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Cadaola

What Is Wrong With This Recipe?

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Hi!

 

Look at this glaze recipe for 1230°C (cone 6)

neph sye 30 / wollastonite 8 / colemanite 21 / EPK 10 / silica 31

 

I tested it and resulted very nice! But then I glazed an object and the glaze spit on the kiln shelf.

The pictures attached show how is the testing tile, how resulted the object and what was on the kiln shelf...

 

For spitting problems I already asked the forum several times, because sadly I continue to have this problem. So following your previuos advices I tried to decrase boron content, to apply thinner the glaze (and sometimes it worked) and to cool slowlier.

 

In this case I double layered the glaze (but it was not too thick) and I slow cooled. I know the boron content is high in this recipe, but do you think is the only problem?

 

Thank you a lot!

Paola

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post-4274-0-02459500-1430753114_thumb.jpg

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Paola, the forum glaze experts will certainly weigh in, but I don't see anything in the recipe that would make me think it was the recipe's fault.  Cooling shouldn't have anything to do with it, since the glaze probably came off on the way up.  I would suspect application problems-- maybe dusty bisque, or too fast firing in early stages.  How are you applying the glaze?  Sometimes dipping layers of glaze will result in the first coat loosening up and popping off, if the second layer goes on well after the first layer is dry.  How hot do you bisque?

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There is a lot wrong with this glaze.  KNO low CAO high Al2O3 high B2903 high, also colemanite is a generic formula for Gerstley Borate.  It would be better to supply B203 with a frit.  You would have to abandon this glaze and start over  or make a up a new formula and keep the formula within limits for cone 6.  It probably spit because of the high amount of B203 supplied by colemanite.

David

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David, I'm dubious that the boron is at fault, in regards to the spitting.  I've seen raku glazes that are half colemanite and they don't do this.  What would the physical mechanism be that would cause a glaze with 20 percent colemanite to blow so much of the glaze off?  There's enough clay in the recipe to cause the glaze to be fairly well-behaved in terms of application.

 

I agree that it isn't a very good glaze, but this is a problem not often seen to such an extreme extent.

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Looks more like the glaze fell off the pot than spitting. I agree with Ray that this looks like either an application issue or firing while pots are very wet. Do you wipe your pots down before glazing to remove any dust? Do you use hand lotion?

 

I have seen a lot of glaze recipes with the boron around the range yours is and they don't have this problem. I don't think the alumina level is high. Also, it doesn't matter if the KNO is low if the glaze is fluxed with other oxides. You didn't mention which frit you tried subbing for the colemanite. Did you sub it 1:1 or rework the recipe? 

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You might try calcining some EPK which means in your next bisque firing put a bowl full of EPK in the bisque. clay and gertsley borate sometimes bond together and fall off the pot.Calcining the epk and using 5% epk and 5% calcined epk will help, maybe enough to solve the issue. I'd also cut the gertsley down and add back a frit like 3134. There are several glaze calc online free programs that will help you balance it out.

Just a guess but cut the gerstley to 10% add 11% 3134 and decrease the silica by about 3%.

I don't have a program in front of me so this is just a wild guess.

Hope this helps

Wyndham

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I think you're overfiring it.  You've got got a lot of flux there, not just boron.  I've got a mid-fire high boron glaze that will behave similarly.  Someone once explained to me that if you've got a lot of boron in higher temp glazes, it will want to separate itself out, causing blisters and bubbles.  It's clear that your glaze was doing some heavy duty bubbling and spitting in the firing.

 

If it were me, I'd up your clay content, lower your silica content, and see if you can get away with less boron.  

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Cadaola posted in the Studio Operations section with another glaze with a similar crawling look. Colemanite is approx 1/2 the amount in that one yet still problematic. Maybe a horrible batch of colemanite? 

