The glass-melting crucibles of Derrière Sairoche (1699–1714 AD, Ct. Bern, Switzerland): a petrological approach
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].
Section snippets
Crucibles from Derrière Sairoche
In the Jura region, dozens of pre-industrial glassworks (17th–19th century) have been localised by historical and archaeological researches [2], [17], [18], [19]. Excavations carried out by the Archaeological Survey of the Canton Bern in the glassworks of Derrière Sairoche (1699–1714 AD), one of four glassworks situated in the Chaluet Valley (Court) (Fig. 1), brought to light the workshop area and the glassmakers' dwellings [17], [18], [19]. Between 2000 and 2003, in the workshop area, a great
Original mineral composition
Under the petrographical microscope, monocrystalline quartz is the only recognizable component of the originally non-plastic fraction of the crucibles. Table 1 shows the analysed samples with their relative mineral compositions, obtained by XRD analysis. XRF analyses revealed that SiO2 and Al2O3 occur as major components, followed by TiO2 which rarely exceeds 1 wt.% (Table 2). The low content of alkali and alkaline earth elements (<1 wt.%) points to an original clay matrix consisting mainly of
Summary
By analysing the crystalline core of the crucible, an original mineral composition of quartz sand (∼80 wt. %) and kaolinite (∼20 wt.%) was determined. The homogeneous chemical composition points to a single source of raw materials. Furthermore, petrographical and chemical features allow to exclude recycling of crucible fragments as temper. These interpretations are in agreement with findings by Eramo [14], who reported a good correspondence between the crucibles and some of the local refractory
Acknowledgments
This paper is part of a Ph.D. thesis under the direction of Prof. M. Maggetti and of Dr. G. Thierrin-Michael (University of Fribourg). The author is grateful to them for their invaluable guidance in my research and constructive criticism in the preparation of this paper. Dr. V. Serneels for X-ray fluorescence analyses and his enlightening remarks. Thanks are also due to Mr. C. Neururer for his assistance on EDS analyses. Prof. B. Grobéty (University of Fribourg) is kindly acknowledged for his
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