Linseed oil is a natural product extracted from flax seed. It has been used for centuries as a preservative and protectant for wood and other things such as masonry surfaces, rope, and as an additive for paint. The use of linseed oil as a wood treatment has been declining in recent years because of its one major drawback -- it can take weeks or months to fully cure. Few people are willing to wait that long to use their new deck or chest of drawers.
What is linseed oil?
Linseed oil is extracted by squeezing flax seeds, much like olive oil is pressed. This raw oil was the first form of the product available but is rarely used any longer because of the long drying time. It was discovered that boiling linseed oil changed the chemical make-up and caused it to dry faster. Modern "boiled" linseed oil is not cooked. Chemical dryers are added to the oil to speed the drying process.
Using linseed oil
Only boiled linseed oil should be used on woodwork today. It is a penetrating finish that seeps into the wood as it is applied and the oil hardens inside the fibres. Linseed oil provides only minimal protection against water and none at all against abrasion. It will never dry to a hard finish like a polyurethane so it is not a good choice for wood that will receive a lot of handling.
Linseed oil as a sealer
Though it is not useful today as a protective wood finish, linseed oil is useful to wood finishers as a sealer when using shellac or lacquer. When applying these finishes over a dye stain, the solvent or brush can sometimes pick up the colouring and change the look of the piece. The dye stain can be sealed by a light coat of boiled linseed oil that is allowed to dry before the lacquer or shellac is applied.
As wood ages, most species darken naturally to the deep wood tone that people find attractive in their furniture. Exposure to sun and air will take freshly cut wood and tan it to a deeper brown. A thin coat of boiled linseed oil applied to the wood will expedite the tanning process. Exposing the wood repeatedly allows it to reach the target hue.
Since linseed oil offers little protection for wood and takes so long to cure, it has largely been replaced by modified finishing products. Woodworkers who desire a soft, satin finish inviting to the touch, have switched to using oil-varnish blends. These offer the penetration of oil with the protective properties of the varnish. Oil-varnish blends can be built up on the surface to offer protection from water penetration and abrasion and they dry relatively quickly.