Thin-section petrography and SR-μXRD for the identification of micro-crystallites in the brown decorations of ceramic lead glazes

Breaking Preconceptions: Thin Section Petrography For Ceramic Glaze Microstructures

“…However, due to the fact that glass is opaque to electrons, SEM might not be the most suitable technique to investigate crystals embedded in glazes. Our pioneering studies focusing on the use of thin section samples have showed this limitation in contrast with OM that produced images much more useful to identify and interpret the processes that formed the crystallites [34][35][36][37][38]. Indeed, a very common situation in glaze microstructures is that the crystallites (either relics or neo-formed) can be found under the polished glaze surfaces.…”

“…This is a kind of decoration relevant in archaeology and interesting from a scientific point-of-view. In Post-Medieval glazed tablewares, braunite crystallites (Mn 7 SiO 12 ) have been identified by using X-ray microdiffraction (µXRD) [35]. When observed in thin section by using transmitted PPL (Figure 3a,b), they show two different morphologies: dark-brown bipyramidal crystallites (br1) and thin brown needle-like ones (br2).…”

“…The first type of braunite (br1) appears as a light spot on the polished surface (Figure 3c), while the second kind (br2) is hardly visible (Figure 3c,d) and the process of nucleation from the euhedral braunite (br1) cannot be appreciated ( Figure 3c). [35] and Di Febo, 2016 [48].…”

“…In these decorations there are crystallites constantly recurring in Medieval and Post-Medieval potteries and tiles. Kentrolite, Pb 2 Mn 2 Si 2 O 9 , melanotekite Pb 2 Fe 2 Si 2 O 9 , braunite, MnMn 6 SiO 12 , bustamite, (Ca,Mn) 2 Si 2 O 6 and hematite, Fe 2 O 3 , are phases often found in brown decorations as well as lead-calcium arsenates in blue ones [35,37,38,[40][41][42]45,47,[52][53][54][55][56][57]. We have recently characterized these minerals in glazes using the described approach [34,35,37,38,48], and from now on they could be identified in other glazes using almost exclusively the affordable OM.…”

“…The possibility to gain further knowledge on the glaze microstructures from the combined use of Raman microscopy and thin section preparations is illustrated in Figure 7. Hematite and cristobalite have been identified in ceramic glazes by µRaman and µXRD [35]. Obviously, the identification of these mineral phases could also be easily performed through Raman measurements directly on the ceramic glazes without sectioning them.…”