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Old 06-30-2008, 06:47 AM
Pete VanderLaan's Avatar
Pete VanderLaan Pete VanderLaan is offline
The Old Gaffer
 
Join Date: Apr 2002
Location: Chocorua New Hampshire
Posts: 22,261
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The binder for tabular alumina in this instance is kyanite which converts to mullite at 2150F. Naturally occurring mullite is really rare. You could add calcium aluminate as a cement binder if you wanted to. It is the binder in castables. I actually sell both products.

We didn't "find " the product, we developed it in partnership with High Temp. It has been in the works for quite some time. We are a small project in a very big company. They have made me the exclusive distributor for the product line. The dedication to quality control at High Temp amazes me.

The bigger the pot, the lower the sales, making it more expensive to justify the costs for developing the mold which are extensive if you do it right. I sell 20 270 lb pots for every 1 400 lb pot. The risks involved in firing big things is far greater, involving really long soak times before 212F and also 1000F. Currently the price of alumina is changing every month where it used to be priced once a year. Finally, the big pots take up far more room in the kiln and take longer firing times which cost more as well. These kilns are not small. Forklifts can drive into them and we only work in the small casting shop!

Finally, for the glass shops which find these larger pots appropriate, their true cost is in the cost of going to high temperatures to melt, coupled with the down time for production that they have when they are melting , not working. One shop I supply with the 34 inch pot runs two of those furnaces and they are charged twice a week each. It's a matter of scale. Lino will go through one of the those pots in a single sitting.

But I do agree with you that the 270lb 24 inch pot is the single most efficient for the small studio owner. When people want the 22 inch pot, which is rare, I recommend buying the 24 inch instead and simply not filling it all the way. The real time spent charging a pot is in the last two inches of glass, which always seem to take forever. I prefer putting a last big charge on, setting the controller and going home to sleep. If the glass is down a bit, I still have 225 lbs of good stuff.

Finally, there is no separation of material types in a crucible. There are coarse tabs and very fine particles. When the pot is vibrated, the fines come to the surface of the pots in the molds. The big stuff that prevents thermal shock is hiding out in the middle of the pot. When a pot begins to fail, the fines are being consumed by the glass itself and the big stuff gets exposed. This creates a disproportionate surface area exposing the glass in the pot to far more alumina surface than the new smooth pot had. Subsequently, cords start to form and they are pretty much unavoidable. The pot is failing . This is basically why I don't like investing a crucible. The investment stops an actual leak but the glass is still pretty crappy by my standards. Pots are not engineered to be permanent installations. 70 to 90 charges is what to expect if you want continually good glass which is the whole point of a pot furnace over a tank.
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