Here's a good thread from 2005:
http://www.thehighroad.org/showthread.php?t=125959
Cabinetman gave some good hints:
"The truth about Cosmo cleaning is that you're only dealing with the very surface of the stock when using any cleaning agent. If exposed long enough, cosmo will soak to the core of the stock. If you don't believe me, look at these old stocks which were ruined and not retrievable.
When cosmo gets that deep and you only clean the surface, eventualy it will precipitate right out as soon as the stock gets warm.
So, cleaning the surface with caustic stuff won't do very much. I won't use any lye-based materials. It's really not doing anything more than a good solvent like acetone or lacquer thinner will do and you're not using any water. Water is the enemy of any wood....especially old stocks.
So, heating the stocks in black plastic bags with kitty litter in the sun works as do 'heating tubes" or oven bakes as some have mentioned. The goal is to draw as much oil as possible to the surface. Since it's warmer outside than in, the colder oil will flow to the surface and get absorbed by the litter. Keep doing that as long as it comes out. Truthfully, you will NOT get it all out.
After you've heated the stock, clean the surface with acetone or lacquer thinner and let it sit until all of it is evaporated. The stock will appear much lighter. DON'T use mineral spirits. It's too oily and will only slow down the process.
When the stock is finally ready to refinish, you can dye back any color that's missing. Cosmo tends to neutralize the pigment if left long enough. After coloring with an alcohol based dye, the next step is important. I recommend that you shoot it with a coat of shellac. Spray shellac from a can is fine. Shellac is impervious to oil and will seal in any remaining cosmo. Hang the stock and shoot it twice with complete but light coats of shellac. Then sand just the finish lightly with some stearated (lubricated) sanding paper....220 works great. Finally go ahead and finish the stock with either oil or shellac depending on the original finish. When you're done, you can then burnish the finished surface with some 0000 steel wool dipped in oil (never use steel wool dry!) . That will produce a very authentic looking finish in either case unless you want a really shiney stock.
Never, ever, bath a stock in anything. It will absorb anything you bath it in and I've heard of many cases where gasoline or keroscene was used and the stocks become "water logged" with those items as they replace the cosmo.
Cosmo is a bugger but it can be mitigated with these suggestions and you can reproduce a great, authentic finish using modern dyes and finishes.
Rome"