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Longterm Glaze Issues | Sometimes Runny, Sometimes Breaks The Pots


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Dear community,

I have this wonderful glaze i use for my porcelain cups (slipcasted), BUT
its been a long time struggle, as this glaze is sometimes doing fine, but sometimes! it breaks the cups,
sometimes it runs, leaving the lip almost with no glaze, sometimes its really nice looking blue, but sometimes its green.

I thought some of you might know how to adjust the glaze, as I really really love the way it looks when everything works fine.

So the glaze is a crackled blue/green transparent one. Its supposed to be for the cone 6.
I applied thick - it breaks the cups, but this time I applied it thin it still broke them and run down to the foot.
I also noticed its not as runny when i put it in the lowest shelve in the kiln.. hm, but thats not the way out..

It has 0.54 of copper carbonate 

Do you have any ideas of how to adjust the glaze so its not that runny.
Is it about adding more of Bentonite and Kaolin?

Can copper carbonate make the glaze be more runny?

it has
Neph Sy.
Dolomit
bentonite
flint
kaolin
whiting
+ tiny bit of copper carb.

Every tip is highly appreciated! 




 

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Hi Nat:

 

Without percentages of each glaze ingredient: no one will be able to make adjustments. Nep Sy, Dolomite, and whiting are all fluxes: dolomite has other properties as well.  The third picture is most telling: it appears you have clay/glaze interface checking: a tell tale sign that your glaze does not fit the clay body (mechanical stress). So let me say what all the others will" more info please.

 

Nerd

 

Glaze run indicates over fluxing.

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Thank you,

I will post the recipe tomorrow, don't have it now by hand.

By saying the glaze doesn't fit the clay body you mean there is no way to adjust the glaze so it could fit?

 

 

 

Hi Nat:

 

Without percentages of each glaze ingredient: no one will be able to make adjustments. Nep Sy, Dolomite, and whiting are all fluxes: dolomite has other properties as well.  The third picture is most telling: it appears you have clay/glaze interface checking: a tell tale sign that your glaze does not fit the clay body (mechanical stress). So let me say what all the others will" more info please.

 

Nerd

 

Glaze run indicates over fluxing.

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Hi,  > I applied thick - it breaks the cups, but this time I applied it thin it still broke them and run down to the foot.

 

I can clearly see crackle in the photos, but no breaks. Can you supply a picture of a couple of the broken cups?

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I will go ro the studio tomorrow and take some pictures.

Yes the pictures you see now are of the cups that did survive the firing,

Which is usually 3 out of 10

 

Hi,  > I applied thick - it breaks the cups, but this time I applied it thin it still broke them and run down to the foot.

 

I can clearly see crackle in the photos, but no breaks. Can you supply a picture of a couple of the broken cups?

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Yes, the glaze can be adjusted to fit. At cone 6, Nep Sy is a vigorous flux; fully expect to see a high percentage when you post. As I also suspect that dolomite is making the color shift between blues and greens. But we will see.....

Nerd

 

Dolomite: 32% calcium  22% magnesium.  The magnesium in that percentage would modify color.

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Copper isn't what is causing the glaze to run.

 

Here is a recipe I used in New Zealand but haven't tried since returning to Alaska. The results (and some of the problems!) are similar enough. I did experience crazing, slight runs, and some blistering and generally found it to be beautiful but unreliable. I was working with Macs White. The color of this glaze is darker but you could reduce the copper to 0.5 for a lighter result.

 

Zam Celadon

Silica 17.6
Kaolin 5.6
Standard Borax Frit 20.8
Nepheline Syenite 45.6
Wollastonite 6.4

Bentonite 2
Copper Carbonate 2.0

note: original recipe called for Frit 3124 instead of standard borax frit. Original recipe may have other changes, I don't remember.

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Are you using the crackle glaze on the inside and outside of the cup.  Crackle glaze on the inside of a cup will stain and I don't believe it is food safe.  I understand that using a crackle glaze on the inside and outside can cause stress from the crackle glaze and break the piece.  I haven't done this myself,  maybe you could do a test with your crackle on the outside Zam Celadon on the inside.  I leave the Copper Carbonate out of it and use it as a clear glaze, very reliable.   Denice

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Denice,

thank you, I will try it out!

