Analysis of change in the decorative properties of granites under thermal exposure

Abstract

We investigated samples extracted in Ukraine from nine granite deposits experimentally. We performed tests of granite samples at high temperatures of 200, 400, 600, and 900 °C. All presented granites showed a change in the color of surface at a temperature of 200 °C and higher. The behavior of granites under heating depends on their mineral composition, structure and texture. Surfaces of all samples became lighter and some granite samples lost saturation of their color. The largest increase in L component (image of stone samples brightens) of the CIELab color system occurred on Cardinal Gray and Carpazi granite samples of natural stone under heating to 900 °C. The increase made up 42 and 44 %, respectively. The smallest increase in L component was on Gray Ukraine and L granites under heating to 900 °C. It made up 4 and 8.5 %, respectively. The effect of temperature was less visible on red granite, since both fresh and heated samples had a similar red color. Flower of Ukraine granite samples acquired a uniform violet-pink color at the temperature of 900 °C due to the content of apatite and fluorite. Red spots appeared on gray granites under heating. The red spots located mainly around mica and other minerals, which were rich in Fe. Reddish-brown spots appeared at the temperature of 200 °C on green Verde Oliva granite. Red spots occupied 67 % of the sample area under heating to 900 °C. We observed the greatest color change on granites, where the phase transition of dark-colored minerals (biotite and pyroxene) into polymorphic minerals took place. This gave granite samples a light color, as the minerals changed color from black to gray or white. Quartz provided the shades of white. White microcracks appeared under heating of quartz. Noticeable aesthetic damage appears at temperatures from 200 to 400 °C at the surfaces of natural stone samples. Thus, one can consider a fire with temperatures lower than this threshold as a “safe” fire in terms of aesthetic damage, if we take into consideration the heating coefficient of fire only and exclude ash and gases.