A little heat will loosen just about anything. Use a medium-size welding tip on an acetyline torch, get the screw head medium-red, and loosen it before it cools off. The heat stretches the thread a little, and relieves the tightening torque, plus it dislodges any caked-up crud around the threads. Letting the screw cool naturally, insead of dumping it in water or something, won't affect the strength, or make it brittle if you decide to reuse it.
Jerry
