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Old 04-04-2005, 10:20 AM
Dave Bross Dave Bross is offline
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Location: Archer FL(near Gainesville)
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Along the lines of suggested simple and quick in studio "ballpark"sorts of tests, here's a little back yard/shade tree upgrade for the thread pull test that has done good things for me in helping repeatability.

I've been using approx. 1/4' wide strips of sheet glass that match the expansion (and are pretty close on viscosity because they're the same general sort of glass) of what I'm shooting for, Spectrum 96 for 96, Bullseye for 90, Moretti for 104.

With a torch, I can easily heat, apply, and flatten out with tweezers, an equal amount of the mystery glass to the glass that is below it in the strip of known expansion. From there, heat it until it balls up and pull. I shoot for the same length of pull to also help consistency. I'll also pull a thread test between the actual glasses I'll be using because even though glass A may fit glass B, and glass C will fit glass B, it doesn't mean that glass A will fit glass C for all the reasons we have been discussing in this thread.

I've got it to where with my spreadsheet results, this technique, and whatever other dark of the moon factors reside locally, I can pretty reliably say that 1mm of bend in 200 mm of thread is just about .6 to .65 in expansion in my spreadsheet, which is conveniently just about what a pound of sand (in a 100 # initial recipe) will move the expansion in a basic soda lime sort of glass. Your mileage may vary.

Final check is a homemade polariscope (two polarizing lenses from cameras, cost $20 from Ebay) used on a section of glass containing the two glasses cut from whatever application they are destined for.

I've come to know from experience how much stress will pass in which application and I'm always working to dispose of even small stresses in the next go-around. There's an article over at the warm glass board on compatibility that has pictures of a chip and bar test in various stages of stress. This was what I initially used to "calibrate" my eye as to what I was looking at in terms of stress levels.

I can't get good results with this if one of the glasses is lead. I'm trying to wean myself from lead anyway, so not terribly critical for me.

I can get pretty good results in the striking glasses if I simulate all the strikes that will happen in the course of working out the glass by reheating the sample as many times as it will strike in the particular process where it will be used, including one for the anneal.
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