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Old 10-08-2017, 02:46 PM
Antti Torstensson Antti Torstensson is offline
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Pot and furnace corrosion and bad glass

We’re going through a furnace maintenance and some minor rebuilding. After opening up the furnace I saw it was in worse shape than I had thought. The furnace is three years old and the pot is just over a year old. Refer to the pics to see the shape that it’s in. The pot looks like swiss cheese.

We’ve never really gotten a great glass out of this furnace. Even with a new crucible, there’s always these very small microbubbles in the glass that won’t rise up. Cords also start to accumulate quite fast. We’re currently using Cristalica’s nuggets. In the past we have used Glasma’s pellets and nuggets. Glass quality has been pretty much the same with all of them, as well as pot corrosion.

The pots we’re using come from Magma Ceramics. You can check the material composition at http://magmaceramics.com/products-ma...ure/materials/ under studio glass and material ref SGP-W.

I’m pretty sure the poor glass and excessive pot corrosion are very much related. Just can’t understand why that is.

The last photo is just to show the furnace design. There’s also one set of three elements on top of the crucible.

Any thought on what could be causing these problems?

I’m sure I didn’t give all the info necessary for troubleshooting so please ask for more specific information. I would greatly appreciate any help.
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File Type: jpg pot.jpg (47.1 KB, 85 views)
File Type: jpg inner_wall.jpg (48.8 KB, 69 views)
File Type: jpg furnace_opened.jpg (38.2 KB, 91 views)
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Old 10-08-2017, 02:52 PM
Jordan Kube Jordan Kube is online now
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What temperature do you melt and work at? Where is the thermocouple located? Are you using their sillimanite crucibles?
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Last edited by Jordan Kube; 10-08-2017 at 02:55 PM.
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Old 10-08-2017, 03:18 PM
Antti Torstensson Antti Torstensson is offline
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Quote:
Originally Posted by Jordan Kube View Post
What temperature do you melt and work at? Where is the thermocouple located? Are you using their sillimanite crucibles?
I work at 1130 C (2070 F) and usually melt at 1160 C (2120 F). I marked the thermocouple location in red in the attachment. And yes, it is a sillimanite crucible.
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Old 10-08-2017, 04:07 PM
Jordan Kube Jordan Kube is online now
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Quote:
Originally Posted by Antti Torstensson View Post
Cords also start to accumulate quite fast. We’re currently using Cristalica’s nuggets. In the past we have used Glasma’s pellets and nuggets. Glass quality has been pretty much the same with all of them, as well as pot corrosion.
Change crucible type or manufacturer.
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Old 10-08-2017, 05:22 PM
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I agree with Jordan. The pots from Dyson were always what I suspected, not really good to work with soda limes Consider Fafnir given where you are. Try to fire the pot to 2500F prior to melting. I think some of the pots on the market today are suffering from being fired too cold Believe in Sintering.

Cristalica continues to contain borax which I really have made noise about at the Dobern factory to no avail. Simply put Borax does poorly with alumina pots as well as anything else you can think of.
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Old 10-08-2017, 10:18 PM
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Quote:
Originally Posted by Pete VanderLaan View Post
...Borax does poorly with alumina pots as well as anything else you can think of.
For what it's worth, I second this.
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Old 10-09-2017, 01:01 AM
Peter Bowles Peter Bowles is offline
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My experiences with Dyson pots has been quite different. We had a couple out of the Indonesian factory when that was operational and they were nothing short of miraculous. I think it was the SGP - W (was there ever an SGP - Z?)

The winning thing for us was having absolutely no flame impingement of the pot, everything was radiated down from the crown with a NG flat flame burner. Was a lovely thing.

Two things come to mind reading your post - firstly melting at 1160 seems very low if you are working at 1130. That may be whats giving you all the seed - it really depends on your schedule and filling practice - maybe you could elaborate on that.

Also, can't help but thinking that the element placement around the lower part of the pot is making the hottest part of the furnace that interface between the pot and the glass, pretty much everything you don't want. Hard to tell from the pics, but is there any suggestion that the pitting is any more defined along the two sides of the pot where the elements are closest?
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Old 10-09-2017, 01:40 AM
Scott Benefield Scott Benefield is offline
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This is from some correspondence I had with Magma fairly recently about crucible composition. There's a trade-off between resistance to glass attack and ability to withstand thermal shock:

- Silimanite SGP - Used for melting basic cullet. Not good for corrosive materials.

- Silimanite SGPW - Same as above but has superior thermal shock, recommended when furnace is regularly turned on and off.

- Zirconia Mulite Z7G - Used for corrosive glass. Kiln cannot be cooled and heated regularly.

If you run long campaigns, only shutting down once or twice a year, it sounds like their zirconia formula may be better suited for the borax-containing Cristalica. Better yet, get in touch with Magma's technical support and they should be able to offer more informed guidance.

Pete, were you recommending the German Fastner pots (http://www.fahaf.de)? I don't know a lot about them, but I believe they are all green pots that must be sintered before use, and thereafter kept up to working temperature--you lose them when you shut down the furnace.
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Old 10-09-2017, 03:31 AM
Antti Torstensson Antti Torstensson is offline
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Quote:
Originally Posted by Peter Bowles View Post
Two things come to mind reading your post - firstly melting at 1160 seems very low if you are working at 1130. That may be whats giving you all the seed - it really depends on your schedule and filling practice - maybe you could elaborate on that.

Also, can't help but thinking that the element placement around the lower part of the pot is making the hottest part of the furnace that interface between the pot and the glass, pretty much everything you don't want. Hard to tell from the pics, but is there any suggestion that the pitting is any more defined along the two sides of the pot where the elements are closest?
We have tried many different melting temperatures and schedules. Raising the temperature doesn't help at all. The bubbles may rise to the top but new ones will form. It must be the pot that is dissolving into the glass. Letting the glass sit for however long also doesn't affect the microbubbles. It will, however, introduce heavy cording in just a couple of days.

The pot is uniformly pitted on all sides. There's also a set of elements at the top. The top elements do diminish a lot faster than the lower ones though. It is true that most of the heat comes from the bottom. Is this a huge problem? I'm thinking of those little wire melters where all the heat goes straight to the crucible walls. I know the glass quality on those isn't the best but I'm quite sure it's better than what I have had.

I get that the pot might not be ideal for the glass we use or any glass for that matter. But the corrosion we're seeing seems a bit excessive. I know a lot of people around Finland and Europe use Magma's crucibles as that's pretty much all we've got. I also believe the SGP-W to be the most common material composition used. The pot lifetime does seem to be generally shorter than what I read here.

Too bad I already have two of the SGP-W crucibles and really no time or even the money to order different ones at this time. I will try to fire the pot to 2500 F as Pete suggested. Anything I should take into account when doing that?
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