Annealing

Annealing is the process of relieving stress built up the a piece of glass as the result of uneven cooling. In modern times, electric kilns with digitally-controlled cooling programs are used to cool the glass at a sufficiently slow rate after the glass has been held at its annealing temperature for long enough that all stress has been relieved.

Previous to the advent of electric kilns, gas- or wood-fired kilns were used to anneal the glass. Such kilns required constant attention of someone to both monitor and control the temperature of the glass at any given time. In some cases, temperature of the glass was changed simply by incrementally moving the glass further from the heat source inside of the kiln (see photo.)

Unannealed glass must be considered unstable, as even subtle changes in environmental temperature or pressure could potentially trigger the latent stress in the glass to increase just enough to trigger cracking or, sometimes, even explosive shattering of the glass.

Following is a photo of goblets being annealed in a gas-fired kiln at a glassblowing studio in Murano, Italy. The forked pole is used to pick goblets up and move them further away from the heat source inside of the kiln as the temperature evens out inside of each goblet.

File:Http://www.wesleyfleming.com/murano 2005/DSCF2291.jpg Photo credit: Wesley Fleming, 2004

This photo shows a wider view of the bank of kilns in this Murano studio. Although these kilns are now gas-fired, they were previously wood-fired. A fireplace is visible underneath each kiln - the exhaust (chimney) was directed up through the kilns to heat them.

File:Http://www.wesleyfleming.com/murano 2005/DSCF2294.jpg Photo credit: Wesley Fleming, 2004