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BRICK. Though a common structural material, brick is the most remarkable one in use in the building industry. It is remark able historically, technically, and artistically, and, it might almost be added, socially. Its history goes back to the earliest times in the civilization of man, and it bears on its surface written records of early ages that are most valuable. Excavations at Ur of the Chaldees, the city of Abraham, have yielded brick tablets inscribed imperishably with information about the life of a people living 6,000 years ago. The Sumerian palace at Kish, in Mesopotamia, built 3,500 years B.C., is another example of the ancient use of brick. It was discovered in 1925-6, and was found to be built of small sized unburned bricks, set with mud joints. The paving was executed in burned brick. A peculiarity of these bricks is that they are. plano-convex or cushion shaped, the curved face being laid upwards.

By some it is averred that brick was manufactured years ago, and undoubtedly the tower of Babel was constructed of brick, if Old Testament records are to be believed. The walls of Babylon were built of bricks, burned and unburned. The former were 13in. square and Sin. thick, and from the ruins of the great city, burned bricks were removed and used for building more modern cities. Egypt has yielded numerous examples of brick, manufactured in the earliest and subsequent dynasties, and usually the bricks were large and unbaked. The step pyramid of Sakkara is one of the most ancient tombs in that region, and one of the most recently explored. The exterior of it was faced with stone or marble, long since removed, so that the brick structure stands revealed, and has endured through the ages. In Egypt, bricks were fre quently impressed with the cartouches of kings for whom they were made, and by these their age can be known. Compared with these earliest examples, the bricks made by the Romans at the beginning of the Christian era, and by the Chinese, to build their great wall, at about the same period, seem almost modern, and yet the art which was developed in those days to a high pitch of usefulness was lost in Europe when the Roman empire fell.

Beautiful examples of brick have been found in the loth century in India. The Paharpur temple in eastern Bengal was built in the 8th and 9th century, of small bricks laid in mud mortar, and was decorated with rows of terra cotta plaques of considerable artistic merit. This interesting discovery shows that the art was not lost throughout the world while Europe passed through the dark ages.

The excellence of the Roman manufactures is to be seen in England and other countries in which the Romans made bricks. They even developed the art of coating them with beautiful col oured glazes, of which many examples exist. About the time they introduced the art into England, they seem to have lost it in their home country, and none were made in Europe between the end of the 4th and middle of the 13th centuries. In England the Romans made bricks towards the end of the 4th century, and the manu facture ceased when the Roman occupation ended, until the Flem ish came over to East Anglia in the 14th century.

Revival After the Dark Ages.

In Europe, the art began to be revived from the 12th or 13th century onward in Holland, Flanders, Germany, Italy, and England. From those days till the 19th century much beautiful brick and terra cotta architecture was executed, and has come down to the present day in monuments testifying to the artistic ability of their creators. Brick was used in Tudor, Jacobean, and Queen Anne architecture with an effect suited to and typical of the countryside and the climate of Eng land, that has influenced the domestic architecture of the loth century.

Introduction of Machinery.

About the middle of the 19th century brick manufacturers began the development of mechanical production. This progress was inevitable; increased population, increased industries, increased charters of liberty, all helped to cause an enormous expansion of a trade which henceforth was to be the prime factor in the housing of the people. Progress in mak ing bricks was greater in England than in any other country be cause forests became denuded and stone was scarce in great areas of the land where cities and towns grew.

Brick manufacture was commenced in America about the mid dle of the 17th century, but it was not until the latter half of the 19th that the industry began to develop in that continent, when English machine-makers went over to seek a market. In Holland, Belgium, and France no real technical progress was made in the manufacture of bricks, until after the World War, although clay working in other artistic branches reached a high state of perfec tion in France before the end of the 19th century.

The Nature of Brick.