 

( I know, I know every bag of colemanite is a bad bag of colemanite  ;) )

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Paola, the forum glaze experts will certainly weigh in, but I don't see anything in the recipe that would make me think it was the recipe's fault.  Cooling shouldn't have anything to do with it, since the glaze probably came off on the way up.  I would suspect application problems-- maybe dusty bisque, or too fast firing in early stages.  How are you applying the glaze?  Sometimes dipping layers of glaze will result in the first coat loosening up and popping off, if the second layer goes on well after the first layer is dry.  How hot do you bisque?

 

BISQUE:

60°C (140F) /h to 120°C (248F), soak 30 min.

100°C (212F) /h to 600°C (1112F)

80°C (176F) /h to 980°C (1796F), soak 30 min.

 

I always clear my bisqued objects with compressed air and sponge them with just a little water. In this case I brushed the glaze, but it was not falling apart from the pot.

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Looks more like the glaze fell off the pot than spitting. I agree with Ray that this looks like either an application issue or firing while pots are very wet. Do you wipe your pots down before glazing to remove any dust? Do you use hand lotion?

 

I have seen a lot of glaze recipes with the boron around the range yours is and they don't have this problem. I don't think the alumina level is high. Also, it doesn't matter if the KNO is low if the glaze is fluxed with other oxides. You didn't mention which frit you tried subbing for the colemanite. Did you sub it 1:1 or rework the recipe? 

I leave my pots drying 1-2 days or more before glaze firing. Don't use hand lotin. I tried a boron calcium frit and another different boron frit i have, calculating and balancing the recipe correctly...

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You might try calcining some EPK which means in your next bisque firing put a bowl full of EPK in the bisque. clay and gertsley borate sometimes bond together and fall off the pot.Calcining the epk and using 5% epk and 5% calcined epk will help, maybe enough to solve the issue. I'd also cut the gertsley down and add back a frit like 3134. There are several glaze calc online free programs that will help you balance it out.

Just a guess but cut the gerstley to 10% add 11% 3134 and decrease the silica by about 3%.

I don't have a program in front of me so this is just a wild guess.

Hope this helps

Wyndham

Thank you! I will try to do as you say and let you know how it works.

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I think you're overfiring it.  You've got got a lot of flux there, not just boron.  I've got a mid-fire high boron glaze that will behave similarly.  Someone once explained to me that if you've got a lot of boron in higher temp glazes, it will want to separate itself out, causing blisters and bubbles.  It's clear that your glaze was doing some heavy duty bubbling and spitting in the firing.

 

If it were me, I'd up your clay content, lower your silica content, and see if you can get away with less boron.  

I suspect this too... I will follow your suggestion and let you know the result. Thank you!

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Cadaola posted in the Studio Operations section with another glaze with a similar crawling look. Colemanite is approx 1/2 the amount in that one yet still problematic. Maybe a horrible batch of colemanite? 

 

( I know, I know every bag of colemanite is a bad bag of colemanite  ;) )

Yes, exactly :(:angry:  :(

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If you want to use less colemanite, or get B203 into limits. You could try Neph Sy 33,Wollastonite 15, colmanite 12, EPK 10, Silica 30. This has less fluxing power, but still is cone 6.

David

Thank you David, I will try this possibility too. Hope I will find a solution with boron: only boron containing glaze (supplied as colemanite or different kind of frits) cause me firing problems.

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Does anyone has already tried to "calcinate" colemanite as Wyndham is suggesting me to do with EPK? Could it work?

 

I really want to thank you all a lot for helping me, I live in a small town, I know some potters around the region, but none of them has interest in experimenting or sharing... they all buy commercial glazes because they don't want problems (and I don't blame them for that, sometimes I feel like I should do so too!), so I really am alone in my way and you are the ones who encourage me to go on and even understand my poor english :-D Your enthousiasm, competences and kindness are overwhelming.

So thank you, with love,

Paola

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Does anyone has already tried to "calcinate" colemanite as Wyndham is suggesting me to do with EPK? Could it work?

The reason for calcining clay is to reduce problems with glazes having high clay content from cracking after drying and chipping/falling off the wares before firing. Clay in the glaze shrinks as it dries. Calcined clay (EPK) will not shrink as much.

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It's strange that you get the same results when substituting frits for the colemanite, but only get the problem with glazes containing boron.  That seems to argue that it is not an application problem.

