Brick kiln

“ Brick Kiln. Mr. Jesse Wenger and Helpers.
The Brethren Taylor and Wenger, with the assistance of the schoolboys and some other natives, moulded and burnt a large kiln of brick. Brother Taylor attended to the moulding and Brother Wenger to building the kiln. This gave the boys training in another line of industrial work, and at the end of about six weeks a lot of excellent brick were ready for building. Unfortunately there was no money on hand to build a house, for the Board had not been informed of our needs in this respect, since brickmaking had been undertaken rather suddenly. ”

“ Make a mould for bricks. Build a brick-kiln and make bricks. Build a smelting-house, blast-furnace, kiln for cleansing ore. Meditations. Build water-wheel and fan-wheel, and set my machinery for an air-blast to reduce the ore.
In the first place I went to work, and with my knife and hatchet fashioned out two quite smooth pieces of wood about four feet long, three inches wide, and perhaps one inch thick. I smoothed these on one side with a great deal of care, and finished them off by means of dry shark's skin, which stood me admirably in place of sandpaper. ”

“ We will assume that the brick has been either naturally or artificially dried, that no cracks have made their appearance, and that it is properly stacked in the kiln ready for burning. Now comes a most important part of the process. It is possible that any microscopic cracks will be closed by fusion or agglutination; but it more frequently happens that in unskilled hands the kiln is responsible for many cracked and “starred” bricks. To know exactly how to introduce the heat so gradually that the bricks shall not be impaired, is an art begotten only of considerable experience. ”

“ Down-draught kilns usually give a more regular fire and a higher percentage of well-fired bricks; and they are more economical in fuel consumption than up-draught kilns, while the hot gases, as they pass from the kiln, may be utilized for drying purposes, being conducted through flues under the floor of the drying-shed, on their way to the chimney. ”

“ The old method of burning wood, or soft coal, or charcoal at the bottom of a small old-fashioned cylindrical fire brick kiln attended to by hand, and heating the articles of pottery arranged on shelves in the chamber above, is done away with to a great extent in large manufactories for the making of stone and earthenware—although still followed in many porcelain kilns. ”

“ The natural mixture of clay and sand, called loam, as well as marl, which consists of lime and clay, with little or no sand, constitutes also a good material for making bricks. The poorer the marl is in lime, the worse adapted it is for agricultural purposes, and the better for the brick manufacturer, being less liable to fuse in his kiln. When a natural compound of silica and clay can be got nearly free from lime and magnesia, it forms a kind of bricks very refractory in the furnace, hence termed fire-bricks. ”

“ In one sort of kiln, the bricks themselves make the kiln. They are piled up in arches, but left a little way apart so the hot air can move freely among them. The sides of the structure are covered with burnt brick and mud, but the top is left open to allow the steam from the hot bricks to escape. The fires are in flues that are left at the bottom. They must burn slowly at first, but after a while, some forty to sixty hours, the heat becomes intense. ”

“ The raw brick is often composed of pieces of chalk or other limestone, in limestone districts and in areas where boulder clays are largely employed for brickmaking. On being subjected to the heat of the kiln these pieces are promptly reduced to the condition of lime. During the process of conversion considerable expansion takes place, and subsequently contraction, leading to the formation of cracks radiating from the fragments of limestone, the homogeneity of the bricks being at once destroyed. ”

“ Kilns are of two types, open and muffle. In the open kiln the flames pass through the firing chamber and the ware may be exposed to their action, as in stoneware and brick; or it may be enclosed in the fire-clay cases, called saggers, as in the many forms of pottery, dishes or faience. The muffle kiln is a closed chamber which is surrounded by flames but which is not entered by them. These kilns are used in the manufacture of terra cotta and heavy enamel wares, and the portable kilns made for studio use are of this type. ”

“ Where no sand is available, it frequently happens that brick-dust will answer the purpose, though this may be at the expense of homogeneity in the long run. In the semi-dry process of manufacture the initial causes of cracking are not present, the block having to contract so little that it may be taken from the press and stacked in the kiln for burning. ”

“ By leaving two or three holes in the door-way, which can be stopped with loose brick, a rod may be run in, from time to time, to take out specimen tiles from the hottest part of the kiln, which shall have been so placed as to be easily removed. The best plan, however,—the only prudent plan, in fact,—will be to employ an intelligent man who is thoroughly experienced in the burning of brick and pottery, and whose judgment in the management of the fires, and in the cooling off of the kiln, will save much of the waste that would result from inexperienced management. ”

“ The gas and oil kilns described in Chapter IX are excellent as far as they go, and indeed the only practicable kind for indoor schools. But their restricted size soon becomes irksome to a craftsman, whilst the expense of firing makes only the finest work remunerative. For over-glaze work they are excellent, but for some reason glazes fired in them seem to lack some of the richness and maturity the same glazes exhibit when fired in the slower and more soaking fire of a brick kiln. ”

“ The kiln for burning the limestone into lime is a cup-shaped concavity, surrounded with solid brick-work, open at the top, and terminating below by an iron grate. It has a stone door that may be opened and closed for charging and emptying the furnace when required. This furnace I formerly charged with alternate strata or layers of small coal and limestone, the latter being broken previously into pieces not larger than a man’s fist, until the kiln was completely filled. ”

“ In the first place, a great deal depends on the way the clay has been mixed or prepared; and in the second, the46 method of burning and the temperature employed, taken in conjunction with the general composition of the earth, are all important. This much may be said, however, that without the iron (or some mineral colouring matter possessing similar properties in the kiln) a red brick would not result. An even colour is the effect of thorough and homogeneous incorporation of the iron with the brick-earth ”

