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Old 01-13-2004, 10:26 AM
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Richard Huntrods Richard Huntrods is offline
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Originally posted by Pete VanderLaan
I think it's worth noting that we normally talk about a glass having a linear expansion of 90 or 96. What never gets thought about is that this is in ten thousandths and is only measured up to 300C. The fact is that the expansion keeps on going when you pass 300C as Richard has observed. It actually really adds up. I find a pot that is brimfull at 2350 will shrink about 3/4 of an inch being brought back to 2050F.

Also, some of the expansion is also due to the entrained air bubbles (seeds) in the melt expanding as you heat the glass from 2150 to 2250. I monitored the furnace during the night, and by 8:30 am the glass level had dropped 1/4 inch - and the bubbles were fewer and larger.

Pete - how do you tell when the glass is "cooked" and ready to squeeze?

When I started cooking, there was a stiff froth of bubbles about 1/4 to 1/2 inch thick on the surface. When I say froth I mean the bubbles were less than 1/8 inch in size, and plentiful.

It took 5 hours in my furnace for the entire glass mass to heat from 2150 to 2250, so it cooked at 2250 from 1am to now (currently 9:30). At 8:30 the bubbles were fewer, a thinner layer (less than 1/4 inch) and larger (most between 1/8 inch and 1/4 inch).

I can happily cook till noon, supper, evening or midnight (or longer)- but there's no point cooking longer than necessary, IMO.

What are the key indicators that glass is ready for the squeeze?

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