CHAPTER VII

SETTING OR CHARGING.

METHODS of setting or charging goods into the kiln vary with the nature of the goods themselves, but are roughly divisible into three classes :

(a) Setting direct in the kiln itself (bricks and unvitrifiable ware).

(b) Setting in cupboards built of quarls, quarries, or slabs.

(c) Setting in saggers or fireclay boxes erected in piles with a roll of clay between each sagger. This method is chiefly used when the goods would be spoiled by the direct action of the flame, and when it is not desirable to fire them in a muffle kiln.

A description of each method of setting would be out of place here, but the following points should have constant attention :

(1) As many goods as possible should be put into the kiln so as to secure economy of fuel, but, on the other hand, if the goods are unduly crowded (and in particular if they are coloured) some of them will be spoiled. When kilns are set on piecework there is a great likelihood of their being incompletely filled.

(2) When various sizes and shapes of goods or saggers are set, care should be taken that they are well and solidly built together, as the expansion on heating causes movement amongst the goods and this is increased by the contraction of the clay, so that unless skilfully set they may easily fall and suffer damage.

(3) Although damp goods should never be placed in a kiln, yet if this rule be broken in special circumstances, they should be placed in such positions that they cannot harm other goods. For this reason they should be set where they will be heated last.of all the goods in the kiln, so that the steam they evolve may not condense on other and cooler goods.

(4) Glazed goods, when not burnt in saggers or in muffle kilns, should be so placed that their glazed faces are together and are protected as much as possible from direct contact with the

flame.

Under no circumstances may glazed surfaces be placed opposite to unglazed ones or the former will be spoiled.

(5) For glazed goods, the inside of the saggers or the muffle should be covered with a coat of glaze (waste glaze will do), as otherwise the beauty of the glaze on the goods will be spoiled.

(6) Coloured glazes must be placed according to their composition, as a slight variation in the heat will often make a considerable difference in the colour. The best positions can only be found by trial, but, as a general rule, the lighter and more delicate the shade the more moderate the heat which will be required. At the same time it is not wise to place coloured goods too near the walls of the kiln, as crazing and chilling are thereby facilitated.

(7) In brick setting great care must be taken to leave sufficient spaces between the bricks to secure a good draught, and at the same time these spaces must be carefully regulated so as to obtain as even a draught as possible in the different parts of the kiln. Finger-room" is correct, but rather vague; two stretchers to five headers is much more accurate.

It is best to put a double row of headers on the bottom of the kiln (fig. 3) so as to ensure sufficient draught. If the bricks are sufficiently well made they may be set in this way throughout, otherwise headers and stretchers must be set alternately one above the other. Towards the top of the fire-screen the bricks may be set rather closer, so as to secure sufficient heat at the top of the kiln.

In order to ensure the fireboxes being directly under the fireholes in

FIG. 3.

a continuous kiln, a lath of 1-4 inches diameter may be passed into the kiln through each fire-hole to act as a guide to the setters. Laths for setting the bricks horizontally are also desirable in the absence of really skilled setters ('placers').

If the bricks are to be glazed later, their faces must be set together so as to prevent damage by exposure to the flame. Where difficulty is experienced in getting sufficient top-heat in a tinuous kiln, Niendorf's plates may be used. These are small tiles placed vertically in the fuel shafts in such a way that they may be knocked level one after the other as the different parts of the kiln get sufficiently heated, and in this way raise the fuel in the shafts.

(8) Careless handling of the goods by the placers is a source of

great loss in some factories, especially in brickworks. The greatest care should be taken to keep the arrises clear when setting or they will be chipped, and the bricks spoiled for best work. The workman should use a board wrapped with several thicknesses of cloth or felt when standing on the bricks, so that his boots do not damage them.

(9) A careful record of all goods placed in each kiln should be kept, and used as a check when the goods are discharged.

