In all of the artist materials books out there that I have heard of and some that Ive had the pleasure of reading thru state that mixing Clove oil into your oil paints will keep them moist and usable. This usually is followed by the warning that if you mix too much in your paints will never dry. Well, I'm hear to tell you that from what I've heard and recently tested you do not need to mix the oil in, the fumes are what does the job. So if you look in the above images you will see my well used EasyL palette with a palette dipper cup that i glued into the corner (back when you could see the wood grain). well that orangeish chunk you see to the right is a small piece of sponge that was wedged in between the cup and the box wall then soaked with clove oil. This little soaked sponge chunk has been keeping fast dryers like Burnt Sienna skin free. I also have a small container that i drilled holes into the lid, placed a sponge inside and basically did the same thing. it works great! I should say that the container or paint box should be as sealed as possible to make it work as good as possible. Please, let me know how it works for you.
In your search for Clove Oil, you will and can find it at your local Pharmacy for a steep price in a small container. One of Clove oils most useful uses is as a temporary reliever of toothache pain. You can get it on Ebay for about $5 for a 4oz like the one pictured above. Good Luck!