Linseed oil and Tung oil are plant-based oils that cure by exposure to oxygen rather than by evaporation. Both of these oil finishes darken as they age.
Linseed oil (also known as flaxseed oil) is available in three formats. Raw linseed oil is not useful as a finish because it takes a long time to dry to the touch and can take weeks for each coat to cure. Boiled linseed oil (BLO) contains drying compounds (petroleum-based or heavy metals) and is not as environmentally or personally safe. Polymerized linseed oil (PLO), which is oil heated in the absence of oxygen, is pure, non-toxic, dries quickly, and produces a sheen finish with a slight yellow tint that is more visible on light-colored woods.
Tung oil is also available as Pure tung oil and Polymerized tung oil. Pure tung oil can be used as a finish—apply thin coats, wipe down each coat thoroughly, and sand between coats. At least 5 or 6 coats are recommended, allowing a week’s drying time between each coat. Similar to PLO, polymerized tung oil dries faster than the pure version. Some brands contain drying compounds. Polymerized tung oil tends to impart a warm, amber hue to wood surfaces and cures to a natural matte to low-satin finish. Tung oil dries somewhat harder than linseed oil and has better water resistance. Beware that some finishes labeled as “tung oil” are actually varnish thinned with mineral spirits.
Teak oil is not a plant-based oil. It is a generic name for any oil that contains a mixture of mineral oil, linseed oil, tung oil, or an oil and varnish blend.
For more information about tung oil read: Tung Oil: debunking the myths