No general rules can be laid down for the proportion of breeze required, as so much depends on the way in which the clamp is built. If too little is used, the bricks will be imperfectly burned, whilst an excess of breeze will tend to run the bricks to a slag. On this account it is desirable, in lighting a very large clamp, to begin with only a few live-holes and to light the others as the firing progresses. The time of firing varies from two to six weeks, according to the spacing of the flues.

Muffle Kilns are employed for burning glazed and other goods which it is essential should be kept entirely free from all contact with the flame or fuel. As is well known, they contain a lining of fireclay bricks which effectively keeps off all flame from the goods. The main difficulty in the construction of muffle kilns is obtaining a perfect distribution of heat around the top, sides, and bottom of the muffle, so that a regular and even heat is obtained inside it. Muffles are heated either by up-draughts or by down-draughts, the latter being preferable and more economical of fuel. As the material of the muffle has to be heated anew each time the kiln is fired, these kilns waste fuel when compared with others; but this cannot be altogether avoided. As there is only a very limited circulation of air inside the muffle, it is essential that it should be well filled with goods so that the radiation may be regular.

Muffle kilns are usually fired separately, but they may readily be coupled together so that the first may heat a second or even a third. The connected kilns are usually so arranged that glost goods are placed in the first, the waste heat from which is (if the flues are rightly constructed) sufficient to biscuit the second for most sanitary ware. For china and earthenware, the use of muffles has no particular advantage over saggers. The fires may

be placed at the ends or along the sides of the kilns, the former being more economical and quite as satisfactory as the latter, and in addition they allow the kilns to be built in ranges of three and more, thus saving heat and material.

Gas-fired Kilns have been suggested in many forms, the Dunnachie and Mendheim patents being the best known in this country. When properly constructed they undoubtedly effect a saving in the cost of fuel, as a much lower grade will suffice. They are mostly worked by means of gas-generators which form a kind of producer-gas rich in carbon monoxide, but with a shorter flame than that from coal-gas. Regenerators are, therefore, used to reinforce the heating power and carry it further. The advantages of a gaseous over a solid fuel consist chiefly in the greater ease in regulating the heat, the greater cleanliness of the goods when

fired in an open kiln, and the reduced labour, owing to the much smaller amount of clinker and ashes which are produced when coal is used in a producer as compared with an ordinary fire. Unfortunately, the majority of the gas-fired kilns which have been placed on the market have not been satisfactory in regard to the first of these conditions, especially in the larger sizes, although the Dunnachie kilns have proved excellent for many classes of work. As is to be expected, the most successful gasfired kilns are all connected in series, so that the waste heat from one may be used by another.

Gas-kilns are specially suitable for heating in a reducing atmosphere, which is much more easily obtained with a kiln of this description than an oxidizing atmosphere, which necessarily requires more air.

On the whole, gas-fired kilns cannot be safely recommended to firms with a small capital; but larger ones will often be able to effect a considerable economy by their use, although at the cost of much experiment and labour in the earlier stages.

The Output of Kilns is a matter of very considerable importance in these days of great competition, and the question of how the output of a kiln can be increased is one which is occupying the minds of many clay workers at the present time.

The output of single kilns cannot, as a rule, be increased to any notable extent, but with continuous kilns there is considerable possibility of this being done. The most direct way of doing so is to increase the rate of travel of the fire, and great progress has been made in this direction of recent years. For instance, the older kilns would not permit the fire safely to travel forward at more than 4 or 5 yards a day, but in the more recent continuous kilns a daily speed of 10 or 12 yards is by no means uncommon. This means that a modern kiln has about three times the output of an older one of similar capacity.

In many continuous kilns the capacity of the chimney is the factor which regulates the output, because, unless the chimney is large enough, it is impossible to add to the number of chambers without lessening the draught. It is a curious fact that whilst most boilers are fitted with an excessively large chimney, in most continuous kilns the opposite is the case.

A defective foundation of the kiln (easily recognisable by the arch and upper portions of the kiln becoming red before the bottom) is a frequent source of diminished output. The steam rising from a defective sole admitting damp to the bottom of the kiln occupies space in the chimney which should be occupied by flue gases, and the amount of heat required to volatilise this moisture

prevents the kiln from heating as rapidly as it otherwise would do. Similarly, the capacity of a chimney is often seriously lessened by leaks in the kiln walls through which air is drawn in. This may be detected by means of a draught gauge applied to various parts of the kiln.

