Credit: Marcy Berman |
I have not had much success [with] the Patty Gray mould despite using the recommended firing schedule. I always have holes or bubbles and the edges are not smooth.
The schedule for Oceanside was:
- 111°C/200°F per hour to 537°C1000°F for 15 minutes
- 167°C/300°F per hour to 662°C/1225°F for 30 minutes
- 195°C/350°F per hour to 798°C/1470°F for 20 minutes
- 9999 to 510°C/950°F for 120 minutes
- 55°C/100°F per hour to 371°C/700°F off
I want to ensure the glass has enough time when it is flowing most freely at the top temperature to level out. This requires scheduling a longer soak at the top and observing how well the glass is levelling out. If more time is required you can add it on the “run,” and advance to the next segment when the surface is as wanted. Read up in your kiln manual how to do both these things.
Yes, the rate is one which will enable devitrification to form on flat glass. The soak at top temperature is even more likely to promote it. However, as the glass is flowing, less devitrification has an opportunity to form. The crystallisation – which is what devitrification is - of the glass takes time to form. The movement of the glass surface is sufficient to reduce the formation of those crystals. It is of course likely there will be some devitrification, but not as much as the slow rates and long soaks would lead you to think.
But for these flows there always is
the possibility of devitrification. You have to plan a method of removing it.
Unless the surface is very flat, grinding the top is not a fast way to remove
it. Sandblasting is a quick way to remove devitrification. Another way is to
sift a thin layer of clear glass powder over the surface. This is an
increasingly popular way to deal with devitrification for those without access
to sandblasting facilities. When fired again, the powder melts and forms a new
shining surface. The piece will need to be fired fire again whether sandblasted
or covered in glass powder.
The summary for flows:
- Slow down to top temperature.
- Give sufficient time there to get the flow needed.
- Observe the progress as you near the top temperature.
- Extend the soak or advance to the next segment when the surface is smooth.
- Anneal soak for the calculated thickness.
- Use a three-stage cool – as outlined in the Bullseye chart for annealing thick slabs - to ensure no temporary contraction stresses are created.
- Accept there will be devitrification.