Linseed oil, also known as flaxseed oil, is one of the most popular wood finishes in the world. Like other hand-rubbed oil finishes, linseed oil saturates deep into the wood grain to protect against scratches and changes in humidity. It is easy to care for, eco-friendly, and produces a satin finish that really brings out the color and grain of the wood underneath.
Raw linseed oil is the purest form, but is sometimes impractical as a furniture finish due to the extended drying times- it can take several weeks for each coat of raw linseed oil to cure. Boiled linseed oil is common as a wood finish, but contains some potentially hazardous drying compounds. Polymerized linseed oil is the best of both worlds: pure and non-toxic with quick drying times.
What is Raw Linseed Oil?
Raw Linseed Oil is the purest, most natural form of the oil. It is the kind used in nutritional supplements and as a conditioner for leather (usually mixed with something to quicken curing time). Raw linseed oil makes a great finish for wood products if applied in thin coats and given plenty of time to cure, but it can take 2-10 weeks to fully dry depending on the environment and the thickness of the application.
Many craftsmen migrate toward polymerized or boiled linseed oil or some other alternative like tung oil if they don't have the patience or space to allow for ample drying time.
What is Polymerized Linseed Oil?
Polymerized Linseed Oil is created by heating raw linseed oil in the absence of oxygen to about 300°C (572°F) over the course of several days. During this process, a polymerization reaction occurs, which increases the oil’s viscosity and decreases the drying time. Both polymerized linseed oil and raw linseed oil contain zero VOCs (volatile organic compounds).
What is Boiled Linseed Oil?
Boiled linseed oil, often written as BLO, is not as it sounds– there’s no heating or boiling involved (unlike polymerized linseed oil). The difference between raw linseed oil and boiled linseed oil is that there are drying agents (either petroleum-based or heavy metals) added in order to make it a more feasible option for finishing wood furniture.
Some of the petroleum-based siccative compounds that are added to linseed oil to decrease the drying time are naptha, mineral spirits, and dipropylene glycol monomethyl. Cobalt and manganese are the most common metal siccatives found in boiled linseed oil.
Because of the drying compounds added to boiled linseed oil, it is the least food-safe of the three mentioned and emits some VOCs while drying. Boiled linseed oils often carry the following warning as required by the state of California:
“Warning: This product contains a chemical known to cause cancer and birth defects or other reproductive harm.”
Note: If you’re buying furniture with a boiled linseed oil finish, we highly recommend you check with the craftsman to better understand the composition of the oil. Ask for the brand of the finish used and skim through the MSDS spec sheet to see what sort of chemicals and warnings are associated with the use.