Forest glass
[1] 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.
This article is mainly interested in the production of forest glass in Great Britain, though it was also made in other parts of Europe.
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.
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.
[8] Eastern glassmakers reverted to using sodium-rich plant ash and for a while supplied southern Europe, using existing Roman trade routes.
[1] 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, with inclusions and bubbles in the fabric.
This may 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.
[5] The wide variety of compositions, together with historical accounts of glassmaking,[10][11] suggest that the new method involved melting a complete batch of raw materials, removing the unreactive components as scum.
[5] From approximately 1400 AD, 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.
In Bohemia at the end of the 16th century the decolourising powers of manganese were used to produce a clear glass suitable for engraving.
[1] The amount of carbon left in the wood ash also may affect the colour of the glass by modifying the furnace atmosphere.
In 1120 Theophilus Presbyter, writing in Germany, gave detailed recipes and instructions and in 1530 Georgius Agricola wrote about current glassmaking.
[15] 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 organization.
[15][16][17] Theophilus recommends the use of beech logs,[10] which analysis has shown has a high proportion of CaO when grown on calcareous soil.
'[10][17] 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, consolidating the light powdery ash that might blow about in the furnace causing contamination.
[1] The furnace firing cycle would be optimised for fuel consumption, output, and humanpower, and as the technology improved, larger glasshouses operated on an almost continuous basis.
