Red glass in Kunckel's Ars Vitraria Experimentalis: The importance of temperature

Abstract

The role of the melting conditions and furnaces used to the obtained final colors has always been a question raised when investigating formulations and recipes of historical glasses. The focus of the present work is the reproduction of three recipes of red enamel glass of the manuscript by Neri, L'arte vetraria (1612) following the translation and comments by Kunckel's in Ars Vitraria Experimentalis (1679). The reproductions include the production of each individual compound of the selected recipes following instructions, and the final glass production in electric and wood‐fire furnaces to assess the effect of different melting conditions. A multianalytical approach was used to fully characterize the produced samples allowing the study of the enamel chemical composition, color, crystals formations, and thermal properties. The results indicate that no significant color differences may be attributed to the melting conditions. However, it revealed that the samples produced in the electric furnace at 1200°C present a high crystallinity degree and the formation of white crystals at room temperature in a short period of time. The formation of crystals on glass is critical, and historically, to avoid it, these recipes must have been made at temperatures between 1050 and 1100°C.

The role of sulphur in the early production of copper red stained glass

“…Sulphur can be present in a silicate melt as S 2− or S 6+ the first predominates when the melt is fired under reducing and the second when it is fired under oxidising conditions. Nevertheless, the solubility of sulphur in the melt is very low, in particular for S 2− and a sulphide-silicate melt partition happens, about ~1100 • C extrapolated from the behaviour of basaltic glasses and also consistent with a recent replication of red glass [18]. As copper is a highly chalcophillic element, copper accumulates in the sulphide partition and during the cooling of the melt (Cu 2 S) particles are formed.…”

“…This study was rendered by our observation of the presence of copper sulphide precipitates in some sandwich red glasses dated to the Medieval (13th − 14th c.) and Renaissance (15th and 16th c.) periods. This has suggested that the presence of sulphur had an important role in the formation of metallic copper nanoparticles in agreement with the work of Vilarigues [18]. However, the role of sulphur in the precipitation of metallic copper nanoparticles has not yet been studied, nor its connection with the role of iron previously sustained by some studies [14].…”

“…Recently several attempts have been made to replicate the method of production described in these treatises. Besides the incongruency between the chemical composition of the glasses obtained compared to the historical glasses, red glass has been replicated following several recipes described in Kunkel's treatise [18]. The main conclusions of the study indicate that the incorporation of sulphur seemed fundamental to attain the red colour and that an adequate control of the temperature (above 1100 • C) was more important than controlling the furnace atmosphere.…”

Historical copper and iron coloured glazes and glasses

“…Renaissance (15th and 16th centuries) periods, the absence of tin and lead (or their presence in trace amounts only) and the presence of copper sulphide precipitates were observed (Figures 6.13, 6.14). This observation seems to confirm the role of sulphur in the formation of metallic copper nanoparticles, in line with the findings of Vilarigues (Vilarigues 2022). However, the specific role of sulphur in the precipitation of metallic copper nanoparticles has not been thoroughly investigated, nor has its relationship with the role of iron previously proposed in some studies , Colomban 2009.…”

“…Valuable insights are also obtained from historical records, which provide additional information about the Recently, several attempts have been made to reproduce the glassmaking techniques described in historical treatises. Despite some inconsistencies in the chemical composition of the glass, it has been possible to successfully produce red glass using Kunkel's recipes (Vilarigues 2022), with firing temperatures of up to 1100ºC. The production process involves combining a frit of a soda-rich mixed alkali glass known as crystal with copper and reducing agents.…”