Forest glass
Encyclopedia
The term Forest glass or the German name Waldglas is given to late Medieval glass
produced in North-Western 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 characterised by a variety of greenish-yellow colours, the earlier products being often 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 glass making centred around the Mediterranean and contemporaneous Islamic glass making to the east.
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 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 NW 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.
Glass consists of four principal components:
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 glass
makers, 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, being often greenish to brownish in colour, 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 (of Si/Na/Ca composition) was of a marked uniformity over a wide area and centuries of time, the medieval glass (of Si/K/Ca composition) is characterised 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. The study of such ternary systems
, together with analysis of trace elements is useful to archaeologists for provenancing
glass.
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 removed and washed of non-reactive components and added to the next batch. Because of the strong way that the Si/Na/Ca compositions affect the melting temperature, the resulting glass was of a fairly uniform composition regardless of the recipe of raw materials used. The melting temperatures of the Si/K/Ca glasses are not so strongly affected by composition, resulting in glasses of more varied composition, so the self-limiting features of the Na system that allowed the traditional partial-batch method to produce consistent compositions ceased to apply and a new way of controlling consistency had to be developed. The wide variety of compositions, together with historical accounts of glass-making suggest that the new method involved melting a complete batch of raw materials, removing the unreactive components as scum.
From about 1400AD, in an effort to compete with the quality of Venetian glass, it was found that calcium oxide (CaO) added as flux to the sand/potash mix in the form of shells, limestone
or marble gave a clearer glass, by virtue of reducing the amount of potash required along with its attendant colorants.
Typical compositions of some historical and ancient glasses. The components are given in weight per cent. In addition to those listed the ancient glasses would also have contained up to one per cent iron oxide
and up to three per cent aluminium oxide, in addition to any colorants and opacifiers.
, Jura) while that in a clay-rich area is olive green (e.g. Court-Chalvet, Jura). Thus a variety of colours can be produced and experimentation allowed the glass-makers to progress from the early muddy green/yellow/brown colours towards clear-coloured and colourless glass. Local conditions allowed some areas to produce finer glass at an earlier stage. In Bohemia
at the end of the 16th Century the decolourising 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 colour of the glass by modifying the furnace atmosphere. The glass in York Minster has been shown to be 90% naturally coloured, without added colorants.
Other clear colours were produce by deliberate addition of metal oxides, often the byproducts of local metalworking; copper oxide
to give green or turquiose, cobalt for strong blue. Red was particularly difficult to produce, using particles of copper under delicately controlled redox
conditions. There is little evidence of antimony or tin based opacifiers being used. or the use of lead to modify other colours.
, writing in Germany, gave detailed recipes and instructions and in 1530 Agricola
wrote about current glass making. Other useful information comes from archaeological finds and experimental and theoretical reconstructions.
logs, which analysis has shown has a high proportion of CaO when grown on calcareous
soil. Whatever wood is used, the amount of potash and CaO it provides, as well as other components that might affect colour and opacity, varies considerably the age and part of the tree, soil chemistry, climate, the time of year when the tree was cut and the dryness of the wood when burned, factors over which the glass maker had little control. This variability explains the problems that glass-makers had in trying to produce glass of a consistent quality. Large amounts of ash would have to be prepared and mixed together to give the homogeneity needed to give a predictable glass composition A typical yield of ash from beech is only about 1% so using Theophilus’ recipe of 2 parts of sand to one part of ash means it would take 63 kg of beech wood to produce one kilo of glass. It has been estimated that, including fuel, 150–200 kg of wood would be needed per kilo of glass.
ting. Theophilus specifies ‘for the space of a day and night.’ This process, which could be monitored by changes in colour as temperature increased, caused a decrease in volume, prior to charging crucibles for the final melting stage, thus minimising the number of times the furnace would need to be opened, and also consolidated the light powdery ash which might blow about in the furnace causing contamination.
into vessels or into cylinders which were then opened into sheets for window glass
.The final stage is to anneal
the finished glass to avoid damage from shrinkage stresses.
into objects. These are placed in the annealing oven to cool. The whole structure is enclosed in a wooden building, and it is likely that wood was stored and dried above the furnace. Remains of a similar structure from the late 15th C have been found in Eichsfeld
in Germany. Another design found archaeologically from the 17th Century is the ‘butterfly furnace’. These furnaces were made from stone and the crucibles from imported highly-refractory clay. They differ in style from the Islamic furnaces of the east, and those of southern Europe, the 'beehive' style where the annealing chamber is above the main oven rather than on the same level.
