The reason oil paints dry is because of something called oxidative polymerization, during which oxygen is absorbed, peroxides are formed and a polymer film is created. This process has been termed “oil drying”.
It has been found that light, temperature and humidity all strongly influence the rate at which oil dries. Oil dries several times faster in the light than in the dark, when the process can almost come to a complete standstill. It is the ultraviolet rays in light that are most actively involved in the process of drying oil: the effect they have is just as strong as that of siccatives. If oil paints are left to dry in direct sunlight for a long time, however, the oil layer may be damaged.
The ambient temperature around the painting also has a significant influence on the process. By way of example, oil paints containing a siccative will take about a day to dry out if left at room temperature (20 – 22 ° C), but at a temperature of 80 – 90 ° C, the drying time is reduced to 1 hour, and at a temperature of 120 – 130 ° C, the drying time is 15-20 minutes. Oil paintings should not be dried at high temperatures, of course, but a reduced temperature will significantly slow down the setting of the layer of paint.
Cold delays the drying of oils, and so too does high humidity. Normal air humidity (65 – 75%) affects the process favorably, because at the same time that the oil is absorbing oxygen from the air, it also absorbs a certain amount of water, which is necessary if the drying process is to take its normal course.
If oil paints are left to dry in air that is not sufficiently humid, the process will happen quickly, but it will not happen properly, as a great deal of the oil’s volume will be lost, due to the formation of a substantial amount of gaseous matter as it breaks down. If you try to dry oil paints in air that is saturated with water vapor, you end up with a swollen film with a sizable water content on the surface, and this film will be covered with cracks when fully dried out.
Among the substances used to accelerate the drying of oils, known as drying agents, are special coordination compounds called siccatives. These compounds facilitate the absorption of oxygen by the oil and speed up the formation of peroxides.
The Old Masters often added mineral siccatives to their varnishes: lead, in the form of lead white pigments, minium and lead-tin yellow; manganese, in the form of umber; iron, in the form of iron sulfide; zinc, in the form of zinc sulfide; and calcium, in the form of lime.
When it comes to delaying and slowing down the drying process, the key role is played by water and the polysaccharides contained in oils, along with certain essential oils that are added to ‘fat’ oil paints. These essential oils themselves require oxygen in order to dry out, and thus prevent the fatty oil from absorbing it.
The material onto which the oil paint is applied also affects the acceleration and retardation of the drying process. Oil and oil paints dry faster on a surface made of lead than on other metals, for instance. On wood, the paints are absorbed more quickly, but the polymer film takes longer to form.
Once the oil has hardened and solidified, it gradually loses its elasticity (as its mass and volume decrease), and that is why old oil paintings become brittle and fragile.
The hardened films of oil sometimes become soft again and lose their stickiness. Softening of this kind, after the oil paints have dried, is mainly observed in poppy, walnut, and sunflower oils, and also, on very rare occasions, in linseed oil. The films will become hard again if left in the light and exposed to dry air.
