Crazy Beautiful Crazing: Uncovering the Mysteries of Snowflake Crackle Glazes

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I have been messing around with crazing as a deliberate decorative effect lately. Though it is technically a glaze defect, crazed surfaces can actually be quite beautiful and I have really been enjoying the depth that crackling can create.

But the crackle surfaces I have been creating pale in comparison to the Snowflake Crackle glazes John Britt writes about in the November 2011 issue of Ceramics Monthly. As you can see here, these crackled surfaces are pretty spectacular! Today, I am giving you all a sneak peek at that article, which includes lots of snowflake crackle glaze recipes! – Jennifer Poellot Harnetty, editor.


Side view of cracked bowl showing thickness of fired glaze versus the thickness of the clay body. Glaze thickness is between 1/8 and 3/32 of an inch thick after being fired.

I only had a brief glimpse of this glaze some 15 years ago in a museum gift shop. It was on a little Sake set in a traditional Japanese wooden box. It was glazed with the most beautiful crackle glaze — not the usual crackle glaze that is common in Raku.

It had a conchoidal fracture with the crazes layered on themselves like a stack of books that had slid over.

Only later did I find out that this glaze was called Snowflake Crackle; actually, it has many names (Snowflake Crackle, Fish Scale Crackle, Ice Crackle, Ice-like Crazing, and Tortoise Shell Crackle), which is generally a sign of how much people like something.

Regardless of what you call it, it’s a crazed glaze that is applied so thick that the fractures run not just vertically but horizontally. These crackle patterns can be large or small, and they are distinct hexagonal shapes. When they are large, it is easy to see why it is sometimes called Tortoise Shell Crackle. It is often said that, if you don’t like it, you call it crazing; if you like it, you call it crackle.

Crazing is often thought of as a glaze defect, but as Nigel Wood describes in his book Chinese Glazes, the Song dynasty potters are thought to be the first to treat crazing as a decorative effect. Commonly called Guan (Kuan) Crackle, the Ru, Guan and Ge ware were all beautiful examples of crazing as a decorative technique. In a specifically beautiful type called “iron wire and golden threads,” or “golden floss and iron threads,” the larger cracks, or primary cracks, were stained black while the pots were still hot. The smaller, secondary cracks developed over months or years as the delayed crazing occurred, leaving them brown.

Crazing

Almost everything (except rubber and ice) will expand when heated and contract when cooled. When a clay body and a glaze are fired on a pot, they fuse together. As they cool, if the glaze contracts more than the clay body, it cracks or crazes. If the opposite occurs and the clay body contracts more than the glaze, the result will be shivering, where the glaze will actually pop off of the pot. But if there is just the right amount of compression between a clay body and a glaze, it adds strength to the piece.

Crazing is usually thought of as a glaze defect because the piece can be approximately 75 % weaker than its uncrazed counterpart. It is also thought that the craze lines can harbor germs or bacteria. For these reasons, dinnerware suppliers like to provide uncrazed ware.

Snowflake crackle is the most extreme kind of crazing, where the glaze is applied so thick and the fit with the body is just right so that the crackles appear to lay on top of one other. The glaze can be twice as thick as the body, just like during the Song dynasty. According to Nigel Wood, in order to get the glaze thick enough, Song Dynasty potters often bisque fired pieces between glaze coats.

Detail of a bowl with 1234 Glaze on Orangestone clay, fired to cone 10 in reduction.

A crazing pattern is always the result of the relationship/marriage between the expansion and subsequent contraction (governed by the coefficient of thermal expansion, or CTE) of a clay body and a glaze. By knowing a little about the CTE of oxides, you can easily control the crackle pattern in glazes to create any range of crazing from very small to very large crackles. Since a glaze is a collection of oxides with a variety of CTEs, knowing that sodium oxide has a high CTE and magnesium oxide has a low CTE allows you to control the overall CTE of the glaze by adding one or the other.

