Bubbles on Single Layer Fusing

“I'm making 3mm French Vanilla sconce covers; …

·        [initially they were] fine, but now 1.5" bubbles form during the full fuse.

·        I pop the bubbles and fill the holes with frit and refire,

·        [The]… edges draw in and distort the design…

·        The shelf is flat,

·        I fire on Bullseye paper, and

·        the 13.5 hour long firing schedule [in F] is:

200 to 1150, hold 30 minutes.

50 to 1225, hold 30 minutes.

300 to 1490, hold 30 minutes.

9999 to 990, hold 60 minutes.

100 to 750, hold 1 minute.

Does anyone know what I can do to avoid the large bubbles? 

A critique of the schedule. 

 This is for a single sheet of 3mm glass, so the hold at 621˚C/1150˚F is unnecessary as is the slow rise to and hold at 663˚C/1225˚F, because it is a single sheet and does not need the traditional bubble squeeze. 

 The hold of 30 minutes at 810˚C/1490˚F is excessive. 

·        The temperature may be too high.

·        Ten minutes at top temperature is sufficient in most cases. 

·        A soak of 1 minute would be enough. 

·        The anneal soak at 990˚F is most probably a misprint for                          516˚C/960˚F. 

·        The anneal soak is longer than the half hour necessary, but not a             bubble creating problem.

 It means the schedule could have been:

111˚C/200˚F to 796˚C/1465˚F for 5 minutes

AFAP to 516˚C/960˚F for 30 minutes

83˚C/150F˚ to 370F˚/700F˚, 0 minutes

Off

 

Different firing strategies are possible.

•         Reduce the time at top temperature to no more than 10 minutes. 
•         Reduce top temperature by 55˚C/100˚F or more and extend the soak to 20 minutes, if necessary.  Peek frequently to see when the kiln work is complete.
•         Fire on fibre paper covered with Thinfire to allow air out from under the glass.

These strategies can be mixed as desired, and the reasoning for the strategies is:

• Excessive time at the top temperature allows the glass to thin as it migrates to form thicker areas/edges. This makes the glass too thin to resist the air pressure from below.
• Reducing the top temperature will increase the viscosity, so              resisting the migration of the glass, and maintain the original            thickness. 
• Also, single layers are prone to dog boning, but there are ways of reducing it.

Ways to reduce the risk of bubbles appearing in general are:

•    Reduce the time at the top temperature,
•    Reduce the top temperature,
•    Provide ways for the expanding air to migrate from under the glass.