Boiled linseed oil isn’t raw linseed oil that has been boiled. You can’t make boiled linseed oil simply by heating it.
Boiled linseed oil, which is sometimes abbreviated “BLO,” is raw linseed oil with metallic driers added. These driers act as catalysts to speed the introduction of oxygen into the oil, which causes the crosslinking and thus the curing to go faster. So boiled linseed oil dries or cures much faster than raw linseed oil.
In fact, raw linseed oil cures so slowly (it takes weeks even with all the excess wiped off the wood) that there’s really no legitimate use that I know of for raw linseed oil in interior wood finishing. Boiled linseed oil cures overnight in a warm room with the excess wiped off.
The term “boiled” probably comes from a previous time when the linseed oil was heated to incorporate the lead driers. Lead is no longer used as a drier in common linseed oils, only in some artist’s and other specialty oil paints. But the term “boiled” continues to be used.
Instead of lead, manufacturers now use synthetic driers that come in liquid form. An example of a widely available consumer drier is “Japan” drier.
