A specially prepared St. Peter sand, known throughout the world as Standard Ottawa Testing Sand, is used to test the strength of cements and as a laboratory standard in physical tests of other sands. MOLDING SAND (18) MOLDING SAND is a mixture of sand and clay or other bonding material and is used to make molds in which metal is cast into various useful shapes. There are two kinds of molding sands, synthetic and natural bonded. The first is an artificial mixture of silica sand and clay; the second is a naturally occurring mixture of sand and a bonding material. For casting, molding sand is first moistened with water and shaped into a mold of the metal part to be reproduced. The mold is then allowed to dry and the molten metal poured into it. The bonding material in the sand must be strong enough to keep the mold in shape during these operations. Natural bonded molding sand is produced in Fayette, Bond, Bureau, Carroll, Kankakee, and Rock Island Counties, Illinois. TRIPOLI (19) TRIPOLI, called amorphous silica in southern Illinois, is a white or light brown, powdery substance that rubs off on the hands like chalk. It consists mostly of very small particles of quartz that result from the weathering of calcareous chert or highly siliceous limestone. It is finely ground and used as “white rouge” for polishing optical lenses, as a filler in paints, in making ceramic products, as a component of buffing compounds, and as a fine abrasive. Tripoli occurs in Alexander and Union Counties and is milled at Elco and Tamms in Alexander County. FULLER’S EARTH (20) FULLER’S EARTH is clay or silty clay material that contains very fine silica. It is soft, nonplastic, opaque, has a greasy feel when wet, and does not readily break up in water. Its color varies from blue-gray to yellow or buff. Fuller’s earth is valuable for its unique property of absorbing and decolorizing substances. The material was first used to “full” or remove grease from woolen cloth, hence its name. It also has been used to filter and bleach mineral and vegetable oils by absorbing dark organic matter. In Pulaski County in extreme southern Illinois the Porter’s Creek Formation contains deposits of clay that were at one time the source of fuller’s earth and still afford clay whose absorbent properties make it useful as litter and as sweeping and cleaning compound. QUARTZ (21) QUARTZ is the most common of all minerals, making up about 12 percent of the earth’s crust. There are two main types of quartz—crystalline quartz and dense, crypto-crystalline (submicroscopic) quartz. Many dense varieties occur in Illinois, probably the most common are chert or flint. [Illustration: uncaptioned]