if too much, shields must be put up to break the wind. The general form of the heap, which is cone-shaped at starting, becomes of a flattened circu'ar t p in course of being worked. It should not be made too high, as that increases the labour of wheeling the clay. When all the clay is used, such portions as are not sufficiently burnt are raked off and thrown up to the top to the greater heat: the heap is then trimmed off and left to cool. When well burnt, the ballast may be worth the trouble; when badly done, as is usually the case, it is not much better than rubbish, in fact not nearly so good as the usual dry brick rublish of which roads should be made. When well burnt, ground fine, and mixed with an equal portion of sand, and a less than the ordinary proportion of good lime. it makes a mortar which will set as hard as cement. The ballast may also be sifted through a 65 or 70 wire sieve, and the fine stuff, hard and clean, used for mortar or for the plasterer; the coarse and the rough for concrete, in addition to gravel. When used as core for a road, it should be at once covered with the Cowley or other gravel, or the clay beneath it rises up with traffic, and much rain will soon render the road as bad as though the soil had not been covered; in fact it turns into mud, and the scamping builder finds it pays to mix his bad lime in the roadway dirt, and to use the mixture for mortar. 1833. Coke breese or breeze is akin to the above in the use now made of it. It is extensively used for mortar; ground in a mill, in lieu of sand or burnt ballast, it is said to set harder, being cleaner and sharper than sand, and requires less lime or cement. It is employed in artificial stonework, in concrete, and in paving. In ballast burning it burns the clay harder, and is cheaper than small coal. For roads and pathways it is clean, not picking up in wet weather, and is good for surface drainage. In some places it may be cheaper than sand or ballast. TILES. 1834. Tiles, which in their constituent parts partake much of the nature of bricks, are plates of clay baked in a kiln, and used instead of slates, or other covering of the roofs of houses. The clay whereof tiles are formed will always make good bricks, though the converse does not hold, from the toughness required on account of their being so mu h thinner than bricks. The common kinds are made of a blue clay, found in many parts about London, and mostly deeper seated than brick earth. The best season for digging it is in September and October, and it should then lie exposed during the winter. It may, however, be turned up in January, and worked in February; and, as in brick, so in tilemaking, the more care bestowed on beating and tempering the clay, the better will be the tiles. In 1477, 17th Edward IV., c. 4, it was enacted that clay should be dug before November, and be stirred and turned be ore March. Tiles are burnt in a kiln constructed on the same principles as the brick-kiln, but with the addition of a cone, having an opening at top round the chamber of the kiln. They require much care in burning. If the fire be too slack, they will not burn sufficiently hard; and if too violent, they glaze, and suffer in form. 1835. Plain or crown tiles are such as have a rectangular form and plane surface. They were made 10 inches long, 6 inches broad, and of an inch thick, by the statute. They are manufactured with two holes in them, through which, by means of oak pins, they hang uron the laths. In using all coverings of this species, one tile laps over another, or is placed over the upper part of the one immediately below; that part of the tile which then appears uncovered is called the gauge of the tiling. Terro-metallic tiles for roofs, with two projections at the back to catch on the laths in lieu of pegs, are now in use. Terrometallic Staffordshire goods in red, blue, and buff colours; also blue and red, plain, capped and rolled ridge tiles in 18 inch lengths. Broseley roofing tiles in various colours and patterns. The best pressed roofing tiles are of superior manufacture and quality, of very hard metal, impervious to moisture, and will not allow of vegetation growing on them. The Kennington and Naccolt tile yards, Ashford, Kent, supply a dark brown tile, about 9 inches by 6 inches, of which 1,400 go to a ton; the timbering of a roof is not more than for slates It was there that the abbots of Battel manufactured tiles for their own use and for sale. On stripping old roofs these tiles have been found sound and were used again; the heart of oak laths had perished from age. Italian tiles, which were made about 1840, by Brown, of Surbiton, differ somewhat from their first prototype, as, instead of being flat, they are slightly curved, fit easily one into the other. with a horizontal indentation across the upper part, to prevent the wind drifting the rain over the tile head; they have either wide or narrow vertical rolls. Such tiles are usefully employed in picturesque buildings in the country. Taylor's new roofing tiles have a plane surface and a slight turned up edge at the sides, a lump on the surface near the upper edge prevents the upper tile slipping; a cover tile is of a similar size and form; these tiles were used about 1872 at the new railway station in Liverpool Street. They are recommended as being half the weight of ordinary plain tiling, each tile weighing under 4 lbs., and as light as slating; they may be laid to as flat a pitch as slates; and that 180 will cover a square of roofing. 