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

 

[Jennifer Porah]

Is there a kiln pattern or certain holds to these glazes?

[Norm Stuart]

A kiln firing with slow-cooling will tend to eliminate the crazing "defect" in crackle glazes, which is not what you're trying to achieve with a "crackle glaze".

This is a similar high sodium glaze which reliably crackles, even with a very slow-cool. The secret is the cryolite, which is also responsible for the pinholing.

	Albino Python  ^5/6
80%	Nepheline Syenite
10%	Ball Clay
3%	Feldspar - Kona - Soda
3%	Cryolite

Without a slow-cool it crawls and crackles far more.

[Jennifer Porah]

Ah! Brilliant~! Thanks for this...I've been moving too slow then... The ramp style I had was more for crystal glazes, rather than crackle.  Thanks for the help! :D

[Joseph Fireborn]

Bumping this. I am going to start trying this out. I will post some results in a few weeks.

[Penny Berglund]

I tried this with a 100 gm test and made up 1000 gm. Test had a hole but OK I thought, pots were another story.The coefficient of expansion of the glaze vs the clay is key and I don't think a slow cool would hurt the crackle.I did fire the pots to cone 6 with a 10 minute hold, but the kiln cooled very rapidly. Pots were bisque fired to 05, so more porous nature would allow thicker glaze application. Snoflake crackle #4

What do you think?

[Norm Stuart]

The top tile looks typical, often with this variation in thickness.

The bottom photo I would guess something in the clay is still off-gassing /decomposing. Thickly crystallized glazes like snowflake are relatively unable to heal pinholes caused by gas from the clay body.

We had this with the last  batch of Laguna B-Mix paper-clay. The clay was not stable at Cone 6.

[Tom Anderson]

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

[Penny Berglund]

I visited my supplier over lunch today and will modify my bisque program for this clay and retest. I will then refire tests with my normal profile and the profile that Tom has just suggested. I will keep you posted and send pictures in the coming week or two.

I have seen these glazes fired to Cone 10 over a black slip with great results. When I get it dialed in, that will be the next set of tests.

Penny

[Tom Anderson]

Penny:

is that a dark or red bodied stoneware I see in your pics? Are either of these bodies used in the pinhole issue? 

[Penny Berglund]

It is a dark brown clay body. I spoke with my supplier and I will modify my bisque schedule. I also got some neph sy from a different source (Thanks Paul!) and will test that too.

I think the darker clay needs to be bisqued in a way to ensure burn out of carbon, etc and I will run another test load and post pictures with new samples.

Ceramics Arts Network posted this schedule which will be similar to what I plan to use.

https://ceramicartsnetwork.org/daily/firing-techniques/electric-kil...

[Norm Stuart]

I think Tom may be suggesting the iron from the clay might be migrating through the glaze, as with an oilspot glaze.

This might be possible we've used snowflake glazes on dark brown clays before and never had a pinjolonh or iron migration problem.

But we do bisque to a full Cone 04 with a half-hour hold at the top to make sure the bisque is fully off-gassed.

But even with this schedule we still have problems with an occasional batch of clay, most recently Cone 6 B-Mix Paper Clay which shed glaze leaving large bald areas. And although people at our studio like working with Greystone, they're often not happy with some glazes changing color which I suspect is caused by a high level of magnesium (Talc) in the Greystone clay recipe.

[Penny Berglund]

I am pretty sure I did a poor bisque job on this batch. John Britt uses this on iron bearing bodies and Reid Ozaki uses a cone 10 version over a black slip with iron and manganese which has some gold color migrating through but no holes.

If the red/brown clay is the problem, I will ask clay art center in Tacoma which of their white stone ware clays has a similar coefficient of expansion. I am not sure at all the clay is the problem. 

Back out to throw some more samples!

Penny