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simatai33

Cone 6 Glaze Problems - Turner White

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Hello, I am new to this forum and new to mixing and firing my own cone 6 glazes in an electric kiln. Boy, what a learning curve! I have much to learn. I'm grateful to you guys for posting so much useful information here:)

 

I just fired a small kiln load of pcs of different stoneware clay bodies with Turner White recipe that I found on Ceramic Arts Daily. I thought it would be a stable fool-proof glaze so I regrettably did not test. 

 

My kiln has been overfiring, so this firing I made the adjustment of firing to cone 5, holding for 15 minutes. My pyrometer cone reached cone 6, so temperature is good...but....maybe some improvements still need to be made. 

 

The turner white came out different on different clay bodies. On Venus White clay (which has some grog) and B-mix clay, it came out an undesirable chalky, super matt. The surface almost feels like clay body. There was also some crawling happening on the Venus White clay body pcs. On red b-mix and electric brown, the finish was a tiny bit more satin matt...but not much more.

 

I also noticed a lot of crackling, popping well after the pieces had cooled down. On some of the Venus White pcs, lots of crazing where I had dripped Leach Blue over the Turner White. 

 

What do you think the problem could be? Could some of this have to do with kiln temperature and/or cool down? And could I save these pieces by coating with clear glaze and re-firing? Or mix a new batch of turner white glaze with additional chemical to increase sheen? 

 

 

 

 

SmartsyArtsy likes this

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Sounds as if the glaze underfired.  Were there any other glazes used in that firing and, if so, did they turn out okay?  Or was the entire load Turner White?  You may need to fire up to cone 6 or fire to cone 5 and increase the hold more than 15 minutes to get melting of the glaze.  Almost by definition, a matte or semi-matte glaze results when you get an immature melt. 

 

You can fire the exiting pieces as they are; no need to add anything.  They likely just need to be refired a bit higher in temperature . . . but only try one or two pieces to see how they withstand refiring before doing all the pieces and ending up with a kiln full of shattered fragments. 

simatai33 likes this

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Hi and welcome to the forums!

 

I'm not to keen on the idea of using a ^10 glaze at ^6, for it be mature at the higher temp it's going to be underfired at the lower.

 

If you haven't already got it a good book to learn the basics of glaze chem is Mastering Cone 6 Glazes by Hesselberth and Roy. There is a chapter in there about figuring out what COE or CTE (coefficient of thermal expansion) will fit your clay without crazing or shivering. Glaze calculation software is a big help too but more of a learning curve than reading the above book.

 

Test tiles are your best friends! Make lots and lots of them, and take the time to test your glazes on each of the clays you use. 

 

If you are looking for a white glaze for functional food bearing surfaces then a glossy clear plus about 10 - 14% zircopax will give you a clean crisp white. Zirconium (the stuff zircopax is made from) also has an extremely high melting point so that when you add it to a glaze it actually helps reduce crazing. If you want a softer white then tin oxide, or a blend of zircopax plus tin oxide. 

 

I would chalk that glaze load up to experience and move forward. 

 

I'm not big on handing out random recipes since your materials, clay and firing schedule are going to be different from mine but here are a couple basic white glazes that fit mid expansion stoneware at ^6 quite well if you want to include them with your tests.

 

Gloss White ^6

Talc 11.20
EPK KAOLIN 20.40
SILICA 17.00
WOLLASTONITE 9.70
Frit 3134 19.50
Zinc Oxide 2.40
Custer Feldspar 19.50

Bentonite  2

Zircopax 11.60
total: 113.30

 

Hill's White Satin ^6 (summer 2014)

EP Kaolin    13.70
Silica      19.80
Nepheline Syenite  27.50
Frit 3124     12.20
Talc 14.10
Whiting 10.00
Zinc Oxide 2.70

Bentonite  2.00
Zircopax 10.00
total: 112.00

simatai33 likes this

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Bciskepotter and Min, THANK YOU for your feedback!!  :)

 

The entire load was not Turner White. I had pcs in semi-matt light grey, pale turquoise, black and leech blue (denim matt). None of the colors came out right actually - the black came out clear and so did the Leech Blue. The pale turquoise and light grey were inconsistent. I suspect something is up with the temperature. I have a digital temperature controller on my Paragon kiln, and after a couple overfirings, I watched it carefully and after doing some reading, fired it up to cone 5, 2167F, then put it on hold for 15 minutes because I saw that the Pyrometer Cone 6 had already bent to the correct position. Maybe I need to hold another half hour?

 

Bciskepotter, I will take your advice and just re-fire some of the white pieces and see what happens.

