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vent holes reduce the possibility of big bubbles
The position of the vent holes is more likely to prevent
bubbles than simply the number. A ball
mould only requires one at the centre bottom.
A rectangular bowl with sharp curves needs the holes in the corners, not
the centre.
The holes in a mould that are intended to allow air to
escape should be at the places where the glass will last touch down on the
mould. When placing the holes, you need
to think where the glass will last conform to the shape of the mould.
In a square or rectangle mould, the corners are the last
places the glass will stretch into. So
the vent holes in the mould need to be there rather than in the centre, or
along the straight edge of the bottom.
If it is a square slumper, it may be that there is no actual need for a
vent hole, as the curve is gentle, but it is safest to have one at the
centre.
If the firing is too hot or too long in any but gently
sloping moulds, large bubbles will be created even though there are adequate or
multiple vent holes, as explained by the glass slipping down the mould and
pushing the bottom up.
More information on big bubbles is available here.
More information on big bubbles is available here.
All myths have an element of truth
in them otherwise they would not persist.
They also persist because people
listen to the “rules” rather than thinking about the principles and applying
them. It is when you understand the
principles that you can successfully break the “rules”.
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