Zwischengoldglas: Technology of production, material analyses and conservation

Zwischengoldglas (gold between glass) objects represent a special category of glass with the decoration placed between two or more layers of glass. The main production centre was located in Bohemia, where most of these objects were made during the eighteenth century. Zwischengoldglaser were extraordinary objects, extremely difficult to produce. Their production demanded high glass-blowing, cutting, grinding, gilding and engraving skills. Each production step meant a risk and many pieces were damaged while assembling glass parts with hot resin. Recently a comprehensive survey of the Zwischengoldglas collection of the Museum of Decorative Arts in Prague was conducted. Four hundred objects were photographed and condition assessment was performed. Organic materials were analysed using micro-Fourier transform infrared spectroscopy (micro-FTIR), which showed that the objects were assembled using a natural pine resin, colophony. The red resin, typically found on the base and on the medallions, is colophony stained with madder. Prussian blue and copper resinate colourants were also identified. In specific cases, restoration of Zwischengoldglaser was considered and in a very few cases was carried out. The treatment of four objects is described, representing typical damage and illustrating decisions and practice for restoration.