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Cone 6 glaze falling off pot (see picture)


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Hi there... I've been firing my pots to cone 6 with an 008 hold on my Skutt kiln for years. I've introduced a new glaze combo to my repertoire a few months ago.  I dip the entire pot in my white glaze, then dip the lip in my grey glaze. Approx 50% of the time the grey glaze slips off of the pot in areas. I dip the second layer after the first layer is dry to the touch (a few hours). Some of the pots turn out fine and some with "slipping" problems in the same load. The grey recipe contains:

Neph Sy,  Dolomite,  Gerstley Borate, Whiting, Zinc oxide, EPK, Flint, and Mason stain

Could it be my hold in my firing? I've tried many, many other grey recipes and they don't turn out grey.  I'm inserting pictures... you can see the problems are usually on the outside of the bowl and all different areas of my kiln. Perhaps its the interaction on my white glaze (which I'm not eager to change).  

Thanks for everything!!--- Margie

IMG_1497.jpg

Edited by Marge
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Could be from a few things, first if the base glaze was too dry when the top glaze was put on the glaze can lift and peel from the pot. You usually see this before the pot is dry and going in the kiln though. If this is the cause then apply the gray glaze as soon as you possibly can.  Second thing is the double glaze layer might  just be too thick and the glaze is crawling while firing. (It's hard to tell in the photo but the gray glaze isn't coming right off the pots in sharp slivers is it?) To check for a thickness issue glaze a scrap piece of bisque, or test tile, as thickly as you glaze your pots with the layered glazes and see if you can see cracks in your unfired test pieces once the glaze has dried. And lastly, some glazes just don't layer well together.  It's not your firing schedule.

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Thanks Min... I think (unfortunately) the 2 glazes just don't like each other.  Although it fires beautifully with a different mason stain (2%) less.  Could 2 different mason stains react differently on the pot... or do you suggest trying 8% of the "troublesome" stain vs 10% in my 5 gallon pail? Does that make any sense?

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You layer the same glaze but with a different stain overtop of the white with no issues? 

I really don't think using 2% less of the same stain will make a difference. I'm leaning towards a thickness or timing issue if you are firing these same two glazes together but with different stains in the top glaze. Try some test tiles of clay that are close to the same thickness as the rim area of your bowls and test glaze thicknesses plus timing of application of top glaze.

I wouldn't add anything to your 5 gallon pail until you nail down what the issue is. 10% stain is too expensive to take a chance with. 

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10 hours ago, Marge said:

Hi there... I've been firing my pots to cone 6 with an 008 hold on my Skutt kiln for years. I've introduced a new glaze combo to my repertoire a few months ago.  I dip the entire pot in my white glaze, then dip the lip in my grey glaze. Approx 50% of the time the grey glaze slips off of the pot in areas. I dip the second layer after the first layer is dry to the touch (a few hours). Some of the pots turn out fine and some with "slipping" problems in the same load. The grey recipe contains:

Neph Sy,  Dolomite,  Gerstley Borate, Whiting, Zinc oxide, EPK, Flint, and Mason stain

Could it be my hold in my firing? I've tried many, many other grey recipes and they don't turn out grey.  I'm inserting pictures... you can see the problems are usually on the outside of the bowl and all different areas of my kiln. Perhaps its the interaction on my white glaze (which I'm not eager to change).  

Thanks for everything!!--- Margie

IMG_1497.jpg

Don't wait so long between dips. When the moist look just vanishes do your second dip.

Is image prefiring?

Odd peeling. Does the glaze have a gum or CMC content?

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Interesting ! I have seen this happen with relatively different melting points of the two glazes. If we assume the additional mason stain makes the overlay glaze more refractory and slows it’s melt a bit then we have a situation where the base glaze begins to melt early enough to cause the overglaze to crack, sheer and lose adhesion  locally, maybe only slightly. Thickness can obviously affect this if indeed it is mechanical in nature as well as when these are dipped.

