Description
In German: Ätzung. Sometimes erroneously called 'polished', which, though, is characterised by an iridescent effect, due precisely to the ceramic glaze.
It is an ancient technique, used as early as the Roman period in Egypt, in the 3rd century AD. It is characteristic of some glass in the first period of Islamic glassmaking, produced in Syria and Egypt from the 7th to the 12th century and the decorations of figurative lead glass in the European middle ages. It was widely used in Bohemian glass-making in the Biedermeier period and even later.
The cooled glass which has been removed from the furnace is decorated on the surface by applying a pigment made of silver salts or copper. The silver or copper ions, when the object is heated in a low temperature muffle furnace, react with the glass surface and substitute (replace) the sodium ions which make up the glass, while the ions migrate to occupy various positions in the network of silicates of the glass structure.
Basically, during this exchange of ions, the pigments are 'embedded' by the glass, and colour the very thin surface layer.
The pigment can be used to paint decorative motifs or to cover whole surfaces, then engraved with decorative motifs. In this case it is adopted as a surrogate to the overlaid engraved glass.