Have You Reached the Strain Point?
When fusing glass, there is a strain point of the glass you need to respect.
This point needs to be recognized as you are heating up the kiln, as well as cooling down the unit.
It plays an important part in the internal stress of your project and assuring you have a successful anneal of the piece.
Glass like wax starts expanding and softening a long time before it is actually liquefied.
The more it is heated, the softer and more fluid it becomes.
The outside of the glass gets heated up before the inside of the glass.
It needs to be heated up slow enough that the inside temperature has a chance to catch up to the outside temperature.
As glass is cooled, the inside doesn’t cool off as fast as the outside of the glass.
The inside stays molten and expanded while the outside is starting to get stiff and begins to contract.
If the glass is cooled off too fast, this expansion and contraction are frozen into place and this creates stress in the piece.
The glass will eventually break to relieve this built up stress.
When cooling down a fused piece, the strain point is the temperature at which internal stress in a piece of glass is substantially relieved.
Annealing glass occurs between the upper and lower strain points, therefore finding the annealing point of a piece of glass is important.
The temperature of the glass needs to be slowly reduced to below the lower point.
In the annealing process, glass is slowly descended at a predetermined rate until it reaches a temperature below the stain point.
This also involves a soaking time, which ensures the glass is all the same temperature.
The soak is done at just above the strain point temperature.
The annealing is accomplished by a slow cooling of the glass to and beyond the lower stain point of the glass.
Below the strain point molecules are trapped where they are, even if the assembly still contains stress.
If we had a way of reading the true glass surface and core we could control and find the exact point.
But when reading the temperature of the kiln, you are actually reading the air temperature and not the temperature of the glass.
When heating up a piece of glass, the glass temperature is typically about 50 degrees Fahrenheit below the air warmth, while cooling down the piece, the glass temperature is typically about 50 degrees Fahrenheit above the air warmth.
Just remember that the air temperature that is recorded is always behind the glass temperature on the way down, so you must control the air temperature to well below this point.
Cooling a piece of glass slowly below this important point ensures that all the glass has reached the same temperature and the strain has been released.
The stain that is built up in a piece of glass must be removed to ensure that the glass is annealed.
Once the piece is below this point, the piece can be sped up a little then at couple of hundred degrees cooler the cooling can be sped up faster.
A general program for annealing needs to cover all the variables and allow the glass to obtain a range.
It is better to get it wrong on the low side of this point than on the high side.
It is wise to aim for a point that is below the strain point in the glass.
Determine what glass you are going to use for your projects.
Find out the manufacturer’s stated annealing range, which provides the excellent temperature to soak the glass.
Check the schedules for Bullseye or Spectrum for some guidelines.
They will give the exact descending speeds to room temperature.
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Strain Point to Glass Fusing Made Easy