Linseed oil and boiled linseed oil can be used for a wide variety of applications, but they should not be seen as interchangeable. Commonly known as flax seed oil, linseed oil is a drying oil that, over time, will dry into a solid, hard form. Boiled linseed oil contains driers to speed up the process, and gives more hardness to the dried material.
Linseed oil is known for its slow drying time, absorption into wood to help prevent cracking, shrinkage, and checking, and its ability to help repel water. It will yellow over time, and helps seal pours, but is still prone to mildew. Cold-pressed raw linseed oil (without solvent) is also an edible oil that is commonly used as a nutritional supplement.
Boiled linseed oil is also in the category of modified linseed oils. It starts with the same oil, but driers are added, then heated to accelerate the drying process, which makes it a toxic material. It is commonly used as a wood sealer, but in many cases it's used in conjunction with other additives for specific applications.
Raw linseed oil, even food grade, is used as a leather softening solution. Many people have made blends with other chemicals, oils, or solvents to optimize the linseed oil for certain applications. Also, it is not recommended to use boiled linseed oil for leather work.
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