Solvents are added to oil paints to temporarily change the way they work and are designed to evaporate evenly and totally as the oil paint dries. Solvents are also used to dissolve resins, making mediums, cleaning up, and for cleaning brushes. It is essential to use solvents in a well-ventilated room and remember that they are flammable.Turpentine is the traditional solvent used in oil painting. It's based on tree resin and has a fast evaporation rate, releasing harmful vapors. It can also be absorbed through healthy skin. Use only artist quality turpentine as the industrial variety you find in hardware stores probably contains impurities; it should be colorless, like water. Also known as spirit of turpentine, oil of turpentine, genuine turpentine, English turpentine, distilled turpentine, double rectified turpentine, or simply turps.
Mineral spirits is based on petroleum and has a moderate evaporation rate, releasing harmful vapors. It is said not to absorbed through healthy skin, but it's sensible to take precautions, especially if you've sensitive skin. Mineral spirits is less expensive than turpentine. Some people react less to mineral spirits than to turpentine. Mineral spirits is a stronger solvent than odorless mineral spirits. Also known as white spirits.
Odorless mineral spirits is based on petroleum and has a moderate evaporation rate. It is said not to absorbed through healthy skin, but it's sensible to take precautions, especially if you've sensitive skin. Odorless mineral spirits is, unsurprisingly, more expensive than normal mineral spirits as it has had some of the harmful aromatic solvents removed. Brands include Turpenoid, Thin-ex, Gamsol.
Despite the more pleasant smell of citrus-based thinners, don't simply assume they don't give off any harmful vapors -- check what the product is made from. Look for something like Zest-It, which is made from food-grade citrus oil combined with a non-toxic, non-flammable solvent. (Of course, if you get migraines from oranges, this would not be a good thing to use!)
Alkyd-based Mediums: If you want to speed up the drying time of your oil painting, consider using an alkyd-based medium such as Liquin (W&N) or Galkyd (Gamlin).
Tip: Test the quality of a solvent by putting a little on a drop of paper and letting it evaporate. If it doesn't leave any resident, stain, or smell, it should be good enough for oil paintings.
Resins are used to increase the gloss of oil paint, reduce the color and drying time of a medium, and add body to drying oils. The most commonly used is a natural resin known as Damar, which should be mixed with turpentine as it will not thoroughly dissolve when mixed with mineral spirits. Damar can also be used as a varnish.