What is scale?

“Scale is the precipitate that forms on surfaces in contact with water when the calcium hardness, pH, or total alkalinity levels are too high. Results from chemically unbalanced pool and spa water. Scale may appear as grey, white, or dark streaks on the plaster, fiberglass or vinyl. It may also appear as a hard crust or calcium deposit around the tile waterline.” IPSSA Basic Training Manual 2006, pg 188, Glossary