The furnace firing cycle would be optimised for fuel consumption, output and manpower, and as the technology improved larger glass houses operated on an almost continuous basis. It has been estimated that a large glass-house might typically use 67 tonnes of wood a week operating for 40 weeks a year.
The vast amounts of wood needed to produce glass in this way dictated that glass houses be located in forest areas and that the woodland be carefully managed by coppicing
and pollarding
to maximise the wood resource and to optimise 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. One of the main uses of forest glass was for ecclesiastical stained glass
windows.
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
produced in North-Western 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 characterised by a variety of greenish-yellow colours, the earlier products being often of crude design and poor quality, and was used mainly for everyday vessels and increasingly for ecclesiastical stained glass
Stained glass
The term stained glass can refer to coloured glass as a material or to works produced from it. Throughout its thousand-year history, the term has been applied almost exclusively to the windows of churches and other significant buildings...
windows. Its composition and manufacture contrast sharply with Roman
Roman glass
Roman glass objects have been recovered across the Roman Empire in domestic, industrial and funerary contexts. Glass was used primarily for the production of vessels, although mosaic tiles and window glass were also produced. Roman glass production developed from Hellenistic technical traditions,...
and pre-Roman glass making centred around the Mediterranean and contemporaneous Islamic glass making to the east.
History
While under RomanRoman Empire
The Roman Empire was the post-Republican period of the ancient Roman civilization, characterised by an autocratic form of government and large territorial holdings in Europe and around the Mediterranean....
rule the raw materials and manufacturing methods of Northern Europe were those of the Roman tradition, using the mineral Natron
Natron
Natron is a naturally occurring mixture of sodium carbonate decahydrate and about 17% sodium bicarbonate along with small quantities of household salt and sodium sulfate. Natron is white to colourless when pure, varying to gray or yellow with impurities...
. For several centuries after the fall of the Roman Empire
Roman Empire
The Roman Empire was the post-Republican period of the ancient Roman civilization, characterised by an autocratic form of government and large territorial holdings in Europe and around the Mediterranean....
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
Carolingian Empire
Carolingian Empire is a historiographical term which has been used to refer to the realm of the Franks under the Carolingian dynasty in the Early Middle Ages. This dynasty is seen as the founders of France and Germany, and its beginning date is based on the crowning of Charlemagne, or Charles the...
in NW 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
Caliphate
The term caliphate, "dominion of a caliph " , refers to the first system of government established in Islam and represented the political unity of the Muslim Ummah...
(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.
Glassmaking
It is important to distinguish between glass making 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 quartzQuartzQuartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
, and from Roman times onwards in the form of sand. - An alkaliAlkaliIn chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...
fluxFlux (metallurgy)In metallurgy, a flux , is a chemical cleaning agent, flowing agent, or purifying agent. Fluxes may have more than one function at a time...
– 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 sodaSodium oxideSodium oxide is a chemical compound with the formula Na2O. It is used in ceramics and glasses, though not in a raw form. Treatment with water affords sodium hydroxide....
(Na2O) as flux. In Roman times the mineral natron was used, a naturally occurring mixture of alkaline sodiumSodiumSodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
salts, sourced from the Wadi-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 potashPotashPotash is the common name for various mined and manufactured salts that contain potassium in water-soluble form. In some rare cases, potash can be formed with traces of organic materials such as plant remains, and this was the major historical source for it before the industrial era...
(K2O)as flux. Calcium oxideCalcium oxideCalcium oxide , commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline crystalline solid at room temperature....
(lime,CaO) can also act as a flux. - A stabiliser - to stop the glass dissolving in water and increase corrosion resistance. The most effective is lime (CaO) but alumina(Al2O3) and magnesiaMagnesium oxideMagnesium oxide , or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium . It has an empirical formula of and consists of a lattice of Mg2+ ions and O2– ions held together by ionic bonds...
(MgO) can achieve this to some effect. These minerals may already be present in varying quantities in sand. - A colourant or opacifierOpacifierAn opacifier is a substance added to a material in order to make the ensuing system opaque. An example of a chemical opacifier is tin dioxide , which is used to opacify ceramic glazes and milk glass; bone ash is also used....
