Credit: Janet Wager
"My snowflakes are fragile. Will more glass make them stronger?”
My experiments for the e-book Low Temperature Kilnforming showed more frit does not make these sintered pieces stronger. Sintered pieces made from frit are made stronger with a combination of packing, time and temperature.
Packing
The amount of packing makes the whole more dense by linking more particles together. The most dense packing is done by putting down thin layers and packing each layer into the previous one. This allows more connections to be made between the pieces, which is important to the resulting strength.
Temperature
The temperature at which the frit is fired is important too. Firing at 650°C/1202°F will produce a weaker piece than one fired at 690°C/1274°F, if both are soaked for the same amount of time.
Time
Generally, the soak at the top temperature will be about two hours. The interplay of time and temperature are particularly important when sintering frit. The connections at the atomic level take more time to form at low temperatures than at high ones. The piece fired at 650°C/1202°F can be strengthened by extending the time to 4 hours instead of 2 hours, which will give it about two thirds of the strength of firing at 690°C/1274°F, for 2 hours.
Rates
The ramp rates need to be slow to achieve maximum strength. For example. A rate of 150°C/270°F to 690°C/1274°F needs a 2 hours soak, but a rate of 600°C/1080°F needs 6 hours to achieve the same strength. An alternative is to use a fast ramp to the strain point and then 50°C/90°F to the top temperature for a 3 hour soak.
Texture
The texture of the sintered piece will vary according to time and temperature. A firing at 690°C/1274°F for two hours will give a shiny top surface, but will be more textured underneath. A two hour soak at 650°C/1202°F will give a sugar grain appearance, but be weak. Extending the soak time to 4 hours will approximately double the strength. If you want to retain the sugar texture and have a strong piece, a long soak at a top temperature of about 650°C/1202°F for at least 3 hours will be needed.
Annealing
Sintered pieces need to be annealed for about 2.5 – 3 times longer than their thickness to be strong and the cool rates need to be related to the soak time to be as stress free as possible.
The e-book Low Temperature Kilnforming is available from:
and me: Stephen.Richard43@gmail.com
