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Old 04-03-2005, 05:48 PM
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Being only interested in confusion rather than in answering Lani's question, I want to add a question to the viscosity discussion. Why does anyone think that measuring viscosity in the 17 to 300C range tells the story of what happens over the entire annealing range? Do all glasses remain linear above that temp up to their annealing point? And isn't a 40 or 50 degree difference in annealing points (or strain points) sufficient to create a fairly large difference in the total contraction that each glass experiences? I favor the behavioral tests over the theoretical because all of the factors that we can or can't measure are included. The ball test is one that no one has mentioned. Take your color and gather a clear layer over it. Shape and anneal. Observe cracking if any. Look at through a polariscope. Most problems will show up at this point. If you want lots more evidence or reassurance do the following. Saw it in half. Observe. Saw a slice. Observe. Look at through a polariscope again. Polish one face (this micro-heating can set off latent strain). If there are problems, compare to a pull test tt determine what the problem is. I think Lani hit one thing right on the head. There is skill in doing a good pull test and they should be repeatable. And there are limits to what they mean, but a dilitometer test of expansion over the lower and of the cooling range doesn't tell the whole story either.
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