“ Turning to a smaller matter, the hand-barrow coming from the drying stacks to the kiln is unprotected, which97 often means that a good brick is spoilt. Of course, we are not alluding, in this connexion, to what takes place during clamp stacking; the brick produced by such a process must take its chance. The method of stacking in the kiln or clamp is very often responsible for damage to the bricks. A common method is to build them sloping outwards, and all sorts of strains and stresses are thus set up, which have their effect in producing lines of weakness, if not of actual visible cracks. ”

“ A lime-kiln is about three meters high, and is lined with fire-proof brick. An opening at the bottom serves for taking out the lime when the firing has continued long enough. In filling the kiln it is the usual practice to begin by laying large pieces of limestone so as to form a sort of rude vault over the fireplace, and on this vault are piled smaller fragments until the entire cavity is filled. The fuel used may be fagots, brushwood, turf, or coal. ”

“ Fires are kindled in the fire-mouths, and the clamp is allowed to go on burning until the fuel is consumed throughout. The clamp is then allowed to cool, after which it is taken down, and the bricks sorted; those that are under-fired being built up again in the next clamp for refiring. Sometimes the clamp takes the form of a temporary kiln, the outside being built of burnt bricks which are plastered over with clay, and the fire-mouths being larger and more carefully formed. ”

“ It is especially necessary that all the joints should be carefully filled, as any small open spaces would admit air, and would materially decrease the yield of the kiln. The floor of the kiln was formerly made of two rows of brick set edgewise and carefully laid, but latterly it is found to be best made of clay. Any material, however, that will pack hard may be used. It must be well beaten down with paving mauls. The center must be about six inches higher than the sides, which are brought up to the bottom of the lower vents. ”

“ It is considered to improve in quality the longer it is kept.
The kilns in which nishiki-de and saishiki-de are baked are one-celled, and built of clay upon a foundation of brick, with walls about six Japanese inches thick. Fire is kindled in the mouth of a passage which projects from the front of the kiln, and the hot air passes up this to the chamber, where it can circulate freely round the muffle, in which the biscuit is deposited. The largest of these kilns have the following dimensions:—
A space of four inches in width is left between the muffle and the inner wall of the kiln. ”

“ Here the kiln was perfect. It was circular, and measured six yards in circumference, its shape being well-defined by small hand-formed masses of red brick-earth. The floor, about two feet below the natural surface of the ground, was paved with a layer of sand-stones, some of them cut into a circular shape, so as to fit the kiln, the upper surfaces being tooled, whilst the under remained in their original state. As at Anderwood, the ware was broken into small fragments, and was scattered round the kiln for five or six yards. ”

“ His theory is that to make the best use of nature's lavish gifts in the way of wood products, an iron or brick still should be erected, on the inside of which the heavy tarry products would naturally accumulate, and so find their way to the base of the kiln where they could be collected and run out into casks for utilisation, whilst the lighter vapours are condensed in the hood of the still to be chemically treated later for their highly valuable properties, and the charcoal itself would be a more certain production from these brick or iron kilns than it is from the present heaps. ”

“ Iron, in large proportion, acts as a flux to other constituents when the brick-earth is subjected to great heat in the kiln, and on that account must be carefully watched. But, to the average brickmaker, the ferruginous constituent is far more interesting as a colouring medium. At a later stage we shall have something to say concerning the colouring of bricks, &c., but it may now be remarked that red bricks, in practically all cases, owe their colour to the effects of firing on iron. ”

“ We may ignore the presence of a number of minerals such as rutile, augite, and hornblende in brick-earths, as they only exist therein in such small proportion, and have no appreciable effect in the kiln. ”

“ To successfully treat a raw earth, so that all these points may be taken into account, and to produce a thoroughly uniform red brick, that shall not vary in tint from kiln to kiln, is a matter requiring considerable skill and attention, though fairly good bricks of that character have been produced by sheer accident in burning natural earths fairly rich in thoroughly disseminated iron oxides. ”

“ The slag and sand having been mixed together till most of the gases have escaped, the whole is pressed into a mould, and punctured frequently to let out the gas. A close-fitting cover then compresses it. The red-hot brick is next taken to the kiln, covered with powdered coal, and left to anneal. Four men make 500 bricks in five hours. In Silesia these bricks cost 25 per cent, less than ordinary bricks. The lead-slag of the furnaces in the Hartz Gebirge gives bricks of inferior quality, being very brittle. ”

“ Unless a brick is extremely well burnt it is not uniform102 in colour throughout. A considerable proportion of a “draw” is often ruined in regard to tint by the adoption of an unsuitable form of kiln. Where the brick is actually burned (as distinguished from being baked) , the contact of the flame from the fires is almost sure to lead to uncertainty in that respect along the flues. Impurities in the coal, such as iron pyrite, are the chief delinquents, and there is sure to be a certain amount of “flash.” ”

“ The manufacturer may test this for himself by heating a good, sound medium burnt brick to the temperature usually found in his kiln when unloading, and suddenly plunging it in snow. It is not, perhaps, that any one of these things is especially dangerous to the brick, but it is the combined effect of all of them trending in the same direction. We desire to be clearly understood on this point. The cracks produced may not seriously impair the strength of the brick; they may be merely superficial, and they mostly are. ”

“ The rolled sheet is left on the casting-table until it has set sufficiently to be pushed over a flat iron plate without risk of distortion; meanwhile the table has been placed in front of the opening of one of the large annealing kilns and the slab of glass is carefully pushed into the kiln. The annealing kilns are large fire-brick chambers of small height but with sufficient floor area to accommodate four or six large slabs, and the slabs are placed directly upon the floor of the kiln, which is built up of carefully dressed blocks of burnt fireclay resting upon a bed of sand ”