Thread: Phosphate Opal
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Old 12-05-2006, 01:01 PM
Dave Bross Dave Bross is offline
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Many thanks to John for participating too.

Here's the original thread on phosphates from here:

http://talk.craftweb.com/showthread....&threadid=3284

and...

Here were my last discoveries on phosphates before all the turmoil that shut me down.....

Sodium tripolyphosphate ended up being the best phosphate source for the least amount of chunkiness. It's a common ingredient in cleaning compounds and easily available at the chemical supply outfits.

The amount of calcium has a large effect on the density and strike of the melt, more calcium seems to equal less strike/density.

Viscosity does seem to be a key issue. I got a fairly decent and quite dense 6% phosphate melt in the last melt recipe listed in the old thread with more lithium and zinc. The low viscosity melts strike quite a bit right out of the pot. They also digest your pot.

I used 1.5% alumina in the melts but in retrospect I wonder if that was adding to the chunky/devit problems. I would think all the zinc would substitute nicely for the durability enhancements alumina brings but Volf warns that this is not so. Unfortunately, Volf didn't explain, just listing a reference to another publication.

I decided to stay away from fluorine due to the electric melter but after reading John's take on Volf I think I'll at least try to match what might have been in the original bone ash.

I'm stuck at low melt temps. with the wire melter but I would definitely like any suggestions on lowering the devit temp. other than with lead. I suspect that the high zinc percentage helps that? Time to go trolling in Volf again.

I think the low toxicity of these glasses will be important in the future.

along the lines of lower toxic glass...

I'm curious about the old glasses using talc (magnesium silicate) for an opacifier too. Probably not much more than an alabaster sort of opaqueness due to the 5-6% limits on the talc.
I tried a tin opacified glass and it was terrible, so short it wasn't workable.
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