CAUSES OF DEFECTS IN HISTORICAL CERAMIC MATERIALS WITH Ca-RICH BODY

The aim of the present work was to study and identify the causes and sources of defects in the glazes and decorations of archaeological ceramics containing a calcium component. As part of systematic archaeological research in the Hradcany area, several reconstructable objects and some fragmentary sherds of archaeological ceramics with a relatively high calcium content were found. Seven objects of faience were rescued from the waste pits on the premises of the third courtyard of the Prague Castle, a flower pot was found in Lumbe garden, and creamware sherds were excavated from a waste pit between the Salm and Schwarzenberg palaces on Hradcany Square. The first step of the research was the evaluation of ceramic bodies, glazes, and decorations using archaeometric methods. Subsequently, dilatometric measurements of ceramic bodies and stress state calculations of ceramic body-glaze, or glaze-decor systems were used to identify the sources of defects observed on archaeological finds. Both primary and secondary defects were identified in all studied finds. Among the primary defects, pinholes and blisters predominated. As secondary defects, crazing, blackening, corrosion, delayed peeling, and mechanical abrasion were recognised. Thanks to the combination of X-ray and optical methods, together with the determination of the stress states of the two- and three-component systems, causes of secondary defects were described. The main source of these defects were calcium residues causing volume changes in the ceramic bodies.