All About Linseed Oil

Linseed oil is obtained but pressing the seeds of flax plants (Linum usitatissimum). Since we already have a ton of flax plants around to make linen, it's nice to be able to do something with the seeds. The oil is edible and has been used for centuries in Europe as a food additive and a nutritional supplement though it's usually called flaxseed oil in this context.

Linseed oil is a triglyceride that contains an unusually large amount of α-linolenic acid, which makes linseed oil is particularly susceptible to polymerization reactions upon exposure to oxygen in the air. This polymerization (aka "drying"), results in the rigidification of the material. And while rigid, it's not brittle, it retains some flexibility. And that's why it makes a good finish (among other uses). In fact, linseed oil has about a billion other uses outside of a wood finish.

NOTE: the drying process is exothermic therefore rags soaked with linseed oil and dumped in a pile are a fire hazard because they provide a large surface area for rapid oxidation of the oil. The oxidation will accelerate as the temperature of the rags increases. The spiral continues until all the oil has oxidized or you exceed the temperature dissipation rate of said rags, in which case you get a cheery blaze. In your shop.

Spread out your rags to dry (on a non-flammable surface), or place them in an air-tight container, so soak in water. Make sure whatever container you store linseed oil in is airtight. The surface area of the oil in a jar say is way too small to be a fire hazard, but the oil will polymerize and all you'll be left with is a chunk of rubbery...stuff. Or at least a layer of that on top of your remaining oil (that remaining oil is still good, BTW).

Linseed oil is notorious for taking a long time to "dry", days sometimes depending on the conditions. So, how do we fix that? Read on...