 

No, I actually use transparent glaze on the inside

but what I don't understand is why the glaze sometimes is really ok,

looks nice and everything is fine and sometimes it doesn't work.... at all

 

 

 

 

Are you using the crackle glaze on the inside and outside of the cup.  Crackle glaze on the inside of a cup will stain and I don't believe it is food safe.  I understand that using a crackle glaze on the inside and outside can cause stress from the crackle glaze and break the piece.  I haven't done this myself,  maybe you could do a test with your crackle on the outside Zam Celadon on the inside.  I leave the Copper Carbonate out of it and use it as a clear glaze, very reliable.   Denice

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Hi there!

so here is the recipe

 

46.24 neph sy

24.19 whiting

23.12 silica

6.45 zinc ox

5.38 bentonite

0.54 copper carb

 

 

do you have any ideas?

 

Thank you!

 

Hi Nat:

 

Without percentages of each glaze ingredient: no one will be able to make adjustments. Nep Sy, Dolomite, and whiting are all fluxes: dolomite has other properties as well.  The third picture is most telling: it appears you have clay/glaze interface checking: a tell tale sign that your glaze does not fit the clay body (mechanical stress). So let me say what all the others will" more info please.

 

Nerd

 

Glaze run indicates over fluxing.

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Here is the photo of the broken cup

 

 

 

Hi, > I applied thick - it breaks the cups, but this time I applied it thin it still broke them and run down to the foot.

 

I can clearly see crackle in the photos, but no breaks. Can you supply a picture of a couple of the broken cups?

post-63238-0-48078200-1463904210_thumb.jpeg

post-63238-0-48078200-1463904210_thumb.jpeg

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Hi Nat:

The cup pictured above gives the evidence I was looking for. You are casting a thin wall cup mold: and the mechanical stress of the offset COE between the glaze and clay is causing the problem. 1. Notice how the crack pattern appears uniformly where the split is?  In two places the crazing pattern runs through the split and continues on the other side: which indicates the split occurred upon cooling. Then smaller cracks appear on either side of the split which occurred after the split because they do not run through split as the other two do. The crack pattern in this example is not the same pattern as shown in your first examples. Notice how much further spaced they are as compared to the first three? Notice how they are linear and concentric in comparison to the first three? The first three are glaze/clay interface checking: which most call crackle glaze. The last picture is plain ole run of the mill crazing: two entirely different cracking patterns, but caused by the same problem: COE.

2. Fluidity: in this case caused by over fluxing. Notice the pastel green on the body, and the dark green on the foot ring? Copper carb become very fluid if over fluxed, and will run with the excess glaze. You applied thin in this case (if I followed you right)? So it is not glaze application, but rather fluxing. I ran your formula on Glaze Master 3.0 and did a formula limit comparison. Your calcium (whiting) limit is above, but the rest is in align. The other two issues: your silica and alumina are well below limits. Softening the glaze body may be intentional in this case to produce a crackle effect. So I will let that go, as well as the rather large difference in the COE of this glaze as compared to the clay body ( which also helps to produce a crackle effect.) That said, it cannot be entirely ignored because you have reached a boundary: producing both the crackle effect and crazing- both caused by offset COE. So I think it would be advisable to lower the COE of the glaze by some degree in order to solve the crazing issue.

3. Calcium limit: you are above the formula limits in calcium. At cone 6, potassium, sodium, and zinc are in a gaseous state, calcium however is in a liquid state. So lowering the whiting is mandatory in this case. Personally, I would increase the alumina and silica in the glaze as well and rely on the offset COE between the clay and glaze to produce the crackly effect you desire. However, I still think that needs to be lowered by some degree, because it is producing enough stress to crack your pieces.

4. Zinc: your zinc is within formula limits. I have to assume you like the very high gloss it produces ( I understand that :) ). You can produce that same sheen and solve other problems by lowering the zinc to under 5 and increasing the silica. Silica is the primary glass former and additional sheen will be produced because of the refractive index of glass.

So how do you want to fix it is the question?  Do you want to maintain a crackle effect, or forego it for a true glaze fit?

Nerd

 

Edit: In the second picture in your original post; it shows a goldish/tan hue with a goldish band on the rim. That is from the magnesium in the dolomite: if that is undesirable, then you need to lower the dolomite as well.

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oh! Thank you so much for your time and advice!

 

so, there is some work to be done! :)

 

you adviced to lower whiting and increasing silica and bentonite (did I understand it right with the alumina?)

I'm new to adjusting glazes - what is the best way I do it?

if, let's say, I lower whiting to 20 and increase silica for 4 - will the result be seen or should i take bigger steps?

