How linseed oil is made

A pile of rags soaked in this oil can catch fire by itself, because it warms up as it hardens in air.

stations: 7
things upstream: 1
raw source: 1
step from the ground: 1

The short version

Flaxseed holds 38 to 45 percent oil. Mills often cook the seed to rupture its cells, then squeeze it in an expeller press, while food oil is cold pressed raw. Industrial plants soak pre-pressed seed in hexane to dissolve out the rest. Refining strips gums and free fatty acids, and heating or blowing air through it speeds its drying.

Clean the dried seed

Dried, ripened flaxseed is cleaned at the mill. Varieties grown for seed yield more of it than those grown for linen fiber, and flax needs less pretreatment than oilseeds that must be cracked or flaked.

Goes in
Flax
Cook the seed

Stacked cooking and drying vessels heat the seed until its cell walls rupture, which lets the oil escape more easily. High heat harms the flavor, so food-grade lines skip this.
Squeeze in an expeller

A screw press crushes the seed. Without cooking the press works harder, and processors often press twice. Friction alone can warm cold-pressed oil to 104 degrees Fahrenheit.
Dissolve out the rest

Industrial oil comes from heat-treated, pre-pressed seed soaked in hexane. Further processing drives the solvent out of both the oil and the meal. High-protein meal gets heated and pressed into cakes for livestock.
Refine the raw oil

Refining removes free fatty acids, gums and other stray material, and alkali treatment gives one common grade. Other lots are left in the sun to bleach or thicken for painters.
Heat or blow it

Heat sets off polymerization, thickening the oil and cutting its drying time. Boiled oil, despite the name, usually means raw oil and stand oil blended with metallic or petroleum-based drying agents.
Ship by grade

Oil leaves as raw, refined, boiled or blown. Raw dries slowest, and boiled and blown form the hardest films, which suits paints and printing inks.

End of the lineLinseed oil Next it goes into stained glass.

Worth repeating

The 1991 fire that badly damaged One Meridian Plaza in Philadelphia, killing three firefighters, was thought to be caused by rags soaked in linseed oil.
Linoleum, invented in 1860, binds wood dust and cork particles together with linseed oil.
Artists' oil paint is made by grinding raw pigment into linseed oil.