Page:The Pima Indians.pdf/130

] The process is well illustrated in plate, b. Samples of this clay were submitted to Doctor Wiley, Chief of the Bureau of Chemistry of the Department of Agriculture, who ascertained the percentage composition of the material to be as follows:

He also explains the general method of determination.

For industrial purposes an attempt is made to separate the proximate constituents of a clay by what is called a "rational analysis." For this purpose the clay is treated with sulphuric acid, which is supposed to dissolve the clay substance proper and leave insoluble quartz sand and sand composed of feldspar or other minerals.

In order that the composition of this clay might be compared with analyses of other clays made upon this plan, a determination was made of the matter insoluble in sulphuric acid in sample 24881, with the following results: {| style="border-collapse:collapse; width:100%; max-width:45em; margin-left:auto; margin-right:auto" Per cent


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The sand insoluble in sulphuric acid is composed approximately as follows:

Comparison of these results with analysis given by Langenbeck indicate that the material represented by sample 24881 resembles the clays used for the production of so-called "red ware" more than any other class of clays used in pottery manufacture by civilized peoples. The following analysis of a typical clay used for making "red ware" is quoted from Langenbeck for purposes of comparison (The Chemistry of Pottery, 1895, 60): {| style="border-collapse:collapse; width:100%; max-width:45em; margin-left:auto; margin-right:auto"
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Another well-known clay pit is situated on MeClellen's branch, at the northeastern base of the Sacaton hills (pl., c), whence a whitish clay is obtained. The villages about the Casa Blanca ruin obtain clay from pits within a stone's throw of the ruin itself and from the river bottom near the village of Rso’tûk.

The tempering materials used in the clays last mentioned are sand and ground potsherds. The clay from the Skâsŏwalĭk hills is so coarse that it requires no tempering.

Red ocher is employed as a slip, which is applied to the surface of the common utensils just before the drying that precedes burning. The water coolers are usually made without this coating of ocher.

Black gum is used for decoration. This is obtained by boiling in a small earthen pot, or in a segment of a large one, mesquite chips