Seasoned paint professionals today use a solvent called mineral spirits as an alternative to turpentine for thinning paint and cleaning paint brushes. Mineral spirits is a mild, low volatility petroleum distillate that can be used for degreasing and cleaning machinery and, along with cutting oil, as a lubricant for the cutting and reaming of screw threads.
Mineral spirits is also known as Stoddard solvent, referring to its co-inventor, W. J. Stoddard. In 1928, Stoddard, along with Lloyd Jackson, developed this alternative solvent for use in his dry cleaning business. Stoddard solvent remained the most popular dry cleaning solvent in America right through the 1950’s and the term is used most frequently in this context. Outside the U.S. and Canada, mineral spirits is called white spirit although it is actually a clear liquid.
In the distillation process, chemicals such as aliphatic (paraffin) and alicyclic (cyclo-propane) hydrocarbons are combined with alkyl aromatic (benzene) hydrocarbons to produce the three most common types of mineral spirits. Type I has been subject to hydro-desulfurization where most of the sulfur is removed from the distillate. Type II is basically a solvent extraction where hydro-treatment removes most aromatic hydrocarbons. Type III is processed with hydrogenation, the addition of hydrogen. What all this means is that the atomic structure of the various hydrocarbons which form the distillate of each type of mineral spirits have been interchanged for different applications.
Type II odorless mineral spirits is one of the most common types of paint thinner. It has been treated to remove aromatic solvents, allowing it to be used in the manufacture of low-odor oil-based paint. Mineral spirits in its aromatic form is, among its other uses, a basic ingredient in asphalt production.
In addition to typing, mineral spirits are categorized as class 1, 2 or 3. Class 1 is a product with a high flashpoint or burn temperature and, therefore, a low volatility rating. Class 2 has a somewhat lower flashpoint temperature and class 3, the highest volatility with, obviously, the lowest flashpoint of the three classes.
Though mineral spirits is one of the safer alternative cleaners and solvents, it is described by the Occupational Health and Safety Administration (OSHA) as a mild dermal irritant. It also receives a class 3 flammability rating; once its relatively high flashpoint is reached, mineral spirits will burn just like any hydrocarbon-based solvent, thereby emitting dangerous toxins. Additionally, chronic toxic encephalopathy, a brain ailment often found in commercial and residential painters, has been blamed on prolonged exposure to mineral spirits. Therefore, although mineral spirits is a far safer alternative to many solvents in certain applications, it should, like any petroleum distillate, be stored and handled with proper care.