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

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.