Critical Glass Fusing
Stages
Kiln formed glass goes through a number of stages during the glass fusing cycle.
These glass fusing stages are very important when fusing and slumping fused art glass.
Fusing is a process where compatible pieces of glass are heated in a controlled environment until they fuse together.
In this case glass is heated and cooled in a controlled manner in a glass fusing kiln.
It can take many hours for large items to reach glass fusing temperature and many hours to cool down.
Heating and cooling is dependent upon the glass thickness and number of layers.
There are a large range of firing schedules devised to cater for the size and thickness of the projects involved.
Because of its small size, fused glass jewelry moves through the fusing cycle far quicker and is not subjected to the same level of stress and strain that larger fusing glass projects are.
Six main fusing stages are usually present in a basic glass fusing cycle.
Initial Heating Stage
During this stage the binders in fiber/shelf paper and glues are burnt off.
The glass will be heated gradually to avoid thermal shock, which can occur if the temperature is raised too quickly.
Cracking or breaking may happen if thermal shock is encountered.
Breakage can occur long after the fusing process if thermal shock stresses are present in the fused item.
As the glass reaches 1000 F/ 540 C it will begin to soften slightly and will have a glossy look, at this point it will have moved through the thermal shock range.
Rapid Heating Stage
The glass is heated rapidly through this stage and at 1300 F/600-700 C will have become soft enough to conform to the shape of a mold as well as commencing to fuse.
At around 1500 F/820 C fusing will be underway and the glass should be glowing bright red.
The fusing glass is held at this temperature for 5 to 30 minutes dependent upon the firing schedule, this is known as the soaking stage.
Bubble Squeeze
When glass reaches a temperature of between 1100 F (600 C) and 1300 F (700 C) it slowly becomes soft enough to move around.
At this temperature introducing a bubble squeeze segment into the fusing schedule can minimize any trapped air bubbles that may be present between glass layers.
A slow firing segment from 1100 F (600 (c) to about 1240 F (670 C) which is then held at 1240 F for a set period of time will allow air bubbles to hopefully escape from between the glass layers..
Rapid Cooling Stage
This involves dropping the temperature as quickly as possible until the glass returns to its natural color.
Temperature is dropped from the soaking temperature to around 950 F/510 C.
This can be done by carefully opening the kiln, whilst wearing protective gear, and vent the heat from the kiln.
With computer controlled glass kilns this is usually avoided by having the controller quickly drop the temperature without having to open the kiln.
One of the reasons for this rapid drop is to avoid the glass beginning to devitrify, taking on a scummy dirty appearance, which is difficult to remove.
The idea is to minimize the time glass spends above 1300 F/700 C, which is considered to be the danger zone.
Annealing Stage
The glass will be held at 950 F/510 C whilst it returns to a solid state.
Holding time will vary depending upon the type of glass and thickness.
The reason for this is to remove stress from the glass and to ensure that it will not break or crack at a later date.
After the glass has been held for the set time temperature is slowly reduced to 750 F/400 C taking it slowly through the Strain Point, which is where it is unlikely, that any stress changes will occur.
Cool To Room Temperature Stage
In this stage the glass is allowed to cool to room temperature.
Many small pieces can be left to cool naturally after the kiln has been turned off.
Thicker fused art glass pieces may need more attention to ensure that do not cool too quickly resulting in thermal shock.
If the glass kiln cools too quickly it may need to be re-fired to control the cooling process.
You should keep a firing log so that you know what the kiln cooling pattern is.
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