 

Min, WOW thanks so much for the recipes! I had read the Turner White was stable for Cone 6, but perhaps that's not so accurate. I will definitely try your recipe for Hill's White Satin. If I don't have Zircopax, what's the ratio of zinc oxide I should use? I read that I can substitute zircopax with that.

 

I do have a couple books on Cone 6 glazes, but you're right TESTING is the key! I guess my last 3 glaze firings have been test pieces then! :wacko: 

 

Coming from four years of firing in cone 10 reduction, cone 6 is quite frustrating for me because I really don't like that "plasticy", lack-of-depth look that oxidation seems to give. So I try to layer my glazes but half the time it comes out looking drippy and sloppy vs. the organic fusion of color and clay in reduction firing. Gotta keep practicing....

 

I am still trying to figure out what type of clays I want to focus on, so have been experimenting with B-mix, Venus White, Electric Brown, Morrocan Sand and Red B-mix. I find that my glazes look radically different on different clay bodies, and only specific combinations of layering turn out good but are not 100% reliable (maybe because I haven't figured out a stable firing program yet). I just looked up that book you recommended and it's listing USED for $166 on several websites. Is it so expensive because it's out of print?

 

 

 

 

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You can get an ebook version of Mastering Cone 6 Glazes or they have issued a b/w version that you can buy from the authors.  http://www.masteringglazes.com/ The cost of the original out of print versions is ridiculously high. The book was self-published by the authors, so runs were rather limited.

 

Check out the work of Steven Hill -- he made the shift from cone 10 to cone 6 and has written articles and has a dvd on how he applies his glazes and glaze recipes.

simatai33 likes this

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I wouldn't sub zinc for zirco. In reduction zinc mostly burns out but in oxidation it is a flux. You could try 4 or 5 tin but it's about 6 times the price of zirco.

 

I've found layering glazes for ^6 can give some nice effects when you layer 2 dissimilar glazes. A fluid gloss over a stable mat, glazes with different fluxes, different colourants etc.

 

Yeah, it sounds like something's up with your thermocouple, trust the cones. Are you slow cooling to get the mats?

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Min - My glaze recipes for the pale turquoise and light greys are semi-mat. I have not tried slow cooling yet. I'm not sure how to program my kiln to do that. I'm going to call Paragon this week to ask about the thermocouple and see if they can advise me. Thank you!

 

And I used half the ratio amount for zinc to substitute for the zirco, which I ran out of for this particular firing. Could the zirco have caused the chalky finish, or simply that Turner White is really a cone 10 glaze recipe? My last firing of the Turner White batch which had zirco in it (instead of zinc) was glossier but an uneven finish, which I suspect was due to the glaze being too thinned out with water. So this time I made a thicker mixer. Coverage was better, especially on white clay bodies, but way too chalk mat!

 

Thanks again to both of you for all your suggestions and input. It's greatly appreciated! I will check out the e-Book and start studying! 

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For better glazes at cone 6......... FIRE DOWN!  Don't just shut of the kiln...... run the firing profile to extend the cooling section of the firing.  THAT is where the "magic" of "cone 10 reduction happens...... it's not so much the reduction.  It is the slow cooling of the large thermal masss and better insulated kilns that does the trick.

 

Small electric kilns (usually used for cone 6) drop like a rock.

 

best,

 

....................john

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 Could the zirco have caused the chalky finish, or simply that Turner White is really a cone 10 glaze recipe?

 

Yes. Zircopax is refractory, it will stiffen the melt. Since Turner White is underfired at ^6 to start with. it will be even more undermelted with a decent amount of zircopax added. If you add a refratory to an already stiff glaze you need to add some more flux to counter that.  (each gram of zircon opacifier is approx 7/10's zircon and 3/10's silica (glass former). Make any sense? 

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JOHN: Thanks for the tip on cooling down the firing. My Paragon TNF-823 kiln has been overfiring at cone 6, taking 12-14 hrs at medium speed. That's why I decided to fire to cone 5, HOLD for 15 min until I saw the witness cone 6 had bent properly, then I shut it off. If I program slow cooling, what do you recommend I set the cooling rate in degrees per hour? New to this, so not sure what to program. Thanks again! 

 

MIN: I'm sorry I meant in my previous post that the ZINC could have caused the chalky finish. I substituted that cuz I ran out of Zirco. But I remember someone saying zinc has more flux. Ack. I think the real issue is that it's really a cone 10 glaze that was underfired in my kiln. 

 

Will be doing LOTS of tests! Thanks again for all the valuable feedback everyone. :) ALOHA from Hawaii...

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