Making Glass in the Forest - by Vicky

To make glass in the forest environment you need to ensure you can make or source the raw ingredients. I’ve tried to go back to basics or at least to the 13^th Century AD with this recipe for making glass in the forest.

Wikipedia describes the raw ingredients of glass consists of four principle components:

1. 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.
2. 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 (Na₂O) as flux. In Roman times the mineral natron was used, a naturally occurring mixture of alkaline sodium 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 potash (K₂O) as flux. Calcium oxide (lime, CaO) can also act as a flux.
3. A stabiliser - to stop the glass dissolving in water and increase corrosion resistance. The most effective is lime (CaO) but alumina (Al₂O₃) and magnesia (MgO) can achieve this to some effect. These minerals may already be present in varying quantities in sand.
4. A colourant or opacifier - 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. (For a list of which metals to add to achieve a certain colour, check out the reply I sent to Gillian earlier). Opacity can be due to bubbles in the glass or the inclusion of opacifying agents such as tin and antimony.

The most important thing you’ll need is the furnace or several furnaces. To build the firing chamber in our forest setting we would need to dig a trench to size, and layer with a stone and tile foundation and walls. In a forest glass house they would have had three ovens. One for a process known as fritting, combining the raw materials. Frit is a ceramic composition that has been fused in a fusing oven, quenched to form a glass, and granulated. The purpose of this pre-fusion is to render any soluble and/or toxic components insoluble by causing them to combine with silica and other added oxides. The prepared ash and sand are heated together, but not melted, at a relatively low temperature (up to about 900 °C or 1650 °F) for about 24 hours. A second oven is used to melt the frit at a high temperature of up to 1400 °C 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 improve the quality of glass. Lastly the glass blown objects would be put into an annealing oven to cool.

Pictures showing the construction of a furnace.

   

You would need to allow for a vent so that you can continue to fuel the firing chamber.  

                                                                   

Daub is used to make the dome shape structure over the furnace. Romanglassmakers.co.uk made their daub from “ water, powdered clay, sand and grit, and chopped hay in roughly equal proportions by volume, mixed it by hand in two plastic half-barrels, then spread out on a large wooden board in front of the furnace to evaporate some of the water.”

                                   

   

The ingredients to make the glass frit would be put into crucibles (the pots shown in the pictures). A crucible is a container that can withstand very high temperatures and is used for metal, glass, and pigment production. While crucibles historically were usually made from clay, they can be made from any material that withstands temperatures high enough to melt or otherwise alter its contents.

    

The raw materials would have been 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 blown into objects.

Some pictures of the oven turning the frit into molten glass.

   

      

These would then be placed in an annealing oven to cool. The whole structure would have been enclosed in a wooden building, and it is likely that wood was stored and dried above the furnace.

Pictures of an annealing oven:

   

Duncan taught us how to make charcoal in the first lesson we had in the forest. Charcoal is made by heating wood in the absence of air. Angus at Woodlands.co.uk suggests the use of hardwoods such as oak, which have a very slow burn. To make charcoal the burn must be slow and incomplete so that the wood dries out and its structure and capacity for burning remains.  This process – converting wood to charcoal – takes about 14-16 hours and produces ready-to-use lumps of charcoal. I’ve briefly mentioned how we made small pieces of charcoal in the post ‘Quest in Fire Wood (Part One) but for more detailed instructions and to make a larger quantity check out this website for details: http://www.allotmentforestry.com/fact/Charcoal.htm  

Romanglass.co.uk recommends using several species of wood to fuel the furnace such as beech, ash, walnut, chestnut and yew.

But in a nutshell, theoretically, ‘yes’ it would be possible to make glass in our forest environment. Realistically ‘no’ as we do not have the time to make the furnaces, (which according to Romanglass.co.uk took over a month to construct), nor the time and manpower to make the frit which takes 24 hours of continuously checking the temperature and fuelling the fire, or then to melt the frit into molten glass ready to work! Does sound like a challenge though.....

References:

Allotment Forestry (no date) Buying Local Charcoal. Available at: http://www.allotmentforestry.com/fact/Charcoal.htm (Accessed 12 April 2012)/

Angus (2012) ‘How do you make charcoal?’ Woodlands.co.uk. 4 July 2008. Available at: http://www.woodlands.co.uk/blog/practical-guides/how-do-you-make-charcoal/ (Accessed 12April 2012). 

     

Taylor, M. And Hill, D. (2011) Roman Glassmakers. Available at: http://www.romanglassmakers.co.uk/furnace11.htm (Accessed: 12 April 2012).

Wikipedia (2012) Forest Glass. Available at: http://en.wikipedia.org/wiki/Forest_glass (Accessed 12 April).