minerals Article Melting Process and Viscosity of Bohemian Historical Glasses Studied on Model Glasses Karolína Pánová 1, * , Kristýna Jílková 1 , Dana Rohanová 1 , František Lahodný 1 , Dagmar Galusková 2 and Martin Míka 1   Citation: Pánová, K.; Jílková, K.; Rohanová, D.; Lahodný, F.; Galusková, D.; Míka, M. Melting Process and Viscosity of Bohemian Historical Glasses Studied on Model Glasses. Minerals 2021, 11, 829. https://doi.org/10.3390/min11080829 Academic Editors: Stefan Röhrs, Parviz Holakooei and Fanny Alloteau Received: 30 June 2021 Accepted: 26 July 2021 Published: 30 July 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Glass and Ceramics, University of Chemistry and Technology in Prague, Technická 5, 166 28 Prague, Czech Republic; Kristyna.Jilkova@vscht.cz (K.J.); Dana.Rohanova@vscht.cz (D.R.); frantisek.lahodny@vscht.cz (F.L.); Martin.Havlik.Mika@vscht.cz (M.M.) 2 Centre for Functional and Surface Functionalized Glass, Alexander Dubˇ cek University of Trenˇ cín, Študentská 2, 911 50 Trenˇ cín, Slovakia; dagmar.galuskova@tnuni.sk * Correspondence: panovaka@vscht.cz Abstract: Analyzing the chemical composition of archaeological glasses can provide an insight into their provenance and raw materials used in their making. However, to the authors’ knowledge, the historical production process itself and melting characteristics of the glasses have not yet been extensively investigated. The main focus of this paper is to describe the melting process of three main types of Bohemian historical glasses: Gothic (14th–1st half of 16th c.); Renaissance (16th–17th c.); and Baroque (end of 17th–18th c.). The model glasses were prepared from natural raw materials and processes that take place during melting were investigated using optical microscopy, SEM-EDS, XRD, and DTA-TG methods. Furthermore, the viscosity of model glasses and thermal dilatation was measured and used to calculate the reference viscosity points. The results illustrate the complexity of historical glass melting, as well as the technological progress between different periods. Keywords: historical glass; model glass; melting process; viscosity 1. Introduction Central European glass production played an important role in Medieval and post- medieval Europe, competing with the Italian glassworks. The glass produced in Central Europe was made with wood ash and wood ash potash (leachate from wood ash rich in K 2 CO 3 ) as fluxing agents, yielding glass that can be categorized as potassium-calcium type [1–4], as opposed to the Mediterranean sodium-calcium glass produced with natron or sodium-rich plant ashes [5,6]. In the present day, sodium-calcium glass has been exten- sively researched, but there is still a huge knowledge gap about their potassium-calcium counterparts and their production technology. This work aims to expand knowledge about the production process of potassium-calcium glasses by analyzing the melting process of model glasses and their properties. Analytical research on archaeological potassium-calcium glass provides valuable infor- mation about the chemical composition, provenance, and, partly, about the raw materials used throughout history. There have been few attempts to reconstruct the historical glass- making technology using model glasses [1,7–10] or even replicas of historical glassmaking furnaces [11–15]. These experiments usually aim to prove previous theories and confirm information from surviving historical texts about the materials, recipes, and glassmaking technologies in different historical eras and different areas. However, only a handful of published works discuss the actual production process and melting characteristics of historical glasses [16–19]. Sometimes, surviving production waste glass from work sites can provide an insight into the production technology too [20–22]. To reconstruct historical methods of glassmaking, one must start from the analytical studies of archaeological glasses and raw materials that were available to glassmakers in Minerals 2021, 11, 829. https://doi.org/10.3390/min11080829 https://www.mdpi.com/journal/minerals