As a glass, silica has a low CTE (it is amorphous), but as a crystal, it has a high CTE. So adding silica to a glaze can lower its CTE, because the silica melts to its glassy state. While adding silica to the clay body (where it remains a crystal) can increase its CTE. It is important to keep the firing cone consistent as heating the piece a cone higher will result in melting more of the silica in the body to glass, thus lowering its CTE. Similarly, firing to a lower cone will melt less of the silica in the body giving it a higher CTE. The time and temperature of the firing is extremely important to the CTE of the clay body and thus to the crazing pattern.

Since seeing this glaze in the museum store, I tried many potential recipes with only moderate success. Since I couldn’t get anyone to share their recipe with me, and I had no understanding of the principles of how to achieve this effect, I postponed my search, but in the back of my mind I remembered this beautiful glaze.

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The uneven stress of glazing only the inside or only the outside of a piece can cause it to shatter

Fast forward about fifteen years to a workshop I was giving last year. I assigned a student a series of tests to improve a magnesium crawling glaze (a.k.a. reticulated glaze) that she was working on in cone 6 oxidation:

SDSU CRAWL/BEADS – Cone 6

Magnesium Carbonate 25%

Nepheline Syenite 70%

OM-4 Ball Clay 5%

Total 100%

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Snowflake Bowl, 5 in. (13 cm) in diameter, wheel-thrown Brownstone Clay, with Snowflake Crackle Glaze #8, fired to cone 6 in an electric kiln.

I had her make up the glaze without the magnesium carbonate and then add it back in 5% increments up to 40%. She then dipped tiles of stoneware, dark stoneware, and porcelain. Fortunately, one of the clay bodies she was using was a very dark low-fire Redart body that she liked to fire to cone 6 (it is rated from cone 05–6). When the tiles came out of the firing, my eye went immediately to the 5% magnesium carbonate test on the dark body; it was Snowflake Crackle! What a stroke of luck. Unfortunately, I didn’t care about her reticulated surface anymore, as I was so excited about this crackle pattern.

Testing

When I returned home, I tried the glaze on a variety of store-bought cone 6 clays. The best, in terms of CTE, were Highwater’s Earthen Red and Brownstone at cone 6, and Orangestone at cone 10. There very well may be others that work, but these were the best of those I tested.

Since potters today generally buy premixed clay bodies rather than making their own, changes in CTE are usually made to the glaze rather than the clay body. This glaze crawled a bit, so I decided to remove the magnesium carbonate (because it has such a high shrinkage) and substituted talc.

While I was analyzing the glaze in Insight (a glaze software program), I decided to look at the CTE numbers, which were around 8.59 (Insight CTE). I then constructed about four glazes with similar CTEs, between 8.59 and 8.70. I came up with several nice glazes this way, but I also tested the original recipe with a simple progression of Ferro frit 3124, just to see if it would melt a bit more. The result was a nice transparent Snowflake Crackle with 8% frit.

Color

The first thing I noticed when I did color test with the usual colorants and opacifiers (copper carbonate, red iron oxide, chrome oxide, stains, zircopax plus, etc.) was that, as the color improved, the crazing ceased. This was because all oxides have expansion/contraction rates and adding them changed the CTE of the glaze enough to stop the crackle effect. I ran more tests with very low levels of colorants (under 1%), which kept the crazing yet still produced a nice color.

Mixing and Application Notes

• These glazes contain high amounts of Nepheline Syenite, which is partially soluble, so the glaze slop can easily deflocculate affecting application thickness. Adding Epsom salts corrects the problem. Mix to a specific gravity of approximately 155–160.

• These glazes must be applied thick, between an eighth and a quarter of an inch (3–6 mm).

• Snowflake crackle is not limited to cone 6; that is just the first temperature where I discovered it. In the cone 10 versions, watch for the flocculating effect of bone ash. You may need to add a deflocculant (sodium silicate), otherwise the glazes can flake off before you get the pots into the kiln. If they don’t flake off, they usually crawl badly during the firing.

• Be sure to wait a couple of days after firing for the crazing to complete enough to see it (this is called delayed crazing.) One load came out and I thought they were unsuccessful, with no crazing. I went away for a long weekend and, to my surprise, when I came back they were all crazed nicely with the Snowflake Crackle effect.

• These glazes are not recommended for functional work as you can often feel the crazing patterns with your hand.

