is rated as a first-class drying oil rather than as a substitute for linseed, but it cannot be used in artists’ paints on account of its strong tendency to turn yellow.

Tung Oil

Tung Oil (China wood oil) is pressed from the nuts of Aleurites fordii and A. Montana, trees indigenous to China. The Chinese oil, from wild trees, was imported into the United States as early as the 1890s; by the 1920s its use had grown to large proportions and because of widespread adulteration and shortages, the tree began to be cultivated in the southeastern United States.

Tung oil is highly valued as an ingredient of industrial varnishes, where it produces tough, durable coatings. It has little value as a paint binder, and it requires expert thermal processing with driers, resins, and other ingredients to perform well in varnishes. No tung oil product is suitable for use in artists’ materials. Tung oil culture and production have been developing for many years in the southeastern states of this country, and it has become an important agricultural and commercial product.

Oiticica Oil

Oiticica Oil is a Brazilian product of similar properties. The fatty acids and the drying reactions of both of these oils are quite different from most of the other drying oils.

Lumbang Oil

Lumbang Oil (candlenut oil) is obtained from widely distributed tropical sources, principally from the Philippines. When economic conditions make it profitable, it is sometimes used as a substitute for linseed oil.

Sunflower Seed Oil

Sunflower Seed Oil and Hempseed Oil have properties resembling those of poppy oil, and have been used in Europe as Unseed oil substitutes. The drying properties of hempseed oil were known to some of the early writers. They are, however, inferior to those of poppy oil, according to modern investigators.

Safflower Oil

Safflower Oil is obtained from the seeds of Carthamus tinctorius and C. oxyacantha, plants which have long been extensively cultivated, principally in India, but also in East Africa, Egypt, Turkey, and elsewhere, for the sake of the safflower dye which is obtained from their blossoms. It has been used in India in textile decoration and in a sort of linoleum.

It is a fair replacement for linseed oil in artists’ colours but its resistance to age is not expected to equal that of linseed oil.

Stillingia Oil

Stillingia Oil (tallow seed oil) is obtained from a cultivated tree in China and is widely used for many purposes in that country. It has good drying properties and probably is the traditional material used by Chinese artists to grind the Vermilion for the seals which are impressed on their paintings. It was formerly encountered here chiefly as an adulterant in Chinese tung oil.

Tobacco Seed Oil

This oil has a high linolenic acid content and has been applied to the production of synthetic products such as oil-modified alkyd resins, which are of excellent colour retention, as are safflower and soybean oils. More attention to tobacco seed oil has been shown in Great Britain than in America. The seed is gathered principally from wild plants in southern India because tobacco cultivated for the leaf is harvested before the flowers develop.

Minor Drying Oils

About fifty other drying oils are known, but most of them are of minor importance because they are not available in sufficient quantities or because no work has been done on the development of their application to paint purposes. Their sources include the seeds of some of our most common domestic berries as well as seeds from remote parts of the world.

Semi-Drying Oils

A large number of oils which are or could be extracted from seeds, grains, etc., will dry alone with extreme slowness but may be more rapidly dried by the addition of driers, by heat treatment, or b\ admixture with rapidly drying paints. They are occasionally used to adulterate cheap paints, and sometimes to prevent the settling or caking of heaw pigments in mixed paints. Cottonseed oil and corn (maize) oil are familiar members of this group. Their use always decreases the durability of oil paint.

Non-Drying Oils

Another large group of vegetable oils is altogether 11011 drying. In some rapidly drying, rather brittle paints or varnishes, it is occasionally possible to add a non-drying substance, the drying action of the coating as a whole being powerful enough to carry the non-drier along with it and produce a finish which is, to all appearances and purposes, perfectly dry. For example, castor oil, a member of this group, may be added to shellac and lacquers to impart flexibility. No animal oils are employed in artistic painting, but for outdoor use, especially on smokestacks, fish oils have been used in industrial paints.

Frequently Asked Questions