HELP! Severe Shivering White Glaze

[clayenthusiast]

Hi Clay Community,

I am needing some input on possible actions to take to prevent the severity of shivering that has been taking place in our studio. We fire cone 10 gas reduction. We have 4 different clay bodies and our reclaim clay. The white glaze is having some compatibility and fit issues with our stoneware clays. And of course the worst being with our red heavy iron baring stoneware. This combination of stoneware and our white glaze is the only culprit. The white glaze is not shivering on the porcelainuos clays. This combo also has a tendency to not behave if used primarily as a liner glaze especially on just the top of large surface areas like plates and will crack the piece all the way through post firing or during cooling. 

Glaze Recipe - 

Coleman White

Custer Fledspar 38.63

Whiting 14.65

Talc 14.65

Kaolin  34.64

Silica 27.97

Bentonite 2.67

Zircopax 8

Any advice on ways to adjust the glaze to create a better fit or compatibility would be great. Or possible adjustments to the firing to prevent the severity of our recent shivering incidents.

Thank you for any input!

Warmly,
Clay Enthusiast

[Min]

Are you looking to keep the magnesium satin look to the glaze?

[Mark C.]

Either change the body  or the glaze

lots of white glazes out there

[Kelly in AK]

I second @Mark C.’s comment. I don’t think you can fire your way out of this one, I believe you shouldn’t even consider it. You said it’s problematic on the stoneware “clays,” which leads me to think it’s more a glaze issue than one odd clay body. There’s a lot of talc, probably gives you the satin, but very low expansion.

I’m not a glaze expert, so can’t advise in depth that way. My only advice is to respect this principle: If a combo is shivering, pieces that haven’t shivered just haven’t done it yet. Doom the lot.

Edit: After pronouncing doom I figured I’d better consult my “bible,” which is Pioneer Pottery by Michael Cardew, because I recalled it as the chief source of my principle. Digging through the section on glazes I did not find satisfaction, then discovered he talks about shivering at length in the section on clay bodies. From around pages 64-75. Not exactly as I remembered, and Cardew treats it more specifically as a clay body issue than a glaze fault. He also talks about different manifestations of the the same problem, such as “shattering” and spiral cracks. Good reading for clay nerds.

Good luck and, as Mark said, there are lots of white glazes out there! 

Edited May 19, 2023 by Kelly in AK

[Min]

Yup, there are definitely lots of white glaze recipes available but I think we need to know if @clayenthusiast is looking for another dolomite (or talc) high magnesium satin glaze or is okay with a gloss.

[BobMagnuson]

Sorry I'm late to this conversation.  I see 3 problems with this glaze.  First, the clay content is too high, which can cause shrinkage and separation before firing.  It might not be noticeable before firing , but can show up as shivering out of the kiln.  Second, the coefficient of expansion is quite low, which could be an issue with some clay bodies.  And finally, it looks like it might be applied too thick.  The first 2 issues could be resolved by reducing the kaolin by 25% or so.  (Run a few tests in that neighborhood.)  Finally, if you're dipping your glaze, keep the specific gravity below 1.5.  1.45 works for me with a 5 second dip.

If the problem is the clay content, another solution would be to replace some (half?) of the kaolin with calcined kaolin.  That way you can keep the chemistry the same while reducing the pre-fire shrinkage.

[neilestrick]

23 minutes ago, BobMagnuson said:

Sorry I'm late to this conversation.  I see 3 problems with this glaze.  First, the clay content is too high, which can cause shrinkage and separation before firing.  It might not be noticeable before firing , but can show up as shivering out of the kiln.  Second, the coefficient of expansion is quite low, which could be an issue with some clay bodies.  And finally, it looks like it might be applied too thick.  The first 2 issues could be resolved by reducing the kaolin by 25% or so.  (Run a few tests in that neighborhood.)  Finally, if you're dipping your glaze, keep the specific gravity below 1.5.  1.45 works for me with a 5 second dip.

