"Can anyone please tell me why this mould always comes out wonky and devitrifies and pulls in on the edges. I used Primo Primer; my kiln is level, and this is the slump schedule I use for 3mm base with 6mm in places [temperatures in Celsius]: 100/593/30 mins; 66/663/25 mins; 204/482/60 mins; 66/371/10 mins; END.* 12cm square."
The suggestion has been made that having
a 6mm base would lessen the irregular slump in the mould. I am not convinced that making the base thicker
will sort the problems.
1) This
is a very deep mould in relation to the span.
The mould sides are steep.
2) The glass slides down and picks up
marks from sliding down the walls of the mould.
The marks are not devitrification.
3) Deep slumps are prone to going off centre. One fix is to watch and be prepared to reach in with wet sticks to readjust the glass placement on the mould.
4) Deep moulds (deep is relative to the span of the mould) require two or more stages of slumping. Start with shallow a slump, and progress through steeper ones.
5) The sides dog bone on many rectangular
moulds. One way to reduce this is to
round the corners with a 10mm radius.
6) Reducing the forming temperature, and
extending the soak time dramatically, will go some way to alleviating the
previous problems. I suggest trying a 620C slump temperature and soak for 2 - 3
hours. Peek at intervals to see when the slump is
complete, then advance to anneal and cool.
In my view, it is a mould from a maker that does not fully understand glass behaviour.
* Schedule in Fahrenheit for the Americans.
- 198 to 1100, 30'
- 119 to 1225, 25'
- 367 to 900, 60' [ASAP is the recommended rate. As it is a tack fused piece, I would anneal as though 12mm/4 layers. This would use a 2 hour soak, cool at 100 to 800, 180 to 700, off ]
- 120 to 700, 10'
- End
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