SUMMARYThe problem of the conservation of ancient buildings is often the conservation of stone. The decayed stone is exfoliated and contains in general a relatively high quantity of gypsum.Fifty samples of deteriorated stones from buildings of the historic center of Florence were collected and examined. The gypsum percentage in the samples, determined by diffractometric analysis, varied between 0.5 and 10 per cent. The sulphur bacteria were in the range from 250 to 20,000 per g of stone. 111 spite of certain indications it is not possible to establish a direct correlation between gypsum content and number of sulphur bacteria.
Pietra Serena and Pietraforte, the two stones most frequently used in Florentine architecture, have been considered in this study. Through mineralogical-petrographical study and by determining the physical properties of these materials it has been possible to show clearly that the principal factors of their decay are seepage of water and changes of temperature. The simultaneous appearance of these two factors seems to induce the decay with different characteristics in the two stones. Gypsum, thought by some authors to be one of the main factors in the decay of stones, has been detected on materials already deteriorated; it is a product of decay apparently brought on by natural change, and may take part in a process which accelerates the decay.
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