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Old 04-03-2005, 08:07 PM
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John Croucher John Croucher is offline
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When Gaffer was started in the early 90's the biggest headache was what expansion/viscosity standard was out there for glassblowers. Whereas the fused glass manufacturers could totally control compatibility in-house, color bar makers had to somehow fit with a myriad of clear batches around the world. In Australasia alone we were faced with 20 odd recipes with widely varying LEC's and viscosities. The Germans were no help, since their pallet was all over the map.

The other problem was what was the best test to check compatibility in-house. Industry standard dilatometers offered no better than +/- 1-2% accuracy and cost upwards of $20,000 and the info was useless unless the respective strain or set points were concerned.

I have to thank Frank Woolley at Corning for pointing us in the right direction, a direction that the fusing manufacturers had been pursuing for years. Measure the mismatch with polarised light and a trident seal. The fusers use flat glass -cane makes more sense for hot glass workers. The trident seal takes care of differing viscosities up to the respective strain points of the glasses, as well as differing Youngs modulii, emissivities, surface tensions and of course LEC's. The strain, whether in tension or compression can be read very accurately with a polarimeter. After a while the retardation can be interpreted chemically when adjusting a batch. We can look at the results and know something needs adjusting up or down by what we call LEC on our spreadsheets.

That still left the question of what to be compatible with. Looking at what was potentially our biggest market globally, there was an obvious Titanic out there -Spruce Pine 87 batch. Even though we felt it's contraction was a little high compared to most of the German glasses (we didn't want to be in our own universe) we adopted it as our standard. We then created Gaffer Batch which was compatible with SP87. That is now our standard.

We are still totally at the mercy though of peoples melting practice, length of time their clear is in a pot or tank, whether the clear batch has been mixed correctly etc. In Europe we are up against widespread use of Glasma batches which are far too high. The same goes for Japan. Vast numbers of glassblowers think a thread test tells them everything when it doesn't, unless the two glasses are very close re LEC and viscosity. It's time we caught up with the fusing scene, which had all this stuff figured out 20 odd years ago. I agree with Lani -measured COE's have become a lazy way of descibing a glasses compatibility. We are guilty of it ourselves. Unfortunately there isn't a quick way of resolving this problem, because there isn't a standard glass. If there was, then one could compare all the studio recipes to it by quoting +/- retardation variances as nanometres/centimeter
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