What is "scale" or "limescale"?
Hard water contains high levels of calcium and magnesium salts, an essential part of the human diet. But when water is heated in your coffee maker, calcium and magnesium break down and precipitate to form solid limescale, or scale. This affects the boilers, pumps and any interior parts that carry water. Areas where the water sits still for a while, such as the boiler, are worst affected.