Sintering (or laminating) is a special form of low temperature kilnforming
that requires attention to the ramp rates and the length of soaks. The rates and soak times were determined by the strength of the resulting pieces.
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| Credit: Researchgate.net |
Rate
The ramp rate has a significant effect on the strength of
the resulting piece.
A moderate rate (150°C/270°F) all the way to the sintering
temperature of 690°C/1080°F gives the glass particles time to settle together.
It works similarly to a slow ramp rate in slumping.
A rapid rate (600°C/1275°F) - as used in medicine – to the
sintering temperature of 690°C/1080°F is used for float glass particles.
An alternative to both these is to schedule a rapid rise to
the strain point followed by a slow - 50°C/90°F per hour - rate to the sinter
temperature.
Soak
The soak time is extremely important in sintering to provide
strong results. It is loosely related to the ramp rate, but in an inverse manner.
The quicker the ramp, the longer the soak required.
The moderate rate of 150°C/270°F needs a two-hour soak at the
top temperature for maximum strength.
The rapid rate of 600°C/1275°F requires about six hours of soaking
at the top temperature.
The alternative of a rapid rise to the strain point followed
by the slow 50°C/90°F per hour rate requires at least a three-hour soak.
These results show the ramp rate is important to the
strength of the resulting piece. Fast ramp rates require increasingly long soaks
at top temperature. Even slowing the ramp rate after reaching the strain point
requires longer soaking than a steady rate. This is so even though the steady
rate is faster than the two-part schedule to the top temperature.
These results indicate that heat work is put into the glass
throughout the temperature rise. The heat put slowly into the structure below the strain point
still has an effect on the sintering of the glass.
This is shown by the two-part schedule that has a slow ramp
rate after the strain point. And even then, the time required is only 0.3hour shorter
than for the moderate steady rise and soak.
There is no time advantage to rapid
rises to the strain point followed by a very slow rise to top temperature. The
six-hour soak required by fast rises to top temperature show there is a
large time disadvantage with rapid rise scheduling of sintering.
More information is available in the ebook Low Temperature Kilnforming and from Bullseye.
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