Heating Bullseye

The two primary concerns on heating are 1) avoiding thermal shock and 2) preventing devitrification. Thermal shock - breakage due to excessive heat differential within the glass body - can be prevented by a slow rate of heating below the strain point. Devitrification - a crystalline scum on the glass surface - is rarely a problem with Bullseye glasses manufactured in recent years. Using an overglaze or rapidly heating the glass through that temperature range where devitrification may occur (1300 ° - 1400 ° F) will prevent it.

The following chart is a rough schedule for heating Bullseye glasses. It assumes a first time firing in which none of the individual pieces of glass in the lay-up is greater than 25% of the total glass mass. It further assumes a kiln with top-firing elements.

 
HEATING A TYPICAL (12" DIAMETER)
COMPOSITION OF BULLSEYE GLASS
Thickness(inches) Initial Heating
(Room temp.
to 1000°F)
rate(°F/HR)		 Rapid Heating
Rate(A)
For Full
Fuse(From 1000°
To 1500° F
& above)
(B)(°F/HR)		 Soak Time
At Full
Fuse
(1500°F)		 Cooling Rate*
From Full
Fuse
(or Slump)
To 960°
(°F/HR)
1/8" 600 (C) (C) AFAP**
3/16" 525 (C) (C) AFAP**
1/4" 450 1000 ° F 10 mins AFAP**
3/8" 375 1000 ° F 10 mins AFAP**
** AFAP = As fast as possible
*Most kilns will not cool this rapidly due to residual heat in the refractories. Allowing the kiln to cool at its own rate between 750 ° F and room temperature is usually adequate for the final cooling stage. This may result in an actual cooling rate slower than that shown above. Cooling the work by opening the kiln door or large vent hole, however, risks thermal shock.


  (A) If you are having problems with bubbles, try slowing the rate of heating up to full fuse. Inserting a half-hour soak at 1250 ° F may also help "squeeze" air from between the glass pieces before the edges seal and trap bubbles. 

  (B) A "full fuse" - generally considered to have occurred when the surface of the glass is completely smooth and free of bumps - is dependent on both temperature and time. Some kilns will achieve a full fuse of Bullseye at 1450 ° F with a 45 minute soak. The same kiln may achieve full fuse with a much shorter (5 minutes or less) soak when taken to 1540 ° F. At the factory we typically program a full fuse at 1500 ° F with a 10 minute soak.  

  (C) Glass lay-ups of this size (12" diameter) which are less than 1/4" thick should not be taken to a full fusing temperature. They will distort in shape and are extremely prone to bubbles. They may, however, be successfully slumped to a shallow depth or tack fused at ~1375 ° F. See Note (A, SLUMP FIRINGS) above.

 

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