(10) Great care is needed in setting certain classes of goods that the glazed portions do not adhere to the sagger or to other parts of the kiln. The saggers or cupboards should be well sprinkled with flint or quartzose sand, and a little of this material may at times be suitably placed on the goods themselves to prevent adhesion. In supporting articles by means of clay cubes or stilts, etc., attention should be paid as to where the marks formed by these supports will appear; carelessness in fixing these supports should on no account be tolerated.

CHAPTER VIII.

KILNS.

THE Construction of kilns is a subject which, like so many others, must be left largely in the hands of experts. At the same time the subject is one of the most important the clayworker has to deal with, for, if his goods go wrong in the kiln, no amount of care afterwards will remedy the damage.

Many kilns are correctly constructed, but fail in actual use because their construction is not sufficiently understood by those using them. A kiln may, for instance, have too much draught, and so cause the heat to pass so rapidly through the flues that parts of the kiln never become properly heated until the speed is checked by the use of a damper or some other means of partially closing the flues. On the other hand, a flue is sometimes partially closed by the builder in such a manner that it may be opened out if more draught is found to be necessary; but unless this is known, the kiln may be condemned as useless owing to it not heating up properly.

The first thing, then, is to know the construction of the kiln, and by this is meant a practical knowledge as well as a theoretical one derived from books or from studying the builder's plans. Only in this way can serious losses be avoided when the kilns are not working quite satisfactorily, or when some part breaks down. In addition to this, a good knowledge of the construction of the kilns in use will not only enable the proprietor to realise the full value of the maxim that a stitch in time saves nine, but will often be the means of suggesting minor improvements in construction which, although small in themselves, are frequently the means of saving considerable sums of money in the course of time. Thus the saving of half a ton of coal a day represents a man's full wages at the end of the week.

The first consideration in erecting a kiln is the selection of the site, for if this is wrongly chosen it may be impossible to obtain

good results from the firing. The prospective kiln-builder must watch carefully to see where the water runs and stands after rain, and must choose the dryest spot. If the kiln is in the vicinity of a river or of underground springs, it is necessary to ensure that these shall have no effect on the foundations. It is, in fact, essential that the kiln bottom be kept dry, and it must be well drained so as to remain so even in the wettest weather. More than one yard has failed as a commercial success for want of attention to this matter.

The depth to which a foundation must be taken to ensure dryness of the kiln floor depends, to a large extent, on the nature of the ground, but is usually much greater than is generally supposed; and Bock states, as the result of a very long experience, that no kiln should be built on ground in which the water stands at a less depth than six feet in the subsoil, and, where floods are likely to arise, the floor should be raised above ground level and arrangements made for draining beneath the kiln. This matter of raised floors is also important in connection with kilns built near to the banks of a tidal river. Of the various materials used for rendering the floor of the kiln damp-proof, asphalt or bitumen is undesirable because it is affected by the heat, and concrete, although excellent when new, is apt to crack and is then useless.. A simple yet effective foundation consists of a layer of moderate sized stones about 18 inches deep, well drained with pipes and covered with a layer of gravel, over which a bed of clay or loam is well rammed into place. A bed of sand may be added if desired, and the whole paved with hard-fired bricks. Where the ground is very wet properly built flues should take the place of the stones, these flues being connected so as to drain from one channel. Small wooden chimneys are also added in some cases to secure the proper ventilation of these flues, but if the chimney draught is strong enough the flues may be more conveniently connected to the chimney-stack.

Again, the outside masonry is often too thin, and the various dampers and caps fit so badly as to be almost useless. Such defects have all to be paid for by an excessive fuel consumption.

The strength of the arches is a matter often needing special care, for it must be remembered that the masonry must not only be sufficiently refractory to withstand the high temperatures, but must also be possessed of such resisting power that it can bear the strains set up by the continual expansion and contraction. Flattened arches are, therefore, to be avoided, as are also those with a very pronounced point. In almost every case the true semicircle is the best form of arch.