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

 

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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

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.

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

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

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

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

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.

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