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Pam
Allow me to jump in here--this is my first post on the Stroke of Genius forum. Pardon me for this being perhaps a little more lengthy than you would like it to be.
I would concur with pretty-much everything that John said. Certainly it is true that Gamblin's "Gamsol" seems to be the best odourless mineral spirit currently on the market. There is such a wide variety of mediums out there, too, that it can become quickly confusing. Plus, what I like to use, or what anyone else likes, might not end up being suitable for you. Thus, we do have to do at least some experimenting.
However, two important things I would keep in mind when delving into mediums: First; always obey the "fat-over-lean" principle. That is, one should never put a leaner (ie: less oil) layer over a fatter one. You can, however, put layers of equal oil-richness or lean-ness--as the case may be--one over the other, as you wish. Second; start out simply. That is, don't try seven different mediums on seven different pictures. Rather, try just straight linseed (plus-or-minus some spirits) for a number of pictures until you are more-or-less comfortable with that. Then, if you want to experiment with more "exotic" mediums, you will have a good basis for comparison. When using mediums, another very good rule-of-thumb is to use as little as possible. A little goes a long way.
Some pigments are naturally ground in more oil than are others. It is important to get to know which are more naturally lean and which are richer. For instance, most blacks and umbers already have a lot of oil in them, whereas most of the white pigments, particularly lead white, require far less oil to manufacture. It also takes more oil to grind a pigment than is necessary for painting with it afterwards. To compound things, many manufacturers use too much oil in all events, even for pigments that don't need so much. It is therefore a good idea to drain some of the oil out of your paints by putting the globs of paint you are about to use onto a piece of paper such as a business card first. A few minutes on the paper will soak out at least some of the oil. Then, when you add medium to them later, you will be adding as little as is possible in order to achieve your goals. Make sense?
Note: The reason the "fat-over-lean" principle is important to follow is because of the differential in drying times between a less-oily, and a more-oily paint layer. The more oil, the longer it takes to dry. If you put a leaner layer over a richer one, the lower one cannot dry properly because the one on top dries faster and impedes the drying of the lower layer. This leads to cracking and other potential harm to the picture. Oil paints dry (or cure) by a process of oxidation from the top down. They don't, in other words, dry throughout all at once. They need some "breathing" space. The Fat over Lean practice allows this to occur.
Hope this helps.
All the best,
Juan
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