Forest Glass (Edited from Wikipedia)

Forest Glass (Edited from Wikipedia) SUMMARY Forest glass is late Medieval glass produced in North-Western and Central Europe from about 1000-1700 AD using wood ash and sand as the main raw materials and made in factories known as glass-houses in forest areas. It is characterized by a variety of greenish-yellow colors, the earlier products being often of crude design and poor quality, and was used mainly for everyday vessels and increasingly for church stained glass windows. Its composition and manufacture contrast sharply with Roman and pre- Roman glass making centered on the Mediterranean and contemporaneous 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 glass-making skills declined. With the rise of the Carolingian Empire in north-west Europe around 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 glass-making technology. Archaeologically, numerous medieval glass-houses 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 glass- making and working were often done on the same site. In post-Roman times political problems in the Wadi El Natrun area (in Egypt) 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. The rest of Europe, north of the Alps, had to find another way of producing glass. 1 The “former” and “stabilizer” 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, colorless and clear glass of consistent composition. The earlier surviving Forest glass vessels are characterized by a wide variety of compositions and lower quality, being often greenish to brownish in color, thick-walled 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. Whereas Roman and earlier glass was of a marked uniformity over a wide area and for centuries of time, the medieval glass is characterized by a variety of compositions. This can be explained to some extent by examining how the melting temperature of glass depends on the relative proportions of its components, which for simplicity are reduced to three. In practice glass contains many more components which complicate the system. In pre-Medieval times it is believed that the batch of raw materials was heated to a temperature where it partially melted. The unmelted parts were removed, washed, and added to the next batch. The resulting glass was of a fairly uniform composition regardless of the recipe of raw materials used. The wide variety of compositions, together with historical accounts of glass-making in Medieval times suggest that the new method involved melting a complete batch of raw materials and removing the unreactive components as scum. GLASS MAKING Glass consists of four principal components: A former , which provides 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 that lowers the temperature at which the silica melts, making it possible to melt the sand using the technology available at the time. In ancient times, the ash of sodium-rich plants growing in arid areas around the eastern Mediterranean provided soda as flux. In Roman times the mineral natron was used, a naturally occurring mixture of alkaline sodium salts, sourced from the Wadi El Natrun area of Egypt. Post-Roman Islamic glass-makers reverted to using sodium-rich plant ash, while in Northern Europe, a method using ash from wood was developed to provide potash as flux. 2 A stabilizer , which stops the glass dissolving in water and increases corrosion resistance. The most effective is lime but alumina and magnesia can achieve this to some effect. These minerals may already be present in varying quantities in sand. A colorant to change the glass color. These can be naturally present in the glass due to impurities in the raw materials or can be deliberately added to the melted glass as minerals or as slag from metalworking processes. The most important contributions are from iron, copper, cobalt, manganese, tin, antimony and lead. Opacity can be due to bubbles in the glass or the inclusion of opacifying agents such as tin and antimony. The resulting color and opacity from a given composition can also be controlled by the temperature and redox conditions inside the furnace. The vast amounts of wood needed to produce forest glass dictated that glass houses be located in forest areas and that the woodland be carefully managed to maximize the wood resource and to optimize the size of wood pieces used. Even so, periodically the glass-house would have to relocate as the woodland was depleted. The glass industry had to compete for wood supplies with other industries such as mining, and domestic demand. In 16th-century England, an embargo was placed on the use of wood for fuel for glass-making. Glass houses were often located in forests owned by the church. Experimenting with the new technology, the forest glass makers found it difficult to achieve the high standards of clarity and color of the Roman methods due mainly to the great variability of color-controlling elements in the raw materials. European sand and soil is generally higher in iron and manganese. Iron gives a blue/green tinge to glass under usual furnace atmosphere conditions but can also give a yellow color. Manganese has its own purple color which can balance out the iron color to make colorless glass. For instance, glass made from beech wood grown on lime-rich soil is high in manganese and thus nearly colorless while that in a clay-rich area is olive green. Thus, a variety of colors can be produced, and experimentation allowed the glassmakers to progress from the early muddy green/yellow/brown colors towards clear- colored and colorless glass. Local conditions allowed some areas to produce finer glass at an earlier stage. In Bohemia at the end of the 16th Century the decolorizing powers of manganese were used to produce a clear glass suitable for engraving. The amount of carbon left in the wood ash can also affect the color of the glass by modifying the furnace atmosphere. Other clear colors were produce by deliberate addition of metal oxides, often the byproducts of local metalworking; copper oxide to give green or turquoise, cobalt for 3 strong blue. Red was particularly difficult to produce, using particles of copper under delicately controlled redox conditions. STAINED GLASS WINDOWS The term stained glass can refer to colored glass as a material or to works created from it. Throughout its thousand-year history, the term has been applied almost exclusively to the windows of churches, mosques and other significant buildings. Although traditionally made in flat panels and used as windows, the creations of modern stained glass artists also include three-dimensional structures and sculpture. As a material stained glass is glass that has been colored by adding metallic salts during its manufacture. The colored glass is crafted into stained glass windows in which small pieces of glass are arranged to form patterns or pictures, held together (traditionally) by strips of lead and supported by a rigid frame. Painted details and yellow stain are often used to enhance the design. The term stained glass is also applied to windows in which the colors have been painted onto the glass and then fused to the glass in a kiln. Stained glass, as an art and a craft, requires the artistic skill to conceive an appropriate and workable design, and the engineering skills to assemble the piece. A window must fit snugly into the space for which it is made, must resist wind and rain, and also, especially in the larger windows, must support its own weight. Many large windows have withstood the test of time and remained substantially intact since the late Middle Ages. In Western Europe they constitute the major form of pictorial art to have survived. In this context, the purpose of a stained glass window is not to allow those within a building to see the world outside or even primarily to admit light but rather to control it. For this reason stained glass windows have been described as 'illuminated wall decorations'. The design of a window may be abstract or figurative; may incorporate narratives drawn from the Bible, history, or literature; may represent saints or patrons, or use symbolic motifs, in particular armorial. Windows within a building may be thematic, for example: within a church - episodes from the life of Christ; within a parliament building - shields of the constituencies; within a college hall - figures representing the arts and sciences; or within a home - flora, fauna, or landscape.

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