The time-dependent change of those properties of a precipitate, e.g. loss of water, growth of crystals, @[email protected], decrease of the specific surface, loss of coprecipitated substances, which generally improve the filtering properties. The process of ageing is very often promoted by maintaining the precipitate and @[email protected] medium together at elevated temperatures for a period of time. The terms chemical, physical and thermal ageing may be used in cases in which some of the (usually combined) effects named above are to be emphasized specifically.