KILN. Lime, a sort of kiln constructed for the purpose of burning lime. The operation called burning lime, consists in exposing marble, limestone, chalk, oyster-shells, or any other carbonate of lime, for some time to a white heat. by w hick means the carbonic acid and water contained in these substances are expelled ; and the earth, which has the pecu liar characters assigned to lime, is left behind in a mass which has little coherence, and is therefore easily reduced to pow der. It is usually called quick-lime after calcination. Newly prepared, it absorbs water with great avidity; it will absorb one.fourth of its weight of that fluid, and still remain per fectly dry. If a sufficient quantity of water be poured upon it, the lime falls into powder ; some of the water is converted into vapour by the disengaged caloric of that part which unites with the lime ; this is called the slacking of lime ; it' the quantity slacked he considerable, and performed in a dark place, light will be observed as well as heat. Kilns are built of different forms or shapes, according to the manner in which they are to be wrought, and the kinds of fuel to be employed. It may be remarked, that, in places where mate rials are dear, from their being fetched from a distance, and where the fuel is coals, and also expensive, the form of a kiln is mostly that of an inverted cone. a form which has its in conveniences; but in districts where the art of burning lime is practised with superior attention and correctness, the form has of late years been gradually changing from conical to elliptical. Some writers are of opinion, that the best form of a lime-fu•nace, in the established practice of the present day, is that of an egg placed upon its narrower end, haying part of its broader end struck off; and its sides somewhat compressed, especially towards the lower extremity ; the ground-plot or bottom of the kiln being nearly an oval, with an eye, or draft-hole, toward each end of it. It is supposed that two advantages are gained by this form. over that of the cone. By the upper part of the kiln being contracted, the heat does not Ily off so freely as it does out of a spreading cone. On the contrary, it thereby receives a degree of rever beration, which adds to its intensity. But the other, and still more valuable effect, is this: when the cooled lime is drawn out at the bottom of the furnace, the ignited mass, in the upper parts of it, settles down, freely and evenly, into the central parts of the kiln ; whereas, in a conical surface, the regular contraction of its width, in the upper as well as the lower parts of it, prevents the burning materials from settling uniformly and levelling downward. They hang upon
the sides of the kiln, and either form a dome at the bottom of the burning mass, with a void space beneath it, thereby endangering the structure, if not the workmen, employed ; or, breaking down in the centre, form a funnel, down which the underburnt stones find their way to the draft-holes. And the contraction of the lower part of the kiln has not the same effect ; for, after the fuel is exhausted, the adhesion ceases, the mass loosens, and, as the lime cools, the less room it requires. It therefore runs down freely to the draft-holes, notwithstanding the quick contraction of the bottom of the kiln or surface.
Lastly, with respect to the lime-furnace, the fire requires to be furnished with a regular supply of air. When a kiln is first lighted, the draft-holes afford the required supply. But after the fire becomes stationary in the middle, or to wards the upper part of the kiln, (especially of a tall kiln,) while the space below is occupied by burnt lime, the supply from ordinary draft-holes becomes insufficient. If the walls of the kiln have been carried up dry or without mortar, the air finds its way through them to the fire. In large deep kilns that are built with air-tight walls, it is common to form air-holes in their sides, especially in front, over the draft holes. But these convey the air, in partial currents, to one side of the kiln only, whereas that which is admitted at the draft-holes passes regularly upward to the centre, as well as to every side of the burning mass ; and, moreover, tends to cool the burnt lime in its passage downward, thereby contri buting to the ease and health of the workmen. Hence, it is to be observed, the size of the draft-holes ought to be pro portionate to that of the kiln and the size of the stones taken jointly, (air passing more freely among large than among small stones,) and the required supply of air should be wholly admitted at the draft-holes. By a sliding or a shifting valve, the supply may be regulated, and the degree of heat be in creased or diminished, according to circumstances.