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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 furnace_opened.jpg (38.2 KB, 76 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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Old 10-09-2017, 05:35 AM
Peter Bowles Peter Bowles is offline
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How often are you melting?
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Old 10-09-2017, 06:37 AM
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Quote:
Originally Posted by Peter Bowles View Post
How often are you melting?
******
Great question.

From my chair, I see what is essentially a silica/alumina pot and it looks remarkably like the old Ipsen pots that were mullite= a 76 percent alumina/silicate of (3Al2.2Si02). Sillimanite I believe is (Al2Si05). So, thats a lot of silica in that pot to dissolve easily. Much more than in Mullite. The honeycombing is the silica being dissolved leaving the alumina standing proud from the dissolution. I could never get excited about Dyson more due to iron leach. I have never seen zirconium based pots do well with Soda Limes. Lead, Yes.

I had problem with the boron in the cristalica and I did tell them that. It reminded me of the sys96 stuff that Spectrum had but was not as bad. The cristalica is completely melted which makes it a good deal better than the Sys96 but the boron keeps coming out. I'm at the point where I just won't use borax anymore.

As to the Fastner, indeed it is a green pot and does need to be fired. I only mentioned it because you have so little else available on the continent. Perhaps Corhart (SEFPRO) in France?

The GLASMA formulation is a high barium and can be tough to melt if it is overheated. It too can throw stones.

I would be interested in what the temperature really is in the lower set of elements. I would suspect it's a lot hotter than Anti thinks. That doesn't account for the corrosion at the door.

Whenever this stuff happens, it speaks volumes for me about simply mixing your own glass. Then you're in control, not your supplier. I have had Spruce Pine mix my formula and that works pretty well. I have written formulas for others which Spruce pine mixed for them.
As to sintering, yes, I think it's a good idea. I'm recommending that the High emp pots get taken to 2500F before loading it with anything. I would actually think it would do better even hotter but most folks would have trouble getting to that. Ed Skeels used to sinter all his Engineered Ceramics pots when I was selling those to him and he was melting fluorines for the lighting fixtures.

There's quite a number of moving parts here. Melting at 2175F is what Cristalica offers up. I find that to be cold.
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Old 10-10-2017, 03:08 AM
Antti Torstensson Antti Torstensson is offline
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I'd say on average I melt only once a week.

I do have a spare thermocouple that I could install in the lower part of the furnace to see what the temperature difference is.

So would the main suspect at this point be a wrong kind of crucible getting too hot?
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Old 10-10-2017, 04:03 AM
Antti Torstensson Antti Torstensson is offline
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On their website Cristalica lists the advantages of using their glass, one of which is:

"CRUCIBLE/FURNACE LIFE: Cristalica glass does not consume your crucible. At least 4 times prolonged crucible lifetime is detected at users of pre-molten material."

I was in contact with Cristalica a few months earlier and they actually recommend Magma's crucibles. But then again there's not much choice.
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Old 10-10-2017, 06:37 AM
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Has this been persistent with all the Dyson pots you've used? Or does this seem to be happening as an outlier?

If bubbles keep on forming in the pot, that's not silica doing it. It is possible that you have a bad pot or a contaminated glass that you're trying to melt. If you were to short term, melt GLASMA, would you have the same experience.

I view the statement from cristalica as salesmanship and nothing else .

Pitting indeed does trap bubbles in that honey comb matrix your photo shows so well. Those bubbles do tend to travel in and out of those crevices. The cording after a few days does suggest a silica melt going on.

I'd change out the pot.

We managed to make a few defective pots based on contamination way back at the inception of High Temp. It was limited to one rubber bag of the mix and at first I simply could not understand it. When I looked at our process for storage, it was obvious and nothing like it has occurred in almost ten years. Embarrassing? You bet.
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Old 10-12-2017, 02:59 AM
Antti Torstensson Antti Torstensson is offline
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This has been persistent with all the pots so far. Maybe not as severe before.

I don't think it's the pitting trapping bubbles. If it were it shouldn't be a problem with a new smooth pot. Shouldn't the bubbles in that case eventually rise to the top anyway? At least if you crank up the heat.

We did have a better glass with Glasma's pelletized batch. Bubbles weren't really a problem but there were some issues with cords. It too did eat up the crucible quite eagerly.

