What are the differences between linseed oil and stand oil? How do these differences affect the properties of paint? The key differences result from two crucial physical properties of drying oils: the degree of polymerization and the acid value of the oil. These two properties are affected by the treatment of oil—typically heat—that changes one or both of them. Heat treatment of oil makes what is called "bodied" oil, which is the more accurate term for what many call "stand oil."
|Cold-Pressed Linseed Oil
|Refined Linseed Oil
Raw and Refined Oil
Raw and refined linseed oil has good brushing properties. Paint made of essentially raw or refined linseed oil has a short, buttery consistency that lends itself to easy brushing. However, the flow of such paints is poor, and it leaves brush marks. Raw linseed oil has an acid value of 4–7, while alkali refined linseed oil is less than one. Exceptions to this are specially refined oils with high acid values to obtain better pigment-wetting properties.
The outstanding property of linseed oil is its excellent durability. It is used more extensively in the paint than any other drying oil.
Bodied oil is polymerized oil made by heating refined linseed oil at a high temperature for a certain amount of time. Where color and low acid numbers are essential, it is heated in a vacuum or under a blanket of inert gas. Bodied linseed oil has an acid value in a wide range, depending on how heat is treated.
Blown linseed oil is partly oxidized 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.
Boiled oils (also called "drying oils") are made by 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 affect the complete solution.
Properties of Bodied Oils
Bodied oil has better color retention than 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 when 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 oxidation during the blowing process and poor color retention due to further oxidation while the film is drying.
Flowing and Leveling
Bodied oil has excellent flowing and leveling properties but is not as great as blown oil. Brushing is more difficult with bodied similar to blown oil.
Due to the viscous nature of bodied and blown oils, they tend to be more difficult to brush because they pull or feel sticky. It is more challenging to separate large molecules in viscous oil than smaller molecules in thin, unbodied oil.
Bodied oil has a much higher gloss than raw or unbodied oil, 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 painting 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. Adding 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 to blending oils to derive specific properties in paint. Paint formulators take advantage of these properties to achieve specific effects in paint, something informed artists could do by better understanding these qualities.
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