For anyone that gives a $#|^ about it...

The reason that you can have a fire start in pile of rags damp with linseed oil, or a linseed oil based paint is that linseed oil is typically at least 50 percent linolenic acid, which is shown in this diagram:

http://upload.wikimedia.org/wikipedi...nd_formula.PNG

The double carbon=carbon bonds between the 9th and 10th, 12th and 13th and 15th and 16th carbon atoms means that the 9th, 10th, 12th, 13th, 15th and 16th carbon atoms will only have ONE hydrogen atom bonded to them, whereas, the carbon atoms in the /\/\/\ part of the molecule will each have two hydrogen atoms bonded to them. By definition, that means that linolenic acid is an "unsaturated" fatty acid cuz you can fit more hydrogen atoms onto it. And, each of those double carbon=carbon bonds is an "unsaturated site".

There's a reaction that occurs in nature called "auto-oxidation", and what that means is that if an oxygen MOLECULE (O2) passes between two unsaturated sites in close proximity to one another, that oxygen molecule will spontaneously break apart to form two oxygen atoms, and each oxygen atom will form a C-O-C crosslink between those two unsaturated sites. It's that spotwelding together of unsaturated sites within a linseed oil molecule and between linseed oil molecules that causes linseed oil to transform from a liquid to a solid in about a month's time; or about 2 or 3 days in the case of "boiled" linseed oil, PROVIDED it's expose to oxygen.

Now, as it turns out, that auto-oxydation reaction is an exo-thermic one. Energy is released when an oxygen molecule breaks apart and becomes a pair of C-O-C crosslinks, and it's that accumulation of exothermic heat that can raise the temperature inside a pile of rags damp with linseed oil above the kindling temperature of cotton. When that happens, the pile of rags spontaneously starts burning.

PS#1: Window glazing putty is nothing more than linseed oil mixed with clay. And, just in the same way that a gallon of linseed oil based paint will form a solid film on it's surface during storage, the surface of glazing putty that's exposed to air during use will harden up by that same auto-oxidation reaction. Oil based paints will absorb up to 17 percent of their weight in oxygen molecules while you're painting. Once the oxygen has been absorbed into the paint, a film will form on that paint in storage. About the only way of avoiding the formation of that film would be to keep the paint cold (by putting it in your freezer) to prevent that auto-oxidation reaction from happening (or slow it down dramatically).

PS#2: If you ever hear some monkey on TV talking about "Omega Three" fatty acids, all that means is that the THIRD carbon atom from the end of the fatty acid has a double carbon=carbon bond. It's just a different naming system. Chemists start counting the carbon atoms from the carboxyl group (which is why the furthest carbon atom to the left is labeled "1") while nutritionists and food scientists start counting the carbon atoms from the other end (which is why the furthest carbon atom to the right is labeled "omega" or "w". So, looking at the diagram of linolenic acid, the third carbon atom from the end marked with an "omega" or "w" symbol has a double carbon=carbon bond on it, so linolenic acid is an "omega-3" fatty acid. It's also an omega-6 fatty acid and an omega-9 fatty acid as well. So, what seemed to be so highly technical turns out to be pretty simple.

Obviously, I'm not very busy right now...