The glass-melting crucibles of Derrière Sairoche (1699–1714 AD, Ct. Bern, Switzerland): a petrological approach

Abstract

Forty-three sherds of crucible found in the pre-industrial (1699–1714 AD) forest glassworks of Derrière Sairoche (Ct. Bern, Switzerland) were studied in order to determine the nature of raw materials and the technological aspects of their production and use. The samples were analysed using optical microscopy, XRD, XRF, UV–VIS spectrometry and SEM/EDS.

The crucibles were produced with local natural clayey sand and fired in two steps. After preliminary firing in a low-temperature furnace (800–1000 °C), the crucibles were brought to temperatures up to 1500 °C within the glass-melting furnace, before adding the glass batch. Using the structural order of cristobalite as an archaeothermometer, temperatures between 1370 and 1500 °C were estimated. The crucible surface shows corrosion by glass and furnace gases. Connections between glassmaking and crucible production are emphasised by this study.

Introduction

Crucibles are as important as melting furnaces in glassmaking. Good crucibles provide refractory behavior (high softening point), no chemical contamination (very low concentration of transition metals) and mechanical strength (thermal shock resistance). Since the Middle Ages, many treatises on glass discussed crucibles as fundamental tools in getting good quality glass. Theophilus (12th century) suggested making crucibles with a “white clay” and once dry, putting them in the “incandescent furnace” [23].

In De la pirotechnia (1540) Biringuccio [41] wrote that both the crucibles and the refractory lining of the melting furnace were made from the same fire-resistant clay (i.e. Valencia clay and Treguanda clay). After a long drying period (six or eight months), crucibles were heated in the fritting oven until they became red-hot, and then were moved with tongs, as quickly as possible, to the melting furnace for the last heating. Merret [30] in his translation of L'Arte vetraria by A. Neri (1612), quotes some source of refractory clays in England and describes the way they should be processed to get good crucibles. A distinction between large pots for cristallo and small pots for colored glass was made as well. The Encyclopédie by Diderot and D'Alembert [11] dedicated volume 17 to glassmaking. In addition to a description of the operating chain in glass production, they also showed how to build the furnaces and make pots. The recycling of broken crucibles as temper for new crucibles and for refractory materials was suggested to reduce shrinkage. Crucibles were fired in two steps in a similar way as reported by Biringuccio [41].

These authors recommended maximum care for crucible production and for the construction of the melting furnace. The technology for producing crucibles depends on glass composition. During the Roman period, the glass had lower liquidus temperatures (900–1100 °C) and common ware could be used as crucibles [24], [36], [40], while in the Middle Ages, crucibles were produced starting from silicatic raw materials able to yield during firing a phase association stable at high temperatures. The technological and pyrotechnological evolution caused by the higher liquidus temperatures (1000–1400 °C) of wood-ash glass demanded more performing raw materials and a better exploitation of combustibles [6], [7], [13], [45]. Owing to this technological dependence glass, crucibles and melting furnace can be considered as parts of the same system, as well as the pyrotechnology needed to melt a given glass. For these reasons in northern European glasshouses, each part was managed by glassmakers [7], [31], [34].