|Monthly Tech-Tip |
Alternate Names: Carbondale Red
Description: Plastic high iron smooth fireclay
This is a smooth plastic very high iron smooth fireclay. As far we know this is the same material as Jasper clay from Gladding McBean, but it is air floated at 200 mesh. It is cited as a possible substitute for Newman red, however it is higher in iron.
This material's fine nature and high iron content make it suitable for terra sigillata. This appears to be the same material as Carbonale Red.
These test bars are fired from cone 10 reduction (top) and 10 oxidation down to 6 oxidation. The drying shrinkage and the fired porosity and shrinkage are shown on the chart. This is a refractory clay, even at cone 10. It has excellent plasticity, similar to a typical throwing pottery clay. This can do at cone 10 what Redart does at low fire: Be added to a white burning clay body and turn it red. But 75% or more Redart is needed while as little as 25% if this can make a body red. While Redart is not plastic enough to use alone, this is (so it will have minimal impact on the drying and plasticity of the body). Since C-Red is refractory it will reduce the maturity of vitreous cone 6 bodies. But that is good because achieving red coloration at stoneware temperatures depends on a body not being vitreous (vitrification turns the color to brown). We tried adding only 30% of this to M370 porcelain, that produced a deep red stoneware.
C-Red clay (also called Carbondale Red) has 12% iron oxide. It is also refractory so easily handles cone 10R. However with that amount of iron is it pretty well impossible to prevent black carbon coring in glazed ware. This cup simply fell to pieces when just touched. Each of these small pieces can be broken into many more with just light finger pressure. Yet the unglazed test bars were unbreakable by hand! The glaze-body bond is also strong. What is demonstrated here is the extreme compression under which the glaze is attached to the clay, breakage relieves it. That means the black coring is increasing the thermal expansion of the body, as the piece cools in the kiln the body is contracting thermally more than the glaze, putting the latter under more and more compression.
This body was made from 35% C-Red, 50% ball clay and 15% feldspar. We made it to see if glazes might fire less cloudy than they do with Redart based bodies or our L215. No difference was observed. Our G1916Q and Spectrum 700 transparent are both crazing. The body color of this C-Red based body is not as dark at Plainsman L215 (lower right).
Imco 800 Fireclay
Newman Red Clay
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.
|By Tony Hansen|
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