Here is what Redart fired like in 2010 when evaluated for physical properties using the SHAB test and LDW test. These bars are fired from cone 5 down to 06 (top to bottom). The clay reached zero porosity (water absorption) between cone 1 and 2. Beyond that, it was stable to cone 4. Its drying shrinkage was only 5%, indicating low plasticity (compared to a typical pottery clay). It is thus an ideal candidate for a ball clay addition to create a plastic body for handbuilding or throwing.
However, its plasticity is just right for slip casting; it can be used as is. It has no soluble salts and a perfect score on the drying performance DFAC test. Cone 04 and 06 porosity and fired color are very typical of terracotta clays. The fired color of this material really is stunning. And the stability of that color across such a wide temperature range.

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This slurry is 100% traditional Redart. This deep red color is a product of the high percentage of iron oxide in its chemical analysis. But that chemistry provides a good example of why using the classical ceramic rational-analysis method to derive a theoretical mineralogy can be misleading for an illitic clay (which this is).
The traditional calculation assumes that K2O and Na2O belong to feldspars, then assigns the remaining Al2O3 to kaolinite and the remaining SiO2 to free quartz. However, Redart is known to be an illitic sedimentary clay. And illite itself contains substantial K2O. That 4.1% K2O is a strong chemical confirmation of Redart containing a lot of illite/mica-type clay. The 1.6% MgO is another clue that some of the chemistry belongs to smectite minerals rather than discrete feldspar and MgO phases.

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The objective is to discover the ratio of Redart to ball clay that reaches near-zero porosity (water absorption) by cone 10, with minimal bloating and rich color. And with high plasticity. This is stage 1 in creating a new body; nine more tests eventually achieved L3118J.
These test bars were fired at cone 10R and 11, 10, 9 and 8 oxidation (top to bottom). It reaches zero porosity by cone 8 and blistering is already happening there (which makes measuring fired shrinkage or porosity beyond that pointless). However, the cone 10R bar is not blistering. It clearly needs less fired maturity. A real bonus: Plasticity is fantastic yet drying shrinkage is well below 7%! In the final body I ended up using 20% Redart, it still imparted good color and near-zero porosity by cone 10.

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This company deserves credit for the internal testing they do and for openly acknowledging consistency issues with the RedArt shipment(s) they have received, doing so at the top of their homepage! It enables brown and red color in so many of their bodies (with little or no need for messy iron oxide or even feldspar). They reveal that "Cedar Heights Clay Company is mining the RedArt from a different seam." They recommend that customers perform production testing prior to use.
An old-versus-current oxide and mineralogy analysis from Cedar Heights (along with shrinkage, absorption and color comparisons) seems needed. Plus the lot numbers affected. If you are a tech at Resco, contact me and let's get this done.
| Materials |
Redart
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. |
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