Hardness is caused by the presence of positively charged metal ions. Total hardness is measured as the equivalent of milligrams per litre of calcium carbonate. Temporary hardness only contains metal carbonates and bicarbonates, the most typical being calcium carbonate. Temporary hardness is precipitated from water when heated and is responsible for the furring of heating elements and metal pipes. Permanent hardness in drinking water consists of non-carbonate based metal salts, and does not precipitate out on heating.