Clay - Glaze Interaction?

[Patricia Kaiser]

My problem is between a lovely Buttercup Yellow glaze and Laguna/Miller ^6 porcelain. Sometimes the glaze, when fired, contains white bumps and clumps like scattered small and medium curds of cottage cheese, irregularly spaced on interior and exterior of a piece. This was found most of all on a bowl with thin glaze, but on pieces with thicker glaze there are occasional white spots. One of these spots has a tiny crater, indicating what exactly I don't know.

 

The glaze recipe includes EPK, Frit 3195, Neph Sy, Silica, Wollastonite, Bentonite, and Rutile.

 

On Laguna brown ^6 clay, the glaze is perfect, with no bumps. This is why I think there may be an interaction between the porcelain clay and glaze.

 

Thanks in advance for any helpful information.

 

Patti Kaiser

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[Natas Setiabudhi]

When i see your recipe, i think the glaze failures are caused by rutile. You must substitute a rutile by another oxide (may be you can use titania which has little bit similar character) or decreasing an amount.

 

Good luck!

[Patricia Kaiser]

When i see your recipe, i think the glaze failures are caused by rutile. You must substitute a rutile by another oxide (may be you can use titania which has little bit similar character) or decreasing an amount.

 

Good luck!

 

 

Thanks, Natas, your help is appreciated! In this recipe, rutile is 6% added to the 100% base recipe. Being a real newbie, I am not prepared to mix another batch of that recipe, but I wonder what caused the reaction with porcelain and rutile. What is it about rutile that would cause this problem?

 

Trying to learn, but not wanting to trouble you for readily available information. If you know some resource that can explain more, I would appreciate your help again to direct me to further details.

[Natas Setiabudhi]

I asked that because it's based on my experiences. The glaze failures are not just caused by one thing, it is integrated, formulas of body and glaze, firing schedule, temperature, etc. May be your recipe is good, but your schedule firing is too fast, so failures will happen.

Sometimes, a borax oxide (B₂O₃) may cause pronounces streaks or spots, particularly in those glazes which are opaque and lightly tinted with some other coloring oxide such as copper or iron. So you must ask to your supplier, is there any borax in frit 3195? If any, you can substitute or just decreasing that.

To make you convince, you can read a book which title Clay and Glaze for the Potter, by Daniel Rhodes. That's old book, nevertheless I think it pretentious book to me..

[Patricia Kaiser]

I asked that because it's based on my experiences. The glaze failures are not just caused by one thing, it is integrated, formulas of body and glaze, firing schedule, temperature, etc. May be your recipe is good, but your schedule firing is too fast, so failures will happen.

Sometimes, a borax oxide (B₂O₃) may cause pronounces streaks or spots, particularly in those glazes which are opaque and lightly tinted with some other coloring oxide such as copper or iron. So you must ask to your supplier, is there any borax in frit 3195? If any, you can substitute or just decreasing that.

To make you convince, you can read a book which title Clay and Glaze for the Potter, by Daniel Rhodes. That's old book, nevertheless I think it pretentious book to me..

 

 

Thanks again, Natas! The local library has that book. It looks like I will have some study to do to make some sense of what exactly is going on.

[Patricia Kaiser]

I asked that because it's based on my experiences. The glaze failures are not just caused by one thing, it is integrated, formulas of body and glaze, firing schedule, temperature, etc. May be your recipe is good, but your schedule firing is too fast, so failures will happen.

Sometimes, a borax oxide (B₂O₃) may cause pronounces streaks or spots, particularly in those glazes which are opaque and lightly tinted with some other coloring oxide such as copper or iron. So you must ask to your supplier, is there any borax in frit 3195? If any, you can substitute or just decreasing that.

To make you convince, you can read a book which title Clay and Glaze for the Potter, by Daniel Rhodes. That's old book, nevertheless I think it pretentious book to me..

 

 

Thanks again, Natas! The local library has that book. It looks like I will have some study to do to make some sense of what exactly is going on with this glaze.