Vent Holes - Kiln Forming Myths 3

Vent holes in moulds will prevent big bubbles

The big bubbles found in slumped pieces are normally between the bottom of the glass and the mould at some place near the lowest part of the mould.  The idea behind the myth seems to be that the air space between the mould and the suspended glass will be trapped and so cause the bubble.

In thinking about how likely this is, look at when bubbles are formed in flat pieces.  This occurs at tack fusing temperatures and above.  Applied to slumping which occurs at lower temperatures, it shows that the glass is unlikely to be plastic enough to allow large bubble formation from heat alone.

Of course, this assertion assumes some things. 

• You need have vent holes in the area where the glass will last touch down.  Their placing will depend on the shape of the mould. 
• The vent holes in the bottom of the mould should be clear, with holes in the side or supported on pieces of 1mm or thicker fibre paper to allow the air from under the mould.
• You should be advancing in temperature at a moderate and steady rate.  Fast rates are likely to cause the edge to conform to the mould and close any air escape through the perimeter.

Large bubbles at the bottom of slumps are most often the result of a too high a temperature or too long a soak or a combination of the two.  A high temperature will allow the glass to continue to move.  As the glass is not plastic enough to thicken, the weight of glass higher in the mould causes the glass at the bottom to rise up in a bubble-like form.  If the slump is at a moderate temperature, but with a very long soak, the same result will be observed.

This means that prevention of large bubbles is by observation.  When using new moulds or new layups for the glass, you should observe the progress of the slump from the softening point upwards and through the soak.  This observation should be by quick peeks at regular intervals and recording the results at each peek.  This will tell you the temperature at which the slump is complete. 

If you find that the slump is complete before the set top temperature, or in the early part of the slump, the target temperature is too high.  In a subsequent firing, reduce the temperature while keeping a half hour soak.  Repeat this until you have a complete slump in that time.  If the reverse is true, increase the temperature until the slump is complete in the half hour.

In conclusion, prevention of big bubbles is a combination of elements.

·         Make sure there are vent holes in the mould,

·         Make sure the air can get from under the mould,

·         Use a moderate rate of advance, and finally

·         Use the minimum temperature possible to achieve a complete slump in half an hour or a little more.

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”.