- 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 slagSlag heapA spoil tip is a pile built of accumulated spoil - the overburden removed during coal and ore mining. These waste materials are generally composed of shale, as well as smaller quantities of carboniferous sandstone and various other residues...
from metalworking processes. The most important contributions are from ironIronIron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
, copperCopperCopper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
, cobaltCobaltCobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
, manganeseManganeseManganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
, tinTinTin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...
, antimonyAntimonyAntimony is a toxic chemical element with the symbol Sb and an atomic number of 51. A lustrous grey metalloid, it is found in nature mainly as the sulfide mineral stibnite...
and leadLeadLead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
. Opacity can be due to bubbles in the glass or the inclusion of opacifying agents such as tin and antimony. The resulting colour and opacity from a given composition can also be controlled by the temperature and redoxRedoxRedox reactions describe all chemical reactions in which atoms have their oxidation state changed....
conditions inside the furnace.
The chemistry of ‘forest glass’
In post-Roman times political problems in the Wadi-Natrun area disrupted the supply of natronNatron
Natron is a naturally occurring mixture of sodium carbonate decahydrate and about 17% sodium bicarbonate along with small quantities of household salt and sodium sulfate. Natron is white to colourless when pure, varying to gray or yellow with impurities...
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 glass
Venetian glass
Venetian glass is a type of glass object made in Venice, Italy, primarily on the island of Murano. It is world-renowned for being colourful, elaborate, and skillfully made....
makers, 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, being often greenish to brownish in colour, 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 (of Si/Na/Ca composition) was of a marked uniformity over a wide area and centuries of time, the medieval glass (of Si/K/Ca composition) is characterised 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. The study of such ternary systems
Glass databases
Glass databases are a collection of glass compositions, glass properties, glass models, associated trademark names, patents etc. These data were collected from publications in scientific papers and patents, from personal communication with scientists and engineers, and other relevant...
, together with analysis of trace elements is useful to archaeologists for provenancing
Provenance
Provenance, from the French provenir, "to come from", refers to the chronology of the ownership or location of an historical object. The term was originally mostly used for works of art, but is now used in similar senses in a wide range of fields, including science and computing...
glass.
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 removed and washed of non-reactive components and added to the next batch. Because of the strong way that the Si/Na/Ca compositions affect the melting temperature, the resulting glass was of a fairly uniform composition regardless of the recipe of raw materials used. The melting temperatures of the Si/K/Ca glasses are not so strongly affected by composition, resulting in glasses of more varied composition, so the self-limiting features of the Na system that allowed the traditional partial-batch method to produce consistent compositions ceased to apply and a new way of controlling consistency had to be developed. The wide variety of compositions, together with historical accounts of glass-making suggest that the new method involved melting a complete batch of raw materials, removing the unreactive components as scum.
From about 1400AD, in an effort to compete with the quality of Venetian glass, it was found that calcium oxide (CaO) added as flux to the sand/potash mix in the form of shells, limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
or marble gave a clearer glass, by virtue of reducing the amount of potash required along with its attendant colorants.
Comparative compostitions
| Egypt Egypt Egypt , officially the Arab Republic of Egypt, Arabic: , is a country mainly in North Africa, with the Sinai Peninsula forming a land bridge in Southwest Asia. Egypt is thus a transcontinental country, and a major power in Africa, the Mediterranean Basin, the Middle East and the Muslim world... ian 15th Cent. BC |
Roman 1st Cent. AD |
European 13th Cent. AD |
Syria Syria Syria , officially the Syrian Arab Republic , is a country in Western Asia, bordering Lebanon and the Mediterranean Sea to the West, Turkey to the north, Iraq to the east, Jordan to the south, and Israel to the southwest.... n 14th Cent. AD |
Modern | |
|---|---|---|---|---|---|
| Silica, SiO2 | 65 | 68 | 53 | 70 | 73 |
| Soda, Na2O | 20 | 16 | 3 | 12 | 16 |
| Potash, K2O | 2 | 0.5 | 17 | 2 | 0.5 |
| Lime, CaO | 4 | 8 | 12 | 10 | 5 |
| Magnesia, MgO | 4 | 0.5 | 7 | 3 | 3 |
| Batch Materials | plant ash quartz |
natron sand |
wood ash sand/quartz |
plant ash sand/quartz |
synthetic components |
Typical compositions of some historical and ancient glasses. The components are given in weight per cent. In addition to those listed the ancient glasses would also have contained up to one per cent iron oxide
Iron oxide
Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely utilized by humans, e.g.,...
and up to three per cent aluminium oxide, in addition to any colorants and opacifiers.
