Linseed oil is made from the pressing of the dried ripe seeds of the flax plant (Linum usitatissimum, Linaceae) which gives flaxseed oil. This oil is then exposed to a solvent extraction process to produce linseed oil.
Pure linseed oil is a non-drying oil, and therefore is not a practical finish for wood. This type of oil may be available in health-food stores, but is not intended for wood.
Boiled linseed oil is not actually boiled, but has been processed (by oxidation, or adding metallic thinners) to speed its drying time. The remainder of this page refers to "boiled" linseed oil.
Linseed oil seeps into the grain of the wood, giving it a perpetual wet look that highly accentualizes the grain of the wood, commonly referred to as "making the grain pop". Because of this, the color of the wood is slightly yellowed which darkens slightly with age.
Linseed oil has long been used as a preservative for wood, hemp (natural) rope, masonry, and as an additive in oil paints.
As long as the surface bearing the linseed oil has not been damaged, linseed oil provides the following benefits:
- Water resistant. Water left on the surface may penetrate given enough time
- Flexible. Oil finishes continue to protect as the wood expands and contract.
- Accentuates the texture and grain of the wood
- Easy to apply (rub on, rub off)
- Very forgiving during application
- Easy to re-apply if the original finish becomes worn or damaged
- Linseed oil takes time to dry. Boiled linseed oil dries much faster.
- Lack of any UV inhibitors
- Linseed oil can support the growth of mildew
- On surfaces where abrasion will be frequently encountered, linseed oil may not harden sufficiently, requiring frequent repair
Pure linseed oil can take up to a week for each coat to dry, and is not considered a practical product for woodworker use. The only practical use for pure linseed oil is on products that are to come into contact with food products, and even then you should be properly warned that product labelling is somewhat deceptive, and may not be truly pure (or safe to eat).
One of the main complaints of linseed oil is the time it requires to properly cure once applied. Vendors have therefore produced a product called boiled linseed oil that has been through an oxidation process, or been given metallic additives, to partially complete the molecular process to speed the drying process.
Polymerized linseed oil has been heat-treated, and does not contain any additives. HOWEVER, vendors of polymerized linseed usually use additives to improve the performance of the product, so careful reading of lables and monographs must be done if you are looking for a food-safe product.
Polymerized linseed oils dry faster, harder and are more durable than "pure" linseed oils
As with any traditional product that has been used for many years, there will be many different opinions on how to use the product. What I will present here is the most simple and straightforward technique, and I will identify optional steps as such.
For full instructions on how to apply linseed oil, please refer to the technique for tung oil, as the application of these two products are virtually identical.
Be careful with the handling and disposal of the rags used to apply linseed oil. The oil itself is not a problem, however the solvents used to thin the linseed oil are highly flammable and combustible. Allow rags to thoroughly dry on a non-flammable surface (such as a concrete block), or washed, or soaked with water before placing in the garbage. Solvents can generate heat through an exothermic reaction with the air (oxidation), and this reaction accelerates as the rags get hotter, and this has been known to start unintended fires.
Repair instructions for linseed oil projects are identical to the technique for tung oil.
Sandpaper and elbow grease is your only option. The linseed oil penetrates the surface, which is usually considered a benefit. If you absolutely must remove the linseed oil, you must remove the surface of the wood that it was penetrated into.Lee Valley Tools
Sutherland Welles Ltd®
Please see the Precautions section above for appropriate disposal of rags used to apply linseed oil.
Linseed oil is essentially an oil product, and can be cleaned from hands using soap/detergent and warm water. Thinner (such as turpentine) can be used to remove linseed oil from surfaces that can not be properly washed.Chemistry of oils during the drying/curing process