Lime

a well known building material. Good lime is a nearly pure compound of calcium and oxygen. It is prepared from limestone, marble, chalk, and shells, by burning. The simplest form of a limekiln is a low, wide, chimney-like structure built of brick or even large blocks of limestone. The kiln is filled up with broken limestone. Care is taken, by constructing a rude temporary arch, to leave a large hollow place at the bottom. Fuel is introduced through a door at one side. The doorway and spaces of air leading up through the broken stone afford a fine draft. A small fire, usually of wood, is made under the arch. The temperature is raised slowly for a few hours at first to prevent the arch from crumbling or caving in; then a furious fire is built, and the whole mass is kept at a full red heat for two days to roast out the carbon contained in the limestone. Ordinarily 100 pounds of limestone yields fifty or sixty pounds of lime; but the chunks of lime retain the form of the stone with not to exceed ten to twelve per cent loss of bulk. Freshly burned lime is a caustic, and burns the fingers and clothing. For that reason it is called quicklime. The purer the lime the whiter it is.

When exposed to air, lime slakes, that is, absorbs water and falls into a powder. Lime is usually supplied with water and allowed to slake for a day or two, a week is better, before it is used for mortar. The mason mixes his slaked lime with sand and hair, the former for hardness in the finished wall. As soon as the water dries out the mortar sets, forming a sort of artificial sandstone, in which the grains of sand are cemented together by lime. The plasterer adds hair to give toughness to his wall. When applied to lath, the mortar curves over behind the lath, and when dry binds the plaster to the wall. Ordinary mortar will not harden while damp, but, if the limestone contains a certain amount of clay and is burned slowly, a hydraulic lime is produced which hardens or sets while still moist.

Slaking is accompanied by the evolution of great heat. A cargo of unslaked lime must be guarded from water, for ships, cars, and warehouses are not infrequently set on fire by a leak which admits water to a mass of lime. Emperor William of Germany has his hunting lunches served hot by means of an outer and an inner can, the space between being filled with unslaked lime. In this way, he avoids building a fire in the woods.

In addition to its use in mortar and cement, lime is required in bleaching cotton goods, in purifying illuminating gas, in making soda, in purifying sugar, in tanning, in glass making, in disinfecting, and in the manufacture of many chemicals. It is used extensively to restore fertility to worn-out land. If a piece of litmus paper be wrapped about a handful of moist earth it will indicate the condition of the soil. If the litmus turn red, the soil is sour, and may be improved by an application of air-slaked lime.

Lime may be produced wherever suitable limestone or marble and cheap fuel are to be found.

See Calcium; Bone; Limestone; Gypsum; Alabaster