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...

Boiled Linseed Oil

Linseed Oil comes in two types: Raw and Boiled.

You might think that "Raw Linseed Oil" is what we get straight from the plant. But, no. What you get when you buy "Raw Linseed Oil" is linseed oil that's been boiled. Wait! What? Then what is BOILED linseed oil? I'll get to that.

Raw Linseed Oil is made by heating linseed oil to near 300 °C for a few days in an anaerobic atmosphere. Under these conditions, the polyunsaturated fatty esters convert to conjugated dienes leading to crosslinking. Basically, you are getting the oil as close to drying as you can and still be liquid, just add oxygen and you are good to go. The resulting oil will dry faster, be more flexible, and less prone to yellowing than pure linseed oil.

It's important to note that the optimal heating point for linseed oil is very close to its flashpoint. If you feel like you want to try this, I'd recommend doing it outside and always use a controlled heat source (double boiler, lab heater, etc.).

And BOILED Linseed Oil?

Modern "Boiled Linseed Oil" is a combination of "Raw Linseed Oil" and metallic salt dryers. These driers are lead oxide (in pre-modern times), and either zirconium, cobalt, or manganese metallic salts. These salts act as a catalyst for the oxidation process and greatly reduces the drying time. Though, these additions make Boiled Linseed Oil mildly toxic, so gloves are recommended.

In the Middle Ages, boiled linseed oil was created by boiling linseed oil with lead oxide (litharge). The lead oxide forms lead "soaps" (lead oxide is alkaline) which promotes polymerization of the linseed oil and the heating further reduces its drying time.

Another advantage to the introduction of these toxic driers is that you don't have to raise the temperature of the oil as high, making it much safer to produce.

Due to its more rapid polymerization, Boiled Linseed Oil should be considered more of a fire hazard than the Raw or Pure varieties. This specifically applies to the safe handling of rags which could spontaneously combust if balled up and tossed in a trashcan of, say, sawdust.