Soaking at 1130 is not annealing. Boro is forgiving so maybe it gets misunderstood. Annealing occurs when the glass is brought through the strain point very slowly and evenly throughout. The strain point in boro is roughly between 800 and 950 degrees. Even though brick kilns cool slowly it is not necessarily slow enough. I am no expert on this, but this is how I learned it. I know there is lots of info out there on it for soft glass. The process is the same even if temps and glass is different. Sorry I can't provide a link. in my experience pieces with opals need to be well annealed and fully cooled to not crack.
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Polly
"Confidence, like art, never comes from having all the answers; it comes from being open to all the questions."
-Earl Gray Stevens-
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