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Old 01-11-2004, 06:59 PM
Dave Bross Dave Bross is offline
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Get it full at the lower temp. and then cook it at the higher temp.

You can add more batch as soon as the stuff you added before has gone flat, and is looking sort of like foam. You don't want to pile cold batch on top of unmelted batch. It's also a good idea to let the furnace recover temp. before adding more batch. That all happens about the same time in mine but that may be different in another melter.

You will be making a number of smaller batch additions at the end to get it up close to the rim.

The idea behind charging at a lower temp and then upping the temp to cook it is so that all the sand is dissolved and not making more bubbles for you to deal with while you're cooking it at the higher temp. As long as sand is dissolving there will be bubbles forming.
The fining agents like antimony decompose and produce gasses at the higher temp which cause the bubbles to swell and rise much more quickly than they would otherwise. The antimony also helps by sucking the really tiny bubbles back into solution in the glass as the glass cools on the squeeze below the point where the antimony changes valence around 2150F.

You may need to batch at a higher temp. with Spruce Pine. I've never used it so I don't really know, but I know it's made with fine mesh silica so I suspect it will do just fine at the temps. suggested. If you get silica snots and stones in the finished glass you'll need to batch at a higher temp.

I looked up Frank Woolley's comments on all this in "glass Technology for the Studio", which you can get from Whitehouse books for about $12. He says as long as you are batching at least 50 degrees below the temp. where you will be cooking you're OK. Frank was a Corning Glass engineer who wrote that little book to help studio glass artists.
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