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Crackle glazes are used on decorative ceramic ware. They have a crack pattern that is a product of thermal expansion mismatch between body and glaze.
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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).
These two glazes look the same, they are both cone 6 satin mattes. On the same porcelain. But the matteness "mechanism" of the one on the left is a low Si:Al ratio melted by zinc and sodium. The mechanism of the one on the right, G2934, is high MgO melted by enough boron to also have plenty of SiO2 and Al2O3. The "baggage" of the mechanism on the left is high thermal expansion and crazing (drastically reducing strength and providing a bacteria opportunity). The glaze is "stretched" on the clay (because it has a higher thermal contraction). When the lines are close together like this it is more serious (they have been highlighted with dye). If the effect is intended, it is called "crackle" (but no one would intend this on functional ware). The glaze on the left also calculates to a high thermal expansion so the crazing is not a surprise. The one on the left calculates much lower and would stay uncrazed indefinitely.
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.
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.
Ceramic Glaze Defects
Ceramic glazes are glasses that have been adjusted to work on and with the clay body they are applied to.
|Oxides||KNaO - Potassium/Sodium Oxides|
|Oxides||Na2O - Sodium Oxide, Soda|
|Oxides||Al2O3 - Aluminum Oxide, Alumina|
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.
Ferro Frit 3110
High sodium, high thermal expansion low boron frit. A super-feldspar in clay bodies.
|By Tony Hansen|
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