Page 233
Gray drift covers nearly all of Sibley County, except along Minnesota River, where alluvium is found. The alluvium makes fair red brick. At Henderson 50 per cent of sand from the bed of the river is added to the richer clay for making sand-mold brick. The clay without the additional sand shows medium plasticity with 23 per cent of water and shrinks 7 per cent on drying. The tensile strength is 100 pounds to the square inch. Burning tests resulted as follows:
| Cone No. | Color. | Shrinkage | Absorption. |
| Per cent | Per cent | ||
| 04 | Salmon | 3 | 24 |
| 02 | …do | 5 | 16 |
| 01 | Red | 9 | 3 |
| 1 | …do | … | … |
The clay is hard after heating to cone 03 (1,994° F.) and reaches viscosity at cone 2 (2,138° F.). The plant can produce about 10,000 red brick a day.
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