Put plenty of water in two pans, add a lot of plums in each. Then, start cooking over medium heat. If the plums start to foam up in the water with bubbles on their surface, throw away the water and put in fresh water. Repeat 2-3 times. If you let the water come to a boil the plum skins will tear, so if the water becomes hot to the touch, drain it off and add fresh water.