EPK vs ball clay in a glaze recipe?

[Beebop]

I use EPK in my glazes usually with a bit of bentonite, I don’t have any non-bentonite ball clay on the shelf. But should I? Would ball clay have better working properties for a glaze than EPK+bentonite? If ball clay is a must on the ingredient shelf, why and which one do you recommend? I seem to have issues with suspension and cracking/lifting during drying, not sure if that could be improved with a different clay. Sort of assume not but I’ve never used ball clay before so idk. Looking to place a materials order so if I were to try some, now is the time. Thanks!

[Bill Kielb]

I would suggest: Post your recipe(s) usually if you have 10% or more clay your glazes may not need bentonite to stay afloat. Generally good practice, bentonite is only used up to 2% as well. I think if you post the troublesome recipes here there is a good chance folks may have some simple suggestions.

Edited July 26, 2022 by Bill Kielb

[Callie Beller Diesel]

I agree with Bill. Better than what, and for what purpose?

Sometimes if you’re having issues with cracking, especially if there’s more than 20% clay, calcining half of it will sort that out. But it does depend on the rest of the recipe.

[Beebop]

I’m less asking about how to fix those specific defects and more wondering if there’s a benefit to using ball clay instead of EPK. I added screen shot of some recipes I use, usually EPK below 20% and bentonite at 2%. Thanks!

[Bill Kielb]

Thanks for posting them, Just a question: do they ALL exhibit issues or are one or two of the above worse than the others?

[Min]

6 hours ago, Beebop said:

wondering if there’s a benefit to using ball clay instead of EPK.

Ball clays are of much smaller particle size than kaolins. On the other hand kaolins are whiter and cleaner burning than ball clays. Smaller the particle size the more plastic a clay will be, this means that if you use a ball clay in place of a kaolin then bentonite might not be necessary to achieve the same level of glaze suspension. Glazes with significant amounts of clay will need to have some of the clay calcined to reduce shrinkage of the raw glaze which otherwise could precipitate the glaze cracking upon drying. If your glazes are up around the 20% clay amount I would rebalance the silica and alumina in the recipes if swapping from one to the other. I use both OM4 and C&C ball clays for glazes but for the most part use EPK and bentonite where necessary, usually if the glaze has less than around 15% EPK in them.

[oldlady]

beebop, i rely on other people for recipes.  THANK YOU, MIN AND EVERYONE ELSE!  and have never experimented with more than color.  so i really do not understand why some recipes have 2 decimal points, like 42.49 or 26.18.   if a recipe has to be that precise, how can you possibly measure, mix, add water, sieve and store it without disturbing those teeny, tiny amounts?

what kind of glaze effect are you searching for?

 

[Callie Beller Diesel]

3 hours ago, oldlady said:

.   if a recipe has to be that precise,

In a way, it doesn’t have to be that precise, but it depends for what. For most things, it’s all about proportions, and as long as you keep the ratios the same, it works out. 

That said, the smaller batch you’re making, the more precise you want to be. If you’re off by a few grams in a 10 kg/5 gallon bucket batch, the changes won’t be noticeable. But if you’re off by 0.1 of a gram in a 100g sample, it might show up more. And some materials will matter more than others. You’ll notice the absence of 0.1 %of cobalt, but probably not the absence of 0.1% silica. And just because there are exceptions that prove the rule, if you’re making a chrome tin pink, yes, that 1/100th of a gram of chrome difference will matter. 

Honestly, I just round any more extended decimal places to the nearest 0.1 of a gram. It’s impractical for me to measure smaller amounts for tests.

[Min]

4 hours ago, oldlady said:

so i really do not understand why some recipes have 2 decimal points, like 42.49 or 26.18.   if a recipe has to be that precise, how can you possibly measure, mix, add water, sieve and store it without disturbing those teeny, tiny amounts?

I'm doing a little demo to show one reason why some recipes go to 2 or 3 decimal points. I'll use the first recipe Beebop posted as an example that pertains to the question in the thread of using a ball clay in place of kaolin in a recipe. 

I took the original recipe (example 1) then did a one to one swap of OM4 for the kaolin (example 2). See how the total is still 100 but them chemistry isn't the same. With OM4 ball clay the silica is now too high and the alumina is too low. (blue and red boxes) plus a tiny difference in the calcium. 

So to get the silica, alumina and calcium to be the same as in the original recipe I need to increase and decrease certain materials then retotal the recipe to equal 100. This is recipe example 3. Numbers are now down to the third decimal point to get the total to 100. Some programs will automatically round the numbers off, some don't but it's simple enough to do.

I then took the 100 gram batch and increased it to a 10,000 gram batch. Example 4. Decimal points are now down to tenths. If I was mixing this glaze I would just round them up or down and get rid of them altogether. 

