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Old 09-10-2017, 07:20 AM
Pete VanderLaan's Avatar
Pete VanderLaan Pete VanderLaan is offline
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Join Date: Apr 2002
Location: Chocorua New Hampshire
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When I wrote the section in Glassnotes IV for Henry, he asked me a question about calculations and I answered "Depending on local conditions" and he really did not like that answer but I meant it and it's a caution here:

Availability and humidity. This goes beyond what Dave has said about size of furnace. Most suppliers are going to have one source of feldspar. Almost inevitably it will be a fairly local spar and will normally be either a potash spar of a soda spar. Spodumene is a lithium spar and many pottery supplies do have it, Each plays very differently in your glass. Screen them or you'll get tiny stones since they tend to clump . The other aspect is humidity which changes your readings at the scale and that includes the scales at East Bay or Spruce Pine. Dave being in Florida would I suspect have a fair amount of water in his materials. When I worked in Santa Fe, the humidity was always under 40% so my formulas reflected that on the scales. Having 1% moisture in the silica is easy to get and that's one pound of water on the scale. Anyone using those formulas in a humid environment is a "Local Condition" Henry understood what I was saying but I'll keep saying that mixing a large amount prior to testing is dancing with the devil.

Dave can call the tools "Toys" but I bet he wishes he had one. My testing methodologies include the dilatometer but I don't necessarily use it unless I have a major problem and that can happen with high potash and fluorine mixes. The dilatometer is the only tool that is not making simple comparisons. It says what it is, no more, no less. Comparisons are really important but sometimes you are just wild ass guessing and that happens with fluorines all the time. Otherwise, I start with a simple pull test, then make a blown cup with the color inside and the clear outside, saw and score it, Then an actual Hagy seal which is well outlined in glassnotes. No single test is definitive. A Hagy seal that's out much more than 1.5 is going to break up coming out of the annealer. A pull test is only workable with similar glass formulations. Occasionally a mismatched ring test survives the saw but not often. I've seen 24 hour delays on them. The real point is that in all instances, the indicator arrow should be pointing in the same direction. Then comes the ultimate test. Sell the piece. If it's off in the slightest it will shatter at an awkward time.
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