Air pollution in recent years has accelerated the decay of different limestones and marbles. The cleaning methods and technologies used for deteriorated stone surfaces depend on the type, quality, condition, degree of contamination and the structure of the stone. According to Hungarian experiments, the chemical cleaning of limestone, in particular that of our soft limestone, causes harmful salt efflorescence. The high pressure lance, using cold and warm water -- if necessary, steam, too -- can be used with sufficient result. Irreparable losses may be detected as a result of dry and wet abrasive blasting of Hungarian soft limestone. A mild alkaline gelatinous cleaner does not cause any harm to limestone and marble statues, ornamental parts of buildings, etc. In this way the gypsum and other impurities can be removed simply by washing. See also 21-950.