View Single Post
Old 03-31-2005, 08:28 AM
Lani McGregor
Posts: n/a
Viscosity and expansion

Originally posted by Pete VanderLaan
As to all glasses that have 96 C.O.E. fitting Sp87, no I don't think that but I think it based on the thickness of the two glasses that are present. I make opals at 96 that saw and grind with SP87 just fine if they are under one inch thick. Go over that and they don't fit. Why? my hunch is annealing range, not point.
Pete, the Annealing Point of a glass is a very precisely defined indicator of the viscosity of a glass (and of course, it determines the RANGE). So we are agreeing: the compatibility problems you sometimes encounter when you match measured COEs relates to their VISCOSITY differences.

You can try to compensate for the incompatibility problems of COE-matched glasses by better annealing practice, but you’re still screwing with glasses that are basically badly matched because they haven’t taken viscosity into consideration at the start. And you can very likely get away with it in blowing (or torchwork), but it comes back to bite you in the butt when you get thicker or grind – which is what lots of people are doing currently. At the same time that they’re kidding themselves that their matching COEs guarantee they’re working with compatible glass.

The annealing RANGE of SP87 is about 890 to 945 depending on thickness
Pete, I’m asking for the Annealing POINT. It’s a laboratory measure of viscosity that we can use to compare the relative viscosities of different glasses. It’s not an “about” number and it’s not a range (although I’d guess there might be slight variation depending on the lab used for the measurement).

For Bullseye, for instance, the AP of our clear is 990F, our white opal is 937F, our lead-based gold-pink transparent is 927F. Because of these annealing point differences, we have to adjust the expansion (to 91, 88 and 85 respectively) in order for the glasses to fuse together without visible stress in the daily compatibility tests. If we adjusted all those glasses to the same COE (believe me we’ve done this), they’d break apart.
Reply With Quote