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Crackle glaze

Crackle glazes have a crack pattern that is a product of thermal expansion mismatch between body and glaze. They are not suitable on functional ware.

Key phrases linking here: crackle glaze - Learn more

Details

A type of ceramic glaze that is intentionally crazed. Crazing is a crack pattern caused by thermal expansion mismatch between body and glaze. After the glaze solidifies (as the kiln cools) it shrinks more than the body. To relieve the tension of being stretched, it cracks. Crackle glazes are typically found on ware fired at low temperatures. Stains and other colorants are often rubbed into the crack lines to heighten the effect.

Crackle glazes are best understood in terms of their oxide makeup, or chemistry. They almost always have very high levels of Na2O, and possibly K2O (collectively referred to as KNaO). These two oxides have the highest thermal expansion, by far, of those commonly found in ceramic materials. Feldspar is the key source. Not surprisingly, high feldspar glazes often crackle. Na2O is also present in the majority of frits. Some frits are formulated to have a high thermal expansion, these are invariably very high in Na2O (much higher than feldspar). Ferro Frit 3110 is an example, some raku crackle glazes have up to 90% of it! As firing temperature increases more kaolin and silica must be added (to source Al2O3 and SiO2) to reduce melt fluidity (and thus the tendency to run down off the ware).

You can control the amount of crackle (proximity of the crack lines) by varying the amount of KNaO-sourcing materials in the glaze recipe. For example, for low temperatures, you could employ a blend of Frit 3110 and 3195 to make up 85% of the recipe (the rest being kaolin). The more Frit 3195, the less the crazing will be. For even better control enter your recipe into your account at Insight-live.com (making sure materials are named correctly so they link to the database), it can display the unity formula. Move the KNaO up (while holding the proportions of other oxides constant) to increase crazing, down to reduce it.

Crackle glazes typically severely weaken ceramic ware, especially if it is thin-walled (to the point it can be easily torn apart with your bare hands). Crackle glazes are definitely not suitable for functional ware (because of bacteria growth and leaching).

Related Information

A crackle glaze by using a cone 6 body at cone 04

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This glaze is Spectrum 700 fired at cone 04 on Plainsman M370 medium temperature porcelain. The clay was bisque fired at cone 04 also. It is more crystal clear and higher gloss than would be possible to attain at cone 6. If better ware strength is needed you can bisque fire pieces at any cone desired, but keep in mind that the more dense the body the longer it will take to dry each coat of glaze. If a white crackle is needed Spectrum #701 can be used.

What is the simplest, most practical raku base crackle recipe?

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A glazed tile showing the raku crackle effect

Many people suffer high-percentage Gerstley Borate "bucket-of-jelly" raku recipes they find on line. Don't do this. There is a common Ferro frit that is perfect for this application, frit 3110 (or Fusion frit F-75). All it takes is 15% kaolin (e.g. EPK) to produce and easy-to-use recipe that is guaranteed to craze. We have assigned it a code number of L4264, a raku base transparent recipe. We have also catalogued some common recipes that people use and outlined the issues they have: L4264A, L4264B, L4264C, L4264D. Do you need a white? It is a simple matter of adding 10% Zircopax to this.

Links

Glossary Glaze Crazing
Crazed ceramic glazes have a network of cracks. Understanding the causes is the most practical way to solve it. 95% of the time the solution is to adjust the thermal expansion of the glaze.
Glossary Ceramic Glaze
Ceramic glazes are glasses that have been adjusted to work on and with the clay body they are applied to.
Glossary Ceramic Glaze Defects
Materials Ferro Frit 3110
High sodium, high thermal expansion low boron frit. A super-feldspar in clay bodies.
Materials Feldspar
In ceramics, feldspars are used in glazes and clay bodies. They vitrify stonewares and porcelains. They supply KNaO flux to glazes to help them melt.
Oxides Na2O - Sodium Oxide, Soda
Oxides KNaO - Potassium/Sodium Oxides
By Tony Hansen
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