John Britt's Snowflake Crackle - The cone 6 oxidation glazes

[George Lewter]

John had a great article in the Nov. 2011 issue of Ceramics Monthly which was excerpted and republished on the Ceramic Arts Daily Website as "Crazy Beautiful Crazing: Uncovering the mysteries of Snowflake Crackle Glazes" Click Here to go to the online article. I contacted John Britt and he said it was OK to reprint the recipes here so our interested members can carry the conversation forward.

Here are the recipes.

Recipe Name:  Original Snowflake Crackle
Cone:  6/7     Color:  Translucent     Firing:  Oxidation     Surface:  Glossy

Amount     Ingredient

  4.26          Magnesium Carbonate

 89.36          Nepheline Syenite

  6.38          Ball Clay--Old Mine #4
100         Total

Additives

2.00          Bentonite 

The glazes seem to work best on dark brown bodies.  Use only small amounts of colorants, as a lot will kill the crazing.

 

Recipe Name:  Snowflake Crackle #4
Cone:  6/7     Color:  Translucent    Firing:  Oxidation     Surface:  Glossy

Amount     Ingredient

   7.86 Talc

   5.77 Ferro Frit 3124

 86.37 Nepheline Syenite

100 Total

Additives

   2.00 Bentonite

   0.20 Copper carbonate for Turquoise

 

Recipe Name:  Snowflake Crackle #8
Cone:  6/7     Color:  Translucent    Firing:  Oxidation     Surface:  Glossy

Amount     Ingredient

   3.94 Magnesium Carbonate

   7.41 Ferro Frit 3124

  82.74 Nepheline Syenite

   5.91 Ball Clay--Old Mine #4

100 Total

Additives

   2.00 Bentonite

   0.20 Copper carb for turquoise

   0.50 Superpax for white

   0.066 Cobalt carbonate for blue

   0.50 Degussa stain 239416 for yellow

   0.50 Red iron oxide for rust red

 

[Penny Berglund]

I will test this as well. My friend Paul suggested Neph Sy might be the source as well. 

Penny

Tom Anderson said:

Norm:

Nep Sy has been the flux of choice for porcelain and now stoneware bodies are incorporating it. The critical temperature for either body is 2050F: when metakaolin converts to spinel. In pottery speak: when the porosity of the body begins to close, and vitrification is beginning. To remedy pinholes: from 2050F up to 2190 ( with hold) or preferably 2230F: slow the ramp down to 125F an hour rate climb. This ramp cycle will cure most all pinhole issues. Nep Sy off gases much more vigorously than potassium: the nature of the beast! 

Tom

[Tom Anderson]

Actually I was looking at the sulfide content: which in Western clay comes in the form of FeS2- iron disulfide. It takes a slightly different bisq schedule to burn off sulfides rather than carbons. Most carbons are organic and are burnt off rather easily.  For sulfides: 1250-1750 F is the key temperature range. Dark stoneware and red bodied clays can have in excess of 700PPM sulfides, even though iron content is lower. The culprit: lignite coal particles primarily.

run the pinhole program first. 

Tom

note: In pottery as Norm points out: we look at iron. Actually it is the chemically bonded sulfides attached to the iron. Better known as iron pyrite.

[Tom Anderson]

Read here:

https://ceramicartsnetwork.org/ceramics-monthly/ceramic-supplies/ce...

Tom

[Penny Berglund]

I broke a couple of the plates/bowls to see what I might learn. I did not find any evidence of bloating in the clay. The problems seemed to be confined to the glaze layer and certainly could be due to off gassing.

I have had bloating problems with a white porcelain like stone ware though. The info in the article was not something I had considered before, especially with white clay bodies.

Penny

[Tom Anderson]

Penny:

that is actually very pure porcelain that I specifically formulated for these tests. I added controlled amounts or iron sulfide, and then additional sulfides later: surprising little actually. I will have another article out shortly " Clay Shoppers Guide" that will teach how to decipher and apply all those numbers associated with clay. COE, shrinkage, and absorption all tell the story of how it was formulated and how plastic it really is.

