Oil

By Douglas Purdon, SCA

Doug Purdon is Educational Advisor for Winsor and Newton in Canada and talks on the history and properties of pigments and material at universities and colleges.

Breaking News! Fat Boy...Afraid of Dark!

You have a coward in your paint box...Oil Paint! While it has been used by artists for 500 years and has stood the test of time it does have problems if not applied correctly. When you visit art galleries you will notice that paintings done in the 16th and 17th centuries are in better condition than those done in the past 100 years? This isn't due to the quality of paint used; in fact materials have improved so that today's material are far superior to those used in the past; the problems are due to improper application by artists.

The title of this article was meant to grab your attention but also to tell you a couple of interesting things about Oil paint that you might not know. Oil paint is a glutton! Not for food but oxygen! Oil Paint doesn't dry be evaporation, as does acrylic and watercolour but by oxygenation. During the curing process a painting can gain up to 28% in weight as the oil combines with oxygen. (NB. Use stronger picture wire!). Additives such as Cobalt Drier, Japan Drier, or Alkyd Mediums are oxidizing agents and speed drying. Many pigments also have properties that act as catalysts for oxidation; that is why Burnt Umber will dry quicker than Alizarin Crimson. Using heat to aid drying will only 'cook' the oil, and alter the proper curing process, setting the scenario for cracking, yellowing, and other problems later, it won't speed drying.

While oxygen is one factor in the curing process, light is the other. Oil paintings need light to dry. Ultraviolet light acts as a catalyst in the oxidation process and also acts as a bleaching agent on the oils to prevent yellowing and help in producing a clearer, brighter and stronger paint film. Oil paintings kept in the dark rapidly yellow, however once they are exposed to light the process is usually reversed. You don't have to expose the painting to direct sunlight just normal room light will be sufficient.

I know I am starting to sound like a workshop for 'Jenny Craig' or Weightwatchers but here is more discussion of fat and lean. Most oil painters know the rule of 'Fat over Lean', however something artists tend to forget is that this rule also applies to the pigments that are used and not only to the practice of adding more medium to each new layer of the painting. Some pigments absorb more oil in the manufacturing process and hence are 'Fat' colours...Alizarin and Ivory Black being a couple of the 'fat boys'. When painting in oils the 'fat' pigments should be reversed for the final layers of the painting and never for underpainting. One of the reasons that older paintings were much more stable was, that the only whites available to artists at that time were Flake White and Zinc, both of which are very 'Lean' colours. They are excellent for underpainting as they suck the oil out of the following layers forming a stable paint film. It is interesting to note that the cracking in paintings has increased since the introduction of Titanium White in the early 1920s, which is a medium oil absorption colour. If you keep the early layers of your painting lean by using little or no medium and only colours of low oil absorption, then gradually increase the use of mediums and more oily pigments in the final layers, you will construct an oil painting, that will stand the test of time. It will have been built it upon a firm foundations. The application of paint in the earlier stages of the painting should also be kept thin, only applying the impasto work in the final layers. Artists working with a painting knife and applying the paint heavily should use one of the impasto painting mediums, W&N Oleopasto being one, as it will help speed drying and improve stability.

In case you haven't a list of the oil absorption properties of pigments, here is a short list for reference:

Low Oil Absorption

Flake White, Zinc White, Chromium Oxide Green, Venetian Red

Medium Oil Absorption

Cadmium Colours, Thalo Colours, Ultramarine Blue, Titanium White

High Oil Absorption

Alizarin Crimson, Ivory Black, Umber's, Sienna's

Very High Oil Absorption

Cobalt's, Viridian Green, Lampblack

This article appeared in the Spring 2001 issue of the SCA Newsletter.

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