 

 

but still, do you have any idea why the same glaze is reacting so differently

and I wonder if cooling time is also influencing it - as these days it's so hot in the studio that the kiln is cooling slower as usual

 

thank you so much!

I go and read now about adjusting glazes :)

 

 

 

 

 

 

 

Hi Nat:

The cup pictured above gives the evidence I was looking for. You are casting a thin wall cup mold: and the mechanical stress of the offset COE between the glaze and clay is causing the problem. 1. Notice how the crack pattern appears uniformly where the split is?  In two places the crazing pattern runs through the split and continues on the other side: which indicates the split occurred upon cooling. Then smaller cracks appear on either side of the split which occurred after the split because they do not run through split as the other two do. The crack pattern in this example is not the same pattern as shown in your first examples. Notice how much further spaced they are as compared to the first three? Notice how they are linear and concentric in comparison to the first three? The first three are glaze/clay interface checking: which most call crackle glaze. The last picture is plain ole run of the mill crazing: two entirely different cracking patterns, but caused by the same problem: COE.

2. Fluidity: in this case caused by over fluxing. Notice the pastel green on the body, and the dark green on the foot ring? Copper carb become very fluid if over fluxed, and will run with the excess glaze. You applied thin in this case (if I followed you right)? So it is not glaze application, but rather fluxing. I ran your formula on Glaze Master 3.0 and did a formula limit comparison. Your calcium (whiting) limit is above, but the rest is in align. The other two issues: your silica and alumina are well below limits. Softening the glaze body may be intentional in this case to produce a crackle effect. So I will let that go, as well as the rather large difference in the COE of this glaze as compared to the clay body ( which also helps to produce a crackle effect.) That said, it cannot be entirely ignored because you have reached a boundary: producing both the crackle effect and crazing- both caused by offset COE. So I think it would be advisable to lower the COE of the glaze by some degree in order to solve the crazing issue.

3. Calcium limit: you are above the formula limits in calcium. At cone 6, potassium, sodium, and zinc are in a gaseous state, calcium however is in a liquid state. So lowering the whiting is mandatory in this case. Personally, I would increase the alumina and silica in the glaze as well and rely on the offset COE between the clay and glaze to produce the crackly effect you desire. However, I still think that needs to be lowered by some degree, because it is producing enough stress to crack your pieces.

4. Zinc: your zinc is within formula limits. I have to assume you like the very high gloss it produces ( I understand that :) ). You can produce that same sheen and solve other problems by lowering the zinc to under 5 and increasing the silica. Silica is the primary glass former and additional sheen will be produced because of the refractive index of glass.

So how do you want to fix it is the question?  Do you want to maintain a crackle effect, or forego it for a true glaze fit?

Nerd

 

Edit: In the second picture in your original post; it shows a goldish/tan hue with a goldish band on the rim. That is from the magnesium in the dolomite: if that is undesirable, then you need to lower the dolomite as well.

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I will go ro the studio tomorrow and take some pictures.

Yes the pictures you see now are of the cups that did survive the firing,

Which is usually 3 out of 10

 

Hi,  > I applied thick - it breaks the cups, but this time I applied it thin it still broke them and run down to the foot.

 

I can clearly see crackle in the photos, but no breaks. Can you supply a picture of a couple of the broken cups?

 

 

3 out of 10 ... how frustrating!!!

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Giselle" TY for the kind words, but there are many here that know more than I. This just happens to be an area of study and knowledge.

 

Nat:  Lets try this fix:

Nep Sy:         47.0

Whiting          16.0

Zinc Ox.           4.5

Silica              24.0

NZ Kaolin         8.0  (used as a replacement for bentonite).. You do not need a glaze hardener in this case.

Copper Carb  0.50

 

The COE was lowered to 7.93 from 8.55-which still should be offset enough to create a crackle effect. Might need further adjustments.

The alumina and silica are on the low side of limits, but still increased over your original recipe.

The whiting (calcium) is now within limits: coupled with lowered zinc it should solve the glaze run issue.

Fire one (and only one) test cup: lets see how this works. There is still enough offset in the COE to produce a crackle effect. Need to check to see if it is still too high that it produces crazing or cracking.

 

Still confused by your original post plus the second pic. I did not see dolomite in the recipe, and the second pic in the original post is showing magnesium, or possible iron content. So you either omitted the dolomite, or your silica has a higher iron content (impurity.) - or clay issues.