1836. dye roof and hip tiles are formed cylindrically, to cover the ridges of houses. They should be 13 in. long, and girt about 16 inches on the outside. Weight about 5 lbs. Ridge tiles, plain, and with cresting, are now introduced in red, blue, black, and green ware. Plain, flanged, rolled top, and ornamental grooved ridging tiles, are commonly seen. 1837. Gutter tiles are about the same weight and dimensions as ridge tiles, though differing in form, and are for the valleys of a roof. They are now rarely used, their place having been supplied by lead, and lately by zine in common work. 1838. Pan or Flemish tiles have a rectangular outline, with a surface both convex and concave, thus dd They have no holes for pins, as plain tiles have, but are hung on to the laths by a knot of their own earth on their underside, nearest the ridge, formed when making. They are often glazed, and should be 14 inches long and 10 inches broad. The Bridgewater double roll tiles are shown in fig. 614a. Three stubs are formed on the back to catch the lath. They lap over two inches, and afford good ventilation for farm buildings, with good protection from rain and snow. Phillips' patent lock jaw roofing 164 Fig. 614a. tiles, with single grip and double grip, are ornamental, and stated to be wind, rain, and snow proof. The grip consists of each tile locking on to its neighbour by one or two rounded grooves or beads. 1839. The following are the weights of the undermentioned sizes of tiles used for various purposes :— Paving tiles at per 100. 22 × 22 × 23 24 x 24 x 3 = = 133 Ridge tiles 18 in., Į 10 to 14 cwts. per 100. 23 to 20 lbs. each. 12 to 18 cwts. per 100. 12 x 12 x Plain tiles = 2·90= 26 = 5.25= 47 1833a. White glazed tiles of Dutch and English manufacture are used for lining the walls of baths, larders, dairies, butchers' and other shops, kitchen ranges, areas for reflected light, and other such like purposes. For walls of entrance lobbies and similar places, glazed tiles are stamped with a pattern, giving a decorative appearance. Mathemutical tiles are employed for covering the vertical surfaces on the outside of walls, in imitation of brickwork, and to prevent wet being absorbed. 1839h. Ornamental Pavements. The use of hardened clay for pavement is of the highest antiquity. Our own country furnishes numerous examples of the varieties employed by the Romans. The tiles are usually made of the clay found in the immediate neighbourhood in which they have been used; and ornamented, sometimes with colour, but more frequently with merely an impressed or raised design. During the Mediaeval age, encaustic and other tiles were largely employed. Many varieties of plain and ornamental tiles are now made in the Potteries, as at Broseley; also at Poole, in Dorsetshire. The coarser kinds, for streets and doorways, have a red or a baff colour, and are prepared from the Staffordshire clay, which is found associated with coal. By mixing metallic oxides with the finer clays, blue and other colours are produced. The manufacture consists in bringing the clay into a state of fine powder, containing a certain amount of moisture; the mass is then placed in a mould of iron which it completely fills, when the ram of an hydraulic press, exactly fitting the mould, gives a pressure of from 150 to 200 tons, compressing the clay into a comparatively very small space; on being removed from the mould it is polished or smoothed off on the surface, and then it is ready for being baked in the kiln. encaustic or variegated tile is composed, in the body, of ordinary red or buff clay; it is pressed in a mould under a common screw press, the mould not only producing the outer form of the tile, but also certain impressions on the face of the clay, about a quarter of an inch in depth. It is then taken out of the mould, and allowed to acquire a certain state of dryness. Devonshire or Cornish clay, coloured, is then poured over the whole surface, filling the impressions, in the state of a very thick slip; when this has been dried to a certain extent, the slip is scraped until the face of the common clay or body is seen, the im M M The pressed spaces only being filled with the coloured matter. A layer of clay is also applied to the back and is sometimes pierced with holes to prevent the bending of the tiles in the process of baking. 