The same oil that binds oil paint can set a pile of rags alight
Linseed oil hardens because it reacts with oxygen, which made it the backbone of oil painting, window putty and linoleum. That reaction also gives off heat. Leave oil-soaked rags bunched up and they can ignite by themselves; such rags are thought to have started the 1991 One Meridian Plaza fire in Philadelphia.
Pressed from the dried, ripe seeds of flax, Linum usitatissimum, the oil ranges from colourless to yellowish. What sets it apart is its high share of alpha-linolenic acid, a triply unsaturated fatty acid that typically makes up more than half its fats. Those unsaturated chains link together when exposed to air, a process called autoxidation, turning a liquid into a solid that is stiff yet not brittle and that repels water. Paints and putties based on it are kept in airtight tins to stop them drying early. Crumpled rags present a huge surface for that heat-producing reaction, and in the Philadelphia blaze three firefighters died.
Its drying habit made it indispensable. It is the carrier in oil paint and, added as a medium, makes colours more fluid and glossy; its arrival was a real advance in painting technique. Glaziers' putty, a paste of chalk and linseed oil, sets over a few weeks and can then be painted. Frederick Walton invented linoleum in 1860 by binding cork and wood dust with it, and lino covered floors from the 1870s until vinyl displaced it in the 1970s. Since the 1990s it has regained ground as the greener option, and it lent its name to the linocut print.
As a wood finish it has real weaknesses. It scratches easily, lets water through within minutes and does little against vapour, and studies suggest it never fully cures, often darkening or even turning black. Processing changes its behaviour: heating it near 300 degrees Celsius without air yields thick stand oil that dries more elastic and yellows less, while medieval boiled oil was cooked with lead oxide to speed hardening. Raw oil is used on cricket bats to add grip.
Edible flaxseed oil is cold-pressed without solvents and must be kept chilled, as it turns rancid quickly. Some European kitchens pair it with potatoes and quark, and the US FDA has granted high-ALA flaxseed oil generally recognised as safe status.
Source: Linseed oil