A brick is essentially a small unit, for use in the construction of walls, piers, buttresses, and arches. By the massing of bricks in quantity with mortar joints of lime or cement, greatly varied forms of construction are erected, having massive or delicate proportions, but always capable of taking their place as the supporting framework, for the carrying of floors, roofs, bridges, viaducts, and commercial structures involving the erection of machinery. Tall chimneys, too, are built with these small units of baked clay, and withstand the strain of storms and the ravages of climate. Brick is made of clay, a product of the breaking down by weather and other destructive agencies of an cient rocks of all kinds. Granite becomes clay, and man in the loth century moulds this clay and bakes it until it becomes in some instances almost as hard as the granite from which it orig inated. Some clays again are made into bricks of far less strength than the parent rock, but yet are capable of withstanding destruc tive agencies such as water, frost, and fire. These agencies have destroyed the rock but leave the brick unscathed.

Resistance to Fire.

Some of the oldest clays found in con junction with coal in deposits of the carboniferous age, those found as dry shale owing to the enormous pressure to which they have been subjected through countless centuries, form, when made into bricks, a material capable of resisting intense heat. The rocks, from which such shale was originally derived, would have perished and broken up, if subjected to such temperatures as are found in furnaces of many kinds used in industry. The rock could not be heated once in such furnaces without disintegration, but the fire brick may be heated and cooled again and again, and still retain its form and properties. Stone used in building perishes gradually in the acid atmosphere of cities, but well burned brick, made even by the primitive methods of the early manufacturers, remains un harmed, and even improves in hardness by exposure. Brick, then, is a material of unchallenged durability and of unsurpassed use fulness in buildings and in industry.

Seasonal Brickmaking.

In the early times spoken of above, when brickmaking entered upon the period of progress, culminat ing in the highly technical processes of the present time, the manu facture was primitive and tedious.

Andrea Palladio, writing on architecture in 157o, says that dry ing bricks under shelter "can't be accomplished in less than two years." Up to the time when machinery was introduced, it was always necessary to dig the clay in early autumn, so that weather ing under winter conditions rendered it fit for forming, by hand moulding, into brick shape. In smaller works this seasonal treat ment still obtains in some parts of England, where surface clays of recent geological age are dug. Hand moulding of bricks, too, still persists in spite of the advent of the machine.

In the early days, the clay, weathered in heaps through the autumn and winter, was dug down from the heaps spread on the ground, watered and then "tempered" by being trodden by men's feet, till a suitable consistency was reached. This method of pre paring the clay for moulding has been superseded for a century by the pugmill, a machine first driven by a horse and later by a steam engine, and in this the ground and tempered clay is ready for the moulder in far less time than by the old method. The pug mill of this class consists of a large wooden tub, without heads, set upright on the ground, and furnished with an iron spindle at the axis, carrying knives, which cut and compress the clay as it falls by gravity towards ground level. A rectangular hole, cut in the tub at ground level, allows the ground clay to extrude, so that it can be cut away with a spade and conveyed to the moulder.

The First Brick Machine.

One cannot discover who was the genius that realized the mechanical potentialities of this early clay-preparing machine. He saw that the extruded clay resembled in section the hole in the tub. He saw that a spade cut away a lump of ground clay of quasi symmetrical form, which might be considered to resemble a large brick. He, theref ore, conceived the idea of placing the barrel on an iron platform, fitting the knife carrying spindle with gearing, and applying steam power to drive it. He reduced the orifice in the tub to the size of a brick, and arranged rollers under the mouth, on which the clay could run without deformation of its rectangular shape. He used then a wire to cut off pieces of the clay column of the requisite thickness, and so a wirecut brick was made by a machine. Engineers soon per fected the machine thus conceived, and so the vertical pugmill brickmaking machine was produced from the old horse-driven mill of earlier days. Other brains came into play and decided that the tub might lie on its side and the clay issue from the end of it, through a mouthpiece of suitable shape to make the clay column. This constituted the horizontal pugmill brickmaking machine, resembling a sausage machine in design and action. Instead of one wire to cut off one brick at a time, a "cutting table" was designed by which o bricks at once could be cut off by io wires carried by a frame. In due time the cutting table was also fitted with a power drive.

The Use of Hard Marls and Shales.

