These clays are much used in some parts of the country to manu facture road ballast. Windrows of cordwood are covered with a thick coating of clay and fired. The wood burns slowly and imparts sufficient heat to the clay to drive off the combined water. As the clay shrinks badly it breaks up under the influence of heat into angular fragments which are found to be excellent material for ballasting roads.
Loess and Adobe Clays.—§ 47. Loess and its methods of formation have already been sufficiently described. It is largely used in the man ufacture of building brick and to some extent also for vitrified ware.
• Adobe is a very similar material found in the western states as out wash plains formed by streams which, swollen by cloudbursts, pick up great quantities of detritus produced by the decoMposition of underly ing rocks, and as they spread out upon the plains and lose velocity, drop first the coarser particles, sands and gravels, which they are bearing, then the silty material mixed with coarser clay which is called adobe, and carry the finer clay to more distant points.
Adobe is much used by the inhabitants of Arizona and New Mexico in the manufacture of unburned bricks, often reinforced with chopped straw, with which they construct their houses. In that dry climate with but little rainfall such buildings are found to be very satisfactory.
Fullers Earth.—§ 48. Fullers earth was formerly much used for removing grease from woolen cloth. It is a silty clay having little plas
ticity, but carrying a large amount of combined water. Clay has a strong affinity for oils and fats, as may be seen by the following experi ment: Add a teaspoonful of oil to a glass of water, then drop into the water a lump of clay and stir vigorously for some time. If the clay is now allowed to settle it will be noticed that the oil has partially or wholly disappeared, depending on the amount and character of the clay used. Each particle of the clay has been covered by a film of oil and, has carried it to the bottom of the vessel. This experiment explains why fullers earth is now so largely used in purifying oils. It may also be used for taking grease spots from cloth.
Minor Uses for Clays.—§ 49. Clay is also largely used in the manu facture of portland cement, and to a less extent as mineral paint, filler for paper, in confectionery and as an adulterant in various foods, in the manufacture of various soaps, window-cleaning and polishing powders, in the manufacture of medical plasters used to subdue inflammation, as a retardant to the setting of the cements now much used in place of lime mortar, and in the manufacture of alum, but the quantities used in these industries are not sufficient to warrant a separate description in a paper of this kind.