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Raw linseed oil is an extremely versatile oil pressed from flax seed. When oil is cold-pressed from flax seed, it is called flax seed oil and marketed as a nutritional supplement prized for its high concentrations of omega oils. The process used to extract the linseed oil includes the use of solvents, rendering the oil itself unfit for human consumption. When extracted as linseed oil, it is used for a number of different purposes, most often as a wood finish and preservative. “Boiled” linseed oil is the raw oil which has been further treated with chemicals or heat, or both; these processes thicken and darken the oil and reduce its drying time.
The curing period for the raw oil is relatively long, taking days or weeks, but eventually it dries to a rigid, flexible finish. It protects well against loss of moisture, and gives an aesthetically pleasing appearance to wood. It can be brushed, sprayed, or wiped onto the surface to be finished and allowed to soak into the wood. Excess oil is wiped off after 10 or 15 minutes and the surface left to dry, at least overnight and sometimes longer. Raw linseed oil dries to a soft finish but offers the surface little protection, if any, against scratches or dents; additionally, water easily penetrates the finish.
Linseed oil is frequently used in the manufacture of paints as the carrier, or the medium in which pigment is dissolved. Its slow drying time contributes to paint’s aesthetic appearance because in the time it takes to dry, it allows the paint to settle to a level finish, smoothing out brush marks. Artists also sometimes use this oil as a thinning agent for their oil paints; when used in this manner, it makes paints appear smoother and glossier.
For rifle stocks, raw linseed oil is the finish of choice. Only a small amount is necessary, and it is firmly rubbed into the wood grain with the heel of the palm. Typically, numerous coats are applied, with each permitted to dry before the next application.
Raw linseed oil is used in a number of other endeavors. Glazing putty is a mixture of chalk and linseed oil which is pressed into place between a pane of glass and a window frame, securing the glass. It dries over a period of days or weeks into a hard surface for painting. Raw linseed oil is also used in the manufacture of linoleum and as a drying oil for the surfaces of earthen floors. It is also a popular dressing for leather.
Although the flax seed from which raw linseed oil is pressed is noted for its antioxidant properties, these properties are not present in the oil. To the contrary, it oxidizes rapidly in an exothermic reaction, giving off heat, and poses a fire hazard when rags soaked in the oil are stored in a confined space. The oxidation process accelerates, generating more heat, which can lead to spontaneous combustion if the heat is not dissipated into the environment.