1839c. The Tessere are manufactured by a similar process. In Lambeth, clay being properly prepared and stained of the desired colour, as black, red, blue, &c., is made into long narrow ribbons, by means of a squeezing machine. These ribbons are cut into squares, which are placed one on another, 15 or 20 high, previously oiled to prevent adhesion. These piles are then placed upon a frame sliding in two perpendicular grooves, with fine steel wires stretched tightly across, so that by pressing the frame downwards the wires subdivide the slices into the square, oblong, triangular, or other shaped tessera required; these are then dried and baked in the ordinary way. Messrs. Minton manufacture their tesseræ by pressure as for making tiles. 1839d. The mode of forming tessera into mosaic paving slabs is as follows:-The tessera are laid face downwards on a perfectly flat slate, in the pattern or design required. The size and shape of the slab is given by strips of wood or slate fastened round the tesseræ. Portland cement is poured on the backs of the tesseræ, and two layers of common red tiles are added in cement; thus forming a flat and strong slab, which is fitted for laying down as pavement. (Hunt, Handbook, 1851.) The better tiles, and the larger tessera for pavements, are laid separately on a carefully prepared foundation of fine concrete, and then set in fine sand. The durability of a tesselated pavement consists greatly in the solidity of the foundation given to it. With a floor subjected to vibrations such a work will go to pieces. The encaustic tiles with raised patterns should only be used as wall linings, as at Granada, and never for pavements, as is sometimes done. 1839e. STONEWARE is a dense and highly vitrified material, impervious to the action of acids, and of peculiar strength. Until about 1836, when the duty was taken off, this material was chiefly used for common spirit bottles, oil jars, &c. The clay used is found near the coast in Devonshire and Dorsetshire. It is dug in square lumps of about 40 lbs. each, ard transported in ships to London. After being perfectly dried it is ground to a powder, mixed with water, and, after being allowed to become of uniform consistency, the mass is passed through pug mills, and taken to the workmen. For making large articles, portions of the burnt material, finely ground, are mixed with the new clay; also some white sand found in the neighbourhood of Woolwich and Reigate. 1839f. Almost all round articles are formed by the potter, on wheels turning with the required rapidity. The potter's wheel was known in Egypt some 2,500 years B.C., and it remains practically the same. It was worked by hand, then by the feet, keeping a steadier constant motion; larger articles caused the disc to be attached to a large flywheel, worked by an assistant, who was directed by the potter; lastly came the addition of steam and the conical drum, enabling the potter to regulate the speed required. For articles of other shapes, the composition in a soft and plastic state is laid in plaster of Paris moulds; the porous plaster gradually absorbs the moisture from the clay, and when sufficiently firm it is removed. Some thousand articles are frequently made from one mould before it is destroyed. When thoroughly dry, the ware is placed in ovens or kilns, and exposed to a gradually increasing heat, so intense as to become, before finishing, quite white; salt is then thrown in, and, being decomposed, the fumes act chemically on the surface of the ware, and fuse the particles together, giving the glaze so well known. Stoneware differs from all other kinds of glazed earthenware in this important respect, that the glazing is the actual material itself fused together; in other kinds of ware it is a composition in which the article is dipped while in what the potters call the biscuit, or half-burnt, state. (Hunt, Handbook, 1851.) TERRA-COTTA. 1839g. Terra-cotta, that is, burnt earth, embraces every kind of pottery, but the term has now come to be applied exclusively to that class of ware used in building, and is more or less ornamental and of a higher class than the ordinary, or even the better make of bricks, demanding more care in the choice and manipulation of the clay, and much harder firing, hence it is more durable. The best terra-cotta is a species of stoneware which does not after years of use show signs of decay from contact with acids and alkalies. 1839h. Terra-cotta, like stone, may be good, bad, or indifferent in quality, but good terra-cotta will hold its own against good stone as a sound building material. Bad terracotta is that which is imperfectly burnt, and when it is "slack burnt," as it is termed, the material will go back to clay again. Flower-pots are common terra-cotta, and often throw off a scale of red earth each time the plant is watered. A well burnt sto k brick is also terra-cotta; and where is the ordinary stone which is equally durable with it? Good terra-cotta is easily t sted; when struck with steel it should emit sparks and merely show a black line, and ring like a bell. It should be free from file cracks, have true lines, its surfaces be not chipped or rubbed after burning, and each piece should be properly chambered with cross-pieces. 