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Old 01-13-2004, 01:18 AM
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Richard Huntrods Richard Huntrods is offline
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Apologies in advance for the long post, but I think this should prove interesting...

Current charging (SP batch):

It's just after midnight, January 13, 2004 and I *KNOW* what destroyed my last furnace.

I charged my rebuilt furnace at 2150 all day today. By 7:30 pm, I had a nice flat layer of melted glass sitting between 1 1/2 and 1 1/4 inches from the top of the crucible.

At 7:30 I started the cooking process by raising the temperature to 2250F. My plan was/is to cook the glass overnight, followed by a squeeze to 1900 tomorrow morning.

Being cautious (because of the past disaster), I decided to check the glass every hour or so. This is because the furnace will take 1 hour to rise from 2150 to 2250, but the "glass mass" will take longer to reach the new temperature.

At 9:30, I did my first check. The glass was now definitely higher in the crucible - about 3/4 inch below the top.

I checked again at 10pm. The glass was now 1/2 inch from the top.

Why? Thermal expansion. Checking with a punty, the layer of bubbles was not all that thick (and didn't increase in size between 9:30 and 10pm), but the glass had expanded due to the increased temperature.

Using the punty, I gathered some of the bubbles until the level was again 1" below top.

Between 10pm and now (midnight), I've checked every 15-30 minutes, and gathered bubbles three times more. Once (10:45) the glass was 1/4 inch from the top. Again, the thickness of bubbles was NOT the reason (I checked with the punty) - glass expansion was the cause.

The temperature now appears to be at equilibrium in the glass mass - recovery time after a gather has decreased from 5 minutes to about 30 seconds, and the temperature drop has lessened (at 9:30 a gather dropped the temp to 2239, at midnight a gather dropped the temp to only 2248). The level appears stable at 1 inch below the top of the crucible.

I'll check again in an hour, but it looks like I can let it cook overnight now.

SO what killed the first furnace?

SO - what destroyed the first furnace build? Same thing - thermal expansion. That time, I filled the crucible to the brim with cullet, let it melt, and filled it again. When I observed the "stiff foam" (my words from back then), it was about 1/2" to 3/4" below the lip of the crucible. SInce the cullet didn't seem to be very melted, I bumped the temp from 2200F (melting temp) to 2400 F, starting at noon. At 2pm, all was fine. At 5pm, the glass had overflowed. I know know that the expansion would easily have caused the glass to rise more than the 3/4 inch that I had, and so the glass simply had to overflow.

WHY? This is a straight-walled crucible, not a round crucible. Round bottom crucibles increase in diameter as you move up in height, so the expanding glass has "somewhere to go" (the extra diameter). In a cylindrical crucible, the glass can only rise.

In the final analysis, it wasn't a faulty thermocouple, or bad cullet, or foam or crap falling into the crucible - just plain old thermodynamics.

Anyway, this is what I have, and what I've found. I'll live with this crucible and furnace for the next little while, as that's what I have (and it's hot). Later, I might put a round pot into it.

So - if the furnace survives the night, I should have some nice glass by Wednesday morning!


Last edited by Richard Huntrods; 01-13-2004 at 01:20 AM.
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