• Glazing only one side of a bowl, either inside only or outside only, will cause the bowl to shatter from unequal tension. After the pots are fired, the glaze sometimes thins on the rim and you can see the brown clay body. This is called “brown mouth” or “purple rim and iron foot” (the glazed rim will reoxidize differently from the unglazed foot).

• The crackle works best on the inside of bowls as the concave interior accentuates the crackle effect.

After arriving at the cone 6 recipes, I started testing for cone 10. I took four separate recipes and tested each on four clay bodies. But before I threw them out, I poured each cup into another to get a blended test. I poured cup 1 into cup 3 (Glaze Snow Flake Crackle 13), then I poured in cup 2 (which became Ice, Ice Baby) and finally I poured all three together for the glaze Snow Flake Crackle 1234. As it turned out, none of the original four recipes worked very well, but the three blends were great! (That is a super secret way to get more mileage out of your tests!)


**First published in 2011
Comments
  • I have a question. Being new to pottery, how do I determine the amounts, in term of ounces, etc., when I see recipes such as this.

    SDSU CRAWL/BEADS – Cone 6

    Magnesium Carbonate 25%
    Nepheline Syenite 70%
    OM-4 Ball Clay 5%
    Total 100%

  • Hi Gilbert – start by deciding the total weight of dry ingredients you want to mix then use the proportions given to calculate how much of each ingredient you’ll use. For example, if the total is 1000 grams dry weight, 250 gms would be magnesium carbonate (25%), 700 gms would be nepheline syenite (70%), with 50 gms of OM-4 Ball Clay (5%). Its interesting that the proportionate recipe shown doesn’t actually add up to 100 – that often happens when the last and smallest ingredient(s) is/are added to temper the glaze (change its colour or surface or behaviour raw etc).

    We potters generally use a gram scale to weigh out components because its a smaller unit of measurement and exact measurement matters, but really oz or gms doesn’t matter to the recipe, only to the amount you make. Do remember that you’re using dry weight measure if using ounces (ounces can be either liquid – fluid ounces like 8 oz=1cup – or weight – dry like 16 ounces=1 pound – they’re different). My ‘guesstimate’ given the specific gravity he mentions (155 -160) is that 100 gms dry will yield about 3/4 of a cup of finished glaze.

    Mixing your own glazes can be confusing because of all the concepts involved – but is definitely worth learning about. Get a good book – online or from library – test lots without high expectations – protect your kiln’s shelves for tests and GOOD LUCK !!! It really is worth the effort.

  • You measure in grams, ie: 25 grams Mag Carb, 70 grams Nepheline Syenite, and 5 grams of Ball Clay. That will give you a total of 100 grams. Of course, that is for a small batch of glaze.

  • When the article says that you should glaze the piece inside and out to prevent breaking, does that mean that you have to use the sdsu glaze for both, or can you use another cone 6 glaze on the outside?

    Thanks for the article! I am very excited to try this new glaze technique!

  • Fantastic article. I can’t wait to try this beauty of a glaze. Thanks JB

  • Lin, first of all, the SDSU glaze is not a snowflake crackle glaze (John mentions it as background for the beginning of his experimentation). Use the glazes at the bottom of the post in the image (click to enlarge it). You should use these glazes both inside and out, because the cracking is due to uneven stress from one side to the other. If you use a glaze with a lower coefficient of thermal expansion (CTE), on one side and a high CTE Snowflake Crackle on the other, you could still get too great a difference in stresses from one side to the other. These glazes, when applied properly, put great stress on the clay body, and evening out that stress is the goal.

  • Great article, thanks so much for sharing your work on these glazes. I’ve been interested in crackle glazes since I was exposed to them in college, but never ended up doing the testing to figure them out.

  • Question – Snowflake Crackle #8 says blue is achieved with 0.5 Copper Carb. I’m guessing that should be Cobalt Carb?

  • How timely! Just finished an exhibition here in Melbourne, Australia and all my “Fish Scale Chun” pieces sold in the first few days. Very popular. OK, my secret is out now so, here it is from “Fibre Kiln Glazes” book.
    Potash Feldspar 83
    Whiting 9
    Silica 8
    Bone Ash 1.5
    Black Iron 0.5
    The K Spar is Australian so the Silica may have to be pulled back for US.
    Now playing with other colorants myself.

