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Old 04-13-2019, 06:13 PM
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John Croucher John Croucher is offline
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The provenance of this "rule" of 2% alumina oxide in the glass for maximum durability can be found at:

Lyle, A.K., Horak, W., and Sharp, D.E., The effect of alumina upon the chemical durability of sand-soda-lime glasses, J. Amer. Ceram. Soc. Vol 19, 142-147, 1936.

I think the problem of SP devitrifying when it is used in a lost wax casting situation is the high level of Li2O, notwithstanding the substantial amount of alumina in the batch which you might think would counteract it.
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