A Review of Rehydroxylation in Fired‐Clay Ceramics

Abstract

Understanding the hygral reactivity of ceramic materials is essential to understanding the long‐term behavior of building materials and of archeological pottery, especially in relation to dating. We explore the literature on the expansion of fired‐clay ceramics, reviewing strain and mass measurements at the bulk scale, and rehydroxylation (RHX) and dehydroxylation (DHX) processes in clay minerals at the molecular level. We present open questions on the nature of ceramic rehydroxylation and its apparent adherence to a sub‐diffusive kinetic (time)1/4 power law. We discuss measurement of the RHX process through mass gain in relation to a proposed new dating method for archaeological ceramics.

Rehydroxylation (RHX) dating of archaeological pottery

“…The samian ware is composed of quartz (around 20%), and Ca feldspar (around 70%), with minor hematite. We do not identify directly the amorphous clay remnants, the components in which the RHX reaction is likely to take place (Hamilton & Hall 2012). …”

“…• C (for a recent review, see Hamilton & Hall 2012). It appears that this is largely or partly a reversal of the dehydroxylation reaction which occurs in ceramic firing.…”

Romanesque and Gothic bricks from church in Pác – estimation of the firing temperature

“…Th e second reason can be the liberation of OH -which could insert itself into dehydroxylated clay during rehydroxylation. For example, a measurable rehydroxylation was registered in 19 th century bricks [8]. Neither the endothermic minimum on the DTA curve (Fig.…”

“…700 °C) for suffi cient long time, the dehydroxylation was completed. However, dehydroxylation can be also appeared in this case as a result of the escape of rehydroxylated water (originated from hydroxyl groups incorporated into dehydroxylated ceramics) if the historical ceramic body was in a contact with moisture for long time [8]. Th e products of dehydroxylation can not be considered as irreversible in a span of hundreds or thousands years between the fi ring and present analyses.…”

“…3), which could be ascribed to the intensive second dehydroxylation, were registered with these analyses. Rehydroxylation is very slow process [8] and ~300 years, when the bricks were in the open air, was probably not suffi ciently long time for the binding of greater amount of OH -into dehydroxylated clay structure. Aft er that period, the new plaster and pavement were laid onto the bricks, therefore the bricks were protected from atmospheric moisture up to now.…”