LIME, also known as caustic lime, quick lime or calcium oxide (CaO), is snow white in color and strongly alkaline, being one of the strongest bases. In contact with water, for which it has great affinity, it increases in bulk, evolves much heat and changes to the hydrate Unless kept in a dry place it will absorb moisture and carbonic acid gas from the air and change to the carbonate (CaCO3) (see CALCIUM). Lime, except for certain impurities, is entirely soluble in water, the principal insolu ble impurities being silica, alumina and iron oxide. It is never found native, but limestone may contain a very large percentage. For ordinary commercial uses lime is obtained by heating limestone, shells, or other material com posed of calcium carbonate, to a temperature high enough to drive off the carbonic acid gas. As the materials used vary in purity, so is there a corresponding difference m the purity of the lime produced. Owing to its property of hardening, by change to calcium carbonate, and its comparative cheapness, lime is the most important of building materials. For commer cial purposes the many varieties of lime are grouped into, (1) common or fat limes con taining less than 10 per cent of impurities, (2) poor or meagre limes containing 10 to 25 per cent of impurities, (3) hydraulic limes con taining 15 to 40 per cent of impurities and (4) hydraulic cements which may contain as high as 70 per cent of impurities.
Common or fat limes in slaking evolve much heat and increase in bulk from two and a half to three times; they are soluble in water. In hardening they shrink, and hence in making mortar require the addition of a large amount of sand. As fat limes are cheap and abundant and can take a large proportion of sand in making mortar, they are generally used for • masonry. The poor or meagre limes are seldom used in this country. The hydraulic limes, so called from their property of hardening under water, though valuable for certain purposes, are also seldom used in the United States for masonry. They slake more slowly than ordi nary limes with little rise of temperature and little increase in volume. Hydraulic cements (see CEMENT) do not slake and do not require the addition of sand to form mortar. They also set or harden much more quickly than ordinary limes, as the formation of calcium car bonate is a slow process that may under certain conditions take years, while in some quick-set ting cements the formation of the calcium sili cates and aluminates to which setting is due may take place in a few hours.
Limestone, marble and shells are burnt to lime by exposure to a temperature of 850 to 900° C. in a Current of air. The harder the limestone the longer the time required for burn ing, but the better the product. Impurities in the limestone may or may not be injurious, de pending on the quality of lime desired. Mois ture in the limestone, or added to the charge, as a steam jet, helps carry off the carbonic acid and hastens burning. Lime burning may be done in heaps or kilns. Heap burning in
which pieces of limestone are piled on a grate of wood, then covered with fines or clay and fired, is now little used. Of kilns there are vari ous types, intermittent and continuous firing. Of intermittent kilns the old °dug out* kilns built of ordinary brick on masonry into the side of a hill require about 72 hours for each firing. Heat is by a coal or wood fire at the bottom. For continuous firing vertical kilns 20 to 25 feet high with draw openings, prefer ably .below the fire line, are used, the lime being drawn about every six 'hours. To sup ply moisture, water is kept in an iron pan in the ash-pit. In the kilns most commonly used there is no grate, but the fuel, preferably charcoal or i anthracite, is charged with the limestone. The lime is not as pure as from furnaces with a bot tom grate, but the consumption of fuel per pound of lime made is less. In such kilns burn ing is started by partly filling the kiln with lime stone, putting in a thick layer of kindling wood and coal, starting the fire, and then adding alter nate layers of coal and limestone. The type of continuous kiln in most favor is vertical, about 25 feet high, bottle-shaped and made of boiler iron lined with firebrick. Such a kiln may use petroleum for fuel, the oil being infected through openings in the wall just above the widest part of the furnace. Vertical kilns fired with gas have not proved successful. The type of kiln that requires the least fuel for amount of lime produced is the horizontal cir cular Hofman kiln, also used for making brick, drain, tile, etc. The fuel is fine coal or even coal dust, and is put in through openings in the top. The lime produced is lighter than that from vertical kilns, slakes more easily and hence cannot be stored as well.
Most of the lime made is used for building purposes, or mixed with clay for cement (q.v.). but lime is also used for making glass and artificial stone, as an agent in many chemical processes, as whitewash, and was formerly largely used in sugar-making to separate sugar from molasses. The statistics of the amount of lime annually made in the United States show that the value of the 1909 output was over $13, 800,000; but at present the largest use of lime is in the Portland cement industry, which mines vast quantities of limestone, this being now the most valuable stone in the United States. See LIMESTONE; PORTLAND CEMENT.
Lime has sonic medicinal value, counteracting excessive acid conditions. Various preparations of limewater are recommended for dyspeptics and pregnant women. Mixed with milk it often reduces the tendency to vomit. Lime water with glycerine has been found useful in cases of creeping eczema. As an alkaline, lime has many uses in the arts and in chemical processes. It softens °hard° water; neutral izes acids, as in soils, and is indispensable in the making of sulphite for paper and gaseous am monia. Consult 'The Mineral (New York 1916).