Control of colour
Experimenting with the new technology, the forest glass makers found it difficult to achieve the high standards of clarity and colour of the Roman methods due mainly to the great variability of colour-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 colour. Manganese has its own purple colour which can balance out the iron colour to make colourless glass. For instance – glass made from beech wood grown on meagre lime-rich soil is high in manganese and nearly colourless (e.g. KleinlutzelKleinlützel
Kleinlützel is a municipality in the district of Thierstein in the canton of Solothurn in Switzerland. It is an exclave of the Canton of Solothurn, enclaved in the Canton of Basel-Country and Alsace.-History:...
, Jura) while that in a clay-rich area is olive green (e.g. Court-Chalvet, Jura). Thus a variety of colours can be produced and experimentation allowed the glass-makers to progress from the early muddy green/yellow/brown colours towards clear-coloured and colourless glass. Local conditions allowed some areas to produce finer glass at an earlier stage. In Bohemia
Bohemia
Bohemia is a historical region in central Europe, occupying the western two-thirds of the traditional Czech Lands. It is located in the contemporary Czech Republic with its capital in Prague...
at the end of the 16th Century the decolourising 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 colour of the glass by modifying the furnace atmosphere. The glass in York Minster has been shown to be 90% naturally coloured, without added colorants.
Other clear colours were produce by deliberate addition of metal oxides, often the byproducts of local metalworking; copper oxide
Copper oxide
Copper oxide is a compound from the two elements copper and oxygen.Copper oxide may refer to:*Copper oxide , a red powder;*Copper oxide , a black powder...
to give green or turquiose, cobalt for strong blue. Red was particularly difficult to produce, using particles of copper under delicately controlled redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
conditions. There is little evidence of antimony or tin based opacifiers being used. or the use of lead to modify other colours.
Operation of the glass house
There are only two historical descriptions of European glassmaking in medieval times. In 1120 Theophilus PresbyterTheophilus Presbyter
Theophilus Presbyter is the pseudonymous author or compiler ofa Latin text containing detailed descriptions of various medieval arts, a text commonly known as the Schedula diversarum artium or De diversis artibus , probably first compiled between 1100 and 1120...
, writing in Germany, gave detailed recipes and instructions and in 1530 Agricola
Georg Agricola
Georgius Agricola was a German scholar and scientist. Known as "the father of mineralogy", he was born at Glauchau in Saxony. His real name was Georg Pawer; Agricola is the Latinised version of his name, Pawer meaning "farmer"...
wrote about current glass making. Other useful information comes from archaeological finds and experimental and theoretical reconstructions.
Sourcing and collection of raw materials
The sand was likely collected from river beds, where it was relatively clean and of more uniform particle size. The felling, transporting, drying and storage of wood both for ash production and as fuel for the furnaces was labour intensive and required a high level of organisation.Preparation of ash
Theophilus recommends the use of beechBeech
Beech is a genus of ten species of deciduous trees in the family Fagaceae, native to temperate Europe, Asia and North America.-Habit:...
logs, which analysis has shown has a high proportion of CaO when grown on calcareous
Calcareous
Calcareous is an adjective meaning mostly or partly composed of calcium carbonate, in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.-In zoology:...
soil. Whatever wood is used, the amount of potash and CaO it provides, as well as other components that might affect colour and opacity, varies considerably the age and part of the tree, soil chemistry, climate, the time of year when the tree was cut and the dryness of the wood when burned, factors over which the glass maker had little control. This variability explains the problems that glass-makers had in trying to produce glass of a consistent quality. Large amounts of ash would have to be prepared and mixed together to give the homogeneity needed to give a predictable glass composition A typical yield of ash from beech is only about 1% so using Theophilus’ recipe of 2 parts of sand to one part of ash means it would take 63 kg of beech wood to produce one kilo of glass. It has been estimated that, including fuel, 150–200 kg of wood would be needed per kilo of glass.