In the early part of the growth of the industry surface clays only were used for brick making, but when the horizontal pugmill was evolved other types of clay of earlier geological age could be worked. In order to break down these drier and harder clays, and bring them to the necessary plastic condition, heavy cast-iron rollers were intro duced and mounted above the hopper of the pugmill. These rollers took various forms to suit different types of clay. After grinding through the rollers it was found expedient to chop the clay in the presence of water. To do this, horizontal trough mixers were de signed with one or two long shafts fixed lengthwise, and carrying small knives along the whole length. A typical train of machinery for making wirecut bricks, then, came to be composed of two pairs of rollers for a preliminary breaking up of hard clays, then a double shaft mixer in which water is added to the clay, then two more pairs of rollers for finer grinding, and finally the horizontal pugmill with rectangular brick sized mouthpiece or die. Accessory to this train is the cutting table for cutting off the bricks to the required thickness.

Brick

For this plastic wirecut process, in which the clay must contain upwards of 25% of uncombined but thoroughly incorporated water, one other type of grinding machine has been largely adopted in the loth century. It is called the wet pan and consists of a large flat-bottomed cast-iron dish, some 7 f t. to oft. in diameter, with sections of the bottom perforated with holes or slots. A heavy vertical shaft in the centre carries heavy cast-iron rollers on the two ends of a horizontal cross shaft. These two heavy rollers, sometimes called edge-runners, are carried rapidly round and round the pan, and crush the clay thrown into it, forcing it grad ually through the perforations in the pan bottom.

The Need for Careful Clay Preparation.

By whatever mechanical means it is done, the essential of modern practice is the complete and thorough preparation of clay before it is formed into brick shape. An approved practice is to interrupt the series of operations above described, just before the clay enters the pugmill, for the purpose of resting it for one or two days. The chemical or physical effect of this resting is not understood. But the "resting period" of mechanically treated clay is in some cases essential, and in all desirable, if the best results are to be obtained.

The Semi-Dry Method of Manufacture.

Towards the end of the 19th century a new method of making a brick from hard, almost dry, marls was devised. It consisted in grinding the clay, without added water, in a dry pan of construction similar to the wet pan above described. It differs only in that the pan revolves, and the horizontal shaft carrying the two heavy grinding rollers is stationary. An inner width of the pan bottom, on which the rollers revolve, is not perforated, but the outer half, annular in shape, is provided with fine holes. The dry clay is ground on the solid part of the pan bottom, and pushed by scrapers on to the perforated part. The finely ground particles fall through the holes, and the larger are pushed back under the rollers for further grinding. The powdered clay is again screened and the finest portion passed to the hopper of a powerful press. From the hopper an automatically measured portion is fed into a strong iron or steel mould with moveable plungers top and bottom. The top plunger descends, actuated either by cam, lever or toggle motion or in some cases by hydraulic power, and presses the clay particles together till they coalesce into brick form and size. The bottom plunger then pushes the pressed brick upwards out of the mould, and the "green" brick is ready to be burned in the kiln.

The brickmakers of Nottingham were persuaded that the bricks made by this process, from their hard marl, would supersede the plastic wirecut brick, and large quantities of semi-dry made bricks were produced and used in the town. But disaster overtook these manufacturers, for the bricks crumbled. Great pressure, then not attainable, is necessary to make a good brick by this semi-dry treatment. It is now known that for this type of clay from 75 to 125 tons pressure is needed on the brick. Although unsuccessful at Nottingham, the semi-dry method was adopted at Accrington in Lancashire with highly satisfactory results and practised for many years. Even here a third method called stiff plastic has been found more satisfactory.

The Fletton Brick.

The semi-dry method, however, found a home towards the end of the 19th century in a district where the Oxford claybeds are found in purest quality. In the Peterborough area of Northamptonshire some 'oft. of surface clay had long been worked by plastic process, turning out a brick of relatively poor quality. Beneath this surface, however, exists the Oxford clay, called in the district "Peterborough knotts" to depths varying from 3oft. to 6oft. or more. This material is a dry shale, not of such great geological age as the shale of the carboniferous period, but of similar appearance. It is characterized by the presence in it of a small proportion of mineral oil, and from 5% to 8% of carbon. This shale was found to respond readily to the semi-dry process of brick manufacture, and the bricks produced by it were square and hard, light in weight and colour, but cheap to make, because, firstly, they required no drying, and secondly, the carbon present in the clay proved to be almost sufficient fuel to burn them. A large industry gradually arose there. In 1927 the quan tity of fletton bricks produced from the Oxford clay was well over 1,000 millions, or sufficient to build 50,000 modern workmen's houses. Works arose in Bedfordshire, Buckinghamshire and Lincolnshire, in which the same clay was found, all of which adopted the same method of manufacture. The fletton brick is the staple building brick of London, and is produced at such low cost that it can compete with plastic made bricks far from the Peter borough centre.

