LATE-MEDIEVAL GLASS MANUFACTURE IN THE EICHSFELD REGION (THURINGIA, GERMANY)

This paper deals with the chemistry and technology of late-medieval glass of the Eichsfeld region. In the late Middle Ages, the Eichsfeld region was a center for the manufacture of glass in Germany. The four investigated glass works produced a particular kind of woodash-lime glass. This stained glass with a mixed alkali composition has an excellent durability, which was due to its high SiO2 content (about 58 wt.%) in addition to its (for glass of forest glass works) uncommonly high Na2O content (about 3.5 wt.%) and relatively low K2O (3.9 wt.%) concentration. In these late-medieval glass works different glassware was manufactured by a specific raw material mixture which consisted of 20% limestone + 20% beechwood ash + 60% quartz sand. A variety of colours from dark green, green-blue to light green-yellow were produced with only one glass composition by changing the oxygen fugacity of the melt. The FeO/Fe2O3 ratio of the glasses range from 0.3 for the green-yellow to 0.8 for the green-blue glass. Blue colouring elements, Cu and Co, were deliberatly added to the glass. Opaque red glass which contains Cu and Pb was produced under strong reducing conditions. The Eichsfeld region glass recipe represents an advanced development in the medieval glass recipe that was in use for nearly 500 years until the 15th century A.D. This is the second extraordinary change of the glass recipe in central Europe and was chiefly caused by the need for fuel on the part of the iron-works and glass works which denuded large forest areas. The first change had been forced by the decline and fall of the Roman Empire, which had meant that the trade in Mediterrenean soda-lime glass was reduced, and so, had been replaced by potassium-lime glass around 900 A.D.