Watching Oil Paint Dry

UNDERSTANDING THE DRYING TIMES FOR OIL PAINT

Traditional oil paints are bound with drying oils. This is what gives them their unique working properties and makes them much slower drying than water-based media. A drying oil is a vegetable oil which dries by oxidation (explanation below) and there are many types including poppy seed oil and safflower oil amongst others. Linseed oil is the one used in the majority of oil paints because it dries to the most durable film

What effects do drying times have on oil paintings?
The main effects are dependent on how you layer your paints. If done incorrectly, you could create damage to your artwork. For example when underpainting, if a faster-drying layer is applied over the top of oil underpainting, this will be pulled apart as the slower-drying colour contracts. This is also true of colours which only surface dry such as cobalt.
For underpainting, we would recommend an underpainting white, alkyd white or flake white (in linseed oil) because of their quick and thorough drying time.
In addition, paintings made in layers are also less likely to crack if the underpainting is moderately thickly applied – that is, a thin paint film, not an excessively thinned paint film. It then has more time to dry thoroughly.

Which colours dry faster?
Most brands of oil paints contain driers in some colours in order to bring the drying times closer to range between 2 and 10 days. This helps to prevent problems with slow-drying colours and is perfectly safe for the paint film when controlled by experienced chemists.
Fortunately the drying rates of colours are rarely a problem because colours are almost always mixed on the palette and so the drying times tend to equalize to a great degree.

Using Winsor & Newton Oil Colours
Both Artist’s Oil Colour and Winton Oil Colour are combined with Linseed oil and will behave in the way described above. In addition to this, it is worth stressing that, as Artisan Water Mixable Oil Colour is also Linseed and Safflower based (not water based) it too has exactly the same drying mechanism and therefore rate of drying.

Speeding the drying rate
The safest way to accelerate the drying rate of oil colours is to use Liquin, which speeds the drying by about 50%. Thickened linseed oil can also be used and will speed drying by about 10%.
Neat driers (such as cobalt) are not recommended because they can crack the paint. The safe addition of driers depends on each pigment and is best left to the experience of manufacturers.

Retarding the Drying Rate of Oil Painting
Sometimes it is also necessary for artists to retard the drying rate. To achieve this use a 50:50 stand of oil and turpentine mixture to thin it.

The Oxidation Process (drying process) explained
Oil colours, unlike water based colours which dry by evaporation, dry as the result of an oxidative reaction, an oxidative reaction being the absorption of oxygen from the air.  This reaction is a complex one that can be broken down into different stages; The Autoxidation phase, the Polymerisation phase and finally the Stationary phase.

• 1. Autoxidation Phase
  Vegetable oils (such as linseed oil and safflower oil) are made up of a mixture of various triglycerides that differ in terms of their fatty acid constituents. A triglyceride is a glyceride in which the glycerol is esterified with three fatty acids. Structurally this means that these oils contain long chains of hydrocarbons. As oxygen is absorbed during the drying process, it attacks these hydrocarbon chains and produces free radicals.
• 2. Polymerisation Phase
  These free radicals are highly reactive substances due to the presence of an unpaired electron. As more and more reactions occur, further free radicals are produced which start to polymerise and the process terminates when they form a new bond as their unpaired electrons combine. This polymerisation stage takes days and weeks to complete after which the paint film will feel dry to the touch.  However chemical changes in the paint film continue.
• 3. Stationary Phase
  During this final stage the polymer chains begin to cross link. Covalent bonds formed by adjacent molecules result in a molecular network throughout the oil colour. This results in a stable and dry paint film.