Road Paving Bricks.

Since the World War the semi-dry process of brick-making has found application to other clays both in England and on the Continent. By its means the hardest and strongest brick ever produced is now made, designed primarily for the paving of roads. For this brick a plastic clay is used, being first dried in order that it may be in a condition suitable for the grinding and pressing process above described.

Stiff Plastic System.

The stiff plastic process of brickmaking is intermediate in character between the plastic wirecut and the semi-dry. It is much used for working marls and shales in the north of England, and many plants are to be found in Lancashire, Yorkshire, Northumberland and elsewhere making building bricks, smooth faced facing bricks, firebricks and hard engineering bricks. In this process the marl or shale is ground in a dry pan and then moistened in a horizontal mixer till it contains upwards of 12% of uncombined water, as compared with to 25%, in plastic bricks and 6% to 8% in semi-dry bricks. The press is similar in type to that used for semi-dry making, but, instead of dust, damp grains of clay are fed to a very short vertical pugmill, which forces the completed material into a mould in order to form an immature clot. On emerging from this mould, the clot drops into a finishing mould in which it is pressed into final form, ready for setting in the kiln. Large numbers of firebricks are now made by this method, although the old process of hand moulding is still considered to hold the field for the highest class firebricks.

Hand-Moulded Facing Brick.

The account given above of the development of mechanical manufacture, applied to the pro duction of bricks of various classes, has perforce left out the original hand-moulding method of brickmaking, and has not dealt with the machinery used so largely in America, and to some extent in England, to imitate hand-moulding of the sand-faced variety. As stated above, hand-moulding is still favoured by the best class manufacturers of firebricks, and the production of this variety is of immense importance to the manufacturing trades of all countries in which high temperature furnaces are employed. The hand-moulding process of plastic manufacture produces the sand faced red-facing brick, with all its modern variety of colour, a brick which for artistic effect in domestic architecture is as yet unsurpassed. The richness of texture and the gradations of colour cannot be equalled by mechanical means, and it is, and always has been, the characteristic facing brick of Great Britain, produced in no other part of the world. The surface clays and Bagshot and Reading beds which serve for the best of this type are found chiefly in the south-eastern fields. The clays are either weathered or mechanically prepared, and pugged in horizontal or vertical pugmills, already mentioned, ready for the hand-moulder. In its mouldable form the clay is very soft, so that when a lump of it is rolled in sand and thrown into a sanded mould, it readily flows into all parts of it, and when turned out on to a suitable pallet, is of good shape with richly sanded surfaces all round. It is then dried and burned in suitable kilns.

The London Stock Brick.

Another type of brick is made by hand-moulding called the stock brick or alternatively the London stock. The chief centres of its manufacture are Sittingbourne in Kent, Shoeburyness in Essex and Slough in Buckinghamshire, but many small yards are dotted about the south-eastern area which use the same process. In most, but not all, of these, the clay is mixed with washed chalk, which has the effect of changing the red colour obtained in burning to a yellow or brindle tone. In a few smaller yards chalk is not added, and the brick is dark brown in general tone. These bricks are moulded by hand, with sand used in the same way as for red-facing bricks. Very large quantities have been produced and delivered to London. The bricks are somewhat rough in texture, light in weight with porous body, this last due to the admixture with the clay of the fuel by which they are burned. Mechanical moulding of both sand-faced red-facing and stock bricks is now adopted by some manufacturers in England. The method is not so much an economizer of labour as it is a substitute for skilled men, who are scarce and difficult to train. In America enormous quantities of sand-faced common bricks are made mechanically, the type of machine being denom inated the soft mud moulder. This machine is in the form of a pugmill of varied design, which presses the soft clay into a series of sanded moulds, three, five, six, or even nine side by side in one frame. The mould is filled with clay under the machine and then pushed out to a man in attendance, who strikes off superfluous clay, knocks the two ends of the mould frame to loosen the bricks, and passes the frame to a second hand. The mould frame is now placed face downwards on a long pallet and the bricks allowed to fall out of the moulds ready for removal for drying.

