The function of oils in painting

The oils used by pet portrait artists and others are called drying oils. These are vegetable based oils that, when exposed to oxygen, produce a solid film which is resistant to the atmosphere and many solvents. Oils dry out really slowly and although the paint film is touch dry inside a few days, it may well take a few years for the paint to get completely dry. At first this film is pliable but, in time, becomes gradually more brittle. All dryings oils have their own colour to some degree and, initially, this has a fairly minimal effect on the paint. Overtime it becomes more pronounced, contributing to the yellowing effect in older paintings. The types of oil available to the artist are:

Linseed Oil
This is the most thorough drying and durable of the drying oils. It becomes ‘touch dry’ within 3 to 5 days. A extremely refined version known as ‘cold test linseed oil’ is produced for artists using a procedure that removes undesirable waxes. It has the propensity to yellow as it dries but is unlikely to crack due to it’s ability to dry thoroughly.

Poppy Oil
Poppy oil is slower to dry out than linseed oil, it’s not as durable and doesn’t dry as thoroughly however, it has the benefit of not yellowing as much.

Safflower Oil
This is very similar to poppy oil. It has the advantages of reduced yellowing, is very pale but is also less durable than linseed oil and is slow to dry. Although, it does seem to be more stable than poppy oil once dry.

Walnut Oil
This oil is thinner than the previous oils so making the paint more fluid. It takes 4 to 5 days to become touch dry and doesn’t suffer from yellowing as much as linseed oil, however, it does not keep well, tending to go rancid if not stored properly.

Stand Oil
Stand oil refers to a process that drying oils undertake. This usually is linseed oil which has undergone heat polymerisation, resulting in a more viscous oil that is also lighter in colour to untreated oil. Stand oil improves the flow rate of paint, gives a glossy finish and is resistant to brush marks making it an excellent medium for glazes. Stand oil can also be thinned using genuine turpentine. It produces a tougher, more elastic paint than the rest, resists the atmosphere and moisture well with minimum yellowing, however, it does take 7 days to become touch dry and even then remains tacky.

Sun-thickened Oils
This is a traditional variant of stand oil and, as it can be made-up by the artist, the exact properties of it will depend on the method of preparation. It can vary from a light straw colour to nearly colourless, it’s drying time can be between 1 to 3 days and possesses many of the benefits of stand oil. Sun-thickened oils can also be prepared with metallic dryers, such as lead, cobalt or manganese, which can aid the drying time and improves clarity.