We all know that fused glass is fired in a glass kiln, right? Have you ever wondered what this actually means? This is the first in an occasional series of posts relating to the process of fusing glass.
Health Warning: This is NOT a definitive guide, more of a loo k-see!
We use 'Bullseye' brand sheet glass, which come from Portland, Oregon, USA. The fusing process takes place in a kiln similar to the one below.
The kiln is essentially an electrically heated, insulated metal box. The heating elements in the lid enable the interior to reach temperatures capable of melting the glass placed inside. The amount by which the glass is melted - or fused - together is governed by the temperature applied to it, but is often in the range 740-804C (1364-1479F).
Different results are achieved by heating - or 'firing' - the glass to temperatures within this range. Later, the fired glass is returned to the kiln at an even lower temperature to be slumped into a mould. This makes the glass into the desired shape - perhaps a bowl, plate or dish.
The temperature inside the kiln is regulated by an electronic controller, which can be programmed to take the interior up - and down - through an infinitely variable range of temperatures, as required. A typical fusing 'programme' is illustrated below.You can see that the rates of heating and cooling the glass are not constant. This is due to various factors connected with what glass can - and can't - do at certain temperatures. The key thing with glass is to avoid it being subjected to extremes of temperatures at the wrong rate of heating or cooling.
As the graph shows, during 'Initial Heat', the glass is heated at a steady rate to about 677C (1250F). The kiln just maintains (or 'holds') the temperature for a while. This is to make sure that the glass is all at the same temperature, inside and out. After that, it is much less susceptible to thermal shock and so can be heated rapidly up to the temperature required for the next phase.
Once at the required temperature, the kiln holds it again for a short time, to allow the heat to fuse the glass. Then follows a 'Rapid Cooling' phase. Once the temperature is reduced to 482C (900F) the glass undergoes another hold, again to ensure it is all at an even temperature.
Next is the critical 'Annealing' phase. During this time the temperature of the glass is slowly reduced to 371C (700F). Gradual cooling through this temperature range is important because the finished project may otherwise retain stresses, which may result in the glass fracturing. This can happen immediately or, more rarely, even several months later.
After annealing, the kiln is allowed to cool naturally until the interior reaches room temperature. At this point the glass will be sufficiently stable and cool enough to remove, clean and use.
In future posts we'll be looking at what actually happens to glass when it is heated in the kiln. If you're able to visit us here at Connemara Blue we'd be happy to give you a guided tour of our little workshop and to chat further about the processes we use.