Wet fuel lessens the output of the kiln by taking longer to heat, partly because the water it contains has to be evaporated and partly because it lessens the speed of travel of the fire.

The efficiency of a kiln may be roughly calculated by assuming that for every inch of width of the kiln chamber (back to front) 160 bricks may be burned daily, though for some delicate clays only half this number must be used. Thus a kiln 10 feet wide (= 120 inches) may be expected to burn 120 × 160 = 19,200 bricks per day, although, with a specially delicate clay, only half this quantity (8600) may be possible.

Frit Kilns are used for melting materials for glazes, and are of well-known construction. They are mostly on the plan of a reverberatory furnace, though some are heated from below. The brickwork must be good and well laid; in other respects their construction presents no special difficulties to the potter's builder.

CHAPTER IX.

FIRING.

In many ways the firing of boilers resembles that of kilns; the reader is therefore referred to page 65 for information not contained in the present chapter.

In order that the firing may be efficiently carried out, and the kilns, or boiler, heated to the desired temperature with a minimum expenditure of fuel, it is necessary to consider the objects to be attained, and the changes undergone by the fuel in bringing these about.

Air Supply. Taking coal as a typical fuel, the first change which it undergoes in burning is decomposition into a large volume of gas and a mass of practically non-flaming coke. The gas so produced will, in presence of sufficient air, take fire and burn with a brightly luminous or with a blue non-luminous flame, according to the proportion of air which has become mixed with it. The luminous flame is not possessed of such great heating powers as the blue flame, owing to its admixture with numerous particles of unburnt carbonaceous matter (soot) which are deposited on any cooler object which comes in contact with the flame.

Carbon on burning may form one of two compounds with the oxygen of the air—namely, carbon monoxide and carbon dioxide; but the greatest heat is only produced when the dioxide is formed, hence it is the object of the fireman to secure as plentiful a supply of air to the fuel as possible, at the same time avoiding such an excess as would waste heat by cooling the flame. As solid carbon when completely burned requires nearly three times its weight of pure oxygen, or twelve times its weight and 13,000 times its volume of air, it will be readily understood how large a volume of air is necessary for an ordinary fire. Yet seldom anything like this quantity of air is supplied to the fuel. As

the gas is set free somewhat suddenly soon after the fuel is charged into the furnace, it follows that the greater part of this large volume of air will be required very shortly after the addition of fresh coal; and as the effective combustion of the gas depends upon each particle coming in contact with particles of air, it is equally clearly necessary that the air and gas must be well mixed before combustion can effectively take place. In the ordinary furnace it is not possible to mix the air and fuel-gas as intimately as might be desired, as they cannot be sufficiently long in contact with each other for diffusion to take place completely; hence the advisability of dividing the air supply into as many fine jets as possible. This may be accomplished in a manner suggested many years ago by C. W. Williams, viz., by boring numerous holes of about half an inch diameter in the door and front of the furnace, the total area of these holes being not less than half the area of the grate. If too many holes are bored, it is easy to close some by an iron plate; but in practice it will seldom be found necessary to do this, as if the firehole has a cross-section of a foot, and the firebox holds a cwt. of coal, the air needed for its combustion would form a column of the same area as the firehole and one mile long! It is quite impossible for the air and coal-gas to be properly mixed by the common practice of opening the furnace door of a boiler, or if the mouth of the fire is filled with coal, as in many kilns at the present day. It is not so easy to supply sufficient air to kilns as to boilers, and reliance is generally placed on the flues, which are presumably made of such a length as to supply sufficient air for the complete combustion of the fuel before it passes into the chimney.

The coke remaining on the bars after the gas has been driven off from the coal being solid, it is clear that the mixture with air can only take place at the surface of the fuel in contact with the air—that is, near the grate-bars. If these are unduly choked with clinker, the fuel cannot burn for lack of air; if, on the other hand, the fuel lies unevenly on the bars, and in some places does not cover them, air will rush through the uncovered portions and into the flues without properly oxidizing the fuel. It is therefore necessary to see that the bars are as evenly covered with fuel as possible; but the layer must not be too thick, or the air will be unable to penetrate it and incomplete combustion will result; about 7 inches of glowing fuel is usually sufficient if the coal is fed on to alternate sides of the firebox, and the layer kept as level as possible.

The efficiency of the combustion can only be completely ascertained by a periodical analysis of the flue gases (see 'Tests'), as