Thanks TT. I am in the process of letting these sit, it's just so much work Any thoughts on whether the freezer helps? Or should I just keep them at room temp?
The rubber has absorbed the solvents you placement them in and has swelled like a sponge might. Warm dry air will allow them to out gas volatile solvents. The heat and low humidity allow the xylol to depart, leaving the oil behind to keep the rubber soft. Neither of the liquids you soaked the boots in actually restore the rubber cross links. They will never be a strong as new again. But, they may still be serviceable.
My guess is that the formula you used was too heavy on the solvents, or that you just left them too long in the solvent.
Freezing them is NOT a viable alternative. Rubber actually expands with cold, the reverse of most materials. Further, if the absorbed solvents do freeze to a solid, they will contract, and the fighting expansion and contraction forces within the material will likely damage the links between the rubber molecules and further weaken the structural integrity of what you are trying to recover, might lead to cracking.
(High school physical science. Suspend a small weight from a fixed position with a rubber band. The weight should be just enough to slightly stretch the rubber so it is in slight tension. Apply some heat to the rubber band as with a hair dryer and the rubber will raise the weight in contraction. Apply ice to the rubber and the material will relax/expand and lower the weight. Most other materials will expand when heated and contract with cold.)
Cheers,