[EDIT: the dishwashing detergent shown in the photo was fine for the synthetic brushes I've been using lately but not the best thing for sables, badgers, natural bristles, etc. - see the comments to this post for other recommendations. There's a lot of bar soaps and other products out there for oil paints that are kinder on brushes.]
The condition of being "Solvent Free" is not to be confused with the rarer condition of "Solvency", which is another thing entirely, especially in the representational art community.
Going solvent free means getting rid of the solvents in your oil painting working methods. Common paint solvents such as odorless mineral spirits (OMS), turpentine, etc. can be dangerous to your health, especially when you work with it every day in close non-ventilated quarters. Several of my artist friends have also developed allergies to the stuff.
A few months ago I was at an open studio and watched a young man paint a portrait using a jar of walnut oil to clean his brushes instead of OMS. His work looked fantastic - lush paint, expressive strokes, fresh and clean. I went back to my studio and decided to research this. Apparently you can use either safflower oil or walnut oil to clean your brushes - don't use linseed oil because of its flash point. Linseed oil soaked rags are a cause of fires since they can accidentally spontaneously combust. Don't leave oil soaked rags sitting around, dispose of them right away.
Watch this, artists!
Did you know that human beings can spontaneously combust, by the way?
A half hour after I finished writing the above, but before I posted this, I read a story about human spontaneous combustion in the Sept. 7-8 weekend edition of the Wall Street Journal. Apparently, a case of spontaneous combustion took place in Oklahoma in February of this year, so it's nothing to joke about or to sneeze at. Naturally, being the alarmist that I am, I researched this a little further. One theory postulates that energy patterns, the Kundalini, that run up and down your spine become misaligned, triggering a temperature spike and you ignite. ( I'm not so sure about this theory.) Another theory is that there are energy anomalies in the earth itself, since suspected cases of human spontaneous combustion are geographically clustered. Hmm. The real estate agents never tell you that, do they?
Back to our story. I wanted to try painting without solvents because I was travelling to a far away land with zero hope of getting solvents when I landed there.Meanwhile, I really like the way my paintings are turning out using safflower oil and dishwashing detergent as brush cleaners. I use small amounts of Oleogel as my medium lately, but I also use other products as mediums. The brushes are incredibly supple and responsive to varying degrees of pressure. I wipe off as much safflower oil as I can before reusing the brush in another color and it usually works, but if I'm going from dark colors to light I change brushes. At the end of the day I wash them in the soap and shape them.
Many thanks to the marvelous Master of Maastricht, Scott Bartner for his encouragement to pursue anti-solventation, though he has much more to say about this subject than I've covered here.