Chemistry of oil drying phenomenon

The first contact of oils with oxygen results in the formation of peroxides. They react with unchanged oil molecules and produce other oxygen-containing bonds. They are the cause of the combination of glyceride molecules.
In this way, they contribute to the formation of complexes with colloidal characteristics, enabling the subsequent formation of a gel from the oil and then a paint film.
Spontaneous drying of the oil leads to the formation of linoxin, a product with eucoloid characteristics (poppy oil shows deviations from this).
The process of oil desiccation is a very complex process, being a tangle of chemical reactions involving oxidation and enlargement of molecules and, finally, colloidal transformations.
The process of oil drying can be called a colloidal (physical) phenomenon caused by chemical causes.
We distinguish between three types of oil drying:
1. desiccation consistent with linseed oil desiccation.
2. desiccation consistent with tung oil desiccation.
3. desiccation consistent with poppy seed oil desiccation.
The rate and type of drying of oil depends on the number and arrangement of double bonds in its molecule.
The oxidation products of linseed oil are mainly:

• easily and intermediate oliphatic acids,
• formic aldehyde,
• oxide and charcoal dioxide.

Of these products, only the less volatile ones remain in the membrane.
Acids are partially neutralized by alkaline dyads.
The increment representing the difference between the weight of the absorbed amount of oxygen and the weight of the volatile products is called the "apparent oxygen number of a given binder."
The higher the density of the oil, the slower the drying.
From a chemical point of view, the complete drying of the oil over the entire thickness of the paint film means the completion of the transformation of non-solid oxidation products of the oil, especially peroxides, into solid, insoluble and non-fusible combinations, which we call oximes.
According to Drinberg, the dried paint film is not a gel, but an insoluble and non-fusible three-dimensional polymer.
According to recent studies of the oil drying process, chemical phenomena occur only during the hardening and drying of the paint film.
Only physical phenomena occur at a later stage.

Linseed oil

It is obtained from flaxseed by pressing.
The best oil is obtained cold (light and transparent).
Oil obtained hot is brown, has a characteristic odor and is opaque.
Fresh oil obtained cold is odorless. The odor appears during bittering, or decomposition.
Linseed oil has the most unsaturated fatty acid called linoleic acid (58%) and in smaller amounts linolenic acid (20%) and isolinolenic acid (2.7%).
The type and chemical composition of linseed oil (as well as other vegetable oils) depends largely on the climate in which flax grows and matures.
For example, only one type of linolenic acid is found in oil from the north.
The desiccation of linseed oil depends on the unsaturated fatty acids present in it:

• linoleic,
• linolenic,
• isolinolenic.

Linseed oil applied thinly on glass, dries all its thickness.
When laid thicker, it dries first on the surface, forming a wrinkled film and increasing its volume by 20%.
This phenomenon is most characteristic of linseed oil.
In winter linseed oil dries in 11 days, in summer in 3 ÷ 6 days.
It dissolves easily in volatile oils. In spirit - poorly, unless it has been oxidized.
Linseed oil can be colored by the following organic dyes:

• xanthophyll - yellow,
• chlorophyll - green,
• erythrophil - red.

These dyes are not always present in the oil all at once. Sometimes the color of the oil is given by the predominance of one of them.
These dyes quickly decompose when exposed to light and the oil brightens.
Linseed oil has a smaller difference between growth and volume loss during drying than, for example, poppy and walnut oils. Hence the durability of the paint film is greater and less prone to cracking.
It is the most durable oil of all vegetable oils.
Its unpleasant drawback is yellowing and darkening as it dries.
This is affected by its linolenic acid content.
Wanting to prevent more yellowing of the painting, it is necessary to keep the painting in a bright and sunny room during the first stage of drying.
From the lack of light and moisture, the image darkens more.

Poppy oil

The color of poppy oil is affected by the shade of the poppy.
For painting purposes, light poppy oil is best suited.
When poppy seeds are cold-pressed, a colorless, odorless oil is obtained.
Its chemical composition differs from linseed oil. It contains much more, as much as 30%, oleic acid (in linseed oil 4.5%), and linoleic acid about 60%.
Linolenic acid, the most conducive to drying, contains negligible amounts, which is why it does not turn yellow after drying.
The high content of oleic acid makes it less able to dry.
Poppy seed oil takes twice as long to dry as linseed oil. The volume changes during drying are much greater (40%) and therefore the poppyseed oil coating is not durable, cracks more easily, and the cracks are deep.
It is not uncommon for poppyseed oil to become soft again after drying and give a sliver.
The sapwood grated on poppyseed oil does not turn yellow, which is why in the 19th century it was often grated on poppyseed oil.

Walnut oil

It is obtained from well-dried walnuts.
It was known in the fifteenth and sixteenth centuries.Leonardo da Vinci himself prepared walnut oil as follows:he mixed the nut pulp with water into a thick mush, then diluted it with water and exposed it to the sun; oil accumulated on the surface, which was collected with cotton wool.
Peanut oil stands on the border between linseed oil and poppy seed oil. It is better than poppy seed oil because it dries all over its surface, but inferior to linseed oil.
It contains 7% oleic acid, 80% linoleic acid and 13% linolenic acid.
Drying process better than poppy seed, worse than linseed.
Like poppy seed oil, it does not darken or yellow, but tends to soften after drying. It quickly becomes bitter.

Sunflower oil

Little known in Western Europe. It is obtained from sunflower seeds.
Obtained cool, it has a light yellow color, which quickly disappears in the sun.
Sunflower oil contains 60% linoleic acid and 39% oleic acid.
It dries slowly and does not wrinkle.
When resins and volatile oils are added, it gives a normal paint film.
Polymerization of this oil gives particularly good results.

Tung oil

Used mainly in China, India and Asia.
It is obtained from the poisonous fruit of trees of the wolfberry family.
Inside such a fruit there are 2 ÷ 3 oily seeds from which the oil is extruded.
Tung oil has a very interesting property:
It dries from the inside - not from the surface.
Therefore, this oil was introduced as an additive for oily varnishes.
It causes the entire paint layer to dry evenly, making it flexible and preventing it from cracking.
Tung oil was used in China to make ink (oil coppice).
When used in large quantities for painting, it causes the paint layer to darken.
However, it is effective and good when combined with resins.