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Old 10-13-2017, 08:42 AM
Pete VanderLaan's Avatar
Pete VanderLaan Pete VanderLaan is offline
The Old Gaffer
 
Join Date: Apr 2002
Location: Chocorua New Hampshire
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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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