Fritting
The prepared ash and sand were then heated together, but not melted, at a relatively low temperature (up to about 900 °C or 1650 °F) in a process known as fritFrit
Frit is a ceramic composition that has been fused in a special fusing oven, quenched to form a glass, and granulated. Frits form an important part of the batches used in compounding enamels and ceramic glazes; the purpose of this pre-fusion is to render any soluble and/or toxic components insoluble...
ting. Theophilus specifies ‘for the space of a day and night.’ This process, which could be monitored by changes in colour as temperature increased, caused a decrease in volume, prior to charging crucibles for the final melting stage, thus minimising the number of times the furnace would need to be opened, and also consolidated the light powdery ash which might blow about in the furnace causing contamination.
Melting
The final stage was to melt the fritted material in crucibles in a covered furnace to give molten glass. The furnace needed to operate at as high a temperature as possible as quick melting and the need for less flux improved the quality of glass. The change from natron to potash required an increase in melting temperature of about 200 °C to around 1350 °C, necessitating a fundamental change in furnace technology and the development of high-temperature ceramics. At this higher temperature, normal clay would react chemically with the glass.Working
Once melted the glass would be blownGlassblowing
Glassblowing is a glassforming technique that involves inflating molten glass into a bubble, or parison, with the aid of a blowpipe, or blow tube...
into vessels or into cylinders which were then opened into sheets for window glass
Stained glass
The term stained glass can refer to coloured glass as a material or to works produced from it. Throughout its thousand-year history, the term has been applied almost exclusively to the windows of churches and other significant buildings...
.The final stage is to anneal
Annealing (glass)
Annealing is a process of slowly cooling glass to relieve internal stresses after it was formed. The process may be carried out in a temperature-controlled kiln known as a Lehr. Glass which has not been annealed is liable to crack or shatter when subjected to a relatively small temperature change...
the finished glass to avoid damage from shrinkage stresses.
Furnace design
Besides the descriptions of Theophilus and Agricola, the only depiction of an early forest glasshouse is from Bohemia in about 1380 (The Mandeville Miniature) This shows a furnace where all the high temperature processes of glass-making were performed in the one structure containing several ovens whose varying temperatures might be controlled to the necessary extent by constant attention. The raw materials are mixed at a pit nearby and carried down in pans to be fritted in one of the ovens, optimum temperature up to 1100 °C. The frit is melted at high temperature up to 1400 °C in crucibles in a second oven, and when ready the glass is being blownGlassblowing
Glassblowing is a glassforming technique that involves inflating molten glass into a bubble, or parison, with the aid of a blowpipe, or blow tube...
into objects. These are placed in the annealing oven to cool. The whole structure is enclosed in a wooden building, and it is likely that wood was stored and dried above the furnace. Remains of a similar structure from the late 15th C have been found in Eichsfeld
Eichsfeld
The Eichsfeld is a historical region in the southeast of Lower Saxony and northwest of Thuringia in the south of the Harz mountains...
in Germany. Another design found archaeologically from the 17th Century is the ‘butterfly furnace’. These furnaces were made from stone and the crucibles from imported highly-refractory clay. They differ in style from the Islamic furnaces of the east, and those of southern Europe, the 'beehive' style where the annealing chamber is above the main oven rather than on the same level.
The furnace firing cycle would be optimised for fuel consumption, output and manpower, and as the technology improved larger glass houses operated on an almost continuous basis. It has been estimated that a large glass-house might typically use 67 tonnes of wood a week operating for 40 weeks a year.
Location of glass houses
Coppicing
Coppicing is a traditional method of woodland management which takes advantage of the fact that many trees make new growth from the stump or roots if cut down. In a coppiced wood, young tree stems are repeatedly cut down to near ground level...
and pollarding
Pollarding
Pollarding is a pruning system in which the upper branches of a tree are removed, promoting a dense head of foliage and branches. It has been common in Great Britain and Europe since medieval times and is practiced today in urban areas worldwide, primarily to maintain trees at a predetermined...
to maximise the wood resource and to optimise 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. One of the main uses of forest glass was for ecclesiastical stained glass
Stained glass
The term stained glass can refer to coloured glass as a material or to works produced from it. Throughout its thousand-year history, the term has been applied almost exclusively to the windows of churches and other significant buildings...
windows.
Topics for further work
- Forest glass products - vessels, stained glass
- Typography of Vessels
- Working techniques
- Decorative techniques