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A very extensive brick industry has been developed at Wrenshall, in Carlton County, where gray laminated clays occur in immense quantities. The existing brick plants are within a short distance of the station, but similar clay lies along the railroad half a mile farther north and for some distance farther east.
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The gray laminated clays washed into Lake Duluth from the west contain few boulders. They are most abundant in the vicinity of Wrenshall, where five large, successful plants have been in operation, each utilizing essentially the same type of clay, which has been developed to a depth of about 50 feet and explored to a depth of 80 feet. In the uppermost few feet the clay is more or less disturbed and mingled with pebbles, but this condition gradually disappears with increasing depth and the stratification becomes very regular. Gentle undulations such as are seen in the laminated clay north of Minneapolis (see p. 114) occur here also (See Pl. I, p. 20) [the clay pit photograph shown in the Wrenshall pictures section]. The blue and gray parts of the clay burn cream-colored; but the top part of the deposit, which has not only been disturbed physically but has apparently been chemically altered by leaching, is red and burns red. In the upper 10 to 15 feet the disseminated lime has apparently been
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segregated into concretions, which, however, are in few places abundant enough to be a serious handicap. The clay extends generally under the flat region and is seen along the sides of many deeply cut ravines draining into the St. Louis River. It extends up the slope about 300 feet above the flat on which Superior and West Duluth are built. The detailed structure of the clay is similar to that of clays along Minnesota and Mississippi rivers, but its occurrence in a lake basin in a place where no river could have flowed, as well as its wide extent, indicate that it is a lake deposit rather than river silt.
The clay slakes [quality by which a dry lump of clay tends to absorb water and fall to pieces when immersed] in three minutes and shows fairly high plasticity [capability of being molded], requiring 23 per cent of water for molding. Its air shrinkage is 4 per cent and its tensile strength [maximum stress it can withstand before breaking] 175 pounds to the square inch, even when rapidly dried.
Test by the Bureau of Standards show that it burns buff at low temperatures but becomes greenish yellow when well vitrified. It has a short range of vitrification [through intense heating, the clay particles fuse to a very hard, glass-like state], the porosity [ability to absorb fluids] dropping from 42 per cent to nearly zero in about 100°. It is not safe to try to vitrify this type of clay, but it makes excellent common brick. An analysis by A. W. Gauger is as follows:
Analysis of clay from Wrenshall.
| Silica | 48.79 |
| Alumina | 12.08 |
| Iron Oxides | 4.60 |
| Magnesia | 5.54 |
| Lime | 12.10 |
| Soda | 2.22 |
| Potash | 2.05 |
| Loss on ignition | 12.02 |
| Moisture | 1.26 |
| Titanium oxide | 0.29 |
| 100.95 |
The plants have capacities of 40,000 to 140,000 brick a day. Some of them find the soft-mud process more favorable, but most of them use stiff-mud machines. Very little hollow ware is produced. Red brick are obtained only when the upper layers of clay are used separately. If clay from the whole deposit is used in proportion as it is exposed, the one-fourth of red-burning clay is not sufficient to affect the color of the three-fourths of cream-burning clay. The quality of the products as tested by the experimental engineering department of the University of Minnesota is as follows:
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Results of tests of Wrenshall brick.
Crushing strength
(pounds per
square inch). Modulus Absorption
Nature of brick. Wet. Dry. of rupture. (per cent).
Soft-mud, cream 2,809 2,166 982 29.4
Stiff-mud, cream 2,834 4,762 926 21.7
Stiff-mud, sewer 4,474 5,888 1,409 18.5
Soft-mud, red 3,354 3,233 679 14.9
Stiff-mud, red 3,770 5,247 1,189 13.3
Source:
Clays and Shales of Minnesota, United States Geological Survey, Bulletin 678
By Frank F. Grout, With Contributions by E. K. Soper
Washington, Government Printing Office, 1919