Calcium and sulphur distribution in fired clay brick in the presence of a black reduction core using micro X-ray fluorescence mapping

A new finding of the observation of an elemental gradient or zoning of calcium and sulphur in fired brick bodies is described, that does not appear to have been reported in the literature before. Many raw clays used in brickmaking and pottery contain sulphur and calcium evenly distributed in low amounts in the unfired clay body. However, when the clay body is fired in the kiln at 1050 °C, the elements sulphur and calcium appear to combine to a compound, most probably calcium sulphate. The formation of calcium sulphate, visualised using energy dispersive imaging micro X-ray fluorescence (μ-XRF), only occurs around a so-called reduction core, also known as ‘black core’, caused by reduced magnetite Fe3O4 in the centre of the brick body. The presence of a black reduction core appears to cause the formation of a calcium sulphate layer around the black reduction core. This research contributes to the understanding of phenomena like salt formation, efflorescence and durability in solid clay brick bodies. Large sums are spent on building conservation by national economies and improvement of bricks is of key interest to them.

Research highlights
► The phenomenon in solid bricks, known as ‘black coring’, is investigated.
► A small Ca and S layer around the black core, probably calcium sulphate, was visualised in a brick with micro X-ray fluorescence.
► Apart from Ca and S, no other large scale elemental gradients were found across a brick slice.
► With powder X-ray diffraction, hematite was found in the outer red rim and modified magnetite was found inside the black core.
► Black brick colour develops in the absence of oxygen.