Petrographic features and thermal behaviour of the historically known 'pietra ollare' from the Italian Central Alps (Valchiavenna and Valmalenco)

Thermal and porosimetric properties of different lithotypes of 'pietra ollare' (magnesite-bearing talc-shists, chlorite-schists, tremolite-bearing chlorite-schists and serpentine-schists) from the Italian Central Alps (Valchiavenna and Valmalenco) have been investigated. Some cross-correlations are established among the main mineral-petrographic and textural features, thermal behaviour and historical utilization of these lithotypes for the production of stoves and cooking pots during the Middle Ages. All the analysed samples show (1) low total open porosity (0.73-2.85%) with meso- and micropores prevailing over macropores; (2) regular linear expansion up to c. 700 degrees C, good thermal stability up to 1200 degrees C and negligible weight loss (< 1%) to c. 500 degrees C; (3) high thermal expansion (5.57 x 10(-6) degrees C-1 < C (alpha 25-100 degrees C) < 8.89 x 10(-6) C-1). The results indicate that, under the thermal conditions typical of the traditional medieval 'open fire system' (T < 600 degrees C), the Italian 'pietra ollare' from the Central Alps was an excellent fire-resistant geomaterial, which did not undergo any significant transformation as a result of thermal shocks.