Kiln Forming
Myths 2
Wet
moulds cause bubbles in glass
This is difficult, as with most myths it is true in some
cases and not in others. The case where
it is true is that casting with wet plaster/silica moulds causes water vapour
to move toward the glass. Casting
practice has alleviated some of the problem, by having an extended steam out at
about 200°C, or pouring the glass into the hot dry mould from a reservoir.
In pate de verre, the mould is most often packed while
wet. The small particles allow any steaming of moisture to pass through, and so
be dry at forming temperatures without blowing any bubbles.
In kiln forming, the moisture resulting from recently
applied kiln wash is considered by some to be a cause of bubbles. The water in the mould will be evaporated by
around 250°C in any sensible slumping programme. At this temperature the glass will not have
begun to move, so the moisture can move out of the mould through any vent holes
at the bottom of the mould, or past the glass as it rests on the edge of the
mould.
The circumstance when a damp slumping mould could cause
difficulties is when using an extremely fast rise of temperature. This is
detrimental to the mould, as the rapid formation of steam is more likely to
break the mould rather than the glass.
It is also unlikely to result in a good slump conforming to the mould.
Bubbles at the bottom of the glass are much more likely to
be the result of too high a process temperature. This allows the glass to slide
down the mould. The glass is not plastic
enough to thicken and form a puddle at the bottom at slumping temperatures. Instead, it begins to be pushed up from the
lowest point due to the weight of the glass sliding down the sides.
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”.
No comments:
Post a Comment