I've just had another thought about the use of anhydrous Epsom salts: the hydrated bark will be subjected to a much greater osmotic shock from the higher concentration that can be achieved using this. There could be something to this - more than one aspect to it as well... It becomes a question of what you think about using more energy roasting magnesium sulphate to get higher yields more quickly versus taking more time and possibly less energy to get... what? Of course, if you use a solar oven, the energy footprint doesn't matter! B)