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 Byzantine and Islamic glass making to the east. History 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. As the Carolingian Empire expanded in northwestern Europe approximately 800 AD, its demand for glass increased but the supply of traditional raw materials was costly and sporadic. An imperial desire to surpass the product quality of the declining Byzantine Empire and the sophisticated Islamic Empire 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. Glassmaking 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: A former – to provide the network of atoms forming the matrix of the glass. This is Silica (SiO2), which in ancient times was added as crushed quartz, and from Roman times onwards in the form of sand. An alkali flux – to lower the temperature at which the silica melts, making it achievable using currently available working temperatures. In ancient times, the ash of sodium-rich plants growing in arid areas around the eastern Mediterranean provided soda (Na2CO3) as flux. In Roman times the mineral natron was used, a naturally occurring mixture of alkaline sodium