Fired Clay Ceramics Research Articles

Gravimetric and spectroscopic studies of the chemical combination of moisture by as-fired and reheated terracotta

The application of microbalance measurements to investigate the chemical recombination of moisture with fired clay ceramics is demonstrated. The kinetics of mass gain at constant temperature and relative humidity (RH) are studied for terracotta fired at temperatures between 800 °C and 1200 °C. The experimental results show that mass gain proceeds in two stages. The second stage mass gain is taken as a measure of the capacity for long-term chemical combination with moisture based on the (time) 1/4 law. A maximum rate of mass gain is obtained for terracotta fired at 1000 °C. It is shown that reheating a sample of fired terracotta at any temperature between 500 °C and the original firing temperature will return the material to its as-fired state. This is supported by Raman spectroscopy. Exceeding the original firing temperature during reheating alters the subsequent rate of reaction to moisture, suggesting a method for determining the original firing temperature of ceramic artefacts.

Dating fired-clay ceramics using long-term power law rehydroxylation kinetics

Fired-clay materials such as brick, tile and ceramic artefacts are found widely in archaeological deposits. The slow progressive chemical recombination of ceramics with environmental moisture (rehydroxylation) provides the basis for archaeological dating. Rehydroxylation rates are described by a (time) 1/4 power law. A ceramic sample may be dated by first heating it to determine its lifetime water mass gain, and then exposing it to water vapour to measure its mass gain rate and hence its individual rehydroxylation kinetic constant. The kinetic constant depends on temperature. Mean lifetime temperatures are estimated from historical meteorological data. Calculated ages of samples of established provenance from Roman to modern dates agree excellently with assigned (known) ages. This agreement shows that the power law holds precisely on millennial time scales. The power law exponent is accurately 1 4, consistent with the theory of fractional (anomalous) ‘single-file’ diffusion.

Moisture expansion and mass gain in fired clay ceramics: a two-stage (time)1/4 process

We report the first measurements of mass gain and expansive strain in clay ceramics immediately following firing. The results show that both mass gain and expansive strain follow a well defined two-stage process, each stage of which is linear with (time)1/4, with a transition from the first to the second stage commencing at t ∼ 1.5 h. The first stage mass gain and strain rates are ∼3.5 times greater than the second stage rates. The strain is found to show the same linear relationship with mass throughout both stages providing evidence that the underlying physical process is the same in each stage. The early time measurements are unique in providing experimental evidence for t1/4 kinetics for both strain and mass gain over times ranging from minutes to days and are consistent with earlier strain data showing t1/4 kinetics over times ranging from days to centuries.

Sintering effects on the development of mechanical properties of fired clay ceramics

This paper addresses the effects of processing techniques on the development of mechanical properties of fired clay ceramics. The influence of thermal treatment, both the rate of heating and firing temperature, were investigated in order to optimize the fabrication process and to obtain the enhanced mechanical properties. Single clay raw material was used for this study. Samples were hand pressed in a rectangular mould and sintered to temperature ranging from 900 to 1200 °C at various heating rates. The variation of flexural strength and elastic modulus starting from 200 to 1200 °C was also presented with heating rate 10 °C/min. In addition, the density, apparent porosity and shrinkage of the fired samples have been determined. Analysis of the firing transformation and microstructure was carried by X-ray diffraction and SEM. Mechanical properties characterization was performed by means of three-point bending tests as well as microindentation tests. In this study, the results revealed that fast heating rate in the early stage of firing has profound influence on densification and the resulting mechanical properties.

Kinetics of moisture expansion in fired clay ceramics: a (time)(1/4) law.

We report the results of experiments to estimate the moisture expansion of several modern and ancient clay brick ceramics from the time of manufacture. From these data we propose a new expansion law, in which the expansive strain increases as (age)(1/4) approximately. Such time dependence is consistent with a mechanism in which expansion arises from a diffusion-controlled rehydration reaction on a linear or low-dimension structure. Our results provide new guidance for the engineering design of masonry and suggest a possible new method for archaeological dating of ceramics.