Tom

[Norm Stuart]

A Cone 04 bisque with a 30 minute hold is solidly above 1,900 F, well above the 1250-1750 F range you've identified for off-gassing sulfides. Cone 04 prevents a lot of problems at Cone 6.

It would be nice if some ceramic organization recognized a test or classification of clay plasticity. But as Jon Brooks of Laguna Clay told me, it depends on how hydrated the clay is, which varies over time, what portion of the pug mill it came from, and every batch of a clay recipe is somewhat different.

What Laguna offers on some clays as a substitute is "Penetration", Which is how deeply a spring loaded shaft penetrates the pug of clay.  I guess that's a measure of softness or stiffness and density perhaps.

Tom Anderson said:

Penny:

that is actually very pure porcelain that I specifically formulated for these tests. I added controlled amounts or iron sulfide, and then additional sulfides later: surprising little actually. I will have another article out shortly " Clay Shoppers Guide" that will teach how to decipher and apply all those numbers associated with clay. COE, shrinkage, and absorption all tell the story of how it was formulated and how plastic it really is.

Tom

[Penny Berglund]

Tom,

That is very interesting. It sounds like a great way to run a real test.

I look forward to seeing your guide when it comes out. I am getting more out of my failure than I could have anticipated.

Pennny

[Tom Anderson]

Norm:

ASTM D-4318 based on the Atterberg Limits, is the plasticity/ liquid test protocol used by other industries: except ours of course. Common to see numerical plasticy values used in Europe, not here of course. Some makers in Germany show carbon content, not here of course. The current testing in the US is based on a potter blowing through 10-12 bodies until they find one that does not drive them crazy.

Some clay deposits are going through changes: just like Custer did a few years back. The Feds only require one chemical analysis be done for the entire deposit: many are decades old. I am sure many potters are finding their favorite clays are inconsistent from order to order: too wet or too dry for example. Potters consume less than 3% of natural clays, so we are far down on the pecking order.

for porcelain: watch the shrinkage rates. The higher the shrinkage, the more plastic it is because high plasticity materials absorb more water. 10% is lower plasticity and 13% is high plasticity.

for stoneware: watch the absorption. To lower the absorption in stoneware you have to add much finer sub micron ball clays to close up the porosity of fire clays. The finer the particle, the more plastic it is.

Tom

I would cut and paste some materials I send out to educators on clay: but it would probably crash this site. George would not be happy.

[Penny Berglund]

I did a new test run with a much better bisque protocol and a glaze test firing. I also ran a small test with a new batch of neph sy. I did see improvement in the cratering I experienced before along with large holes. I had problems on the tops of the bowls with crawling, likely due to very thick glaze application and I still had pinholing on the undersides of bowls but not at bad. Glazes were applied on different days to the underside to allow the inside of the bowls to dry and then dried completely before firing.

New neph sy small test was essentially the same.

Have any of you succeeded with this glaze? I will try to spray the glaze on instead of pouring and may even go to bisque between layers of glaze if I need to. Mr. Britt says this needs an application of 3 to 6 mm of glaze.

Penny

Comparisons show items from first firing and from this latest test

[Norm Stuart]

From the photos, I believe your problem lies exclusively with the clay used continuing to off-gas in an interaction with the glaze.

I'd try again with a different clay, as I've never seen this glaze pinhole, although it is an interesting look.

As an example, we've always had a pinhole problem with Laguna "Half and Half" clay when a fluorine-based glaze is applied (Ferro Frit 5301).

There's obviously some raw material in the "Half and Half" clay, which is not present in other Laguna clays, which the Fluorine flux in Ferro Frit 5301 decomposes during a Cone 6 firing.

Even after refiring the "Half and Half' with 5301 glazes three times, pinholes continue to develop, so it's a very long-lived reaction.

The typical look of the Snow Flake glaze

Penny Berglund said:

I did a new test run with a much better bisque protocol and a glaze test firing. I also ran a small test with a new batch of neph sy. I did see improvement in the cratering I experienced before along with large holes. I had problems on the tops of the bowls with crawling, likely due to very thick glaze application and I still had pinholing on the undersides of bowls but not at bad. Glazes were applied on different days to the underside to allow the inside of the bowls to dry and then dried completely before firing.

