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Post by mariewright on Jul 26, 2018 12:15:04 GMT -6
Hi All,
Every now and then my glazes will seem to go on well, but as they dry they crack and sometimes flake off of the surface of the piece. This is happening to a student of mile, and I'm not sure what to tell her to do to correct it.
Her glaze is a 644/silica/zinc glaze with a bit of bentonite in it. She is using CMC and I think Darvan 7 with it.
Any suggestions about how she can make her glaze stick to the pot?
Cheers,
Marie
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Post by adammacmillan on Jul 29, 2018 20:27:46 GMT -6
Why is she using darvan? Also, how much cmc? She should be fine without darvan and 2% cmc
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morgan
New Member
Darn...lot of good info on the old forum. Sad that it got unstable. But whadya do?
Posts: 21
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Post by morgan on Sept 18, 2018 5:49:18 GMT -6
Marie...I was having a heck of a time with exactly the same issue. BAD. What worked for me was to start calcining my zinc to around 013. Seems like zinc has gotten "wet" in the last couple of years and has huge shrinkage. I also use 75% calcined bentonite/25% raw bentonite for suspension...with a pinch of Epsom salts after mixed. It's always a changing game with this, isn't it. lol Hope to see you at a show soon.
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morgan
New Member
Darn...lot of good info on the old forum. Sad that it got unstable. But whadya do?
Posts: 21
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Post by morgan on Sept 19, 2018 3:14:12 GMT -6
My apologies, Marie. I misstated the calcine temp for the zinc. 013 was a bit too much and I had to grind it to get the little pebbles crunched. Now I go to 015. Enough to dry it out, but doesn't make little hard pebbles from sintering.
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Post by bill2015 on Sept 24, 2018 8:18:36 GMT -6
I agree with Morgan that using calcined zinc is a factor with drying glaze cracking/flaking. I buy mine calcined and store in plastic lidded tub with a sheet of plastic between lid and tub for better seal. If I need to calcine I just fire to red heat, 1200F.
I use a CMC solution to wet my dry glaze materials. I make solution adding 20 grams of dry to 1 quart of hot water and let sit a few days. If I need right away I throw both in blender and mix until blended. I mix the solution at about 1/4 cup to 100 grams dry glaze materials. Mix with immersion blender, add little water if needed, then sieve. Using the CMC solution I never have cracking/flaking. FYI - Just read that bentonite can act as crystal enhancer so I plan on adding 1% to my next mixing of glazes....
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Post by mariscal on Sept 25, 2018 0:38:34 GMT -6
Hello, Yes!! I have the solution, change water for "coke". It is not orthodox but works very well
Jose
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Post by tileman2 on Sept 25, 2018 4:57:41 GMT -6
Jose:
Crystalline glaze is unorthodox; that is why we fire it. We crystalliers being the rebellious, non- conformist bunch we are.
Tom
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Post by mariewright on Sept 30, 2018 19:17:41 GMT -6
Hey guys, this is all really great stuff - thanks a lot! The answers on this forum are always interesting. I was not expecting coke!!!
I think that her issue was that her glazes sat around awhile and the CMC degraded, but I'm glad I asked - this is all good food for thought.
Cheers,
Marie
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morgan
New Member
Darn...lot of good info on the old forum. Sad that it got unstable. But whadya do?
Posts: 21
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Post by morgan on Oct 29, 2018 5:37:02 GMT -6
Jose: Crystalline glaze is unorthodox; that is why we fire it. We crystalliers being the rebellious, non- conformist bunch we are. Tom Tom, I've NEVER been described as orthodox. Heck...just barely above heathen caveman. Guess I've found my calling in life. lol Jose, I'll have to try that "coke". You mean to just sub it completely for the water, or just add a bit? Original coke, diet coke, caffeine free coke, coke zero??? So many choices. hahaha |
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Post by evan cornish-keefe on Nov 1, 2018 8:26:17 GMT -6
I agree with the idea of calcining zinc. I have some glazes that seem to change over time when mixed up and stored in a bucket, it's hard to tell exactly how it changes after a few months but I've wondered if the zinc sitting in the wet glaze could be an issue over time?
Also, my understanding is that darvan and bentonite will somewhat counteract each other (deflocculant lessens to ability of Bentonite to suspend the glaze), which is why epson salt or another flocculant is often used in combination with Bentonite, though that may just be the orthodox approach.
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Post by bill2015 on Nov 3, 2018 9:09:25 GMT -6
My understanding is that it's the frit that changes when stored wet for long periods of time. The frits we use in our glazes contain a large percentage of sodium and sodium is a soluble material. Even though fritted there is still some that remains soluble, so this changes the glaze over time.
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Post by tileman2 on Nov 4, 2018 7:46:27 GMT -6
Bill:
Correct, soluble salts migrate to form nodules. (hard pebbles). The reason coke works is due to the citric acid titrating the PH down to a level that prevents this chemistry from occurring. Nep Sy has 14-20% soluble salts; which create the beloved "hard panning" of those glazes. Tartaric acid will do the same, been using it for years. Organic, and will mildew if allowed to set for extended periods. Tartaric acid is great for cooking too: commonly known as Cream of Tartar.
Tom
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