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Paul Stokstad

Difficulty applying an ice crackle glaze

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Hi, an amateur potter here posting for the first time in this forum. Sorry in advance for the length, but I've got a ton of questions and would love any advice from the glaze experts out there.

I've been experimenting with the cone 10 Fiske Ice Crackle Glaze from John Britt's article in the November 2011 issue of Ceramics Monthly. I'm getting gorgeous crazing on some test tiles and small test bowls, but I'm driving myself crazy trying to apply it effectively. The recipe is 91 Nepheline Syenite, 5 Bone Ash, 3 Ferro Frit 3134, 1 EPK, and 2 Bentonite, mixed to a specific gravity of about 160. According to the article, the glaze needs to be applied extremely thickly: 3-6 mm.  (I also tested some of the other cone 10 recipies in the article, which are lower in neph sy, but the Matt Fiske recipe crazed the best on the clay body I'm using.)

I initially tried applying this glaze by dipping or pouring, but I've been having several problems. First, the glaze dries  with extreme drip marks--the rims of the bowls form stalactites (which are then stalagmites when I turn the bowl rightside up).  I try to minimize the drips by shaking and rotating the bowls as I hold them upside down, and by wiping the rim as the first coat dries, but it's probably the worst glaze I've ever used in that regard. Second, when I apply a second coat, which I've got to do to even approach 3 millimeters, the glaze is cracking as it dries, especially on thinner-walled pieces. Third, the unfired glaze coat is extremely fragile.  I worked in a shared studio, and unless I can improve the adhesion, I'll probably lose a large percentage of the work before the kiln ever gets lit.

I've tried both flocculating (using epsom salts) and deflocculating (using the sodium silicate) the glaze to improve the application characteristics. I wasn't sure which way to go, because the article warned both about the flocculating effects of bone ash and the deflocculating effect of nepheline syenite. The epsom salts exacerbated the tendency of the glaze to gel in the bucket and worsened the cracking of the raw glaze. Adding a little bit of sodium silicate, which was the article's suggestion for the recipes with bone ash, seemed to improve the cracking and somewhat harden the glaze coat, but exacerbated the drip problem and also caused the glaze to hardpan in the bucket very quickly.  So I feel caught between a rock and a hard place.  I've watched the John Britt videos and read the digitalfire articles about flocculation and deflocculation, but I'm not sure what the right approach is in this situation. Any suggestions? 

Yesterday, I tried spraying the glaze. (It was my first time using the spray booth, but I was desperate.) I was able to get the glaze on thickly without cracking and without drips, but the surface was extremely pebbly and seemed even more fragile than the dipped glaze. My current plan is to try adding some CMC gum or Magma to the glaze to improve the strength of the sprayed coat. But searching these forums and the rest of the web, I couldn't find any much advice about the use of additives for sprayed glazes.  Any suggestions on that front?

My other thought is to try overspraying the glazed piece with starch or something else to strengthen the glaze coat. Does anyone have any experience with that technique that they could share?

Thanks in advance!

Paul Stokstad

 

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I would stick with spraying but add some CMC solution. Would have to mix up another batch of glaze (but wouldn't need a large amount if you spray) since you can't add CMC directly to a ready mixed glaze. I would also try spraying on the glaze as thickly as you can then bisque fire the glaze on then spray a second thick layer on. The CMC will help with both hardening the raw glaze on the pot and with suspension. CMC will slow down the drying time of the glaze so you might have to spray, wait, spray again to build up the thick glaze coats. I wouldn't worry too much about the specific gravity with spraying, just mix it as thick as you can but not so thick that it won't spray. If you have control over the bisque schedule you might also try bisque firing a cone or two cooler so the clay is more porous.

Welcome to the forum, would love to see your results when you get it sorted out!

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Just now, Paul Stokstad said:

Thanks for the suggestions! I've usually seen copper carbonate suggested as a preservative (though I haven't tried it myself). Does cobalt carbonate work too?

Sorry, typo, I meant copper carb. I've fixed it.

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Copper carb is weak in color so that will not alter the color-Cobalt carb is an unknown to me-you can try it? the copper works well.

You can use hairspray when done glazing to lock it in place.I never have but read it works

Edited by Mark C.

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.25% cobalt carbonate will give your glaze a lovely soft blue colour. Which isn't so lovely if that's not what you were going for. To my knowledge, it doesn't work as a preservative.

I have used both hairspray and laundry starch to overspray glazed pieces back in the days when I was still transporting all my work. Both help, but it isn't a total fix to the problem. You still have to handle everything carefully. Hairspray is cheaper than laundry starch.

 

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