 

[Beebop]

16 hours ago, Bill Kielb said:

Thanks for posting them, Just a question: do they ALL exhibit issues or are one or two of the above worse than the others?

Good question. I have lots of kinks to work out in general, some are better than others. The first and the third one I posted (CGB2 from Hesselberth and Roy and a SiC-Jun in test/development phase) are two of the most problematic as far as crawling on the finished product. The fourth one cracks a lot on drying but rarely crawls, been trying out different EPK levels on that one from 0% to 20% to limit running. The second one is a kaki I’ve been developing, it rarely has issues with application or final defects. 

[Beebop]

16 hours ago, Min said:

Ball clays are of much smaller particle size than kaolins. On the other hand kaolins are whiter and cleaner burning than ball clays. Smaller the particle size the more plastic a clay will be, this means that if you use a ball clay in place of a kaolin then bentonite might not be necessary to achieve the same level of glaze suspension. Glazes with significant amounts of clay will need to have some of the clay calcined to reduce shrinkage of the raw glaze which otherwise could precipitate the glaze cracking upon drying. If your glazes are up around the 20% clay amount I would rebalance the silica and alumina in the recipes if swapping from one to the other. I use both OM4 and C&C ball clays for glazes but for the most part use EPK and bentonite where necessary, usually if the glaze has less than around 15% EPK in them.

Good advice, I will work on sourcing silica and alumina elsewhere or calcining clay for problemtic recipes with high clay content. Thanks for the suggestions!

[Min]

17 hours ago, Beebop said:

The first and the third one I posted (CGB2 from Hesselberth and Roy and a SiC-Jun in test/development phase) are two of the most problematic as far as crawling on the finished product.

If these were my glazes I would calcine some of the kaolin in the first recipe. For the ones with bone ash, I'm wondering if the bone ash (real or tricalcium phosphate?) is gelling the glaze and tricking you into adding more water which in turn would cause more shrinkage as the raw glaze dries causing the crawling. Have you tried different SG's of those ones and seeing if raising it reduces the problem? Worst case scenario I would get some Magma (Brackers sells it) and add a small amount of it to those bone ash glazes. (Magma needs to be made into a jell then a small amount of that is added to glazes. It helps stop glaze cracking and crawling but does slow down the drying time. It is also a great floc.)

 

[Mark C.]

Another Magma vote -it cures all settling issues with a very small amount needed.It can float a brick

[Bill Kielb]

22 hours ago, Beebop said:

Good question. I have lots of kinks to work out in general, some are better than others. The first and the third one I posted (CGB2 from Hesselberth and Roy and a SiC-Jun in test/development phase) are two of the most problematic as far as crawling on the finished product.

Just a comment here, crawling can often be the result of the fired glaze surface tension preventing its ability to heal so cracks developed early on can exacerbate the issue. Calcining part of the epk often can help with the shrinkage and help with the early formation of cracks. Interestingly ball clay is know to shrink more,  but does not necessarily translate to more early formed cracks maybe because it is more plastic or has a more favorable fired glaze surface tension or both so you might have an idea worth testing.

[Beebop]

5 hours ago, Bill Kielb said:

Just a comment here, crawling can often be the result of the fired glaze surface tension preventing its ability to heal so cracks developed early on can exacerbate the issue. Calcining part of the epk often can help with the shrinkage and help with the early formation of cracks. Interestingly ball clay is know to shrink more,  but does not necessarily translate to more early formed cracks maybe because it is more plastic or has a more favorable fired glaze surface tension or both so you might have an idea worth testing.

That’s very compelling and makes a lot of sense. I’ll definitely give that a go. Thanks!

[Beebop]

10 hours ago, Min said:

If these were my glazes I would calcine some of the kaolin in the first recipe. For the ones with bone ash, I'm wondering if the bone ash (real or tricalcium phosphate?) is gelling the glaze and tricking you into adding more water which in turn would cause more shrinkage as the raw glaze dries causing the crawling. Have you tried different SG's of those ones and seeing if raising it reduces the problem? Worst case scenario I would get some Magma (Brackers sells it) and add a small amount of it to those bone ash glazes. (Magma needs to be made into a jell then a small amount of that is added to glazes. It helps stop glaze cracking and crawling but does slow down the drying time. It is also a great floc.)

 

I have never heard of Magma but just looked it up and it sounds like a miracle! I will definitely give this a go! I have run into some really impressive problems with bone ash gelling, but this particularly one rendition has been ok. I’m still learning when something needs more/less water versus floc/defloc though.

[ATauer]

On 7/27/2022 at 7:14 AM, oldlady said:

beebop, i rely on other people for recipes.  THANK YOU, MIN AND EVERYONE ELSE!  and have never experimented with more than color.  so i really do not understand why some recipes have 2 decimal points, like 42.49 or 26.18.   if a recipe has to be that precise, how can you possibly measure, mix, add water, sieve and store it without disturbing those teeny, tiny amounts?

what kind of glaze effect are you searching for?

