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Clays from the Dakota

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The Dakota formation of central Kansas is the State's most important source of commercial fire clay for use in the manufacture of refractories, structural clay products, pottery (including porcelain, whiteware, stoneware, earthenware, and art ware), and similar high-grade products.

For the past 20 years the Ceramics Division of the State Geological Survey has been engaged in a detailed study of the ceramic properties of clays from the Dakota formation, and data on the behavior of more than 500 samples from about 200 localities have been collected. This work has resulted in the building of three potteries and two well-established brick plants with two more planned for construction by 1955.

Within recent years the application of powerful analytical tools, including differential thermal analysis, modern chemical and spectrochemical analysis, x-ray diffraction, and electron microscopy, to the study of clay materials has made possible much better understanding of clay mineralogy.

Variation between beds in the Dakota formation is pronounced; the deposits include kaolin, ball clay, plastic fire clay, siliceous fire clay, and highly siliceous fire clay. For the present report, four representative clays were selected for a study of the relations between their fundamental or basic properties (such as chemical and mineral composition and particle size and shape) and their ceramic behavior.

Correlations found between the basic properties of the clays and their behavior under ceramic tests are definite aids to the understanding of ceramic properties and to the application of the ceramic data to problems arising in the ceramic industry. Furthermore, the basic properties, alone or in connection with the ceramic data, can be used directly by the industry as a means of finding the cause of deviations in quality of the product, as a positive check on the quality of clays mined for ceramic use, or as a means of finding new deposits. These correlations between basic properties and the ceramic data also indicate that the easily obtained ceramic data can be used as a method for the rough determination of basic properties.


The clay samples studied were obtained by channeling on the face of pits excavated for the specific purpose of clay sampling (Plummer and Romary, 1947). The samples contain only clean clay free from surface contamination and weathering.

The entire sample (60 to 70 pounds) obtained by this method was mixed thoroughly and pulverized in a 3-foot pan mill. A representative cut, a sufficient amount for the entire investigation, was taken from this sample and screened through a 20-mesh sieve. The over size was pulverized with a mortar and pestle until all the sample had passed the sieve. This cut of the sample was again split to obtain a relatively large sample for the ceramic tests and a smaller one for the basic studies involved.

The size fractions coarser than 8 microns, from 2 to 8 microns, and finer than 2 microns used in the chemical, x-ray, and differential thermal analyses were separated according to Stokes' law by repeated decantations from additional cuts of the four clay samples. It was necessary to treat sample 0-5-6 with a 3 percent hydrogen peroxide solution prior to fractionation. This treatment, which was not necessary for the other three clays, may have caused slight modifications in the size fractions obtained from clay 0-5-6 as compared to the other three.

Methods Used

The four representative Dakota formation clays described were analyzed and examined by every means available in the Geological Survey laboratories. The fundamental or basic properties of the clays were determined by chemical and spectrochemical analyses of both the major and minor constituents, and by petrographic and mineralogical analyses. Pipette analysis, optical examination of thin sections, and electron micrographs were used to determine particle size and shape. Mineral composition was calculated from chemical composition, x-ray diffraction, and differential thermal analyses. The ceramic analyses are empirical tests of the clays under dynamic conditions of wetting and drying when unfired and under conditions of increasing temperature when fired.

The data are presented with a minimum of interpretation except for that needed for an understanding of the specific tests. Interpretations and correlations are discussed in a separate section of this report.

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Kansas Geological Survey, Geology
Placed on web June 22, 2007; originally published Dec. 1954.
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