Forest Glass - Control of Colour

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. Kleinlutzel, 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.