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Polarised-light (or petrographic) microscopy has been widely applied to heritage materials to assess
composition and diagnose damage. However, instead, this paper focuses on the petrographic investigation of
brick and mortar technologies for the production of quality repair materials compatible with their adjacent
fabrics. Furthermore, the paper relates production technologies to the physical properties of the materials
fabricated, and thus their final quality and durability. According to Cesare Brandi´s theory of compatibility (the
20th century architect on whose work modern conservation theory and practice are largely based) existing
historic materials should be replaced with their equivalent. This paper demonstrates that polarised-light
microscopy provides data on the origin and nature of raw materials, and processing parameters such as
blending, mixing, firing, calcination and slaking, and how these relate to the quality of the final product. In
addition, this paper highlights the importance of production technologies as these directly impact the physical
properties of the materials fabricated and thus determine their final quality and durability. In this context, the
paper investigates mortar calcination and slaking, two important operations in the manufacture of building limes that govern the reactivity, shrinkage and water retention of a lime binder which will impact mortar's properties such as workability, plasticity and carbonation speed, and these in turn will determine the ease of execution, durability and strength of a lime mortar. Petrographic analysis also provides evidence of ceramic technology including identification of local or foreign production and processing parameters such as sieving, blending, mixing and firing. A petrographic study of the ceramic matrix coupled to the diagnosis of mineral phases formed during firing allows to quantify sintering and vitrification and thus determine firing temperatures. Finally, certain features of the raw clay such as the grading and the amount and nature of the non-plastic material inform, not only on the raw material's origin, but also impact the physical properties of the ceramic ware.
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