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Crazing
Sept 28, 2015 22:06:36 GMT -6
Post by evan cornish-keefe on Sept 28, 2015 22:06:36 GMT -6
The clay supplier near me recently changed the feldspar used in many of their clays, I assume for the same reason yours changed. Lots of people around here have been having issues with those clays dunting, and the clay supplier has been refunding them. Some people say pieces came out of the kiln cracked and others seemed ok for a week before breaking. If possible I'd look at whats breaking in relation to the feldspar change. What I've heard is that an excessive amount of silica is causing dunting, and that a better formulation should be able to cool quickly through quartz&cristobalite inversion.
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Crazing
Sept 29, 2015 16:47:09 GMT -6
Post by tileman2 on Sept 29, 2015 16:47:09 GMT -6
Arnie:
I have the MSDS sheet for the standard custer feldspar. Going to dig around a little and see if I can come up with a new one for the recent changes. Between your piece, Evans' comments, and email from my supplier: there had to be a significant change in custer. Too many issues for it to be coincidence. Typical analysis: SiO2 72.92% Al2O3 14.95% Na2O 4.13% K2O 3.83% CaO 2.10% CaF2 1.10% Fe2O3 0.03% LOI 0.86%
Melting Point 1100°C to 1300°C
Going to put my money on the amounts of sodium (Na) or potassium (k) changes in the new custer. Will make a guess that one or both is lower than before.
Tom
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Crazing
Sept 29, 2015 23:35:13 GMT -6
Post by evan cornish-keefe on Sept 29, 2015 23:35:13 GMT -6
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Crazing
Sept 30, 2015 8:22:06 GMT -6
Post by billcampbell on Sept 30, 2015 8:22:06 GMT -6
Custer does have less K, and is now a coarser grind. The coarser grind does not seem to be an issue, however.
We have had to change our body to work with the different Custer. The glazes seem to remain the same.
Bill Campbell
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Crazing
Sept 30, 2015 20:56:26 GMT -6
Post by tileman2 on Sept 30, 2015 20:56:26 GMT -6
Arnie:
A suggestion: try adding 15-25% of cornwall stone to your engobe.
Evan- great article- added it to my information library.
Bill: thanks for the confirmation.
Tom
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Crazing
Sept 30, 2015 21:17:45 GMT -6
Post by jfox on Sept 30, 2015 21:17:45 GMT -6
Back in the 70's everybody on the west coast used Kingman feldspar then it started to vacillate in composition and then shortly thereafter the mine shut down and we switched to custer
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Post by billcampbell on Oct 1, 2015 6:59:16 GMT -6
Custer may end up being a problem for a while. It may be that you will have trouble getting it. The mine is now sending a lot of its production to a large ceramic Mfg. Co. To us that means that it is scarce. There was a problem with a big piece of their machinery that has also limited production. Several suppliers have not had any to sell. We used G200 for a long time, but that is now gone. That mine is closed. I do not have any solution to this problem.
I was told that some of our suppliers may be getting a Potspar from Europe. I hope that it works.
Bill
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Crazing
Oct 1, 2015 13:08:36 GMT -6
Post by tileman2 on Oct 1, 2015 13:08:36 GMT -6
Bill: You mean the good stuff like this: Still have access to Custer; if you need help will put you in contact directly. Bill: I have 1200lbs on hold- if you want it I would suggest you PM shortly before it disappears. Tom
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Crazing
Oct 5, 2015 19:23:13 GMT -6
Post by tileman2 on Oct 5, 2015 19:23:13 GMT -6
Arnie: Here is a new issue for me anyway. I have made several dozen of these bowls from a press mold; until I made three new ones from a new box of cone 6 porcelain from a different supplier. The whole right side collapsed including the side walls; the left side kept its form. The bottom is not warped; but the whole right side acted like it was melting. Thinking this is a custer feldspar problem. Pulled if from the kiln at 117F, after a natural cool. Every once in awhile I hear the glaze tinking- got a feeling this one is going to split. Tom
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Crazing
Oct 12, 2015 8:25:20 GMT -6
Post by evan cornish-keefe on Oct 12, 2015 8:25:20 GMT -6
Tom, I'd guess this wouldn't be prone to dunting, looks like a different issue. My understanding is that Mullite starts to develop in clay bodies somewhere above ~2100F, cone 10+ clays develop enough mullite to give the piece structure which allows for more glass formation without warping. At cone 6 not enough mullite has formed so clay cannot be as vitrified without slumping. Also, using soluble fluxes like lithium carb in a glaze may influence how much your clay is melting??? The form and thickness of the piece also can exaggerate the warping.
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Crazing
Oct 12, 2015 19:04:59 GMT -6
Post by tileman2 on Oct 12, 2015 19:04:59 GMT -6
Evan:
I am going to put lithium low on the list; have used the same percentage for some time without this issue. However, I do slurry the cone 6 and then add molochite and #6 tile kaolin. So the clay is more like cone 7-8 when I get done with it. So your point about vitrification may be the most likely issue. I slab roll the clay to nearly a 1/2" thickness before pressing into the mold: so perhaps a bit heavy. So use to thicker slabs because of tile work; never gave that issue much thought. Think I have one box of cone 6 left that I have not beefed up: think I will roll and press it as is: should tell us something. Thanks for the input. By the way, I made three bowls from this slurry mix. The first piece, and the piece I pulled out tonight did not slump. The one above was the second and it did fail: that is what baffles me.
Tom
Tom
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Crazing
Oct 21, 2015 8:24:04 GMT -6
Post by evan cornish-keefe on Oct 21, 2015 8:24:04 GMT -6
Well, let's rule out the piece being too thin. So you've fired this clay to cone 7 & 8? With and without the added molochite and tile 6?
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Crazing
Oct 21, 2015 16:53:44 GMT -6
Post by tileman2 on Oct 21, 2015 16:53:44 GMT -6
Evan:
I fire the straight cone 6 porcelain to 2230F- close enough to maturity. The custom mix I fire to 2232-2236F; so slightly under maturity. Which is why I still cannot explain the extreme slump- but just on one side. I am going to run the program again with 6 cones around just one shelf just in case for some odd reason I have a hot spot. Should not have them, but stranger things have happened inside kilns.
Tom
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