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”.