1839. The clays best suited for terra-cotta are found in the tertiary beds, or those occurring above the chalk, and corresponding with the lower Bagshot sands of the London district. Also those in the oolite and lias formations. It is procurable at Tamworth in Staffordshire; Watcombe in Dorsetshire; Poole in Dorsetshire; Everton in Surrey; Ruabon in North Wales; in Cornwall, and in Northamptonshire. The clay should be as free from iron and limestone as possible, and should be cleansed from all impurities. Natural terra-cotta clay contains 60 to 65 parts of silica to about 28 parts of alumina. The Roman material consisted usually of the following ingredients: Silica, 71-45; alumina, 2-25; protoxide of iron, 12; protoxide of manganese, 017; lime, 8:14; soda, 16:62; magnesia, a trace. Sand is an essential ingredient, and should be free from iron. The chief materials constituting the paste are clay, sand, flint, glass, and phosphate of lime. 1839k. All clays require careful preparation before use, and their after characteristics are often as much determined by this as by anything; the same clay being different under different treatment. 1st. Kneading, or pugging, which consists of well mixing the clay and reducing it to a perfect consistency throughout; this is now done by a pug-mill. Most clays are too fat, and require an alloy to make them more workable; their shrinking is too great, and they are liable to twist and warp in drying and burning, so that rough stuff or burned clay ground fine is added in proper quantities to prevent this, and it gives the potter more certain command over the clay. When mixed it is raised in a dry state into the mixers, water is added, and it is then passed through the pug-mills, when it is ready for use. Sometimes the clay is rendered more homogeneous by being struck continuously with an iron bar, to assimilate the parts and to expel any air, which on being expanded by the heat of the kiln would shatter the work. 2nd. A ball of this clay is supplied to the potter, who proceeds to form the article by hand; or it is pressed into a mould, which is of plaster, when repetitions are required. Care is necessary to have an equal thickness throughout, to prevent unequal shrinkage. This thickness is not much more than one inch. When required of a greater thickness, the blocks are formed hollow with cross webs to strengthen them. When necessary these cavities may be filled with concrete; this filling also prevents the accumuiation of moisture, to which the blocks would be liable were they left open. 3rd. The article so formed in the rough is removed to be dried. Drying is evaporating the water, which must be done very gradually and evenly or there would be a liability to crack and twist. When nearly as hard as a piece of soap it is placed on a lathe and smoothed or polished with an iron tool. If any part is required to be attached to it the part is moulded, and the clay moistened at the point of junction, and the two luted with a very little soft clay. The work is now ready to be burnt. 4th. Burning is a process of the utmost importance, as on it depends the lasting qualities of the material. A chemical action goes on in the firing which changes the whole nature of the clay; it never admits of being worked up again, as in its original state. To accomplish burning successfully requires much experience, skill, and patience. It is now removed to a kiln or reverberatory furnace, and carefully packed in fire-clay troughs called sggars, or placed one over the other. When the kiln is full the doorway is bricked up, and the fires are lighted in the furnace holes around the kiln. Large articles have to be fired very slowly for four or five days, then for about forty-eight hours fired sharply until a heat is attained sufficient to bake the ware, and to flux the ingredients of which the body is formed into a vitreous mass without melting the whole. The intensity usually necessary is stated to be that at which soft iron would melt. The articles have to be protected from the coal flame by the seggars, or by being coated with paper and clay, or by a muffle throughout the kiln, as the flame is apt to crack many clays openly exposed to it, and the vapour of coal is sure to discolour the ware, generally turning it a foxy red. A kiln of large goods takes about a week to cool. 18397. Of late years terra-cotta has been used extensively for the facings and dressings of a building in the place of stone. It is generally made of hollow blocks, formed with webs inside so as to give strength to the sides and keep the work true while drying, whereas when required to bond with brickwork it must be at least 4 inches thick. 1839m. The following result of experiments made by Mr. Blashfield for Mr. Charles Barry, were given in a paper by him on Dulwich College, read at the Royal Institute of British Architects, session June, 1868. A block of Portland stone about 6 inches cube, bore a crushing weight equal to per foot super. A block of Bath stoke, equal to per foot super. A stock brick A solid block of Terra-cotta, equal to per foot super. 