Many vegetable oils will dry to form an adhesive film, either by themselves or when mixed with other ingredients. These oils do not "dry" as we know it by evaporation. They dry by oxidation or absorption of oxygen to form a layer of linoxyn. Other complex chemical changes take place in this process but the oxidation process is the most important.
Types of Drying Oils
Linseed Oil
Linseed OIl is taken from the flax plant. Seeds from different areas of the world have their own characteristics and are rated in quality according to the area from which they are taken. Most linseed oil produced today is not produced for use by artists. Most is used by modern industry, where quantity is desired over quality. Ongoing research is being done on how to extract the most from each seed. As for the production of artist quality linseed oil, little is being done to advance the process.
Cold Pressed
Cold pressed linseed oil is often thought of as the best oil for artists. With such a reputation, it carries a considerably higher price than other oils. It is made by crushing the flax seed with great pressure. The oil can vary in color from a light yellow to a somewhat darker golden yellow, but is always very clean and clear. The drying time is reasonably fast when compared to some other oils.
At one time cold pressed linseed oil was used in the production of most artists' oil colors. The production of cold pressed linseed oil is very crude and uneconomical. Due to these factors, many companies have replaced the use of cold pressed linseed oil with refined steam pressed linseed oil.
Stand Oil
Stand Oil is produced when linseed oil is heated to about 550° F and is kept at that temperature for 16 to 20 hours. When complete, the physical and mechanical properties of the linseed oil change and go through a polymerization process. The oil may be heavy and viscous with the consistency very much like that of honey. It can be thinned to a consistency used for painting by adding several parts turpentine. Stand oil should not be used in the grinding of pigments, because there are other oils that will produce better results.
Stand oil is best used as an ingredient for glazing or painting mediums. Over time stand oil becomes less yellow than many other oils. and when thinned and mixed to create other mediums it is virtually non-yellowing. One of the other most beneficial properties of stand oil is that it drys to an enamel-like film, smooth and free of brush strokes. When it is added to paints and mediums, it tends to extent this property to them as well.
