Raw and Refined Oil
Raw and refined linseed oil have good brushing properties. Paint made of essentially raw or refined linseed oil has a short, buttery, consistency that lends itself to easy brushing. The flow of such paints is poor, however, and it leaves brush marks. Raw linseed oil has an acid value of 4–7, while alkali refined linseed oil less than one. Exceptions to this are special refined oils made with high acid values to obtain better pigment wetting properties.
The outstanding property of linseed oil is its excellent durability. It is therefore used more extensively in paint than any other drying oil.
|[FONT=tahoma]Extra High Viscosity, Vacuum-Bodied Oil[/FONT]||[FONT=tahoma]Pale Drying Oil[/FONT]|
Bodied oil is polymerized oil made by heating refined linseed oil at high temperature for a certain amount of time. Where color and low acid numbers are important it is heated either in a vacuum or under a blanket of inert gas. Bodied linseed oil has an acid value in a wide range, depending upon how it is heat treated.
Blown linseed oil is essentially partly oxidized oil made by passing air through at high temperatures. Since completely oxidized oil would be solid, partially oxidized oil is exceedingly viscous. The typical viscosity is Z-2 to Z-4 in the Gardner-Holt Viscometer standard. The acid number of blown linseed oil is typically high.
A small amount of blown linseed oil may be added to very short paint (called "puffy" paint) that typically grinds very slowly to speed up the grinding time.
Heating raw oil, adding driers and cooking it in an open or closed kettle is how boiled oil was made. Today, liquid driers are added to refined oil and heated briefly at lower temperatures to effect complete solution.
Bodied oil has better color retention than either unbodied oil. This can be understood if we consider that we have an oil that has gone partway toward a dried film via polymerization. Such a film, drying faster than a similar film of unbodied oil, absorbs less oxygen by the time it is dry. Since it is the oxidized film that is mainly responsible for yellowing and since a polymerized dry film has oxidized less than unbodied oil, we can understand why it has better color retention.
Bodied oil has better color retention than blown oil. It is also easier to understand why oil that has been partly oxidized by blowing will end up with a greater degree of oxidation when dry than one in which some of the double bonds (oxidizable bonds) have been removed by polymerization. Blown oil has poor initial color, due to the oxidation during the blowing process and poor color retention due to the further oxidation taking place while the film is drying.
Flowing and Leveling
Bodied oil has very good flowing and leveling properties, but not as great as that of blown oil. Brushing is more difficult with bodied similarly to blown oil.
Due to the viscous nature of bodied and blown oils, they have a tendency to be more difficult to brushing, because they pull or feel sticky. It is more difficult to separate large molecules in viscous oil than it is smaller molecules in thin, unbodied oil.
Bodied oil much higher gloss than raw or unbodied oil, although similar to blown oil.
Bodied oil has good wetting and grinding properties. However, blown oil has better wetting properties. This is because the acid value is higher in bodied oil than in unbodied oil and typically even higher in blown oil.
Due to the large molecule size, paints incorporating bodied and blown oil have much better holdout or non-penetration than similar paints based on unbodied, thinner oils. The large sized molecules have much less tendency to penetrate a porous surface.
An unusual property of blown oils is their tendency in paint to tolerate large amounts of water. Blown oil is sometimes used to make water-sensitive paints less so, and to correct paint that sometimes increases viscosity due to its water sensitivity. The addition of a small amount of blown linseed oil often corrects this problem.
The increased polarity induced by the double bonds of blown oil gives it better moisture resistance properties, and better flowing and leveling properties than unbodied and some bodied oils.
There are advantages in blending oils to derive certain properties in paint. Paint formulators take advantage of these properties to achieve certain effects in paint, something informed artists could also do by better understanding these qualities.