 

I don’t think you should look at it as an all or nothing issue, such as replacing all your EPK with ball clay. There are plenty of recipes out there with ball clay instead of or with EPK, it depends on the requirements of the glaze. Depending on how the glaze was developed, there may have been (hopefully there was) extensive testing with multiple line blends or quadraxial blends to determine which of the possible glazes had the most desirable characteristics and that determined the recipe. I’m aware plenty of recipes seem to be just thrown together or are alterations of other recipes, or blending of two recipes (which is such fun to try! You never know what you might end up with!). So hopefully some thought was put into which clay was used, generally the determination should have a lot to do with how much silica and alumina you want and you can adjust that with ball clay without necessarily changing how much flint or feldspar is added. I will say the recipes I see ball clay in I honestly can’t work out usually why they are using it. But your problems that you are having, are not typical of EPK except of course at high enough levels to have excess shrinkage and need calcining. But that doesn’t always solve things, and it may not be your solution. I wouldn’t just jump to trying ball clays instead in these recipes, you’d have to alter the recipes with glaze calc software and people forget that ball clays have an extremely high shrinkage rate themselves, so I don’t know that it would solve your issues. Especially looking over the recipes you shared, the levels of EPK are quite moderate, no where near I would be concerned or recommend calcining. I would be looking elswhere. Any new bags of materials you’ve bought in the last couple of months? Switch to a new supplier? Are you using a newer talc? 

The other thing I wanted to add is if you haven’t tried CMC with these recipes I would give it a good shot. It will significantly improve the bond between the bisque and the glaze, it literally helps it stick on harder, and tends to help with glazes that are problematic at this stage- we literally use it as a glue in glass fusing and apparently most wallpaper pastes are made out of it! It should help with preventing dried glaze cracks and allow for a smoother application and I think that will maybe help with the crawling. Of course with crawling there are all the typical things about making sure the bisque is super clean, no dust, no fingermarks, no grease to cause the glaze to crawl. It might well help a lot to do a quick dip in water before glazing, that often helps with crawling. I just think looking at your recipes the crawling is not coming form the EPK at all, there just isn’t enough of it to be the problem, so the detective work should shift in new directions. And considering you are having problems with so many glazes…that’s weird. I’m really wondering about any materials you’ve replaced recently. I could see this happening on one or two glazes but not this many unless a material is contaminated…

[Beebop]

On 7/28/2022 at 10:32 PM, ATauer said:

I don’t think you should look at it as an all or nothing issue, such as replacing all your EPK with ball clay. There are plenty of recipes out there with ball clay instead of or with EPK, it depends on the requirements of the glaze. Depending on how the glaze was developed, there may have been (hopefully there was) extensive testing with multiple line blends or quadraxial blends to determine which of the possible glazes had the most desirable characteristics and that determined the recipe. I’m aware plenty of recipes seem to be just thrown together or are alterations of other recipes, or blending of two recipes (which is such fun to try! You never know what you might end up with!). So hopefully some thought was put into which clay was used, generally the determination should have a lot to do with how much silica and alumina you want and you can adjust that with ball clay without necessarily changing how much flint or feldspar is added. I will say the recipes I see ball clay in I honestly can’t work out usually why they are using it. But your problems that you are having, are not typical of EPK except of course at high enough levels to have excess shrinkage and need calcining. But that doesn’t always solve things, and it may not be your solution. I wouldn’t just jump to trying ball clays instead in these recipes, you’d have to alter the recipes with glaze calc software and people forget that ball clays have an extremely high shrinkage rate themselves, so I don’t know that it would solve your issues. Especially looking over the recipes you shared, the levels of EPK are quite moderate, no where near I would be concerned or recommend calcining. I would be looking elswhere. Any new bags of materials you’ve bought in the last couple of months? Switch to a new supplier? Are you using a newer talc? 

The other thing I wanted to add is if you haven’t tried CMC with these recipes I would give it a good shot. It will significantly improve the bond between the bisque and the glaze, it literally helps it stick on harder, and tends to help with glazes that are problematic at this stage- we literally use it as a glue in glass fusing and apparently most wallpaper pastes are made out of it! It should help with preventing dried glaze cracks and allow for a smoother application and I think that will maybe help with the crawling. Of course with crawling there are all the typical things about making sure the bisque is super clean, no dust, no fingermarks, no grease to cause the glaze to crawl. It might well help a lot to do a quick dip in water before glazing, that often helps with crawling. I just think looking at your recipes the crawling is not coming form the EPK at all, there just isn’t enough of it to be the problem, so the detective work should shift in new directions. And considering you are having problems with so many glazes…that’s weird. I’m really wondering about any materials you’ve replaced recently. I could see this happening on one or two glazes but not this many unless a material is contaminated…

