Boiled linseed oil is linseed oil — which, like flax oil, comes from flax seeds, though it is treated differently and is not edible — that has been boiled until it polymerizes and oxidizes, causing it to dry quicker. When boiled linseed oil is named on an ingredients list, it may just be linseed oil, but it more often has additives including petro-chemicals and metal-based dryers to accelerate the drying process to a satisfactory level. Boiled linseed oil is sometimes heat-treated linseed oil, which dries slowly and is very thick, making it difficult to work with. The two main uses of linseed oil are as paint binders and wood finishes. While it is cheaper than similar oil products, it takes longer to dry and may not last as long.
Without any additives, boiled linseed oil is just that: linseed oil that has been boiled. This causes the linseed oil molecules to polymerize — or become a long and repeating chain — and oxidize, introducing oxygen into the oil. This makes the oil thicker, meaning it can be used for more applications, and makes it dry quicker. Regular linseed oil takes many hours to dry, making boiling is a necessity for many projects.
Most linseed oil listed in products is either loaded with additives or heat-treated rather than boiled. Additives are more common, because this makes the linseed oil more useful. By adding petro-chemicals and metal-based dryer catalysts, linseed oil will dry even quicker. This means linseed oil will dry almost as fast as other commercial finishes.
Heat-treated linseed oil is not very useful as a finish. When linseed oil is heat-treated, and not boiled, it goes through the same polymerization process, but it does not oxidize. This means the oil becomes thicker than boiled oil but will take much longer to dry.
Boiled linseed oil is found most often as either a paint binder or a wood finish. As a paint binder, linseed oil binds the pigment to the paint, especially in oil media. When used as a wood finish, linseed oil is able to coat and protect wood from weathering and other damage.
While this type of linseed oil is useful, it is considered lower in quality than commercial binders and finishes. The largest reason is because the drying time is much longer than other finishes, making it useless in commercial projects that require finishes to dry quickly. Linseed oil also is passed over is because it does not offer the same protection as other commercial products; it is, however, about half as expensive.