New neph sy small test was essentially the same.

Have any of you succeeded with this glaze? I will try to spray the glaze on instead of pouring and may even go to bisque between layers of glaze if I need to. Mr. Britt says this needs an application of 3 to 6 mm of glaze.

Penny

Comparisons show items from first firing and from this latest test

[Penny Berglund]

I will try another clay body. I will also ask my supplier what they think. I am getting good crackle even where it is thin which would mean I won't have to go so nuts with the thicker glaze layer.

Is there a Laguna clay body that has worked for you with this glaze? IMCO? I can get clay at Clay Art Center in Tacoma, and I will be in Seattle next week and can get the clay bodies that Seattle Pottery Supply makes up as well. Since I am in the gorge now, it is also possible to make a trip to Portland to try what Georgies makes up.

Penny

Norm Stuart said:

From the photos, I believe your problem lies exclusively with the clay used continuing to off-gas in an interaction with the glaze.

I'd try again with a different clay, as I've never seen this glaze pinhole, although it is an interesting look.

As an example, we've always had a pinhole problem with Laguna "Half and Half" clay when a fluorine-based glaze is applied (Ferro Frit 5301).

There's obviously some raw material in the "Half and Half" clay, which is not present in other Laguna clays, which the Fluorine flux in Ferro Frit 5301 decomposes during a Cone 6 firing.

Even after refiring the "Half and Half' with 5301 glazes three times, pinholes continue to develop, so it's a very long-lived reaction.

The typical look of the Snow Flake glaze

Penny Berglund said:

I did a new test run with a much better bisque protocol and a glaze test firing. I also ran a small test with a new batch of neph sy. I did see improvement in the cratering I experienced before along with large holes. I had problems on the tops of the bowls with crawling, likely due to very thick glaze application and I still had pinholing on the undersides of bowls but not at bad. Glazes were applied on different days to the underside to allow the inside of the bowls to dry and then dried completely before firing.

New neph sy small test was essentially the same.

Have any of you succeeded with this glaze? I will try to spray the glaze on instead of pouring and may even go to bisque between layers of glaze if I need to. Mr. Britt says this needs an application of 3 to 6 mm of glaze.

Penny

Comparisons show items from first firing and from this latest test

[Norm Stuart]

I can't guarantee we've tried Snowflake with all of these clays, but these are our combined clay orders over the past five years.

WC 371	Big Pot Clay 60 mesh sand
WC 373	Dark Brown Clay -- fine sand
WC 376	L.B. Blend Clay 60 mesh sand
WC 381	Big White Clay with grog & sand
WC 384	Dave's Porcelain
WC 398	WS-4 gray/white with fine sand
WC 429	RSMC Red Clay
WC 894	Half & Half SC
WC-353	Paper Clay Cone 10
WC-370	Amador Cone 10 - densifies nicely at Cone 6
WC-390	Laguna Red
WC-391	B-3 Brown Black
WC-395	Porcelain Five
WC-400	Morrocan Sand
WC-402	Half n Half - pinholes with Ferro 5301
WC-412	Stoney White
WC-423	Greystone - contains something (zinc?) which turns many blue and green glazes to brown
WC-436	B Mix 5 with Grog
WC-438	Red B-Mix with grog   Easy Dry
WC-841	Danish White Clay
WC-842	Danish White Clay -- fine sand
WC-851	52 Buff
WC-861	Haggi Porcelain
WC-866	Electric Brown
WC-886	B Mix with Grog Paper - extremely problematic blistering, order only WC-353

Penny Berglund said:

I will try another clay body. I will also ask my supplier what they think. I am getting good crackle even where it is thin which would mean I won't have to go so nuts with the thicker glaze layer.

Is there a Laguna clay body that has worked for you with this glaze? IMCO? I can get clay at Clay Art Center in Tacoma, and I will be in Seattle next week and can get the clay bodies that Seattle Pottery Supply makes up as well. Since I am in the gorge now, it is also possible to make a trip to Portland to try what Georgies makes up.

Penny