If the problem is the clay content, another solution would be to replace some (half?) of the kaolin with calcined kaolin.  That way you can keep the chemistry the same while reducing the pre-fire shrinkage.

I agree. The kaolin, silica, and bentonite make up almost 50% of this glaze. There's just not enough fluxing going on there. I'm wondering if there was a problem with converting the recipe at some point since the numbers add up in a strange way. Maybe someone messed up the math?

[PeterH]

3 hours ago, neilestrick said:

I agree. The kaolin, silica, and bentonite make up almost 50% of this glaze. There's just not enough fluxing going on there. I'm wondering if there was a problem with converting the recipe at some point since the numbers add up in a strange way. Maybe someone messed up the math?

No comment on the "quality" of the glaze, just the numbers.

Looks like a fairly direct conversion of Coleman No Craze White https://glazy.org/recipes/3331
Custer for G-200, kaolin for EPK and silica slightly changed ... with the addition of bentonite and zircopax.

The original recipe added up to 970 rather than 1000, which might account for funny numbers.

PS
Glazy seems to have the same recipe listed under Coleman No Craze White (New Zealand Halloysite) https://glazy.org/recipes/3806
... which claims to be Adjusted for New Zealand Halloysite.

[Min]

1 hour ago, PeterH said:

Coleman No Craze White (New

Which would make sense given the amounts of clay and talc. Given that @clayenthusiast isn't getting shivering on their porcelain type clays, and I'm guessing Coleman created this recipe to use on his porcelain, the extra fluxes from those bodies is likely the reason this glaze doesn't shiver on those bodies.  Increasing the calcium, potassium plus sodium along with decreasing the alumina would raise the expansion and should alleviate the shivering. Or try another recipe. 

Edited May 21, 2023 by Min

[Min]

On 5/18/2023 at 4:12 PM, clayenthusiast said:

Or possible adjustments to the firing to prevent the severity of our recent shivering incidents.

One more thought, do you have a new supply of talc? Is "recent" since a new batch? Any material changes? Glaze still would probably have fit issues for some claybodies, I would be surprised if one glaze fit a wide range of bodies well.

On 5/18/2023 at 4:12 PM, clayenthusiast said:

This combo also has a tendency to not behave if used primarily as a liner glaze especially on just the top of large surface areas like plates and will crack the piece all the way through post firing or during cooling. 

For sure the glaze expansion is too low on the bodies when you get dunting on large flat surfaces. Pots with glaze just on one side and a low expansion glaze fighting against one with a notably higher expansion on the other side of the pot (as with liner vs exterior glaze) will also dunt. Given that calculated COE figures are not accurate with matte (or semi matte) glazes you need to look at what is in the recipe and the properties of those materials.

[Magnolia Mud Research]

Do you have a clear glaze that works OK all the time?
if so, just add some Zircopax to your clear glaze and you will have a white glaze.  
the amount of Zircopax will probably be somewhere between 4 and 8 %. 

LT 

[clayenthusiast]

On 5/18/2023 at 5:27 PM, Min said:

Are you looking to keep the magnesium satin look to the glaze?

Yes I am interested in keeping the characteristics of the glaze and the recipe as close as possible. We have an extensive library of glaze combination tiles that I would like to have them still apply if at all as all possible.

[clayenthusiast]

23 hours ago, Magnolia Mud Research said:

Do you have a clear glaze that works OK all the time?
if so, just add some Zircopax to your clear glaze and you will have a white glaze.  
the amount of Zircopax will probably be somewhere between 4 and 8 %. 

LT 

I love knowing this. Thank you!

[clayenthusiast]

On 5/18/2023 at 7:42 PM, Mark C. said:

Either change the body  or the glaze

lots of white glazes out there

Might be changing the clay body, mainly because we have a incredible library of tiles for all of our glaze combinations.

