O I L S

The Importance of Oils in Oil Paint

The oils used in oil paint serve two purposes. First, the paints are made by grinding pigments together with oil, which acts as an adhesive that attaches the pigment to the support and keeps the particles in suspension. Second, oils are used as a medium to modify paints, either by themselves or mixed with turpentine or resin.

When used as a medium for painting, oils perform several important functions:

1. Executive--Allows the colors to be applied and spread out.
2. Binding--Locks the pigment particles into a film, protecting them from the environment and physical damage and from being disturbed by additional applications of paint.
3. Adhesive--Dries and acts as an adhesive, attaching the colors to the support.
4. Optical--Brings out the depth and tone of the pigment, resulting in a different quality from its dry state.

Oils are classified as either drying or semi-drying. Linseed oil is a drying oil and the most widely used. It has been utilized by painters since the 15th Century for both grinding pigments and as a paint medium.

Safflower, sunflower, and poppy oils are all semi-drying. Because they are paler and have less of a tendency to yellow, they are often used for grinding white pigments. Semi-drying oils are often used to slow down the fast drying times of some pigments and can be used either by themselves or mixed with a drying oil. When using them, care should be taken to make sure that they are completely dry before applying the next paint layer.

The type of processing that oils undergo determines their characteristics and how they interact with paint. Stand oil is made by heating oil without contact with air. This causes a molecular change--polymerization--that gives the oil a syrupy quality. Stand linseed oil is particularly useful in painting mediums. Thinned with turpentine or mineral spirits, it increases the flow properties of oil paint. The resulting paint film will resist yellowing, be tough and elastic, and retain its flexibility as it ages. Because it is slow to dry, stand linseed oil is often combined with damar varnish and a solvent to promote drying.

Sometimes metallic driers--such as lead, manganese or cobalt--are added to hasten drying. This is called "drying linseed oil" and is good to speed drying in the early stages of a painting (and it dries darker than plain linseed oil).

Sun-bleached and sun-thickened oils are processed by mixing oil with an equal amount of water, followed by exposure to sunlight in loosely covered, flat trays. Sunlight temporarily bleaches plant colorings, but the color of the oil deepens when it dries and hardens. The mixture is left outside for a number of weeks until the desired thickness is reached; then it is filtered and the water removed.

Sun-bleached and sun-thickened oils undergo polymerization upon contact with air, so they dry faster than stand oils. They are low viscosity and may be thickened with stand oils or thinned with solvents, as desired.

Sun-thickened linseed oil has been a standard material of painters for centuries. It produces a strong, elastic paint film. Sun-bleached poppy oil is a yellowing-resistant oil which dries more slowly than sun-bleached linseed oil. It is highly recommended for use with whites and pale colors in the final layers of an oil painting.