Bullseye Annealing Chart from manufacturer

ANNEALING

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Annealing, the controlled cooling of a glass, is critical to its longevity. Glasses which are not properly annealed will contain stress which may result in breakage before or at any time subsequent to their removal from the kiln.

The following chart represents a simple schedule used routinely in Bullseye's factory studio. Annealing schedules for thicker pieces (up to 8") are available from the factory upon request.

ANNEALING A TYPICAL (12" DIAMETER) BULLSEYE GLASS PROJECT

*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.

What does an annealing break look like?

On a flat fused project (such as the hypothetical 12" disk suggested above) annealing breaks are most typically S-shaped. If the break runs along the interface of two different glasses, it is more likely due to incompatibility. If the broken glass appears to have moved apart (often with some force) during the firing, it has probably broken due to thermal shock. If the glass is broken into a web-like pattern of many small pieces, it has most likely stuck to the shelf or mold. Proper annealing cannot prevent incompatibility, incorrect heating or cooling in lower ranges, or improperly prepared contact surfaces.

Do tack-fused projects require less annealing than full-fusings?

Absolutely not. A tack-fused work has many more edges than a fully fused shape. Annealing is always more easily achieved with a glass body of uniform mass. A small glass sphere (the marble) is the easiest object to anneal. As the number of edges increase, so too does the annealing time.

Do slump firings require less annealing than full fusings?

No, especially if the glass does not come into uniform contact with the slumping mold (e.g. a dropout mold). Or if the mold itself is not of uniform thickness. In these instances you will need longer annealing times.

Can I over-anneal my work?

Theoretically no. In actual fact, if the heat distribution within your kiln is extremely uneven, yes. If you anneal soak your glass in a space with uneven heat distribution, you can increase the difficulty of cooling that work uniformly. An extremely critical but often overlooked factor in annealing is the quality of heat distribution within the kiln. Check yours. Make sure that all elements are firing properly and that you do not have drafts or cold spots within the kiln chamber.
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Extending the annealing time hasn't seemed to help, what now?

Try insulating the edges of your work. A fiber paper, blanket or board wall around the outer edge of the glass will reduce the rate of cooling at the edges, thus helping to stabilize the cooling rate throughout the glass body.