 

Nerd

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but still, do you have any idea why the same glaze is reacting so differently

and I wonder if cooling time is also influencing it -

Yes, I have some ideas on reactivity- but lets solve the glaze issues first.

When doing crackle: cooling can help or hinder- lets work with the glaze first.

 

Nerd

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Nerd,

 

Thank you! I'm loading kiln tomorrow, so soon we will see some results.

 

no, sorry, there is no dolomite in the glaze, I mixed it with another glaze recipe. I actually like this yellowish color,

But as always, sometimes cups have it and sometimes they don't. It s a mystery. For now :)

Looking forward to new results.

thank you so much

 

 

Giselle" TY for the kind words, but there are many here that know more than I. This just happens to be an area of study and knowledge.

 

Nat:  Lets try this fix:

Nep Sy:         47.0

Whiting          16.0

Zinc Ox.           4.5

Silica              24.0

NZ Kaolin         8.0  (used as a replacement for bentonite).. You do not need a glaze hardener in this case.

Copper Carb  0.50

 

The COE was lowered to 7.93 from 8.55-which still should be offset enough to create a crackle effect. Might need further adjustments.

The alumina and silica are on the low side of limits, but still increased over your original recipe.

The whiting (calcium) is now within limits: coupled with lowered zinc it should solve the glaze run issue.

Fire one (and only one) test cup: lets see how this works. There is still enough offset in the COE to produce a crackle effect. Need to check to see if it is still too high that it produces crazing or cracking.

 

Still confused by your original post plus the second pic. I did not see dolomite in the recipe, and the second pic in the original post is showing magnesium, or possible iron content. So you either omitted the dolomite, or your silica has a higher iron content (impurity.) - or clay issues.

 

Nerd

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Nat:

When you get time, I need some info on the porcelain slip you are using. Is it a commercial mix, or home grown variety? Curious to see the specs in regards to silica to alumina ratios, as well as magnesium or iron content. If you are not adding magnesium to the glaze; then it has to be coming from the slip. It is possible that the silica or Nep Sy has a higher percentage of either than usual.

I assume you are bisque firing these pieces? If so, what color are they after bisq? that will tell me something too.

Nerd

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Hello Nerd,

The porcelain slip is from Limoges, I buy it as a powder and then mix it by hand in the studio.

It's th porcelain they recommend firing up to 1250 and not higher.

Yes, I'm bisque firing the cups and they look white, beautiful white

 

Will try the new glaze today

look forward to the results

 

I was looking at the cups that survived the firing

I wonder why do they have this nice and fine crackle

but on those that broke crackle effect is not the same, cracks are big, breaking through, so you can see a crack appearing in the glaze on the interior.

I actually realized that the problem is not much in the runny glaze but in this crackle thing..

 

 

 

Nat:

When you get time, I need some info on the porcelain slip you are using. Is it a commercial mix, or home grown variety? Curious to see the specs in regards to silica to alumina ratios, as well as magnesium or iron content. If you are not adding magnesium to the glaze; then it has to be coming from the slip. It is possible that the silica or Nep Sy has a higher percentage of either than usual.

I assume you are bisque firing these pieces? If so, what color are they after bisq? that will tell me something too.

Nerd

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What's happening is that your clay and your glaze are shrinking at different rates. The glaze is pulling the pot apart because it shrinks so much more than the clay. If 7 out of 10 pots are coming out of the kiln broken, the other three are guaranteed to be structurally unsound. They will break readily.

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Hi Nat:

I know several crystalline glazers in France and England that use Limoge porcelains. From what I know about them, they seem to be a high quality clay maker.

You described the bisque color as "beautiful white", which I take to mean a bright white color. That would mean that the magnesium is under 0.30% of batch weight, and the iron under 0.50%: otherwise it would not be pure white. Titanium would also be under 1% as well. So for now I will assume that the porcelain slip is not leaching impurities that would cause a goldish/yellow color in the glaze. That also means that either your silica or Nep Sy, or whiting has magnesium/iron impurities at a level which is causing this additional color to appear. We lowered the whiting, so if that color continues: we know it is not the whiting.

After this test firing, post some close ups of the piece please. If the glaze is what you want, but it is still cracking pieces then we might attack this from a different angle. Your slip is a dry powder, so you can also goose it up a bit to give it additional strength.You adding sodium silicate when you mix?

Nerd

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