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Old 06-06-2018, 11:39 AM
Dave Bross Dave Bross is offline
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A dilatometer does not tell you anything about the variations between two glasses on softening point, which is the point where the glass hardens up from a liquid to a solid.

If one sets up before the other is done shrinking you have a mismatch.

If softening points mismatch, then two glasses testing the same on dilatometer expansion will not fit.


To answer the question about weirdness of thread tests on phosphates. I tried to simulate how many strikes the particular work I was going to do would require. I did these strikes at the stage where the two glasses were melted together (pre-pulling) for the thread test. I shot for having a straight thread on final cooling after doing this and pulling the thread.


Further thoughts...the E&T numbers are most useful after you have a thread test showing how far off you are. In my little world in my style of thread test 1 mm of bend over 200 mm is equivalent to .6 in E&T numbers. If my melt was off 1 mm on thread test I would then go to my E&T calculator and change the ingredients to change the final E&T number .6 for mixing the batch for the next melt or to get amounts for any attempts to add things to correct the melt sitting there.

Using E&T numbers prior to having compatibility proof is just a shot in the dark. If you know what E&T number your furnace needs (each furnace is unique on this) for your clear it increases your odds of coming up close by shooting for that number. Those E&T numbers will get you closer than anything else on the first try but rarely will they hit it.

More fun with phosphate - our side trip into decolorizers reminded me of this - try a tiny bit of phosphorous (less than .5 %) in your clear batch. Brightens it up noticeably. Thanks to Scholes for that one.

I never tried it, but wondered if a tiny bit of phosphorus in the chalcedony wouldn't do something interesting. What it might do is screw it up due to all the oxygen in the phosphorus but maybe not in a tiny amount as in brightening the clear. The "glass within a glass" nature of the phosphate might do something fun.

One more - the yellowing Cristallica actually looks like iron green on my monitor. Could it be iron contamination?
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