As glass cools, it crystalizes. After being heated above 950 degrees farenheit (my torches run about 2200 degress), a piece of glass must cool slowly enough that the entire piece cools at the same rate. If one area of a piece of glass cools more quickly than another area, it will crystalize earlier, causing a crack in the glass.
For example, on my pieces, the leaves are much thinner than the branches. If I cool the pieces too quickly, the leaves will cool faster and crack off the thicker branches. If a branch itself is cooled too quickly, the surface will cool faster than the interior and the branch will crack.
The process of soaking the heat evenly into the glass and then cooling it slowly enough to allow the entire piece to cool consistently is called annealing. Given that glass is a poor conductor of heat, the annealing process must go very slowly!
After making the parts for my pieces – the leaves, fruits, branches or vines – I anneal them. Then I make the actual sculpture and anneal that.
Annealing cycles for my work range from eight to fifteen hours. Artists who cast thick pieces of glass anneal for several days!
Annealing is done in a kiln; the same types of kilns that ceramicists use. Most artists have digital controllers into which they enter the annealing cycle, and then let the kiln and controller do their work.
Without a controller, the artist would have to watch over the kiln and manually adjust the temperature settings to make sure the kiln stays on cycle. That was how I annealed before I bought a digital controller. Here was a situation when I got distracted and forgot to turn the kiln down after it reached the highest temperature I needed.
I think Salvadore Dali would have loved this!
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