Fused glass annealing is an important stage in the glass fusing process.
At the end of the glass fusing process glass is cooled rapidly down to the annealing point where the stress in the glass is relieved by holding the glass at a particular temperature for a set period of time.
It is necessary to remove stresses otherwise the glass may break suddenly at some future time.
After the glass kiln has cooled to about 1050 F (566 C) the annealing phase will begin.
Glass will begin to solidify as it reaches the upper annealing point and begins to cool during the annealing phase.
The temperature will stabilize throughout the entire piece.
The thicker the glass the longer it will take for the glass temperature to stabilize.
It follows that thicker glass pieces need to be held in the warm glass kiln for longer periods of time to ensure that they anneal completely.
The fused glass must be held or soaked in the fusing glass kiln at a particular temperature for a set time to ensure that complete annealing occur
Fused glass annealing occurs best with a slow, but steady drop in temperature down to the strain point.
Annealing does not happen at a particular point but rather between the upper annealing point and the strain point.
This is the annealing temperature at which the glass will be moving towards a stabilization of internal temperatures. T
his rate will be determined by the makers' recommendations and by your own experience.
The strain point is the lower end of the annealing range. Art glass at this stage has solidified and any stresses remaining are unlikely to change unless the glass is reheated. No further annealing will occur below this point.
The glass should then cool down slowly in accordance with your glass fusing schedule.
The whole concept of annealing centers on the proposition that by soaking the glass at or about the annealing temperature stress will be relieved.
If unsure of an annealing time always opt for a longer period, as you can do no harm by over annealing.
It is important to remember that the kiln temperature readings are not measuring glass temperature but the air temperature inside the kiln.
Subject to the glass thickness and size some projects may take many hours to progress through the total glass fusing process.
Visit the Glass Fusing Stages page for additional information.
Bullseye and Spectrum have practical annealing information on their web sites to assist people working on smaller kilns.
Your annealing schedule will need to take into account type, thickness and color of glass you are firing.
The type of kiln and its efficiency can also affect your annealing schedule.
Bullseye glass has an excellent technical article on measuring hot and cold spots in your kiln.
It is important to keep a firing log and record each glass kiln firing for future reference.
Once you gain experience in annealing it is possible to adjust your firing schedule to accommodate an annealing range where no information is available.
Your familiarity with glass fusing schedules will allow you to set an annealing range suitable for your project.