My pocket vases keep breaking underneath the fibre paper. What can I do?
If a pocket vase is going to break it most likely will be in the brittle phase of the glass. This is usually from too fast heating. It, more rarely, can be too fast cooling. This happens as the glass is moving from or into a solid. It is an extreme case of shading heat from the lower layers.
The general condition
As the temperature rises, glass becomes a little less brittle. The viscosity of the glass reduces. This can also be expressed as becoming less brittle. Due to its excellent insulating properties, glass transmits heat slowly through its substance. This means that the expansion differences within the glass are greatest during the coolest part of the brittle phase.
The brittle phase is described as glass temperature being below the strain point. Then strain point is the temperature at which the glass exits the brittle phase. This temperature is about 540°C/1000°F for fusing glasses. It is higher for float and bottle glass.
The effect of shading heat from the lower parts of the piece
is to induce different rates of expansion within the glass. The riskiest part
of this temperature range is the lower part of the brittle phase. This is where
the viscosity is highest and the rigid structure is easily broken by different
expansion rates. This has been empirically observed by Bob Leatherbarrow and
others to have greatest effect below 300°C/573°F. So, slow ramp rates are
advisable to at least this temperature. Some continue this slow ramp rate to the
strain point before increasing the rate.
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| Credit: Latta's Fused Glass |
Pocket Vases
The effect of the differential expansion shows most obviously in pocket vases. This is a construction where fibre paper is inserted to create the pocket between two sheets of glass. This leaves the lower part of the glass under the fibre paper insulated from the heat. The other parts are left exposed to the radiant heat. While the exposed glass is getting hot, the covered glass is still cool. This sets up the conditions for the maximum differential in expansion rates. The differential in expansion causes the glass to break – usually it is the bottom piece that breaks. A major reason to schedule for at least double the glass thickness of a pocket vase is this shading effect. Slightly different calculations apply to tack fusing where there is no insulating layer between glass sheets.
The care in scheduling applies both to the first ramp rate
and to the cooling rates of the fired piece. Fast cooling will leave the area
covered by the fibre paper much hotter than the exposed glass. The difference
in contraction can break the glass on the cool down too.
More information is available in the ebook Low Temperature Kilnforming; an Evidence-Based Approach to Scheduling.
