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Double Dip Flakes Off When Drying


Diz

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A question for those who have the experience and knowledge….

 

A couple days ago I mixed up a batch of ‘Cobalt Blue’ for cone 10 and stoneware clay.  I have mixed and used this glaze before and never had problems.  However this time, when the glaze dries, it is extremely powdery.  It adheres well but I can easily brush off the drips while I would usually carefully shave the drips with a tool. My bigger problem is that when I dip the top edge of my cup in another glaze I usually use, the 2nd glaze cracks and easily falls off.  This double dip is a method I have used before and it worked beautifully with the glazes involved.  I also tried the 2nd dip using another glaze, with the same flaky  results, so I assume the problem is with my cobalt glaze.  I did fire a piece with the cobalt glaze only and the color and thickness showed no problems.

Here is the Cobalt Blue glaze recipe:

Custer feldspar             48.2

Flint                               24.1

Whiting                            7.7

Dolomite                          4.8

Gerstley Borate               4.8

Barium Carbonate           4.8

Zinc oxide – calcined       2.4

Tin Oxide                           .9

Copper                            1.9

Cobalt oxide                     2

 

Any educated guesses as to what I did wrong?  Can you suggest an easy fix or is recycle bin my easy fix? 

Thanks for all the help those of you share on this forum!

Diz

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I used the same custer (325) that I have used in the past so it is a several years old but had been used before in this recipe.  Only new bag opened for this mixing was new cobalt oxide. And as I said this is a recipe that I also mixed last year - did not get 200 til after that mixing.  My 325 is older than my 200 also.

 

Calcined zinc oxide is zinc oxide that has been fired to cone 06 before using in a glaze.

 

Guess I'll use this batch without a 2nd dip of any kind as it fires to a nice finish all by itself. Then  I'll mix a couple small test samples using the 200 and 325 and see if one recipe is not flaky - hope that will be an easy solution.

 

Since you mention an apparent difference in 200 and 325 custer - what changes does that imply in mixing glazes?

 

Thanks for your input!

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325 will flux (melt better)I gave up 200 decades ago.

Allowing your 1st glaze to dry completly will help with any double dipping.

Cobalt glazes are notorious for crawling. I know thats not your issue-your glaze has no clay in it to make it stick.So in the big picture it may be your issue.

I have no idea why your calcining zinc?

This is usually done to epk so it does not shrink to much and flake off.

 

I do not think the 200 mesh to 325 will do a thing to your flaking

You have such a high % of feldspar and silica and so many colorants this is bound to get powdery with no clay to stick it.

All I can add is what did you do different?

Mark

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How good does this stuff slurry. I don't fire cone 10 but I do test a lot of glazes and I have no idea how this glaze stays suspended. I bet even a small addition of clay would make the flaking stop, or any suspender like bentonite.

 

I did a quick google of your problem: When glazes 'powder' onto your hands and create dust during handling it can be more than just aggravating. The causes of dusting generally contribute to other problems (slurries settle quickly and lay-down varies in thickness).

 

http://digitalfire.com/4sight/troubleshooting/ceramic_troubleshooting_powdering_cracking_and_settling_glazes.html

 

That article seems like it nails your exact problem down.

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Mark - leave it to me to buy 25# of 200 last year when all I had used before was 325.  Guess I'll just stick with 325 in my glazes.  I obviously did something different - wish I knew what! - then my problem would be solved.  I have never taken a glaze chemistry class so I use calcined zinc oxide as that is what the recipe calls for.

 

Grype - yes that article holds a lot of info and it does appear to deal directly with my problem - will look at it closely tomorrow. I have plenty of EPK and also bentonite so will try one.  Love your location!

 

Thanks!

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Just be aware its very difficult to mix bentonite into an already mixed slurry, its nearly impossible to get it to mix in good, so if your gonna try something with your already existing slurries maybe try EPK first. 

 

You could always mix a small batch of your dry mix with some bentonite and see if it slurries better.

 

Good luck!

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Sorry my question was phrased weird, I more meant why are you calcining it. Never heard of or seen that in a recipe before. Turns out after a little googling it is a thing.

 

An interesting looking glaze I bet, if you are planning to use barium and copper in a glaze you really want to learn some glaze chemistry. As a retro fix 5% kaolin could help, around 15% is good but if you add that now it will stop the glaze melting. If the glaze is very glossy 5% kaolin will probably be accepted into the melting.

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Might be totally off base here, but I had a similar problems with low clay glaze in the past (chun/jun types). I was letting the glaze totally dry before the second dip to get a thicker second coat. It all ended up on the shelf/flaking off in sheets. I found that you had to do the second dip while the first coat was still slightly damp (ie hasn't turned powdery looking yet). I'm not sure if this is the same problem as what you're having, but it might be worth a try...

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Tonight I finally just washed the glaze off the cups..  Glad I did as it was so powdery that I could get some of the glaze off the cup simply by shaking it in the bucket of water.  Coming off that easy, I could just see it sloughing off the cup and landing on the shelf - then a grinding job - no thanks. I think I'll mix another small batch of glaze and see if it has the same problem. Since I have used this recipe before I think I must have made a gigantic booboo - guess I better wear my glasses when I mix this batch!  I have another cobalt glaze without the barium - perhaps I'll use that one.

 

Thanks for the help - I love this forum - lots of info and lots if inspiration!

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