When specifying cast copper alloys, for instance for pump or valve bodies, there is sometimes excessive concern to prevent dezincification by not allowing any alloys containing zinc. This is because an alloy known as manganese bronze is, in fact, a high tensile brass, and it is very susceptible to dezincification in seawater. Gunmetals are alloys of copper, zinc, tin and lead that have given excellent service in both fresh water and seawater for many years without de-alloying. This is because they all contain less than 10% zinc and this is not sufficient to allow dezincification to occur, even in seawater. Zinc-free alloys like phosphor bronze are difficult to make pressure-tight as castings, while the lead in gunmetals ensures good pressure-tight castings.