 

When I said that about firing too fast, I meant in the glaze firing.

 

What does the glaze look like after application, but before firing?  Does it have a lot of little cracks?  That might argue for excessive shrinkage of the glaze before firing, but that seems unlikely to me.  10 percent of EPK is not a lot of clay, really. 

 

Maybe the glaze is too fluid, but the pattern in the last picture doesn't resemble what happens when a too-fluid glaze runs off a pot.  Usually if that happens, the pot is firmly attached to a big puddle of glaze.  That looks to me like the glaze fell off the pot in little chunks and collected around the pot, before it melted.

 

Glaze experts: is it possible that something in the glaze is causing it to have a very high surface tension when molten?  The blobs of glaze on the shelf are standing up pretty high.  Usually my too-fluid glazes are a lot flatter when they hit the shelf.

 

It's an intriguing problem, but it must be very frustrating for Paola.

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I think you're overfiring it.  You've got got a lot of flux there, not just boron.  I've got a mid-fire high boron glaze that will behave similarly.  Someone once explained to me that if you've got a lot of boron in higher temp glazes, it will want to separate itself out, causing blisters and bubbles.  It's clear that your glaze was doing some heavy duty bubbling and spitting in the firing.

 

If it were me, I'd up your clay content, lower your silica content, and see if you can get away with less boron.  

 

So it is all to do with phase separation? http://digitalfire.com/4sight/glossary/glossary_phase_separation.html

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It's not just the boron containing recipes that are the problem. Cadaola posted recipes in the "Glaze Squirt" post that are also causing crawling and spitting. The Tenmoku / Chun overlap also is doing this and zero boron. Too much of a coincidence that he has these, plus clear, plus Blue / Green plus the one from this thread all doing the same thing.

 

How thick are the glazes going on? 

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Does anyone has already tried to "calcinate" colemanite as Wyndham is suggesting me to do with EPK? Could it work?

The reason for calcining clay is to reduce problems with glazes having high clay content from cracking after drying and chipping/falling off the wares before firing. Clay in the glaze shrinks as it dries. Calcined clay (EPK) will not shrink as much.

 

 

Ok, now I know the difference between calcinating and fritting (maybe).

I thought that calcinating and fritting was the same process: a kind of bisque firing of a material in order to give it better qualities. I mean by fritting you made less soluble a material (for example the boron) and now I know that by calcinating you eliminate extra water. I imagine that fritting needs a different and more complex process than calcinating and it is true, it is a pity, because I have plenty of colemanite and it would be perfect if I could simply transform it in a better material just by bisque firing!

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It's not just the boron containing recipes that are the problem. Cadaola posted recipes in the "Glaze Squirt" post that are also causing crawling and spitting. The Tenmoku / Chun overlap also is doing this and zero boron. Too much of a coincidence that he has these, plus clear, plus Blue / Green plus the one from this thread all doing the same thing.

 

How thick are the glazes going on? 

 

True: the problem in that case was actually the thickness of application. I solved the problem with tenmoku+chun blue and blu-green matt glazes by thinnig the layers, I fired then in the same kiln of the recipe of this topic (also same layer thickness), which on the contrary still give me problems...

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"When I said that about firing too fast, I meant in the glaze firing." Probably true...

 

"What does the glaze look like after application, but before firing?  Does it have a lot of little cracks?" No cracks.

 

"Maybe the glaze is too fluid, but the pattern in the last picture doesn't resemble what happens when a too-fluid glaze runs off a pot.  Usually if that happens, the pot is firmly attached to a big puddle of glaze.  That looks to me like the glaze fell off the pot in little chunks and collected around the pot, before it melted." 

Because I had several times spitting problems I don't trust my glazes and when I suspect a glaze (boron glaze are among the most suspected!) I put the pot on a ring to elevate it from kiln shelf and avoid to stick to it.

"The blobs of glaze on the shelf are standing up pretty high." True! :angry:

 

"It's an intriguing problem, but it must be very frustrating for Paola." I agreeeeeeee

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