710

THEORY AND PRACTICE OF ENGINEERING.

Book II.

introduce the necessary quantity of wood for the purpose of maintaining the fire; when theflame has made its entire way through the whole of the layers, and appears at the top, thefuel is more sparingly applied, and in a short time the fire is suffered to go out, and thekiln gradually cools. Forty-eight hours is sufficient to burn off a kiln of bricks, if care ispaid to the raising and slacking the heat, which requires some skill on the part of theburner.

Marls , or malms, stocks , and place-bricks are so named, according to their fineness orquality. The first is of bright uniform yellow colour, partly obtained by washing chalkwith a fine clay ; from these are selected the cutters for arches, wdiich, from their fine tex-ture, are calculated for rubbing.

Seconds is another term for a second quality of malm, which are used for gauged work,and the facings of walls.

Stocks are either red or grey, the first being the kiln-burnt, the other that usually fromthe clamp.

Place-bricks are of an inferior quality, and, from the fire not having thoroughly burntthem, are generally fit only for purposes of building where soundness is not a requisite.

Floating bricks, in imitation of those made by the ancients, were formed by M. Fabbroniout of a material which consisted of 55 parts of siliceous earth, 15 of magnesia, 14 of w’ater,12 of alumina, 3 of lime, and 1 of iron: this kind of brick does not become altered byfire, being infusible, and although it loses § part of its weight, it is not in any waydiminished in size : as these bricks are found to float on water, they have been very muchused where lightness of construction was desirable.

Fire-bricks , used for furnaces and the lining of stoves where great heat is generated, aremoulded in various parts of England. At liedgerley, near Windsor, where the loam con-tains a considerable amount of sand, they were made in large quantities; Welsh lumpsand Stourbridge bricks are a similar quality, and will stand the action of great heat.

Iletorts for a variety of purposes are made with a mixture of clay and iron, particularlyfor the manufacture of gas; the iron retort receiving a casing of prepared clay, whichpermits the fire to act first on the clay, and thus prevents the rapid destruction of themetal.

Retorts or bricks made of one part pure clay and three parts of coarse and pure sand,slowly dried and annealed, will resist a very high temperature, and are not readily fused ;but if in contact with any metals in a fusible state, which are suffered to oxidise, they willthen act upon the earthy matter, and cause it rapidly to fuse.

A long continued white heat will soften the compound made of any of the siliceous andaluminous earths ; therefore clay and sand are not so well adapted to bear a great heat as anentire clay ; coarsely powdered and burnt clay being substituted for sand, the vessels whichcontain glass in the furnace, and which are subjected to intense heats, are thus made, andresist for a length of time the action of the saline fluxes they contain.

Windsor loam, or a mixture of clay and sand, made by beating a thin paste, is employedas a lute to unite the joints of fire-bricks, or to set them in instead of common mortar ; andif it is required that vitrefaction should take place with the clay so used, borax or red lead,mixed in small portions, will produce the effect; such a compound will destroy the porosityof earthenware, when exposed to high temperatures.

The strength of brickwork depends entirely upon the manner in which the bricks arelaid; at present the practice in England is confined to what is called Old English andFlemish Bond. The first, which is the preferable mode, has a course of stretchers, andthen a course of headers; the latter is alternate header and stretcher.

In the Old English the stretching courses bind the wall together in the longitudinal,and the heading courses in the transverse direction; so that if any fracture occurs, it doesnot break at the joints, but as a solid mass.

The Flemish bond, introduced into England about the reign of William and Mary , is,however, frequently preferred, because it presents a more regular face ; wherever strengthis an object it should be rejected, and the old English practice followed. Each coursebeing alternately headers and stretchers, should have every brick in the same course laid inthe same direction, and in no instance is a brick to be placed its whole length along theside of another, but to be so situated that the end of one may extend to the middle ofthose it adjoins, except in the outside of the stretching course, where three-quarter bricksmust be used, to prevent a continued upright joint in the face of the work : and where awall crosses another at right angles, all the bricks of the same level course should lie inthe same parallel direction, in order that the angles may be completely bonded.

In Flemish bond, to prevent the wall splitting, it is necessary to make use of iron hoops,in the horizontal joints between the two courses, and some bricklayers, to attain the sameend, lay diagonal courses at certain heights from each other, but the latter system is far fromefficacious; others lay all heading courses within the outside Flemish bond, making theface-work alternately of 9 and 4 inches in thickness; this prevents splitting, but destroysthe stretching bond.