Verna Clay

The Verna Brick Co. is developing the deposit at Verna, near Warba station on the Great Northern Railway.  The portion of the deposit that is considered available is about 10 feet thick and extends over about 10 acres.  It is yellowish gray in color and has apparently been leached.  Underlying it and extending over a great many acres of the surrounding country is a blue laminated clay containing a few limestone concretions.  The gray clay burns cream-colored and the blue clay is said to burn red and to show a much greater shrinkage.  Any attempt to mix the blue and gray clays encounters the difficulty common to all attempts at mixing a stiff, plastic clay with a lean, sandier clay.  The plastic lumps remain suspended in the more fluid mass and require very thorough pugging or some other form of mixing to get a satisfactory structure and avoid auger laminations.  The yellowish-gray clay, which is being used, slakes [quality by which a dry lump of clay tends to absorb water and fall to pieces when immersed] at once and shows very low plasticity [capability of being molded], requiring 23 per cent of water for molding.  The air shrinkage is 3 per cent, and the tensile strength [maximum stress it can withstand before breaking] is well above 100 pounds to the square inch, even after rapid drying.  As burned at the Minnesota School of Mines experiment station it gave the following results:

The clay becomes hard at cone 02 (2,030° F.) and reaches viscosity at cone 3 (2,174° F.).  The plant has a capacity of 35,000 brick a day and makes a common brick of cream color, using wood as fuel.

Source:
Clays and Shales of Minnesota
By Frank F. Grout
With Contributions by E. K. Soper
United States Geological Survey, Bulletin 678
Washington, Government Printing Office, 1919
Page 175