Bowl drop failure?

[Mark Wright]

4 layers of glass 8 3/4" square over a 6 1/2" square hole in ceramaguard with a drop of 4 1/2". When I peeked at 1300 degrees It had fallen through the hole?
First failure I have had. I think I violated some rule of thumb. Too many layers and the glass was too heavy? Not enough glass outside the hole? Both?
Schedule:
Top firing Skutt kiln so I go very slow
100 to 500 hold 15
100 to 1100 hold 15
100 to 1300

[Brad Walker]

A good rule of thumb is 1" larger than the hole on all sides. Looks like your piece had no problems there.

But the bigger the hole the lower the temperature at which it will drop. It probably dropped through around 1200F. And a piece with an even larger hole will drop through even earlier.

[KaCe]

So does that mean one needs to fire very slowly if one has a large hole? Just asking...

[Brad Walker]

So does that mean one needs to fire very slowly if one has a large hole? Just asking...

'
'To a lower temperature. A sink, for instance, might only need to be fired to 1100F/600C.

[DonMcClennen]

You are going to have to be there to watch it from 1100F and up.