Linseed oil is extracted from the seeds of the flax plant by exposing them to steam and passing them through a crusher. The oil is filtered to remove any plant waste from the crush and is known as "raw linseed oil". The "boiled linseed oil" of common use is this raw oil combined with a metallic salt as a drier. Usually this drier is cobalt octoate.
Raw linseed oil can be further treated to take advantage of its natural drying properties and prepare it for varnish making. The oil can be washed with water to improve clarity. The "foots" which are organic and inorganic phosphates are removed by heat or the addition of a mild acid or alkali solution to "break" the oil. This produces very clear oil with minimal tendency to react with ambient humidity. The oil can now be used for varnish.
Linseed Oil is a drying oil, that is, it will convert under the proper conditions from a liquid to a solid. In its raw and processed forms the oil is soluble in mineral spirits, turpentine and similar solvents. As a solid it is a very insoluble substance. It is impervious to water, resists ordinary solvents, mild acids, and mild alkalis. The polymerization of the oil is the process of curing which moves the oil from a liquid to a solid phase.
When linseed oil is exposed to heat and or atmospheric oxygen a thickening occurs and it eventually becomes a solid, rubbery mass. If the exposure is to thin layers the result is a clear hard solid. The process of oxidization and subsequent polymerization forms aggregates of higher molecular weight. The final product is a solid, homogenous lattice-work of three dimensional molecules of exceptional stability.
Curing Linseed Oil
When linseed oil is exposed to the proper conditions of heat and available oxygen it begins to absorb oxygen at a rate that builds to a peak and then rapidly diminishes. The early rapid absorption of oxygen is known as the period of induction.