In France and Belgium one other system of moulding a brick has existed for many years and is still practised. It has not found favour in other countries. In the summer season only the surface clay found in great abundance in the north of France and in Belgium is dug, and without any further preparation fed to hand or mechanically operated presses, which press the damp clay into brick form. In some ways this method bears a resem blance to the stiff plastic, but it is very primitive in character.

The Drying of Bricks.

The drying of bricks before burning is, in modern days, a process of great technical difficulty. The old way, still existing in many seasonal works, apart from the earliest method of laying the bricks out in single layer on the ground, was by means of sun and air, no artificial heat being employed. Large space is required for this method, an acre to a million annual production, and it has been almost entirely super seded by artificial dryers, except in small works and the large stock brick fields. The newly moulded bricks are set on edge in long single rows, and when one row is completed a second row is placed on it, and so on till the hack is eight or ten bricks high. The hacks, each with two rows of bricks, are arranged on flat open ground with space for barrows between and the bricks are pro tected from rain and wind by pent shaped covers and side boards. The bricks require from three to six weeks to become dry accord ing to weather. The labour cost, loss by storms, and damage by handling render this system more costly than it would seem.

In large works in the Midlands and North of England recourse was early had to spacious shedding with floors, on which the bricks are laid, heated by exhaust steam from the power plant, and live steam from the boilers at night. This was found less costly than outdoor drying, and could be and is practised throughout the year in the majority of works making wirecut bricks.

Tunnel Dryers.

At the end of the 19th century, tunnel dryers were proposed and put on the market in Europe and Amer ica. In these a series of rail tracks of small gauge are laid in chambers upwards of z oof t. long and 6f t. high. Cars carrying wet bricks are closely loaded with spacing sufficient to allow the pas sage of hot air. The cars are pushed at regular intervals from one end to the other, while hot air is blown into the chambers in the reverse direction, drying the bricks on the cars progressively in their passage. At first this invention appeared to be technically correct in principle and economical in working. Experience, how ever, showed that, except for some types of clay which will with stand rapid drying without cracking or warping, this type of plant was much more costly than steam floors. The tunnel dryer is only economical if the bricks can be dried in 24 to 48 hours. Com paratively few clays will admit of such rapid treatment, and if longer time is occupied the capital charges and working costs become too great in proportion to output.

The Humidity Dryer.

The most recent advance in the diffi cult technical problem of rapid drying of bricks in the mass, is the adoption of a new principle termed humidity drying. It has been found that if a brick is heated in the wet state in an atmos phere saturated with moisture, until it attains a temperature of 120° to 150° it loses no moisture. It may then, however, be sub jected to dry currents of air at the relatively high temperature of about 200° and will dry rapidly and completely in a very short time, without cracking or warping. Modern chamber dryers adopting this principle, combined with efficient apparatus for hand ling the bricks, are highly satisfactory for drying on a large scale and are far less costly than tunnel dryers.

The Burning of Bricks.

For the burning of bricks, in the early days of civilization, in Roman times, and in the period of revival in the 13th to the i8th century, small brick-built kilns or ovens were used, and the heat necessary was produced by wood burning in furnaces situated below the bricks set over them in open formation. Kilns of this type are still used with coal firing in many small works, and are generally known by the name of Scotch kilns. Much waste by over and under burning results in them, and the fuel consumption is as high as 20% of the weight of the burned goods. The down draught kiln, highly perfected in design, is a modification of this intermittent type of brick kiln. In this type an arched-over chamber is provided with furnaces in the external walls. The heat from them passes into the cham ber over bag walls in the interior, and then downwards through the goods, descending finally into underground flues leading to a small chimney. Large numbers of this type of kiln are used in this country, not only for brick burning, but also for tiles, pipes, Qaving quarries, and other varieties.

