From Gamblins website on solvents

"Solvents in history.
Painters have been using turpentine for hundreds of years because it was commonly available. Pure 100% odorless mineral spirits (OMS) is an innovation of the late 20th century so it is no wonder that many painters are just beginning to understand how safe and available OMS is.

Too bad artists of 50 years ago did not know that before they created huge canvases of oil colors that were diluted with turpentine. It is not surprising that these artists experimented with, then later switched to water based media. Turpentine, a known respiratory irritant, has a fast evaporation rate and a low permissible exposure level. It causes nausea and lightheadedness, dermatitis, kidney and bladder disease, and asthma. Turpentine is the only solvent commonly available to painters that is absorbed through healthy, unbroken skin. Turpentine is toxic.

Rembrandt used no painting mediums. He did not need mediums because hand made paints are thin and very fluid. Since the 19th century and the invention of the three roll mill, oil colors have been made into stiff pastes. Using these luscious pastes lead to the dominance of direct painting in the 20th century. Painting mediums are used only to increase fluidity of oil colors when using this technique. Extending oil colors with only solvent can lead to failure of the paint film. Adding more than a small amount of linseed oil can increase the tendency of oil paint films to wrinkle. Recently more painters are interested in creating unusual surfaces and optical effects so there is more interest in different kinds of painting mediums."