292 tons 104 82 "" 523 A ho'low block, unfilled, but made with thicker walls 1839n. The shrinkage of terra-cotta clay in burning is very uncertain; it is one-eighth to one-twelfth. To obriate the risk of warping, large pieces should only be used where absolutely necessary. Blocks may average from 1 to 3 feet cube; never more than 4 feet; the block is usually from 12 ins, to 16 ins. long, by 6 ins. to 15 ins. high; 4} ins. to 9 ins. on the bed; if hollow, from 1 to 2 ins. thick. Larger blocks should have a division or web of terra-cotta across them. Joints should be joggled, stopped ends made solid. beds even, and samples should show extreme limit of colour and evenness. In the old Continental and English examples, brick dimensions are as much as possible adhered to. Large blocks require corresponding extension of time to be allowed in their manufacture. 18390. As regards economy it compares favourably with good stone, while it is much more durable, stronger, and cheaper; for the use of the mould allows, where there is great repetition of parts, of most elaborate work, produced at a cost less than that of stone; as much as one-third is saved. The cost of the raw material of terra-cotta is only half the cost of Portland cement, and not one-fourth the cost of good stone. Mouldings having a girth of two feet can be bought at two shillings and sixpence per lineal foot; tracery for parapets, 4 inches thick, at three shillings per foot superficial. 1839p. Its lightness is a source of economy in comparison with stone, by which a saving is effected in carriage and lifting; the filling of the blocks can be done on the site with the broken bricks lying about. In a district where stone abounds, the saving in cost would not be so advantageous. In London it would be on an average, say 20 per cent. less than Bath stone, and 40 per cent. less than Portland stone. The subject will be further treated in the next book. SECT. XI. LIME, SAND, WATER, MORTAR, CONCRETE, AND CEMENT. 1840. Lime has not been found in a native state; it is always united to an acid, as to the carbonic in chalk. By subjecting chalk or limestone to a red heat it is freed from the acid, and the lime is left in a state of purity, and is then called caustic or quicklime, which dissolves in 680 times its weight of water. It is not our intention here to enter into any account of either of the theories relative to the formation of lime, facts being of more importance to the architect in its employment than the refined fancies of the scientific chemist. The calcareous minerals are mostly distinguished by their effervescing with, and dissolving in, an acid, as also by their being easily scratched or eut with a knife. In respect of the lime obtained from chalk, Dr. Higgins (in his work on calcareous cements, Lond. 1780) says, "It should be observed, that the difference between chalk lime and the lime obtained from the various limestones, chie y consists in the greater retention or expulsion of the carbonic acid gas contained in them." 1841. An account of the stone from which lime may be obtained in the different counties of England would unnecessarily extend this article; we shall, therefore, after observing that the use of marble for burning to lime would be too expensive, state the varieties of limestone as, 1, the compact; 2, the foliated; 3, the fibrous; and 4, the peastone. The compact limestones are of various colours, in hues inclining to grey, yellow, blue, red, and green, and to a smoky sort of colour besides. It is usually found massive, often compounded with extraneous fossils, particularly shells. Its internal appearance is dull, the texture is compact, the fracture small, fine, and splintery; fragments indeterminately angular, more or less sharp-edged; semi-hard, sometimes soft, brittle, and easily frangible. Specific gravity varies from 2.500 to 2·700, and it is composed of lime, carbonic acid, and water, mostly with a portion of argyl and oxide of iron, and sometimes of inflammable matter. 1842. The foliated limestones are such as calcareous spar, statuary marble, &c.; the fibrous limestones, such as satin spar; and the peastone, another species of spar. It may be remarked, that the various sorts of marble, chalk, and limestone may be divided into those which are nearly pure carbonate of lime, and those containing in addition from onetwentieth to one-twelfth of clay and oxide of iron. "Though the best limestones are not such as contain the greatest quantity of clay, yet," observes Mr. Smeaton, none have proved good for water building, but what, on examination of the stone, contained clay; and though," he continues. "I am very far from laying down this at an absolute criterion, yet I have never found any limestone containing clay in any considerable quantity, but what was good for water works, the proportion of clayey matter, being burnt, acting strongly as a cement; and limes of this kind all agree in one more property, that of being of a dead frosted surface on breaking, without much appearance of shining particles." 1843. Among the strongest limes, such as will set under water, those most in use in the metropolis are called grey stone limes, and are procured from Dorking, Merstham, and the icinity of Guildford in Surrey. The Dorking and other limes of that part are burnt from |