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The oils used by pet portrait artists and others are called drying oils. These are vegetable based oils that, when mixed with oxygen, produce a solid film which is resistant to the atmosphere and many solvents. Oils dry very slowly and although the paint film is touch dry within a few days, it in fact takes a few years for the paint to become fully dry. At first this film is pliable but, in time, becomes gradually more brittle. All dryings oils possess their own colour to a certain level and, initially, this has a reasonably minimal effect on the paint. Given time 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 under 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 long-lasting 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. However, it appears 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 begin yellowing as much as linseed oil, however, it does not keep well, tending to go rancid if not stored carefully.

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 helps paint flow, gives a shiny finish and does not tend to leave brush marks making it a effective medium for glazes. Stand oil can also be thinned using genuine turpentine. It produces a stonger and more flexible paint than the others, 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.