Barium carbonate, also slightly soluble in water, is able to eliminate gypsum scumming very effectively. The dissolved barium carbonate reacts with calicum sulphate becoming calcium carbonate and barium sulfate. Barium sulfate and calcium carbonate are much more relatively insoluble in water, preventing the calcium from migrating with the water to the surface of the clay. After the barium sulfate comes out of solution, more barium carbonate dissolves. Despite its desirable qualities, barium carbonate is poisonous.
Soap scum is the bane of the soap user with hard water.
Soaps are sodium oleate and sodium stearates. When washing with water hard with calcium salts, these calcium oleate and stearates form a scum on sinks, clothes, and washing machines. Because they are immiscible with water, these ornery compounds require more than simple rinsing to remove them. Mold soap makes use of this reaction. Soap is lathered onto the plastic mold, building up a layer of immiscible calcium soaps on the surface.