Bricks in historical buildings of Toledo City: characterisation and restoration

Abstract

ABSTRACT.-Two different types of ancient bricks (XII-XIVth centuries) collected from historical buildings of Toledo (Spain) were characterized by Optical Microscopy, SEM/EDS, Electron microprobe, XRD, DTA and 57 Fe-Mössbauer spectroscopy. Physical properties such as water absorption and suction, porosity, density and compression strength were also determined.Several minerals found in the brick matrix, such as garnet, let us infer raw material sources; calcite, dolomite, illite and neo-formed gehlenite and diopside phases, on temperature reached in firing; secondary calcite, on first cooling scenarios; and manganese micro-nodules, on late pollution environments. XRD and DTA of original and re-fired samples supply information about firing temperatures. Additional data on firing conditions and type of the original clay are provided by the Mössbauer study. Physical properties of both types of bricks were compared and correlated with raw materials and fabric and firing technology employed. The physicochemical characterization of these bricks provides valuable data for restoration purposes to formulate new specific bricks using neighbouring raw materials.

Archaeological ceramic amphorae from underwater marine environments: Influence of firing temperature on salt crystallization decay

“…The absence of illite and the presence of gehlenite, wollastonite and, in a lower extent, anorthite and diopside in the samples is a consequence of the reactions occurring when calcium or magnesium-rich clays are fired above 900ºC. 27,42,43,45,47 Because gehlenite starts to be developed from 800ºC, 48 its coexistence with illite, micas and calcite on both Tarraconensian sherds (Fig. 4e-f), suggests firing temperatures of around 800ºC.…”

“…The contacts between aggregates and the paste are sharper and the color of the matrix or paste is more golden brown compared with the blurry contacts and dark red brown matrix of sample AIbe-1, which denotes lower firing temperature in the former and higher in the latter. 27 At lower firing temperatures (800ºC) the individual mineral temper grains are easily distinguishable from the 8 clay matrix; at higher firing temperatures (1000º-1050ºC) the sintering process produces an increase in the interconnection among these grains and the matrix and the porosity decrease. 28,29 The clast grain size distribution is mainly bimodal in Italic and Tarraconensian samples ( Fig.…”

“…It is not formed during firing as is also common in other cases from the transformation of dolomite and silicates. 27 However, the diopsides display coronitic reaction rims due to its amphibolization (hornblende) (Fig. 3b) with their cleavage planes parallel to the foliation, indicating that these have been affected by the firing temperature.…”

“…27,[40][41][42] The presence of micas (biotite and muscovite), illite and some calcite in A-Ibe-31 (Fig. 4b), in turn, would denote much lower firing temperatures, under 800ºC 5,27,43 . The Italic amphorae (Fig.…”

“…4c) which may denote firing temperatures above 900ºC. 27,29,[44][45][46] However, as was mentioned before, in this case diopside seems to come from the raw material and not developed during firing. The absence of illite and the presence of gehlenite, wollastonite and, in a lower extent, anorthite and diopside in the samples is a consequence of the reactions occurring when calcium or magnesium-rich clays are fired above 900ºC.…”