Forest glassworks

Forest glass (Waldglas in German) is late Medieval glass produced in northwestern and central Europe from approximately 1000–1700 AD using wood ash and sand as the main raw materials and made in factories known as glasshouses in forest areas. It is characterized by a variety of greenish-yellow colors, the earlier products often being of crude design and poor quality, and was used mainly for everyday vessels and increasingly for ecclesiastical stained glass windows. Its composition and manufacture contrast sharply with Roman and pre-Roman glassmaking centered on the Mediterranean and contemporaneous Islamic glass making to the east.

While under Roman rule, the raw materials and manufacturing methods of northern Europe were those of the Roman tradition, using the mineral Natron. For several centuries after the fall of the Western Roman Empire, around 450 AD, recycling of Roman glass formed the major part of the local industry and glassmaking skills declined. With the rise of the Carolingian Empire in northwestern Europe approximately 800 AD, increasing demand for glass and problems with supply of traditional raw materials, together with an imperial desire to emulate the more sophisticated culture of the Islamic Empire (which was producing high quality glass) led to experimentation with new raw materials and the development of a totally new glassmaking technology. Archaeologically, numerous medieval glasshouses have been found in western and central Europe, particularly in the mountains of Germany. Due to later reuse of the building material, most are poorly preserved, but there is evidence that both glassmaking and working were often done on the same site.

It is important to distinguish between glassmaking from raw materials and glass working, which is the production of finished articles by melting pieces of raw glass or cullet which may have been made elsewhere or by recycling old glass. Glass consists of four principal components:

In post-Roman times political problems in the Wadi El Natrun area disrupted the supply of natron so alternatives had to be developed. Eastern glassmakers reverted to using sodium-rich plant ash and for a while supplied southern Europe, using existing Roman trade routes, however, the Venetian glassmakers, who had inherited the Roman glassmaking skills, monopolised the trade in plant ash and banned craftsmen from working outside the city. The rest of Europe, north of the Alps, had to find another way of producing glass. The former and stabiliser components of glass occur in all regions as sand or quartz and as lime of various forms. The northern Europeans experimented with using ash from wood, ferns, and bracken as a source of the alkali flux. At its height the Roman glass industry was producing high quality, thin, colourless, and clear glass of consistent composition. The earlier surviving Forest glass vessels are characterised by a wide variety of compositions and lower quality, often being greenish to brownish in colour, thick-walled, and with inclusions and bubbles in the fabric. This suggests that using wood-ash was not just a case of changing the raw material, but necessitated a whole new technology with attendant development problems.

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