|Monthly Tech-Tip |
Alternate Names: Lizzella, Lizzela
A brown or red burning clay (depending on firing temperature) from Lizella, Georgia (raw color is light green). It is available from Davens or Kickwheel Pottery Supply. It is plastic and usable as a clay body by itself.
This material is sometimes cited as a substitute for Redart in bodies, but the two materials are very different. Compared to Redart, Lizella has a higher drying shrinkage (6.5% vs. 5.5%), it is much more plastic (thus more prone to cracking during drying). The iron content is lower (5% vs. 7%). As far as fired maturity, these materials are not even close. At cone 4, for example, Lizella has around 11% porosity vs. almost zero for Redart, and 4% fired shrinkage vs. 8% for Redart. Lizella stays stable to cone 10 oxidation, Redart is melting by then.
These bars have been fired at cones 4, 2, 02, 04 (top to bottom) using the SHAB testing procedure. We can measure fired shrinkage and porosity in each to get an indication of their fired maturity. The Redart (left) is much more vitreous and reaches almost zero porosity by cone 4 whereas the Lizella still has 11% porosity at cone 4. Lizella also has a much higher drying shrinkage (because it is way more plastic).
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Fired at cone 10, 6, 4 (top to bottom).
Porosities: Cone 10 - 3%, cone 6 - 8%, cone 4 -11%.
Fired shrinkages: Cone 10 - 6%, cone 6 - 5%, cone 4 - 4%.
Somewhere between cone 6 and 10 it turns color from red to brown. That color change probably brings with it tendencies to warp and bloat.
Clays that are not kaolins, ball clays or bentonites. For example, stoneware clays are mixtures of all of the above plus quartz, feldspar, mica and other minerals. There are also many clays that have high plasticity like bentonite but are much different mineralogically.
The most common commercially 200 mesh available raw terra cotta clay in North America. It fires red, has low plasticity and matures a low kiln temperatures.
Ocmulgee Red Clay
|By Tony Hansen|
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