Part 7

Man has used clay in various ways for many hundreds of years. From it he made, and still makes, bricks to build his dwellings, pottery utensils of many kinds, and other useful products.

Everyone knows what clay is, yet it is a substance difficult to define. All clays are earth materials, most of them plastic or sticky when wet but firm when dry. If heated sufficiently (fired) they become hard.

Clays are composed of various minerals. Of these, the so-called clay minerals—complex substances composed mostly of alumina, silica, and water—generally are the most important. They impart the property of plasticity and also cause clays to become hard when fired.

Most clays are what geologists call unindurated (unhardened) rocks. Clay that has been indurated and occurs in layered deposits is commonly called shale. The layers may be from a fraction of an inch to several inches thick. Most Illinois shales are not plastic when dug from freshly exposed deposits, but they become plastic when crushed and kneaded with water. The clays and shales of Illinois are the basis of a huge and important industry.

Early Uses in Illinois

Clays and shales are useful because they can be made plastic by adding water, formed into desired shapes, and fired to a rock like hardness. As a result, various kinds of bricks, drain tile, pottery, and other useful products are made from them. In its early years, Illinois had many widely distributed potteries that used clay from nearby deposits to make a variety of jugs, crocks, and bowls that served in place of many present-day glass or metal articles.

Drain tile has been of major importance in the development of the state. Early settlers found many low lying, swampy areas and tracts of land that drained poorly after heavy rains. Ditches were dug to carry away the water from some areas, but others were drained by means of drain tile—pieces of fired clay pipe several inches in diameter and about a foot long that were laid end to end in trenches below plough depth and then covered with earth. Water seeped into the tile, which discharged it into ditches. Tile factories, built throughout Illinois near clay or shale deposits, did an active business. Gradually, however, as more and more farm land was drained the demand slackened and many tile factories went out of business. Although there are fewer factories, much drain tile is still manufactured in Illinois.

Many of the early tile plants also made bricks to be used for making foundations, buildings, sidewalks, and other structures. The bricks were made by hand-operated equipment. Some of the old hand-molded bricks may still be seen in older buildings. Now the brick-making process is highly mechanized and even though there are fewer plants they produce more bricks.

Clay and Shale Deposits

Illinois shales are a part of the bedrock—that is, they are associated with indurated rocks such as sandstone and limestone. Most clays are surficial rocks occurring in deposits near the surface, where they lie above the bedrock. Exceptions are certain clays found in extreme southern Illinois and the underclays, also called fireclays, that occur beneath coal seams and are part of the bedrock.

The surficial clays are of two principal kinds—till and loess. Till is a deposit left by glaciers. It is a gray, blue-gray, or brown clay containing varying amounts of sand, pebbles, cobbles, and even boulders. Till is found at many places in the state and is used for brick making, especially in the Chicago area.

Loess is a wind-deposited silty clay or clayey silt and is found in many parts of Illinois. It is thickest on or near the bluffs of the Mississippi, Illinois, and Ohio Rivers. It generally is brown and stands in steep faces in roadcuts and other excavations. It once was widely used for making brick and tile.

Of major importance in making clay products in Illinois are the bedrock shales and the clays associated with the coal-bearing rocks that underlie much of the state. The shales, and the clays to a lesser extent, are dug at many places for making structural clay products such as bricks, structural tile, and drain tile. They also are used to make lightweight aggregate for concrete. The underclays of some of the older coal seams are used to make buff-colored brick, stoneware, and a highly heat-resistant brick (firebrick) that is used in industrial furnaces or in other operations involving high temperatures. Some fireclay, ground as fine as flour, is added to molding sand to make it coherent enough to form into molds for metal casting. Sewer pipe and flue lining also are made from underclays.

Clays unlike those found elsewhere in the state occur in extreme southern Illinois. One of these has the property of removing color from oils and was so used at one time by petroleum refineries. Another, kaolin, was extensively used during World War I for making crucibles.

Clay Minerals

The uses of clay and shale are determined to a large degree by the properties of their clay minerals and to a lesser degree by the impurities present. A clay or shale containing the clay mineral illite, and other similar but less important clay minerals, commonly becomes red when fired and gets hard at a relatively low temperature. It therefore is used to make red bricks, drain tile, building tile, and other structural clay products.

Another clay mineral, kaolinite, generally burns to a light color and is difficult to fuse. Therefore, clays composed wholly or mainly of kaolinite can be used for making buff or light-colored bricks and for the manufacture of highly heat-resistant (refractory) bricks.

The clay mineral in the southern Illinois clay that was used to decolorize oil is montmorillonite. This clay is now used in sweeping compounds, as an oil absorbent, as animal litter, and for other purposes.

Studies of Clay and Shale

In view of the significant relationship between the clay minerals and the utilization of the clays and shales in which they occur, the Illinois Geological Survey has investigated extensively the clay minerals in the clay and shale deposits of Illinois. Many samples were studied by means of powerful microscopes, X-ray, and chemical analysis. Most of the surface clays and shales proved to be composed principally of illite or related minerals. The kaolin clay of extreme southern Illinois contains the mineral kaolinite. The older underclays also contain kaolinite, but many of them also contain smaller amounts of illite.

The Survey also has tested many clays to determine their burning properties and color when fired, and hence their potential uses. The bonding capabilities of other clays have been measured to find out whether they can be used as a bonding material for molding sand. The bloating properties of Illinois clays and shales from many deposits have been studied to determine which are suitable for making lightweight aggregate for the manufacture of concrete.

The object of these studies has been to discover the location, character, and possible uses of the state’s clay and shale resources. Special studies are continuing in several parts of the state. Illinois is well endowed with clays and shales that can be used for a variety of purposes and has resources to fill future as well as present needs.

How Bricks Are Made

Conversion of Illinois clays and shales into useful products is an interesting process and is exemplified by the making of building bricks. Mechanical shovels dig the clay or shale and load it into trucks or small railroad cars that take it to the brick plant. There, machines grind the raw material and mix water with it until it has the consistency of stiff mud.

Next, a machine, which operates somewhat like a meat grinder, extrudes a brick-sized column of clay. As the column moves forward, it is automatically cut into bricks by wires. The bricks are then dried in large heated rooms.

From the driers, the bricks go to huge ovens (kilns) and are heated until they are hard and have attained the desired color. This is known as firing or burning the bricks. Temperatures employed are rarely lower than 1800° F.

Three kinds of kilns are used in Illinois for burning bricks—beehive, tunnel, and scove. A beehive kiln (fig. 21) has a round base and a dome-shaped top and somewhat resembles an oversized beehive. Unfired bricks are stacked in the kiln and the doors are sealed with burned bricks and clay. Fires are started in hearths or fire boxes in the wall of the kiln and the heat is circulated into and through the kiln. It usually takes several days to fire the bricks adequately and let the kiln cool so that the bricks can be removed.

Tunnel kilns, made from heat-resistant bricks, are actually tunnels big enough for a man to stand in. The unburned bricks are loaded on flat steel cars on top of a layer of refractory blocks that protect the steel from the heat. The cars enter the kiln and heating begins. As they move through the kiln, they carry the bricks through a firing area, then through a cooling zone, and finally out into the air.