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Old 06-05-2018, 07:41 PM
Dan Vanantwerp
Posts: n/a
So, by walking through the Archives I came across Dave's nice description of getting to his first, best phosphate opal...he had not come across the strontium idea yet. This is how he described the compatibility tests:

Now another interesting thing...

I'm working on matching the expansion to 96 and have been using thread pull tests and polariscope. I know that the phosphate and my clear are really different glasses and that a straight thread will not equal a perfect match, but does anyone remember if the phosphate has to test higher or lower to match? I've come 2 points (E&T calculations and equivalent change in the thread))down in expansion from where the final position of the thread shows the phosphate higher expansion, to where the thread is straight, to about one point (calculated) below where the thread was straight. They all show stress in the polariscope.

I also know there are very different overall expansion curves between the clear and the phosphate. When you first pull the thread test it looks like the phosphate is lower than the clear, then after it cools about 30 seconds or more the thread will bend back in the opposite direction quite a bit. That's a lot of change at the transformation point and I know it's the phosphate because this is the only time I've seen that. I've made a few things from each batch and the stuff within about a half point either way from where the thread is straight will withstand diamond sawing and a few other rudimentary stress tests.

I'll answer my own question. The ones that pull straight on the thread test can be de-stressed by coming through the tail end of the annealing process (around transformation point) more slowly."

I'm not sure how I would achieve this tail-end, annealled thread testing, but I really don't want to try anyway. I'm not sure this glass was ever really fit to a "96" clear. I think it's interesting that Dave noted such weird results when comparing theoretical expansion, thread and stress testing. No matter. The stuff is pretty awesome and would be worthy to me as a standalone glass. That I have fit it to Pete's silver opal is really awesome. I'll probably drop Cristalica anyway and focus on a batched clear. Down to my final 2 boxes of cullet.

Thanks for everyone's help in the village
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