Thread: Phosphate Opal
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Old 12-05-2006, 02:55 AM
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John Croucher John Croucher is offline
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P2O5 over 5% is pretty much asking for trouble. Your recipe comes out at 5.3%. Back off 15% on the STP, substitute for the soda content you have lost and try again. Redox is irrelevant in my opinion (sorry Hugh) -its all about the devit temp of the apatite crystal/globule you are creating at temp within the matrix.
Colorants may change the devit temp, so Hugh's advice has some relevance.

Pb brings down the devit temp if you want to go down that route. According to Weyl a little F2 stabilises the apatite emulsion (think teeth)which is why the old bone ash recipes seemed to work better. A lot of the 19th century recipes used guano (sea bird shit) and incorporated a little Greenland cryolite or CaF2 in a hydrid P/F recipe to accommodate the difficulties of a pure P opal, even though guano probably had suffient F at temp.

Animal bone ash contains F, in maybe the right stabilising proportion.

Pure phosphate opals are a route to madness but can be conquered, albeit with quite a bit of experimentation..

Don't expect a dense white, but phosphate opals are good for stiffness and are user friendly re pollution. There is more color play with a reddish transmission and there are a whole heap of color combos. They certainly need to be melted and worked out hot to stop devit. Theoretically they shouldn't fade with time at temp as against F opals, which do lose opacity owing to F volativity, but you can never know for sure with colored glass making.

The bottom line -opal phosphates are one of the last exciting frontiers of studio glassmaking.
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