Calvert Clay

Description: Red Burning Clay

Oxide	Analysis	Formula
CaO	0.30%	0.02
K2O	2.00%	0.09
MgO	0.80%	0.09
Na2O	0.41%	0.03
TiO2	1.21%	0.06
Al2O3	23.80%	1.00
SiO2	57.79%	4.12
Fe2O3	5.71%	0.15
LOI	7.98%	n/a
Oxide Weight		394.37
Formula Weight		428.57

Notes

A description we have inherited (but do not know from where) is that this is "a red burning material used in the formulation of clay bodies". From the chemistry it appears to be refractory but no a fireclay. It has fairly high Al₂O₃ content so likely has some kaolin content. Thus, Redart, a low-firing material, would not be a good substitute for Calvert in a recipe. Fire-Red from Plainsman Clays, is refractory.

We have no indication of its physical working properties (e.g. plasticity, drying shrinkage, texture) nor can we find a source for any physical data. The physical data shown here is likely not correct (a clay with a PCE of 30 would not have 1.4% porosity at cone 1)! That being said, a clay with this chemistry would be more refractory that is shown by this data also).

Here is a note we received from Professor Susan Grogan, St. Mary's College of Maryland and J.M. Cameroon, artist: "Calvert clay probably refers to the Miocene sandy clays of the Calvert Formation that extend from Virginia to New Jersey--but at great depth except where it is mainly exposed at the bottom of Calvert Cliffs in Maryland. The name might also have been coined in studies trying to replicate and find the clay sources of the bricks used to build St. Mary's Chapel at Historic St. Mary's City. The local clays are mainly fireclays (with iron, there is lot of iron in the well water of the area)."

Related Information

Links

Typecodes	Clay Other 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.
Materials	Fire-Red
Materials	Newman Red Clay
Materials	Redstone

Data

Firing Shrinkage	Cone 1: 8.9% total Cone 5: 9.1 (overfired)
Ultimate Particle Size Distribution	10 microns: 89% 5: 84% 2: 67% 1: 54% 0.5: 43%
Pyrometric Cone Equivalent	29-30
Water absorption	Cone 1: 1.4% Cone 5: 0.3

By Tony Hansen
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