Linseed oil is pressed from the seeds of the flax plant. Flax has been cultivated since the Neolithic period for fiber and for the oil in its seed. The Egyptians used it to preserve wooden sarcophagi. Medieval painters mixed it with pigments to make oil paint. Shipwrights used it to preserve wooden planks above and below the waterline. It is the oldest wood finish in continuous use.
The reason it works is chemistry. Linseed oil is a drying oil -- it polymerizes when exposed to oxygen, hardening from a liquid into a flexible, moisture-resistant solid. Unlike varnish, which sits on top of the wood as a hard film, linseed oil penetrates into the wood cells and hardens there, protecting from inside the material rather than on top of it. A surface treated with linseed oil does not crack or peel; it wears gradually, becoming thinner with use, and can be refreshed with another coat at any time.
The practical distinction between raw and boiled linseed oil matters. Raw linseed oil is exactly what it says: pressed oil with nothing added. It dries slowly -- anywhere from three days to two weeks depending on temperature and humidity. This slowness is an advantage for tool handles and outdoor wood, where deep penetration matters more than rapid turnaround.
Boiled linseed oil (BLO) has metallic driers added -- compounds of cobalt, manganese, or zinc dissolved in petroleum distillate -- that catalyze the polymerization reaction and reduce drying time to 24 to 72 hours. The petroleum-based driers are the only non-natural component of BLO. They are effective. For most workshop applications where the goal is protecting a tool handle or finishing a workbench, BLO is the practical choice.
Polymerized linseed oil, as made by Tried and True, avoids the chemical driers entirely by heating the raw oil in a controlled environment until it partially polymerizes before application. The result is a thick oil that dries in 24 hours, requires no chemical additives, and produces a finish appropriate even for cutting boards and food-contact surfaces. It costs more than BLO and is worth it for applications where the chemical content matters.
Application is simple. Wipe on with a clean cloth, let penetrate for fifteen minutes, wipe off the excess. Repeat until the surface stops absorbing. For raw linseed on a dry tool handle, this might take three to five coats applied over a week. For a finished furniture surface being refreshed: one coat is often sufficient. The cloth used to apply linseed oil must be spread flat or submerged in water before disposal -- linseed-soaked rags generate heat as they cure and can ignite if bunched. This is not hypothetical.