26 November 2013

Making glass--experimental archaeology for southern African materials

My first home made glass.


Doesn’t look like much at the moment, but this is a photo of a cross section of the first glass that I made from quartz and soda carbonate flux, approximating the basic glass recipe.  It was made in the back yard, literally, on a charcoal fire with a forced draft.  It proves that glass can indeed be made quite easily from locally, even universally available materials, and that it is not particularly hard to make glass. It has always been said in the historical literature, and still is, that glass was so difficult to manufacture initially from raw materials that it must have been made only in a very limited number of places, and that the raw glass, once made, was traded or shipped all over the world, or all over trade networks within whatever limits.  This seems not to be true.  Certainly, one has to know how glass CAN be made, and HOW it can be made. It requires a craftsman who is skilled with high temperature technologies such as metal smelting, melting, casting, or working, but apart from that the materials are universally available.  

  •   Especially important is the fact that the quantities of inputs--the recipe--seems to be relatively unimportant.  The glass forms as the fluxed silicon dioxide gradually melts and seems to form eutectic substance as the sodium ions and other ions reach saturation points in the silicon dioxide solution, and excess materials either never dissolve into the substance at all, and remain unchanged, or crystallise out of the glass solution.  
  • )b carbon dioxide form throughout the glass initially.  The way these eeliminated, it appears, is by allowing the first formation of glass (what I made, anas in the picture above) to cool, regrinding it, and melting the eutectic mixture, now with the volitile component already removed, to melt again. This give a relatively bubble-free glass.  Thus, large numbers of bubbles in the glass probably indicate that the glass is made directly from raw materials and has not been reground.  Typically, the second melt would also include cullet, or ‘scrap’ glass from broken vessels in the melt.  this assists in the melting and provides a better glass with fewer inclusions and bubbles. 

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