Holes are blown through single and even double layers on texture moulds when the temperature is taken too high.
This piece was taken to a full fuse at 804°C/1480°F. The bubbles were blown near the edge, causing the glass to fall over the edge of the mould, rather than the blank being too big to begin with. At full fuse temperature the glass viscosity is too low to resist the air pressure from underneath. The air could not escape from the edge as the glass had conformed tightly to the mould surface.
Image
credit: Debi Scoggins Johnson
Although it may be possible that the glass draped over the edge of this texture mould during the same full fuse, it is possible the air gathered closer to the centre, again because the full fuse sealed the edges, restricting air flow out to the edges.
These two examples reinforce the requirement that all texture mould schedules should use slow ramp rates, long bubble squeezes, low temperatures and long hold/soaks at the top temperature. This keeps the viscosity of the glass high, helping to resist bubble formation. This post gives information about the rate, bubble squeeze and top temperature.
Be patient. Let the glass have a long time to conform to the mould.
Many bubbles are blown simply because the top temperature is too high and reached too fast.
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