Spectroscopic investigation of early medieval tiles and bedding mortars from Nonantola (Modena, Italy) excavations

Tiles adapted for flooring a room of a monastic building (second half of the 9th century) were found during excavations of the San Sylvester Abbey courtyard (Nonantola, MO, Italy) and were investigated using vibrational spectroscopies. Greyish-white granular encrustations or chromatic differences were observed: the firsts ones were due to residues of both bedding mortar (comparable relative proportions within the carbonatic and silicatic fractions) and of sprinkled coarse lime powder used to avoid adherence of the ceramic clay to the formwork (a material enriched in calcite). Cross sections allowed investigating chromatic differences between tiles surfaces and bodies and classifying fragments into two groups: 1) with gradual lightening from the body to the surface; 2) with surface layer clearly distinguishable from the body. In the first case different component distributions within internal and the external portions were observed and attributed to a surface treatment with a spatula. Particularly, a lower amount of clay was observed on their surfaces, enriched in coarse minerals and calcite, probably due to the use of a lime-rich phase to facilitate the spatula sliding. The surfaces of the second group displayed instead very low quantities of calcite, indicating a slip obtained with a barbottina-like technique adding a clay-charged material before smoothing.