THE DRYING OF OILS AND OIL PAINT
Oils have been used since antiquity as both decorative and protective coatings. The addition of pigments is a more recent invention and oil paints as a medium of artistic expression have a history of over 1000 years. The original artist's paints were simple mixtures of oil and pigment which could be applied to wood or canvas and allowed to dry. Over time artists made many changes in how the paint was prepared to increase control of the texture, appearance and drying time of the paint.
Various criteria have been used to determine the suitability of oils and paints as coatings. The most practical and easiest test is to determine if the oil will form a dry film when spread out as a thin layer. Simply touching the paint film will tell one if the coating is hard or tacky (sticks to the finger) or still wet. The length of time it takes to dry is important depending on the purpose of the coating.
If a paint is used outdoors then it should dry quickly to avoid picking up dust or dry unevenly due to weather or location. Oil films used as coatings on painting or furniture should dry quickly to avoid picking up dust which would change the optical properties of the coating. They should also form hard surfaces that are resistant to moisture penetration. Artists' paints, however, may not be as useful if they dry too quickly. In preparing an image on a canvas artists may want to take time to produce the desired effect or to change their mind. For each purpose different methods of oil preparation or different additives were used in the oils or oil preparations.
When oils are mixed with pigments to give them color, several things may take place. The oils themselves are slow in drying but the addition of pigments may speed up or in some rare cases even slow down the drying of the mix of oil and pigment (paint). This depends upon the chemical composition of the pigments added to the oil. Pigments that contain lead or cobalt will tend to speed up the drying of the paint while certain organic materials such as alizarin or bitumen will slow down it down.
These paints can be examined by making test coatings on different substrates and waiting until they dry. These test coatings can be examined in a more quantitative or scientific way by understanding and taking advantage of the chemistry of these oils. Oils in coatings become solid by reacting with the oxygen in the air, a process called oxidation. Because the oxygen is being taken from the air, the weight of the oil or paint actually increases for a while. Over time, however, the oxidation process breaks down some of the oil and forms volatile compounds that can evaporate and cause the film to lose weight. By the time of maximum weight increase, the paint or oil is dry to the touch. One can then study the effects of pigments or other additives to oils by making a plot of the amount of weight that is gained against the length of time.
Another factor that affects the behavior of oils is the surface on which the oil is placed. This is not usually a problem with materials such as wood or canvas but on metals the effect is quite different. Painting on iron will have little effect on the drying of oil paints but painting on copper greatly decreases the drying time, in addition to giving a slightly green cast to the oil.
If the only question about oils and paints were "How fast do they dry?", research would be easy. However, once the oils or paints have become dry to the touch, they may still require considerable time to dry completely. Changes after the initial drying process are considered as ageing. Changes in paints, for example, which would not be detectible over 10 years may become very important over 100 or 200 years. Chemical and physical changes may sometimes be slow but are important nevertheless. Beyond drying to the touch, paints are still very active chemically and the polymerization process that starts with the uptake of oxygen will still be active for years afterward, and even affect the physical properties of paint decades after the paint film was applied. Both the chemical and physical processes in paints go on for many years.