The bricks of Hagia Sophia (Istanbul, Turkey): a new hypothesis to explain their compositional difference

The work shows the results of a multi-analytical study performed on twenty-nine brick samples taken from Hagia Sophia in Istanbul (Turkey). Hagia Sophia, one of the most important historical buildings in the world, has a very complex construction history. This complexity is also reflected in the materials that were used for its construction. The main purpose of this work is to verify whether there are any compositional differences in the bricks used in different historical periods, but also to understand the reasons for any compositional differences between one period and another. The samples were studied by optical microscopy (OM), X-ray fluorescence (XRF), X-ray powder diffraction (XRPD) and thermogravimetric analysis (TGA), coupled with a new approach, based on the micro-chemical EDS analysis used to obtain information on the clay fraction of the matrix. The study showed that, most probably, the differences between the bricks belonging to the different construction phases are due to the composition of the clays used for their preparation.