No / To obtain accurate results in the RHX dating of ceramics, it is essential that the RHX measurements are continued until the rate of mass gain is constant with (time)1/4. In this paper, we discuss how the initial stages of mass gain are affected by the specific surface area (SSA) of the ceramic material. The paper provides guidance on experimental protocols to avoid dating results being distorted by relatively early-time mass gain data.

Accurate prediction of long-term moisture expansion in fired clay ceramics requires finding a relationship between the reactivity of a ceramic material with moisture and time. Recently a (time)1/4 law has been proposed which provides a precise relationship between moisture expansion and mass gain in fired clay ceramics and time. However, mass gain studies rather than expansive strain studies provide a more accurate and fundamental measure of the reactivity of fired clay ceramics with moisture. The possibility of using the (time)1/4 law to describe rate of mass gain and consequently to predict moisture expansion in fired clay ceramics with time requires study of the effect of chemical composition and firing temperature on the linear dependence of mass gain on (time)1/4. Pure kaolinite as well as kaolinite mixed with controlled additions of alkali and alkaline-earth metals were employed in this study. These materials were fired at temperatures between 800°C and 1200°C. Mass gain due to the chemical combination of the fired materials with moisture was measured using a recording microbalance under tightly controlled environmental conditions of temperature and relative humidity. The mass gain results show that the (time)1/4 law can be used to obtain an accurate linear relationship between long-term mass gain and time at almost all firing temperatures and at all different compositions. The presence of alkali metals was found to strongly affect the chemical combination of fired clay ceramics with moisture and hence affect the rate of mass gain. On the other hand, alkaline earth metals were found to produce similar reaction kinetics to kaolinite alone. BET surface area and X-ray diffraction results confirm that there is a correlation between the reactivity with moisture and both the specific surface area and crystallinity of fired clay ceramics.