are largely in use at the present time for the manufacture of terra-cotta, bricks, and roofing- or paving-tiles, as well as for the calcining of limestone and similar minerals. Since its first introduction, in 1856, many modifications have been made, and the number of varieties of Hoffmann kilns now in use is very large, but the underlying principle in all is to have a more or less ringshaped tunnel in which the goods are set, the firing being so arranged that whilst part of the kiln is being fired, other parts may be filled or discharged, the kiln being divided up into a number of chambers by means of movable partitions.

In its most compact form the kiln is quite circular, but in the case of large kilns two parallel walls are terminated at each end by semicircular ones. Thus a 12-chamber kiln is usually circular, but with a greater number of chambers the diameter of the kiln remains the same while its 'length' is increased. Providing the ground will allow of it there does not appear to be any limit to the length to which such a kiln may be extended; but very long kilns, when used for clays which may be burned rapidly, are usually worked as though they formed two or more independent kilns. The capacity of the main chimney and its drawing power are the chief factors controlling the length of a continuous kiln.

Reduced to its simplest dimensions, the Hoffmann kiln may be represented as in figs. 4 and 5.

The kiln proper consists of a circular tunnel of any suitable section, which receives the goods to be fired through doors placed in the circumference or outside wall, and built up in the firing. The fuel is fed through apertures in the roof of the tunnel. Flues lead from the tunnel to the 'smoke collector,' which surrounds the base of the central chimney, the communication with which can be cut off by means of bell-shaped cast-iron covers. These flues may be at the base or near the top of the tunnel, according as the kiln is worked with an up- or down-draught. Immediately behind each flue a suitable opening is made for the insertion of a damper-plate, which forms, when in position, a complete partition across the kiln and so prevents the leakage of air or hot gases. As metal dampers are not perfect in this respect many firms now employ so-called 'paper shutters,' or large sheets of a specially made paper which are pasted on the inside of the kiln during the setting and are later burned down by contact with the travelling fire. These paper shutters do away with one of the great sources of air leakage, viz., through the damper-slots, and, although apparently expensive in maintenance, rapidly repay themselves in the amount of fuel saved. The paper used is a kind of coarse tissue-paper which is light and tough.

The fuel falls through vertical columns arranged by the setters in filling the kiln, and some skill is required in so arranging the goods that these columns, with their radiating channels, shall be properly constructed, for on this the success of the firing depends to a large extent. It is usually wise to fix 'setting laths' vertically in the kiln through the fire-holes, and to instruct the setters to use these as guides in the placing of the goods. laths are removed through the fire-holes when the chamber is full.

The

FIG. 4.-Vertical Section of Hoffmann Kiln.

The number of chambers in a kiln is, theoretically, of no importance, although in practice it should bear a definite relation to the time required for warming and burning the clay and to the desired output. If too many chambers are used the goods will be too long in the slow firing, whilst if too few chambers are employed (i.e. if the kiln is too short) sound goods cannot be obtained. It is essential that all the chambers should be in use, if the fullest economy of fuel is to be reached. Twelve chambers

is a convenient number for most brickyards. Referring to fig. 5, it will be observed that all the chambers (numbered 1 to 12) are connected to each other by a free passage, but that a damper across the kiln prevents any passage between No. 11 and No. 12. Assuming that the kiln is in full operation, the doors in chambers

1 and 2 being open, No. 1 will serve for filling with the fresh goods and No. 2 for taking out those already burned. The chambers 3, 4, 5, and 6, which are already filled with burnt goods, are gradually cooling with the air which enters through the doors of 1 and 2, and this air as it passes on becomes warmer and warmer until at last it is as hot as the glowing ware, and conse

quently the fires in the kiln are supplied with air almost as hot as the furnace itself.