In America, until recent years, this type was used almost ex clusively for very large outputs. In this kiln the fuel consumption is still high, but the quality of the goods produced is as perfect as can be desired. Two important types of high-class bricks are burned almost exclusively in them, the blue Staffordshire engi neering bricks, known all over the world as the iron brick, and glazed bricks of various qualities. While common and facing bricks are usually burned at temperatures from 900° to i,000° C the blue bricks and glazed bricks require a temperature as high as 1,250° C.

Common salt is made use of in burning certain kinds of bricks in down draught kilns. The silver grey sand-faced facing brick made at Reading is one variety, which is very highly valued by architects for its artistic appearance. Salt is also used in kilns burning blue Staffordshire bricks, in order to create a reducing atmosphere in the later stages of the burning, and to form a thin glaze over the surface of the bricks. Salt glazed bricks made with a fireclay or stoneware clay are glazed by means of salt in a simi lar manner, and are much used for the plinths of large buildings in towns and cities and internally for lavatories, etc.

Clamp Burning.

A method of burning bricks in what are called clamps in England and county kilns on the Continent, was devised perhaps towards the beginning of the i9th century. No records appear to be available as to its origin. Prior to the World War it was the chief method in use in Belgium and the north of France, but has now been largely superseded by modern kilns. In England the method is still in use to a considerable extent in the stock brickfields described above, and in small works chiefly in the south. Clamps are large heaps of bricks stacked systemati cally as closely as possible. In England they are built of various sizes holding from 1oo,000 to i,000,000 bricks, and generally about 'oft. or z eft. high. On the Continent they are built much larger and higher, containing upwards of 3,000,000 bricks. The fuel for burning is usually incorporated in the mixture of clay and chalk, with a certain variable amount sprinkled between the layers of bricks. Fires are started in small channels at the bot tom, and serve to ignite the fuel in and between the bricks. When once the fire has taken a good hold of the bottom courses no more stoking is needed, and the heat spreads gradually throughout the mass. In England it takes six weeks to burn a large clamp of bricks throughout. In the continental system the firing is more rapid, and two or three weeks suffice for burning the large clamps where erected. The fuel is economically applied in this method but the results are uncertain and dependent on weather conditions. Waste by over and under burning is always considerable.

Continuous Kilns.

At the present time the majority of building bricks are burned in continuous kilns of two kinds. In the Hoffmann kiln, named after its inventor, the bricks to be burned are put in chambers in a massive brick construction. The chambers are contiguous, to the number of 16 to 24 in a kiln. They are disposed in two rows with semi-circular or rectangular ends, forming a complete circuit. The bricks are withdrawn after firing and replaced by unburned goods day by day, and day by day also the fire, which is maintained for the burning of them, travels along from chamber to chamber. The actual firing takes place at a position about f our or five chambers distant from the point where unburned bricks are being set and about seven to ten chambers from the point where burned bricks are being with drawn. Adjacent to the point where bricks are being set, a damper is opened, giving passage for the gases of combustion towards a high chimney, which creates the draught required for combustion. A wall or damper of brown paper, pasted on to the green bricks, prevents external air from having access to the bricks in the last chamber set, so that the air and gases passing to the open damper must perforce be drawn through all the bricks set in the 14 to 20 chambers. The cycle of the burning operation therefore is, that cold air enters the back chamber where burned bricks are being drawn, and passes through a series of, say, seven to ten chambers which have been burned and are cooling. It becomes heated in doing so, recuperating and carrying forward the heat so acquired. It reaches then the zone of, say, two cham bers which are being fed with coal, and serves to promote combus tion. It then continues through the four or five chambers of green bricks set ready for burning, giving up a large proportion of its heat to them and heating them preparatory to being finally burned in their turn. The pull of the chimney draws the heat forward, causing the fire to travel through the chambers succes sively and continuously, so that each chamber in turn becomes heated, burned, and cooled ready for withdrawal and sale. This type was invented in Germany in the middle of the 19th century, and revolutionized the burning of bricks on a large scale. It quickly found users in England. It is exceedingly efficient in the use of heat, not requiring more than 6cwt. of fuel for 1 oocwt. of burned goods, instead of 20% used in intermittent kilns.