I would be curious to analyze the two recipes chemically then test and confirm their melt. I recently experienced this with a group of  three studio glazes that had this characteristic over a matte we designed to melt and move at about cone  4-1/2. Our cone 4’ish test firings confirmed the complete melt of the matte. Turns out the three studio glazes were loaded up with Boron to the point that they melted perfectly at 04.  They were mixed in the trial and error days so adding more and firing was a thing to do.  Unfortunately for a cone 6  glaze this caused other issues such as a propensity to crawl or separate.

Not saying this is the issue, but I would have a look at each recipe and maybe testfire where these actually melt.

Picture below is about how we want it to move with all our glazes, maybe just a tad less. Taking out the excess Boron from the three offending recipes and normalizing them has cured this issue so now students can work in a pretty carefree way and get a decent result.

 

Edited by Bill Kielb
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Thanks everyone for your helpful thoughts. The picture is the finished product... not pre-firing. I have a new load going in tomorrow which I dipped the second layer immediately after the first.  @neilestrick here are the 2 recipes... please let me know if anything jumps out at you. Thanks again!!!!

White                                                                     Grey

18 Gerstley Borate                                         24  Neph Sy

27 Neph Sy                                                          11  Dolomite

12 EPK                                                                   12  Gerstley

8 Whiting                                                              4   Whiting

15 Talc                                                                     2  Zinc Oxide

9 Silica                                                                      7  EPK

9 Zircopax                                                             40  Silica         10 Mason Stain

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On 4/1/2019 at 7:04 PM, Marge said:

Thanks everyone for your helpful thoughts. The picture is the finished product... not pre-firing. I have a new load going in tomorrow which I dipped the second layer immediately after the first.  @neilestrick here are the 2 recipes... please let me know if anything jumps out at you. Thanks again!!!!

White                                                                     Grey

18 Gerstley Borate                                         24  Neph Sy

27 Neph Sy                                                          11  Dolomite

12 EPK                                                                   12  Gerstley

8 Whiting                                                              4   Whiting

15 Talc                                                                     2  Zinc Oxide

9 Silica                                                                      7  EPK

9 Zircopax                                                             40  Silica         10 Mason Stain

Ok I will take a stab at this. I looked over each recipe and what impressed me most was the very small amounts of silica and alumina under UMF.

Anyway, long story short, I made slight adjustments to these for trial using your existing ingredients and attempting to reapportion them  so these glazes behave more uniformly at cone 6. No guarantees but they might be worth mixing up 100 or 200 grams and testing to see if they perform better. 

I am not going to go into the technical thought behind these right now other than to say these glazes have been normalized in composition in an attempt to make them more reliable. They  both exhibit durable chemistry in the trials provided. Only thing left to do is fire them to cone 6 and see if you like their behavior separately and layered.

I did not include your mason stain in the grey recipe, you should still add it to get your depth of grey desired. I just simply left it off the spreadsheet.

good luck- if you test and it works well, then I will be happy to give you the specific how and whys for the minor changes.

Edited by Bill Kielb
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@Marge forgot to add this. Your existing glazes likely fire sort of matte or some level of satin. If the replacements work then it is an easy matter to dial in the amount of gloss (or matte)  you prefer by slowly reducing the silica in each of the recipes until it becomes your desired level. Maybe a 2 - 4 gram progression

Edited by Bill Kielb
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@Marge, if you want to try one more thing while testing Bill's versions of your recipes try using Mason Best Black stain 6600 in place of the 10% gray. In the vase below I've used only 0.6%  Mason 6600. It would save you a fair bit of money versus the 10% gray stain if it works in your base glaze. Sometimes using a black stain to make a gray just gives you a washed out version of the oxides used to make the stain but with some glazes you can get a good gray this way.

IMG_2713.jpg.20cb223092e5690daac9559222232f31.jpg

 

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