During the late Middle Ages and early modern era Spessart had around 150 small glass foundries and was one of the most prominent glassmaking areas. From the 14th century onwards the glassmakers in the large forest areas north of the Alps produced masses of green Forest Glass. Beakers, rummers, „pass,“ glasses, thumb glasses and bottles. The foundries were located in the middle of the forest because they used enormous amounts of wood, which is why the glass that was produced in them is called Forest Glass.
The characteristic green colour of this glass comes from the iron oxide particles in the sand used as the main raw material for the molten glass. Glass was a very expensive article of daily use at the time because of the high-energy requirements needed to produce it and wood continued to be exclusively used for this purpose in Spessart into the 19th-century. The foundries used wood both for powering the furnaces and as potash to reduce smelting temperatures. Potash was made from Birchwood and huge amounts of wood had to be bought at high prices from surrounding forests. Thus territorial lords made an excellent living by selling it.
In order to demonstrate the enormous energy requirements to you the Royal Löwenstein-Wertheim-Freudenberg’sche Administration gave the museum a 150 cm-high beech wood tree trunk with a diameter of 50 cm, weighing 200 kg, with which Spessart glassmakers would have produced just one kilogram of glass: or five small beakers. Nowadays 800 kg of glass, that’s eight times the amount, could be produced from this tree trunk.