The Tunnel Kiln.—The second type of continuous kiln is known as the tunnel kiln, and differs from the first in the facts that the goods travel through the tunnel continuously, and the fire zone remains always at the same position. In other respects the principle is the same as in the Hoffmann type. Substitute the word cars for chambers and the description of the burning cycle is identical. The tunnel kiln is built upwards of 300f t. long, is rectangular in cross section, and at ground level is provided with rails, on which run cars carrying the bricks to be burned. Al though actually conceived before the Hoffmann kiln, it is only in the years subsequent to the World War that the tunnel kiln has become a commercial proposition. Though simple in principle, great difficulty was originally encountered in obtaining uniform burning. An average dimension of the cross section of the tunnel kiln is about 6ft. square. The cars, about 61ft. long and carrying, say, 1,200 bricks each, are pushed through the tunnel by hydraulic or gear-driven rams, a car being introduced every one to two hours and taking from 36 to 72 hours to pass from end to end. Tunnel kilns are fired either by direct burning of coal or mineral oil, or by producer gas generated in efficient gas pro ducers. The amount of fuel consumed is similar to that of the Hoffmann type.

Sand-lime Bricks.—One other method of producing bricks may be briefly mentioned. The sand-lime brick, made largely in Germany, in America, and to a smaller extent in England and France, uses sand and lime as its raw materials instead of clay. About io% of lime in a hydrated condition is mixed with the sand, and bricks are made by the semi-dry method in powerful presses. The bricks are stacked on cars and introduced into large boilers or autoclaves, which when full are closed. Live steam under pressure is then introduced into the autoclaves for a period of about 24 hours, hardening the bricks and rendering them fit for use in buildings.

Value and Production in Great Britain.—The foregoing relatively brief account of brickmaking cannot adequately convey to the reader the vast amount of technical detail involved in the successful carrying on of manufacture. Clays differ widely in characteristics and behaviour, no two kinds being treated in ex actly the same way, or yielding exactly the same products. As the years pass, radical changes are made in the incidence of la bour. From being a trade of somewhat degraded character, brick making has become highly specialized, producing goods of great variety with wide differences in value. A common building brick, for instance, such as the fletton, may be sold at works for 35s. per 1,000, a facing brick at loos., an engineering brick at 15os., and a glazed brick at 300s. or more. The total production in England is over 5,00o millions annually. The cost of installation of modern works is in the neighbourhood of £3,000 to £4,000 per million annual production, so that the capital value of the industry, as apart from goodwill, is upwards of £20,000,000, and the turnover annually reaches approximately the same figure.

The Strength of Bricks.—The strength of bricks and brick work is considered to be of considerable importance, in view of the function of brickwork as the framework of buildings and the support of the superimposed loads. The following table gives the ascertained resistance to thrust or, as it is usually termed, the crushing strength of standard qualities of bricks : Tons per sq.ft.

London stock . . . . . . . . loo-15o Sand-lime brick . . . 16o Hand-made red facing . . . . . . . 225 Fletton . 25o Red marl brick Leicestershire etc. . . . . . 35o Red marl brick North Wales . . . . . . 500 Red marl brick Lancashire . . . . 95o Staffordshire blue . . . . . . 500-700 Red or brindle engineering . . . . . . 600-1,000 Vitrified paving brick . . . . . . 1,75o The strength of brickwork has been found, by the building re search board, to be largely dependent on the strength of the joints made in building, and does not approximate to the crushing strength of the bricks used. A Leicestershire brick, for example, requires 35o tons per sq.f t. to crush it, but brickwork built with it in lime mortar fails at ioo tons, and in cement at 1 so tons. A Staffordshire blue, built with cement joints, fails at 200 tons per sq. ft.

See

A. E. Brown, Brick Drying; Hand Brick Making (1902) ; and A. B. Searle, Modern Brick Making (192o). (A. E. B.)

bricks, clay, burning, method and found