Unique wood ash Co-coloured glass tessera from mediaeval Madonna: Raman spectroscopic study of production technology

Zdeňka Čermáková; David Hradil; Petr Bezdička; Janka Hradilová; Karolína Pánová

doi:10.1016/j.saa.2023.123183

Unique wood ash Co-coloured glass tessera from mediaeval Madonna: Raman spectroscopic study of production technology

Spectrochim Acta A Mol Biomol Spectrosc. 2023 Dec 15:303:123183. doi: 10.1016/j.saa.2023.123183. Epub 2023 Jul 21.

Authors

Zdeňka Čermáková¹, David Hradil², Petr Bezdička³, Janka Hradilová⁴, Karolína Pánová⁵

Affiliations

¹ Institute of Inorganic Chemistry of the CAS, v.v.i., ALMA Laboratory, Husinec-Řež 1001, 250 68 Řež, Czech Republic. Electronic address: cermakova@iic.cas.cz.
² Institute of Inorganic Chemistry of the CAS, v.v.i., ALMA Laboratory, Husinec-Řež 1001, 250 68 Řež, Czech Republic; Academy of Fine Arts in Prague, ALMA Laboratory, U Akademie 4, 170 22 Prague 7, Czech Republic.
³ Institute of Inorganic Chemistry of the CAS, v.v.i., ALMA Laboratory, Husinec-Řež 1001, 250 68 Řež, Czech Republic.
⁴ Academy of Fine Arts in Prague, ALMA Laboratory, U Akademie 4, 170 22 Prague 7, Czech Republic.
⁵ Department of Glass and Ceramics, Faculty of Chemical Technology, University of Chemistry and Technology, Technická 5, 166 28 Prague 6, Czech Republic.

PMID: 37523853
DOI: 10.1016/j.saa.2023.123183

Abstract

While the natron and plant ash glass tesserae may be found on places of importance across the former Roman and Byzantine empires, wood ash glass tesserae are scarce. This is the first time a wood ash glass tessera is studied in detail. It was part of a magnificent 8-metres tall statue of Madonna in Malbork, Poland, created at the end of the 14th century and destroyed at the end of World War Two. It was found to be coloured by cobalt with possible impact of copper, and opacified by Ca-phosphate. Processes previously described in sodium-rich glasses were observed also in the studied potassium-rich wood ash glass tessera, such as diffusion of the respective alkali metal into the Ca-phosphate grains. The elemental composition of the tessera indicates that it is original - mediaeval, from the area north of Alps. Two phases were identified for the first time, to authors' best knowledge, in any glass tessera - leucite (tetragonal KAlSi₂O₆) and pseudowollastonite (monoclinic CaSiO₃). As pseudowollastonite is a high-temperature phase, it may serve as an indicator of production temperature, which was further supported by the study of polymerisation index of model glasses. This study contributes to the knowledge of old technologies and showed that the know-how for opacification was imported from the Mediterranean, while the raw materials employed for the base glass preparation were from the area north of Alps.

Keywords: Production Technology; Pseudowollastonite; Raman Micro-Spectroscopy; Tessera; Wood Ash Glass.