For years I've been using this stuff to clean my brushes. I blush to admit that more than once I have let my wet brushes sit longer than I should and have had to work doubly hard to bring them back to life. This product is a Godsend to absent-minded or just plain lazy artists. You can also leave it on as a final shaping aid to re-point those fine sables. According to "The Masters" Brush Cleaner and Preserver website: "Just wet the brush and work up a lather on the hard cake. Even old, hardened oil paint brushes can be restored to their original snap and luster. Brush Cleaner and Preserver works on most acrylics and watercolors, too. It gives off no harmful fumes or odors, and it's safe for use in the classroom or studio."
I'm not so sure about their claim that it can remove "old, hardened paint," but it certainly works on brushes that have been drying for a day or two. The photo above is a container that I have had for over 15 years (!) so don't let the price stop you since it seems to last forever. It also comes in smaller 2 5/8 oz cakes for about 5 bucks.