Wednesday, 30 September 2015

Kiln Forming Myths 4

More 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.


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”.