  • Yes, the blue should be cobalt carbonate, sorry my error!

  • Hey John,
    About 4 years ago I found a recipe called “Cracked Ice” – same type of glaze, I intended to experiment and misplaced the recipe. Thanks for sharing – I can now get to what I wanted to try!

    Thank You
    Glenn Woods

  • How Spectacular!

    I ahve wanted this glaze recipe for 5 years now as this glaze ha sbeen used in Japan. Last year I orded 2 – 5 kilo sacks of this glaze, one is white and the other is celadon, with shipping $400.00 US this ain’t cheap. Now I can go and mix my own ice crackle and color it how I want thanks for posting this article you have just saved me another $400.00 US.

    Richard

  • May I ask that the formula sheet enlarge to something larger than a quarter. Can’t read it. I would like to print.

  • Mark- Click the image, after it’s enlarged in the floating window, right-click for a pop-up menu, click on “View Background Image”. Save it or print is as you like. It’s 900 pixels tall. Whereas “a quarter” would be about 60 pixels tall. Try not to be too snarky when asking for assistance.

  • The recipe for Snowflake Crackle #8 has been corrected!
    -eds.

  • Yes, thanks so much for sharing. Can’t wait to try the cone 6 recipes.

  • Charles, my apology if I sounded “snarky”. I do appreciate your help. Just needed someone kind like you to get me going. Thanks again.

  • Found the problem. Doesn’t work with CHROME. Explorer works perfectly.

  • Very interesting, but I have one question: You stated that “As it turned out, none of the original four recipes worked very well, but the three blends were great!” Which do you recommend? At least to start with? The Snow Flake Crackle 1234? How are the blends different in fired form? Thanks, Lee

  • the glaze formula in the Sept 21 posting from Fibre Kiln Glazes: what cone does it fire to? thanks a lot – very interesting post!

  • Sorry Ginny, I only leave out the most essential bits of info.
    The fibre Kiln Glaze “Fish Scale Chun” is cone 8 to 10. Reduction on the heavy side is best, but it will work with a lighter copper red type reduction. Just a little less vibrant blue. I am about to unload some line blends of John’s 1234 with Si and colorant variations. Thanks John.

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  • In cone 10 version, you have suggested to add deflocculant(sodium silicate) for avoiding flake off. How much % should I use?
    Without using gravity testing tool, pls provide us a guide line of % using Epsom salts.
    Thank you
    SC

  • Do you have a name and supplier or a recipe for the red earthenware that’s mentioned being fired to cone 6?

  • Thank you very much. I have loved this glaze since I saw a spectacular example at an exhibition of Japanese potters 30 years ago. I made a test of the 1234 substituting frit 3124 for frit 3134 since it was all I had, and it worked great.

  • Ben — that would be Highwater Clays in Asheville, NC.

  • Brian – Like all crazing, it will pick up and hold any sufficiently thin liquid which means it will stain and harbour bugs. Plain water also temporarily destroys the visual effect so it’s not so good for waterbowls etc either.

  • Question? For the Snowflake Crackle Glaze #8 would you suggest Cone 6 or 7? I see in the pic you used 6 but the recipe has both at the top?

  • Hi, I am from Germany and admired the Snow Flake Glazes in September 2011 – they are so beautiful! – but was not able at that time to try them. Now I bought a new kiln with as higher range of temperatures and would like to try at least the cone 6 versions, perhaps the cone 10 versions, too.

    My question: I can’t find out german descriptions of the following:
    > Custer feldspar – what means CUSTER?
    > Superpax (for white) – what is it?
    > Ferro frit (cannot find it in the German stores. Of course they offer frits, but not FERRO frit.)
    > OM-4 Ball Clay – is OM-4 a special kind of ball clay?

    Can you help me? Thank you so much.

  • Hi Louise,

    it is april 2015, 4 years later. I have not been able to follow up on your response for a great variety of reasons. I just wanted to thank you for your clear response. Thank you.

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