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Old 05-01-2019, 06:33 PM
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John Croucher John Croucher is offline
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I think what I would do first with a trident seal test, if I only had a polariscope, is make a seal using all three canes out of the same master glass clear, anneal those and then see what zero strain looked like. Then I would compare that trident seal to a mismatch test using two outer canes of whatever you were testing, against the same clear master glass in the center. If the center cane was showing the same shade of light grey as the first test then there is no strain, no mismatch. If the center cane is much lighter, whiter, brighter and not at all the same shade of pale grey, then strain is showing up. To find out how much and whether it is showing tension or compression, then there is the ring test, thread test or chip test on a strip of sheet glass (made out of your master clear).

The chip test can indicate which way the strain is going by bending the master clear sheet strip either way. Stretching or compressing the chip indicates the extinguishing of the brightness.

This is what Bullseye uses. It objectively gives you a handle on how much strain but only by a lot of comparative experience. The same goes for the ring test. The ring test is good for indicating tension or compression, but not good giving an objective assessment of the degree of strain without a good deal of experience. Furthermore, getting the two glasses to be of equal thickness in the ring test is not easy and that is very important to achieve an accurate assessment.

The thread test is only indicative if comparing glasses from the same family, with similar viscosities, Young's modulii, surface tension, emissivity etc. and again, thickness of the respective glasses.

Unfortunately the dilatometer doesn't provide you with comparative strain points, although, granted, it does yield deformation and Tg temps. It does give objective LEC's though, up to the deformation temp, although even the most expensive dilatometer (upwards of $30,000) doesn't give outputs any more accurate than +/-1 points at best. The bottom line though, is that the dilatometer doesn't reveal helpful mismatch viscosity data.

If you were melting your own color and wanting to get glasses compatible, then repeated melts would eventually yield you a trident seal that showed no strain under your polariscope, as you adjust the chemistry. If you were repeatedly doing this for lots of color then a Strainoptic polariscope cuts lots of corners and starts to look like good value.

Last edited by John Croucher; 05-01-2019 at 06:37 PM.
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