What is linseed oil?

There are several vegetable oils that have the property of drying. They form a tough adhesive film either by themselves or with the assistance of an added ingredient. As explained in the previous post oil doesn't dry in the normal sense of the word, they go through a process of oxidation. Today we will discuss linseed oil as its the most common in use today, and tomorrow we will getting into some of the less common types.

Linseed Oil

Linseed oil is pressed from the ripe seeds of the flax plant (Linum usitatissimum) which is grown in all temperate or cold climates. Each regions seeds have their own characteristics and respectably their quality. The impurities are mostly due to region specific additives, usually deliberately. The production of linseed oil has been under research and development for hundreds of years, but much of the useful information gained from this production are credited to much more modern dates. Most changes to it are to improve the economic efficiency and its uniformity to suit the needs of its various uses. Because of the relative insignificance of quantities in artists materials scarcely will any of this research provide results directly applicable to our purpose.
The seed is crushed in an expeller. The use of extreme pressure and heat will give you the more economical results, while cold pressing with less pressure, especially from the artists viewpoint, gives higher quality. After heat pressing it further extraction can be made through the use of solvents, the solvent then allowed to evaporate and the remaining oil is then added to the heat pressed oil. This mixture can then be put through a purification process. Yet after these processes are complete, when compared to cold pressed, hot pressed is still inferior in quality though it yields higher quantities. The overwhelming demand sense its introduction sometime in the 1800s for the cheaper costs has made hot pressed linseed production increase, limiting the production of the cold pressed to only that of the highest in quality paints. Though this trend will seemingly end soon as more artists realize the mistake of using low quality supplies, and manufactures are forced to supply better quality at competitive prices to meet the demand for the removal of low quality supplies.
Raw Linseed is the product produced after the crude oil is placed in a tank and allowed to be slowly heated for a period of time. The idea being that as the oil heats the impurities contained within will fall to the bottom and be filtered out. While some seeds go through this process rather quickly, other varieties seem to drop impurities indefinitely. After the process is complete the oil is left with a dark yellow, or greenish, tint. While you can buy raw linseed its not really intended for artistic purposes as its generally considered inferior to further refined products. Cold pressed linseed properly aged and filtered will have a pale golden color and does not need any refinement to be used in the highest grades of artists colors.
Further refinement processes consist of the use of sulfuric acid and water to pull out any remaining impurities and improve the color to a pale straw or golden color. This process is typically done on very large scales, and the highest grades are able to be purified from any trace of the acids and water. Many variations of this process exist, some including a use of some bleaching agents and other chemicals. Though it is usually understood that these bleaching affects are inferior as the tend to revert back to the deeper tone over time. Much more modern versions are beginning to produce this affect without chemicals and are showing in an improvement of color stability.
The oil discussed above meets the requirements for use in materials for ordinary painting or paint making, but another class of refined oils is produced for the use in varnishes. The first requirement needed is it should be free from break. When linseed oil is heated to 500 degrees F a cloud of particles will form as if the structure is breaking, oils that remain clear are considered nonbreak oil. Another specification for varnish oil is that it have a low acid number, whereas for maximum dispersion of pigments, oils for grinding paint should have a reasonably high acid number. The common method in use to achieve this is the use of alkali, but some of the high qualities use a mechanical process.
When linseed oil is heated to 525-575 F and held there for several hours it under goes a internal change into what's known as stand oil. This change is molecular, a type of polymerization, nothing is added nor is anything removed. The process leaves the oil as a heavy, viscous material about the consistency of honey. With the use of turpentine it can be thinned out and used as normal linseed oil. The advantage being the pale color it has compared to the latter. While its no longer useful in grinding paint when its thinned its very useful in glazes and as a mixing medium. The most striking characteristic of stand oil is it leveling properties, that is, its ability to dry to a level surface without brush marks, and its ability to impart this to materials its mixed into. The production of this oils requires large scale industrial equipment, under high vacuum or in an atmosphere of carbon dioxide. Formerly, stand oil was produced in open kettles for up to 18 hours. However this process produced product that was generally burnt to a dark tone or ended up polymerizing beyond the point of useful. As far as the artist is concerned this type of oil has yet to become uniform within its production. Some are heavier than others, and require more thinner to get it to the correct consistency. More lab work is still to be done to determine where the highest qualities in stand oil sits, but if mixed and utilized properly little difference in the variety is apparent. There is also what is called blown or boiled oil. These products a much like stand oil, yet inferior in every way. They are produced by forcing air through the oil to cause oxidation until it thickens, and non of these products should be used in permanent painting.
An old process of refining artist oils is one involving the use of the sun. Dating back to the fourteenth century this process of sun refined or sun bleached oil consists of placing the oil and an equal amount of water, usually salt water, into a glass jar or a tray. The vessel is then cover loosely to allow air to enter, but keep dust and soot out. It is then allowed to be exposed to direct sun light for a period of a few weeks depending on the needed of the intended final product. Each day the vessel is given a good shake to make sure it stayed thoroughly mixed. If a small amount of clean sand is put in at the beginning it will also help the impurities to settle to the bottom to later be filtered out through paper or a fine cloth. This process leaves the oil in a similar state as blown oil, and should be for all intensive purposes be avoided in use if possible. However it is the only process that is executable entirely at home as no industrial machinery is needed to complete the process.

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