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Old 01-11-2004, 07:09 PM
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Originally posted by Dave Bross
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.
Excellent dexcription! That's exactly what I'm seeing right now. I've added 3 rounds of batch so far, and the crucible is just over 1/2 full of "flat foam". I just added another run of batch, so it's sitting at 3/4 full - mixed batch and foam (as of this minute). Still all at 2150F

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.
Again, that's what I've been doing. My furnace drops from 2150 to between 2132 and 2140 during additions (I also keep looking )

It seems to take about 5-10 minutes to recover the temp, and I've been waiting 1.5 to 2 hours between additions (no reason, it just works out that way).

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.
Sounds good.

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.
Again, sounds good. I'll be anxious to see what this glass looks like when done. I know from SP cullet that this furnace will produce a very nice glass with a 1900 squeeze and 2150 working temp.

This time I'm going to play with working temp a bit - see what 2050 is like and all that.

Another question (If you've ever done this). If I finish charging later tonight, what about letting it sit at 2150 (charging temp) overnight, and then cooking it tomorrow?

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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