I'm inclined to think that the root of the problem is too much heat distributed right next to the crucible. I'm still wondering how the small wire melters manage to work.
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Old 10-12-2017, 06:12 AM
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Quote:
Originally Posted by Antti Torstensson View Post

I'm inclined to think that the root of the problem is too much heat distributed right next to the crucible.
*********
I would suggest getting a thermocouple down in that clean out area and finding out what the temperature is. I see a lot of crucibles and really am pretty clear about the variety of ways they decompose. You have a classic of silica eaten out of an alumina matrix. It does tend however to trap bubbles. At the temperatures you cite, I would not expect bubbles to rise much.

Don't discount the possibility of something actually being in your cullet.

Further, if there are elements above glass line, don't discount the possibility of silicon carbide shedding into your pot. That would help explain the bubbles but not the dissolution of the pot itself.
I would have to admit that I had a lot of difficulty melting the cristalica cullet. It seemed at the time that I was the only one experiencing trouble fining out the goop. For me, it worked better when I melted far hotter.
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Old 10-12-2017, 12:26 PM
Jordan Kube Jordan Kube is online now
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I think it's the pot formulation. All of those pots have a pretty high silica content. It's just dissolving into the glass. Try a high alumina pot.
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Old 10-13-2017, 07:42 AM
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I'm trying to look at it from the view in the periodic table.
AL is 13
Si is 14
O is 8

So mullite is 3AL2 2Si02 3(26) + 2( 14+ 8+8) or 78AL +28+16
Sillimanite is AL2Si05 (26) + 2(14+40) or 26AL+ 28 +40
I think that's right but I welcome corrections. I get fuzzy in the periodic table.

What it does suggest is a large difference in the Alumina content in the two bodies and Mullite was already a problem in the original Ipsen pots or at least I thought there was. Sillimanite , by the above math has only 1/3rd the alumina that mullite has and it seems to me that mullite would represent about a 76% alumina content with 24% silica , so the sillimanite is around 25 alumina at best.
The stock Alumina pot from either High Temp or Engineered Ceramics is 90% Alumina or so both companies claim. High Tmp has china clay in the mix for elasticity. They are both engineered with coarse grain tabular Alumina which is nicely resistant to thermal shock. La Clede is a very fine grain pot which has poor thermal character but performs better above 2400F than it's competition. I rarely see the need to melt that hot but I know people who do it.
Normally, I like to see 70-90 melts from a pot and then I change it out. If I was using the sillimanite pot, I suspect I'd need a change at about 40-50 Melts based on looking at that photo.
For me, the real question comes down to the amount of time you are down changing pots, not so much the pot cost. I mean, we ship pots to South Africa And Malaysia. Also to Oz and NZ. It is obviously more expensive to do that but I would not want to have poor looking glass most of the time. Either that, or build an AZS tank which is quite the different furnace. Even though the Fastner pots are green, they might be a better choice. I know Gaffer uses them a lot. They also use some Chinese pots as do we as well in the Shanghai shop. I'm curious about what Michael Alhfeldt uses in Sweden. He used the EC pots for a time but stopped years ago.
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Old Yesterday, 08:46 AM
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I think that the Pot looks normal for having been in your furnace for a year. I would expect that it is simply time to change the pot, which I consider a normal (if not frustrating) cost of doing business. I am guessing that your small bubbles are coming from melting at far too low of a temperature and/or the difficulty in fining out cullet.
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Old Yesterday, 09:49 AM
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In looking at the website and their calculation of alumina content, it appears to me they claim your pot is 55% alumina which would be really low. It also list the silica content as 42.5 % which is a lot of silica and that's where the swiss cheese look is coming from.
Although I'm always suspicious about zirconium in a crucible for soda limes, the DCA looks like a better mix for what you are doing. It lists a 13.5% silica content and a 72.5% alumina content. The literature claims it's more suited for "Colored glasses or "more corrosive glasses". It will be shockier.

I don't view 72% alumina as being high but it beats the sillimanite mix by a long shot. You might consider trying it.

I am inclined to agree that you're not going to get bubbles to rise out of your cristalica without getting a good deal hotter. That in turn will increase the corrosive action on your pot. With ultra fine bubbles, I think you're really looking for re-absorption into the glass. Obviously that isn't happening.
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