Oils used in painting are divided into two groups according to their composition and purpose. The first includes the so-called fatty drying oils obtained from the seeds of various plants and related to fats of vegetable origin, such as linseed, poppy, nut and other similar oils.
The second group includes oils of various origins that do not belong to fats, bearing the name of essential oils.
Fatty drying oils have the ability to convert into solids in the air, which makes it possible to use them as binders of paints in oil painting and in painting. On the basis of this, they are called drying oils, unlike a number of other oils of vegetable origin, which either do not show this ability at all in the conditions of painting practice, or they show it only partially.
All vegetable fatty oils can be divided into drying, semi-drying and non-drying. Drying fatty oils include: linseed, poppy, nut, hemp, white acacia oil, lyalemantia plants, pine and spruce seed oil, elderberry, wood (tung), perilla, niger.
Semi-drying include: cotton, sunflower, maize, cretonne, rapeseed, sesame, linden tree oil, etc. Non-drying - almond, castor, olive, coconut, palm, pistachio and many others.
Animal fats, for the most part, do not harden in the air; the only exceptions in this respect are blubber (liquid fat extracted from the fat of whales, seals, etc.) and fish oil, which have this ability and therefore are often used to falsify drying fatty oils.
The most popular greasy drying oils in painting are: linseed, nut and poppy oils.
These oils, like other vegetable fatty oils, are non-volatile colorless or colored more or less thick liquids that do not mix with water and do not dissolve in it, but, being lighter than water, float on top of it. They hardly dissolve in alcohol, easily - in ether, carbon disulfide, chloroform and essential oils.
Vegetable fatty oils, like animal fats, consist of carbon, hydrogen and oxygen. They are complex chemical compounds in which the most important constituent elements are compounds of animal acids with glycerin, on the basis of which they are called glycerides.
These glycerides, under certain conditions, are capable of forming soap and releasing glycerin: on this basis, fatty oils are also called saponifying oils. Essential oils are non-saponifying oils. Representatives of this type of oils by their composition belong mainly to hydrocarbons. Some of them evaporate in the air without residue; others evaporate partly and partly oxidize, tarring and forming solids.
The oils that evaporate without residue include products extracted from oil and coal tar. These are the various types of kerosene, gasoline, benzene, toluene, xylene, tetralin and decalin. Most of them belong to substances that are chemically little active.
Essential oils, which include turpentine (French, Russian, etc.), Copay balsam oils, copal and amber oils, partly evaporate, partly oxidize, i.e. they are tarred, and solids are formed in larger or smaller amounts. They are so-called terpenes - chemically active substances. Essential oils also include oils that are chemically a mixture of hydrocarbons of a number of terpenes with some oxygen compounds. These are: rosemary, spik, lavender, clove oils and elemi balsam oil.
Oils and their solidification process
These oils are obtained by squeezing flax and poppy seeds. Seeds undergoing squeezing are heated to a certain temperature or squeezed cold.
The oils obtained by hot pressing, i.e. with heating of the "pulp" (crushed seeds), acquire a darker color and contain a large amount of foreign impurities; the oils squeezed out by the cold method are much lighter and contain fewer of them. Foreign substances found in freshly squeezed oils include: mucus, starchy and protein substances, salts of inorganic origin, coloring principles, cellulose.
All these impurities in the oil worsen its composition, spoil its color, contribute to its rancidity and slow its drying. Oils in their pure form have neither taste nor smell and are not capable of spoilage. For the purposes of painting, therefore, the most suitable are oils squeezed out by cold pressing and, moreover, from dry seeds. In practice, however, pressing with heating of pulp is more often used. With prolonged settling of oils, foreign impurities contained in them settle to the bottom of the vessel, and the oils become free of them. Thus, the quality of the oils improves over the years.
Drying fatty oils are able to turn into solids in contact with air, moreover, in a fairly short period of time, which is why they have been used for painting and coloring purposes for a long time; as for the essence of the phenomenon mentioned, its scientific explanation began only later.
The drying process of vegetable fatty oils, therefore, differs significantly from the drying of adhesive solutions or alcohol lacquers, where the essence of the solidification process consists only in the evaporation of water and alcohol and is a very complex process in which both chemical and physical phenomena take part.
The solidification of fatty oils is accompanied, on the one hand, by the formation of volatile elements, the presence of which in the air, where fresh oil paint is located, is recognized by the characteristic smell of some of them, familiar to every artist; on the other hand, an increase in the volume and weight of oils. (The discovery of this important phenomenon belongs to the Russian scientist Professor of physics F. To Petrushevsky) . In the first period of oil solidification, the gain in weight exceeds the loss, in the further long period there is only a decrease in its weight and volume, as a result of which the dried oil layer is subjected to compression and tension, under the influence of which cracks form in the layers of oil painting.
When oils and oil paints solidify, a film forms on their surface, which hardly passes air, which is why the process of through drying of the layer is delayed for a long time. Oil paint dries better in all conditions in the depth of its layer than a layer of oil alone, and many of the paints vigorously contribute to the process of solidification of oils, for example, paints of lead origin. Drying of oil and oil paints is a very long process.
Painters distinguish three points in it:
- drying of oil, in which a film forms on the surface of the Layer, giving off;
- drying of oil when observed the disappearance of the cleavage and the formation of a solid surface and
- complete through-hardening of the oil layer.
So, linseed oil, for example, dries up in 3-6 days, dries up in 60 days and hardens in the entire thickness of the layer in 2 years.
After two years from the beginning of solidification of the paint, its compression, although it does not yet reach the limit, becomes almost imperceptible. The decrease in the volume of paint for 2 years, according to Prof. F. Petrushevsky, is equal to 1/20 to 1/4 of the previous volume and even slightly more. The paint shrinks the more significantly the more oil has been injected into it. The origin of the oil is also of great importance in this case, since the degree of compression of different oils, as experience has shown, is different.
Under various conditions, this process sometimes accelerates, then slows down. Experience has shown that light, temperature, dryness and humidity of the air have a great influence on the acceleration and deceleration of oil drying, but besides them there are other factors acting in the same direction.
Light plays a very important role in the drying process of oils, and in the absence of light, drying is almost suspended. So, the oil that dried in the light for 6 days dried in the dark in 66 days. Direct sunlight acts especially energetically in this case, since their warmth is also of great importance here. Indeed, it takes from half to one-third of the time it takes to dry the oil paint in sunlight, which is necessary to dry it in the shade. The most active rays of the sun are ultraviolet rays, which have no less effect on accelerating the drying of oils than the action of the so-called "sicatives".
With prolonged drying of oil paints under the influence of direct sunlight, the oil layer may suffer.
How great is the value of the air temperature for drying oils, this can be judged by the following data. Linseed oil at 25-28 gr. C; dries out already sooner than at 15-18 gr. C; with a significant increase in temperature, the drying process is especially accelerated. So, according to Weger, oil that dried out in winter at room temperature in 5-7 days and in summer in 3-4 days dried out at 50 gr. C in a dark drying cabinet for 12 hours, at a temperature of 95 gr. C - approximately for 1 hour, and at 120 g. C - in 30 minutes. Oil containing a manganese sicative and dried at room temperature for 14 hours, at 95 gr. C dried in 30-40 minutes, and at 120 gr. C - in 15-20 minutes.