[clayenthusiast]

On 5/20/2023 at 8:57 PM, BobMagnuson said:

Sorry I'm late to this conversation.  I see 3 problems with this glaze.  First, the clay content is too high, which can cause shrinkage and separation before firing.  It might not be noticeable before firing , but can show up as shivering out of the kiln.  Second, the coefficient of expansion is quite low, which could be an issue with some clay bodies.  And finally, it looks like it might be applied too thick.  The first 2 issues could be resolved by reducing the kaolin by 25% or so.  (Run a few tests in that neighborhood.)  Finally, if you're dipping your glaze, keep the specific gravity below 1.5.  1.45 works for me with a 5 second dip.

If the problem is the clay content, another solution would be to replace some (half?) of the kaolin with calcined kaolin.  That way you can keep the chemistry the same while reducing the pre-fire shrinkage.

I am also late to the reply on my own thread, thanks for the patience. I was away from my desk for a few days. This right here is the knowledge I was hoping to garner. This it the kind of test I would like to run to see if I can help these issues. I will definitely be running this test. Thank you thank you for sharing your knowledge!

[clayenthusiast]

On 5/20/2023 at 9:32 PM, neilestrick said:

I agree. The kaolin, silica, and bentonite make up almost 50% of this glaze. There's just not enough fluxing going on there. I'm wondering if there was a problem with converting the recipe at some point since the numbers add up in a strange way. Maybe someone messed up the math?

Good to know about the overly dominate clay in this recipe. Also, I've honestly always wondered the same thing about the way the numbers add up. I may do some digging into some of the historical notes for this studio or other versions of this same recipe perhaps. I might be playing around with the percentage of clay and run some tests.

[clayenthusiast]

On 5/19/2023 at 8:00 AM, Min said:

Yup, there are definitely lots of white glaze recipes available but I think we need to know if @clayenthusiast is looking for another dolomite (or talc) high magnesium satin glaze or is okay with a gloss.

I'm really hoping to keep the glaze recipe as close as possible.

[clayenthusiast]

On 5/21/2023 at 10:06 AM, Min said:

One more thought, do you have a new supply of talc? Is "recent" since a new batch? Any material changes? Glaze still would probably have fit issues for some claybodies, I would be surprised if one glaze fit a wide range of bodies well.

For sure the glaze expansion is too low on the bodies when you get dunting on large flat surfaces. Pots with glaze just on one side and a low expansion glaze fighting against one with a notably higher expansion on the other side of the pot (as with liner vs exterior glaze) will also dunt. Given that calculated COE figures are not accurate with matte (or semi matte) glazes you need to look at what is in the recipe and the properties of those materials.

We have received a new supply of talc and have mixed a batch with the new material.  Are there changes in the mining of talc as of late or its properties? I do believe I heard something of the like. One of my mentors also inquired about the talc. But even prior it never behaved well as a glaze but as of late has seemingly worsened.

Thank you for the info on dunting. This helps better my understanding and makes sense. 

[neilestrick]

@clayenthusiast Here's a recipe for Betty Woodman White, which is a very popular magnesium white that I've never seen have any problems on stoneware bodies. I used it for many years on my own work. Give it a try and see what you think.

BW White.pdf

[clayenthusiast]

16 minutes ago, neilestrick said:

@clayenthusiast Here's a recipe for Betty Woodman White, which is a very popular magnesium white that I've never seen have any problems on stoneware bodies. I used it for many years on my own work. Give it a try and see what you think.

BW White.pdf 58.37 kB · 0 downloads

I appreciate it, thank you! Also how amazing that it's a Betty Woodman recipe.

[Min]

3 hours ago, clayenthusiast said:

We have received a new supply of talc and have mixed a batch with the new material.  Are there changes in the mining of talc as of late or its properties?

Yup! Link here to some of the replacements.https://community.ceramicartsdaily.org/topic/24846-talc-shortage-new-talc-cim-talc-and-fabi-talc/ Definite differences in claybodies, depends on how much is in the glaze as to whether it makes a difference.

You might want to consider suppling the magnesium in the Betty Woodman recipe with dolomite. Or try your new talc in one recipe and recalculate it using dolomite to supply the magnesium and see how they compare. (if you don't use glaze calc someone here could help with this)