Drying oils and their drying process

These oils are mainly obtained by hot or cold pressing the seeds.
Hot-pressed oils are darker in color and contain some bystanders.
Cold-pressed oils are lighter in color and less contaminated.
Oil contaminants include:

• mucilage,
• proteinaceous bodies,
• mineral salts,
• coloring bodies,
• cellulose.

The presence of these bodies in the oil changes its color, makes it bitter and stops it from drying out.
Oils in their pure state have neither taste nor smell, and do not spoil.
For painting purposes, oil obtained cold and from dry seeds is best suited.
In practice, however, hot-obtained oil is most common.
As a result of long storage of oil, all contaminating bodies settle to the bottom of the vessel and the oil cleans itself.
Therefore, the older the oil, the better it is for painting.
There are also a number of artificial ways to purify oil.
Drying oils have the ability to turn into solids upon contact with air.
The process of drying oil is very different from the drying of either glue or spirit solutions, where the hardening process involves only the evaporation of water or spirit.
The process of oil drying is a very complex process, in which both chemical and physical phenomena are involved.
During the drying of oil, one can observe the volatilization of many volatile components, such as:

• water,
• charcoal,
• acetic acid

and in negligible quantities:

• charcoal oxide,
• acreoline,
• formic acid,
• acrylic acid,
• butyric acid,

as well as an increase in volume and specific gravity.
The greater the difference between the increase in specific gravity and the loss of specific gravity, the less durable the paint film.
During the drying of oil and oil painting, a film forms on the surface, which hardly allows air to pass through, and therefore the drying of the entire depth of the paint layer is stopped for a long time.

Oil painting

dries better throughout the thickness of the layer than oil alone, with many of the dyes vigorously interacting during the drying of the oil (e.g., lead dyes).
The oil drying process can be divided into three periods:
1. desiccationIncrease in volume and specific gravity (by absorbing oxygen from the air). A thin film is formed on the surface giving odlip (3 ÷ 5 days).
2. dryingLoss of weight and volume as a result of volatile parts produced volatilizing (odlip disappears, surface hardens 60 days).
3. complete drying and hardening of the paint filmFurther loss of weight and volume over a period of 2 years. A thin film called linoxite forms on the surface of the drying paint film.
After two years from the beginning of drying, the degree of shrinkage of the paint layer is minimal.
The paint layer shrinks depending on the oil it contains and the type of oil.
A great influence on the drying process of the oil is exerted by the temperature and humidity of the air.
Humid air retards the drying of oil.
The action of light accelerates the process. For example, oil dries on light in 6 days, without it - 66 days.
The oil dries fastest in the sun, but exposure to ultraviolet rays for too long causes the coating to crack.
Heat also speeds up drying, e.g. oil with manganese siccatives dries at room temperature in 14 hours, and at 120°C - 15 ÷ 20 minutes.
Oil drying in too dry air dries quickly, but loses a lot of specific gravity by rapid evaporation of volatile components.
Drying in too humid air, it forms a swollen film with a high water content and in the final stage of drying is covered with cracks.
The dried oil coating is insoluble in water and hardly soluble in solvents.
A freshly dried coating is spongy and can smear with water, especially if the dye does not reinforce it.
As the oil dries, chemical phenomena occur first, followed by physical phenomena.
The brittleness and fragility of the dried oil coating is the result of physical phenomena.
Parallel to the drying of the oil and the chemical changes that occur in it, there is a darkening and loss of transparency of the paint layer.
The initial yellowing is superficial, but then it penetrates the entire layer.
The degree of darkening depends on the conditions under which the oil film dries and the oil used.
Drying oils (linseed type) are characterized by the following properties:

• they dry quickly,
• they slightly lose their specific gravity after drying,
• do not thaw again,
• they give a non-flammable coating,
• after drying, the coating hardly dissolves under the influence of organic solvents.

Among the drying oils are:

• linseed,
• tung,
• hemp (partially),
• Psilla.