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In Minneola Township the Zumbrota Clay Manufacturing Co.’s plant used a rather massive bed of the shale 15 feet thick. Tests by the Bureau of Standards gave uniform vitrification, with increase
FIGURE 22. – Map of Goodhue County. Dotted area is underlain by Decorah shale.
of temperature, to a very low porosity. The best temperature of burning (as is characteristic of the Decorah shale) is rather lower than for most other good clays. In all physical tests this clay resembles the average Decorah shale. The plant has a capacity of about 50,000 brick a day but makes draintile and hollow brick in addition to common brick. Six hollow building blocks with two holes lengthwise tested on the side and on the edge by the Minneapolis building inspector gave an average strength of 330 pounds to the square inch, and one block tested on the end showed 5,100 pounds to the square inch.
Source:
Clays and Shales of Minnesota
Frank F. Grout with contributions by E. K. Soper
United States Geological Survey
Government Printing Office, Washington, D.C., 1919