Modified Mineral Phases During Clay Ceramic Firing

Abstract

Ceramic clays are among the most complicated of ceramic systems because of the very intricate relationship between the behavior of minerals during ceramic processing and their modifications during heating. A major challenge is to predict the phase changes in clay ceramics. The aims of this study were to establish reference data of ceramic products that can be formed based on the mineralogical compositions of the local raw materials. These data, in turn, can be compared with archeological ceramics in order to study their origins.The mineralogical compositions and modifications during firing (550–1100°C under oxidizing conditions) of seven clayey materials sampled from the main clay deposits of northern Morocco were evaluated by X-ray powder diffraction. Two groups of clays were distinguished according to the type of neoformed high-temperature minerals: non-calcareous clays and calcareous clays. For the non-calcareous raw materials, spinel was produced at 950°C. Cristobalite and mullite were formed at temperatures in excess of 1000°C from clays that contain illite, kaolinite, and chlorite. In clays containing vermiculite and large amounts of chlorite, hematite was formed at temperatures in excess of 950°C. Firing of calcareous clays at temperatures >950°C yielded Ca-silicates (diopside, gehlenite and wollastonite), spinel, cristobalite, hematite, and feldspars. Mullite may also form in the calcareous clay products when the carbonate content exceeds 10%.

Selection of Co-Belonging Ceramic Fragments from Archaeological Excavations and Their Location in Vase Bodies from Theroremanent Magnetization

“…Clay does not have a particular mineral composition because its formation is due to the decomposition of fragmented pyrogenic, sedimentary and metamorphic rocks. Typical components of the raw material according to the "classical" triangular diagram [40][41][42] of Levin (1964) are various aluminosilicate compounds of SiO2 (45-70%), Al2O3 (10-30%) and CaO (<20%) with lesser admixtures of iron oxide (<6%) and magnesium (2-3%). The final mineral composition of ceramics and their magnetic properties are formed during firing (at typical temperatures of 600 o -1100 o C) and depends on the composition of the clay and the heating conditions (heating/cooling rate) and ventilation (oxidative-reducing atmosphere) in the kiln.…”

Sumerian Pottery Technology Studied Through Neutron Diffraction and Chemometrics at Abu Tbeirah (Iraq)

“…It is widely accepted that quartz and feldspar do not undergo significant variations at low temperatures, except for "alpha and beta" transition at~573 • C [5]. These changes in the original clay mineral structure can be considered an indication of the temperature reached by the clay object during firing [6]. In particular, illite/muscovite, minerals naturally present in Abu Tbeirah's clays are stable during the firing process up to 900 • C [6].…”

“…These changes in the original clay mineral structure can be considered an indication of the temperature reached by the clay object during firing [6]. In particular, illite/muscovite, minerals naturally present in Abu Tbeirah's clays are stable during the firing process up to 900 • C [6]. Clay minerals such as illite/muscovite are dehydroxylated close to 950 • [6].…”

“…In particular, illite/muscovite, minerals naturally present in Abu Tbeirah's clays are stable during the firing process up to 900 • C [6]. Clay minerals such as illite/muscovite are dehydroxylated close to 950 • [6]. At this temperature, new calcium minerals, such as diopside (CaMgSi 2 O 6 ), are produced from calcite and dolomite reacting with other components [6].…”

“…Clay minerals such as illite/muscovite are dehydroxylated close to 950 • [6]. At this temperature, new calcium minerals, such as diopside (CaMgSi 2 O 6 ), are produced from calcite and dolomite reacting with other components [6]. Human modification of the clay body should be also taken into account in considering illite/muscovite and diopside as indicators of the firing temperature reached by the clay object.…”