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CHAPTER II.

FURNACES, FUEL, CRUCIBLES, &c.

FURNACES are of two distinct kinds, viz.: blast and wind. In the former, the fire is urged by means of bellows; and in the latter, by a chimney, or common draught. We shall commence with the latter, as they are in most common use. They are of various kinds according to the purpose for which they are required. The three principal kinds are those of fusion, calcination and cupellation. Coal, coke, or charcoal are the fuels employed, and the merits of each will be particularly discussed. Blast furnaces are only employed for the purpose of fusion, although their forms are various: charcoal and coke are the fuels in use.

All furnaces consist of certain essential parts, viz.: 1st. the ash pit, or part destined to contain the refuse of the combustible employed; 2nd. the bars on which the fuel rests; these are sometimes made moveable, or fixed to a frame, the former arrangement is more convenient, as it allows clinkers and other refuse matters to be readily removed. 3rd. The crucible, or body of the furnace in which the heat is produced; and lastly, in wind furnaces, the chimney by which the heated air and gaseous products of combustion are carried off.

Wind Furnaces: Calcining Furnace.—Calcining furnaces are small and shallow, because a high temperature is not required. They may be made square or circular. The former are most readily constructed, and the fuel they contain can be easily stirred without fear of overturning the contained crucibles. Where many crucibles are to be heated at once, they are preferable to the circular, but the latter give the greatest degree of heat with the least possible consumption of fuel, and are to be preferred on that account, where one crucible only is to be ignited.

The crucible, or body of the furnace is best made with good bricks, lined with Welsh lump, fire-bricks, or a mixture of Stourbridge clay and sand. It is also desirable that a plate of iron with a ledge be placed over the upper part of the furnace to protect the brickwork from blows with crucible, tongs, &c., and to keep it in its place when disturbed by sudden alternations of temperature. The bars of the furnace may be either one single piece, or made up of several bars of iron fastened to a frame. They ought to be as far as may be from each other, and not be too large. They must be large enough, however, not to bend under the weight of the fuel and crucibles when they become hot, and they must not be so far removed from each other as to allow the coke or charcoal

to fall through readily. Lastly, the more readily the air can find access to the centre of the fuel, the higher will be the temperature produced in the furnace, and very simple assays occasionally fail, only because the bars are either too large or too close together.

The ash-pit, as before stated, is an open space under

the bars, and serves as a receptacle for ashes, clinkers, &c., produced during the time the furnace is in use. It ought to have the same area as the crucible, and completely open, so that the air may have free access; it is well, however, for the sake of economy, to furnish this opening with a hinged door, having a register plate fixed in it, so that the draught may be reduced, or entirely shut off, in order that the fire may be extinguished and fuel saved, which otherwise would be burnt in sheer waste.

Chimney.-Calcining furnaces generally have no fixed chimney, but are covered with a moveable one when a greater degree of heat is required. This chimney is made of strong plate iron, furnished with a wooden handle. The lower part is provided with a door, by means of which the interior of the furnace may be examined without disturbing the whole arrangement of the chimney, and consequent cooling of the contents of the furnace.

If, during the course of any experiment, noxious or inconvenient vapours are expected to be given off, the furnace must be so arranged that they are introduced into a flue by fastening a piece of iron plate pipe furnished with an elbow joint on to the moveable chimney before spoken of.

Evaporating Furnaces.—The furnaces just described answer exceedingly well for heating small flasks, evaporating basins, &c., when furnished with a tripod stand or sand-bath. The latter is necessary, as many assays by the dry way are preceded or followed by certain operations by the humid method.

The Hood.-In order to prevent certain vapours from

fires, evaporating basins, &c., from entering into the laboratory, a large metal covering, termed a hood, is employed, which hood terminates in a chimney, having a good draught. They are best made of sheet zinc, plate iron, or better still, of galvanized iron, as that is cheaper than zinc and quite as serviceable; it has also the advantage of not being combustible.

Fusion Furnace: Wind Furnace.-The wind furnace, properly so called, is a furnace provided with a chimney, and which is capable of producing a very high temperature.

Wind furnaces are generally square, but if more than four crucibles are to be heated at one time, they may be made rectangular, the chimney being placed at one of the long sides. However, when the furnace is required to hold but one pot, it may be made cir

cular.

The body of the furnace ought to be made of good bricks, solidly cemented with clay, and bound by strong iron bands. The bricks must be very refractory and capable of sustaining changes of temperature without cracking. They are ordinarily made with the clay used in the manufacture of crucibles. In some cases, bricks are not used for the lining of this kind of furnace; for instance, a mould of wood is placed in the centre, and the open space between the surface of that and the outer brickwork is filled with a paste of very refractory clay, each layer being well beaten down. When the space is filled, the case is withdrawn, and the crust of clay dried with much precaution, every crack that may be caused by unequal dessication, being filled up as fast as formed. This method of manufacture is very appli

cable to circular furnaces. In every case, however, it is necessary to border the edge with a band of iron, to prevent injuries from tongs or pots.

The Ash-pit.-On the one hand it is well to have the power of cutting off access of air into the body of the furnace by the lower part, either to put out the fire entirely, or to deaden it whilst putting in a pot, and on the other, to attain the maximum of temperature, we must have the means of allowing the air to pass with the greatest possible facility into the furnace. In order to do this, it is necessary to furnish the ash-pit with doors, or valves, whereby the quantity admitted may be regulated as described.

The bars are made in one piece, or are made up of moveable pieces of metal, which latter arrangement is the most convenient. Wherever a wind furnace is in use, the superior opening is closed by a cover made of refractory bricks, encircled with iron.

The chimney is a very essential part of a wind furnace ; it is on its height and size that the draught depends; and, in consequence, the degree of heat produced within the furnace. In general, the higher and larger the chimney is, the stronger is the draught, so that by giving it a great elevation, exceedingly high temperatures may be obtained; but there is a limit which it is useless to pass in a furnace destined for operations by the dry way, and besides this, the building a very high chimney presents many difficulties and much expence, so that in laboratory operations, where a very strong current of air is required, recourse is had to pair of double bellows. A temperature can be produced in a wind furnace sufficiently strong to

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