In chamber No. 7 the fire is burning, and when its contents Jave reached the desired temperature those of No. 8 will have become so heated by the waste heat from No. 7 that the fuel fed into No. 8 after No. 7 is finished becomes instantly ignited. Chambers Nos. 9, 10, 11, and 12 will be dried off by the waste heat which passes through them, and the damper in No. 12 will turn the gases into the chimney through the small flue at a temperature only just sufficient to support the draught. When No. 1 chamber is filled and closed the damper is removed and placed between Nos. 1 and 2, the bell-shaped damper over No. 12 being lowered and the one over No. 1 lifted and lowered between Nos. 1 and 2. The contents of No. 3 will then become sufficiently cool to be drawn.

In some places a modification of this process of firing is employed, the air from the hot goods being used to dry those freshly set into the kiln and to warm them sufficiently to prevent any condensation products forming upon them when they come into contact with the burning gases. This is a very important matter with clays liable to scum from this cause, but some degree of loss of heat is experienced. This arrangement also offers additional facilities for lengthening or shortening the fire-space, as by the additional flues required the flue-gases may be led from almost any one chamber to any other.

When the flue-gases' are used to dry the freshly-set goods, there is a great danger of the sulphur compounds they contain being condensed on the cold, damp goods and so scumming them with a deposit of sulphates. This may be prevented by using a 'stove' in which air is heated before it enters the kiln, and the gases from the fuel in the main burning zone of the kiln are not used until the goods have become so heated that there is little or no danger of condensation occurring. The preliminary drying and warming generally referred to in this book as 'smoking' or 'smouldering' is sometimes known as 'stoving' from the use of this supplementary stove or air-heater (p. 181).

The use of a stove requires careful attention, or the difficulty of scumming will not be removed. The usual fault consists in allowing the stove to burn too low or not brightly enough; the amount of air heated is then very small, and as the products of combustion from the stove are mixed with the air used for drying, these very products may, if too concentrated, occasion the defect which the stove is expressly intended to avoid.

Many improvements have been suggested at one

time or

another, but up to the present the Hoffmann kiln in this country remains practically unaltered from its original form. At the same time it must be admitted that for some purposes an alteration is desirable, and in others the diminished cost of construction is sufficient to make the modification of the standard type well worth consideration. For example, Otto Bock has patented what is practically a Hoffmann kiln, the walls of which are of earth, the kiln being constructed by excavating out the main part of the kiln-the large circle or 'ring' which holds the goods-and building in the necessary flues in the central piece of ground. This kiln has, therefore, the top of its walls level with the ground, and in this way a considerable amount of heat, which would otherwise be lost by radiation, is preserved. The Bock kiln also differs from most other kilns in having no permanent roof, the 'roof' being composed of a layer of clay and another of sand or soil about 4 inches thick. To fill the kiln, the goods are slid down boards from the ground level to the men inside the kiln.

The Zig-zag kiln is another modification of the Hoffmann kiln, designed to avoid the great length of ground required by the original 'long' form brought out by Hoffmann, in 1870, as an improvement of the earlier circular form. In the Zig-zag kiln the main part forming the chambers is turned repeatedly on itself, as its name implies, so that the goods are loaded in through the ends instead of through the sides of the chamber. The Zig-zag is also one of the numerous varieties having permanent walls to divide the kiln up into a fixed number of chambers. It is specially used for small kilns where the ground is limited.

In order to overcome the difficulty of some of the goods (bricks) being damaged by the ashes from the fuel falling on them, permanent walls or fireboxes are sometimes erected. These un

doubtedly serve the purpose, but at a considerable loss of economy in fuel. Still, for many classes of goods their use is quite unavoidable. In addition to this, the horizontal direction of the flame tends to prevent its proper and even distribution through each chamber. The simplest of these 'continuous chamber kilns' is obtained by building permanent 'flues' for the fuel to fall into when fired, as in the ordinary Hoffmann kiln; but this not being altogether satisfactory, it is more usual to build a complete partition to form each chamber, and to provide close to this a firebox or row of fireboxes to contain the fuel. It is advisable to leave plenty of holes in the lower part of the partition for the gases from the previous chamber to pass into the firebox of the succeeding one. These fireboxes then impart a strong upward tendency to the stream of gases which, being drawn from near