Technical guides

Pigments are inherently categorised as slow- or very slow-drying, through to fast- or very fast-drying, according to their chemical nature.

For example, umbers inherently contain manganese oxides, which make them very fast-drying. Prussian blue is another example of a relatively fast-drying pigment.

Alizarin Crimson, Titanium White and Ivory Black, on the other hand, are very slow-drying pigments.

This information is extremely important for an Artist to know in order to avoid cracking. Cracking can occur when a layer of an oil colour containing a fast-drying pigment is applied over a layer containing a slow-drying pigment before the underlying layer has fully dried.

In such a case, the upper layer dries quickly, restricting the access of oxygen to the underlying layer. The slow-drying oil colour therefore takes considerably longer to dry, if it dries completely at all, and this difference in drying behaviour can eventually result in cracking.

We provide this information for Artists’ guidance. Please check the properties of each Oil Colour before use.

The fat-over-lean rule is essential for avoiding cracking.

Be sure to check the oil content of the colours you are using — very low, low, medium or high — and take this into consideration when building up layers.

The reason behind the rule is the difference in drying times: lean layers dry faster than fat layers. If a lean layer is applied over a fat layer, the upper layer may dry before the underlying layer has properly dried and cured, potentially resulting in cracking.

When colours are mixed, the resulting mixture will have an oil content determined by the respective oil contents of the component colours, in proportion to the quantities used.

For example, if a small quantity of a very-low-oil-content Titanium White is mixed with Alizarin Crimson, the resulting mixture will no longer have a very low — or necessarily even a low — oil content. Instead, its oil content may be considerably higher, depending on the proportion of Alizarin Crimson in the mixture.

It is quite common for oil colours to undergo chromatic changes towards yellow or even black, depending on the materials contained in the medium.

Mastic or damar can result in yellowing, while copal can contribute to darkening or blackening. Oil colours themselves tend to yellow over time; although yellowing can often be addressed by conservators, blackening is a considerably more serious issue. Siccatives can also contribute to yellowing. Make sure you are aware of the composition of any medium you use.

Another important role of a medium is to alter the oil content of an oil colour, making a low-oil-content colour fatter or a high-oil-content colour leaner. The extent of this alteration depends on the proportions and composition of the medium.

This can help maintain the fat-over-lean rule without requiring the Artist to settle for alternative colours. In other words, the Artist can use precisely the oil colour they want while adjusting its oil content as necessary.

The absorbency — or lack of — of the painting surface, whether canvas, wood, board, card or another preferred material, is fundamental to the behaviour of oil colours.

If the painting surface is absorbent, it will draw oil from the oil colour, effectively rendering the colour considerably leaner than it was originally. This can result in undesirable effects such as chalking.

You should therefore know the absorbency of your painting surface so that you can either adjust the oil content of the oil colour or alter the absorbency of the chosen surface accordingly with the appropriate preparation.

We take care to make the vast majority of our Oil Colours single-pigment colours, without fillers, either in name or in essence, so that they retain their pure, clean colour and give the Artist absolute control when mixing.

However, we also propose the technique of layering because it can produce an even clearer and more radiant result.

All Oil Colours are at their purest when used without mixing. Nevertheless, it is possible, for example, to achieve a beautiful purple by applying an Ultramarine Blue layer and then over-layering it with an Alizarin Crimson glaze.

The resulting purple can have considerably greater radiance than one produced by physically mixing Ultramarine Blue with Alizarin Crimson.

Try it if your work permits it, and let us know what you think!

Do not forget that the underlying layer must be completely dry before applying the glaze.

Varnishes should be applied only after the work has fully dried. Based on our experience, this will generally occur 6–12 months after completion.

If varnish is applied sooner, the possibility of cracking is considerably increased because the varnish layer can restrict the access of oxygen to the underlying layers. Additionally, we advise Artists to avoid varnishes containing wax, which are usually matte varnishes, as they tend to attract dust and impurities that can be very difficult to remove.