This is very helpful, thanks for the feedback.  I’ve only been making glazes for a couple years now and haven’t had to rebuy anything yet except talc, which involved switching to Texas talc from another one that is no longer sourced, though I haven’t noticed much of a difference in performance. But now that I’m saying that I should go back and actually look at my recipes and results to see if there’s a correlation. It’s not that all my glazes have the same problem, but that most of my glazes have some problem, many of them being hard panning, pin-holing, crawling, cracking when dry, glaze fail caused by layering. I think a big part of my problem is that I need to work with a lower SG, and I have some other fixes I’m working on as well, though trial and error is slow when you do glaze firings at most every other month! I am definitely interested in using an additive I think that could go a long way for me in problem solving, added a new post with follow-up questions on that. Thanks! 

[Beebop]

On 7/28/2022 at 12:43 PM, Mark C. said:

Another Magma vote -it cures all settling issues with a very small amount needed.It can float a brick

I added a new post with follow up questions. I’m very intrigued by this solution! 

[ATauer]

51 minutes ago, Beebop said:

This is very helpful, thanks for the feedback.  I’ve only been making glazes for a couple years now and haven’t had to rebuy anything yet except talc, which involved switching to Texas talc from another one that is no longer sourced, though I haven’t noticed much of a difference in performance. But now that I’m saying that I should go back and actually look at my recipes and results to see if there’s a correlation. It’s not that all my glazes have the same problem, but that most of my glazes have some problem, many of them being hard panning, pin-holing, crawling, cracking when dry, glaze fail caused by layering. I think a big part of my problem is that I need to work with a lower SG, and I have some other fixes I’m working on as well, though trial and error is slow when you do glaze firings at most every other month! I am definitely interested in using an additive I think that could go a long way for me in problem solving, added a new post with follow-up questions on that. Thanks! 

Texas talc certainly caused a ton of problems, but largely just with claybodies, not usually with most glazes, though I have known some people who were able to isolate that the Texas talc was the problem with a specific glaze. But usually not all of their glazes. And it wasn’t common. That talc significantly changes the claybody color, instead of being bright white it made all the lowfire white clays dirty buffs that were very ugly, and it caused them to be very short and crack a lot, and had a bunch of other flaws I won’t list all of them, but it was a nightmare. But talc doesn’t contribute color to glazes and all the types of flaws it caused in claybodies are things that don’t really apply to glazes usually. I hope you are able to identify what is going on, I would try if you are able to mix up 100 gram batches of your glazes that you are having problems with each with an ingredient changed, and see if the glaze behaves differently. That would help you determine if it is a specific material that is a problem-you could start with just one of the problem glazes and try that and you might already get your answer. The other thing to do is compare all the recipes and see if there is one or more ingredients that is common to all of them and do a test batch without it or with a different brand you might buy a very small amount of from another supplier. It is strange that all of them would be doing all these things, and the variety of flaws. You might want to look at how you mix yours up, whether you need to ball mill (if you don’t have one you can get a cheap rock tumbler and buy the porcelain balls for it for $10, and it works great, that’s what I use) or sieve more. What you use to mix them, you might want to try something else. Make sure you have enough clay in each, definitely measure SG when you have gotten it to a viscosity that is good where it is behaving and monitor it and add water or deflocculant as needed to maintain that SG. 

 

[ATauer]

You also might want to look at your bisque and glaze firing schedules again. A lot of the firing flaws could potentially be fixed by a drop and hold and slow cool schedule, I don’t think you mentioned what you do, I’m making an assumption you fire to cone 5 or 6 so if you lowfire or are cone 10 the exact cones I mention are not as important as the points I make about them.

Make sure you are bisqueing at the right temp for these glazes and consistently using the same temp for the bisque, no over firing and always use the exact same cone consistently without fail (witness cones!). The only exceptions are you can bisque lower if you are doing raku or pitfiring/barrelfiring/saggarfiring. And you might want to look at the glaze temperature you are firing to, according to your cones, maybe that needs to be altered if your kiln’s programmed cone is actually different than the right temp for that cone, it may need adjusting. As Matt Katz is always reminding everyone, cone 5 is super far actually from cone 6 when you look at the degrees, compared to the distance between other cones. Glazes can perform very differently between the two, even though people say cone 5/6 like they are the same thing, and even just with over firing to cone 7 which a lot of people do a lot without knowing it. Getting all the way to cone 6 if you don’t use witness cones with every firing could be a problem, or if you are firing to cone 5 a lot of the glazes may be underfiring. And you’d be surprised sometimes how small of a range some glazes have, where cone 7 is hugely over firing…while others can fire from cone 06-10 (that’s really just the Stroke & Coats and the reverse engineered recipe for them on Glazy, but there are a surprising number of glazes that work from cone 4-10 with minimal visible changes.) Good luck, I hope you find something that helps.