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FIREBRICK

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Originally appearing in Volume V10, Page 419 of the 1911 Encyclopedia Britannica.
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FIREBRICK .—Under this See also:

term are included all bricks, blocks and slabs used for lining furnaces, See also:fire-mouths, flues, &c., where the See also:brickwork has to withstand high temperature (see See also:BRICK). The conditions to which firebricks are subjected in use vary very greatly as regards changes of temperature, crushing See also:strain, corrosive See also:action of gases, scouring action of See also:fuel or See also:furnace See also:charge, chemical action of furnace charge and products of See also:combustion, &c., and in See also:order to meet these different conditions many varieties of firebricks are manufactured. See also:Ordinary firebricks are made from fireclays, i.e. from See also:clays which withstand a high temperature without See also:fusion, excessive shrinkage or warping. Many clays fulfil these conditions although the term "fireclay " is generally restricted in use to certain shales from the See also:Coal See also:Measures, which contain only a small percentage of soda, potash and See also:lime, and are consequently highly refractory. There is no fixed See also:standard of refractoriness for these clays, but no See also:clay should be classed as a fireclay which has a fusion point below i600° C. Fireclays vary considerably in chemical See also:composition, but generally the percentage of alumina and See also:silica (taken together) is high, and the percentage of See also:oxide of See also:iron, See also:magnesia, lime, soda and potash (taken together) is See also:low. Other materials, such as lime, See also:bauxite, &c., are also used for the manufacture of firebricks where See also:special chemical or other properties are necessary. The suitability of a fireclay for the manufacture of the various fireclay goods depends upon Its See also:physical See also:character as well as upon its refractoriness, and it is often necessary to mix with the clay a certain proportion of ground firebrick, ganister, See also:sand or some similar refractory material in order to obtain a suitable brick. Speaking generally, fireclay goods used for lining furnaces where the firing is continuous, or where the lining is in contact with molten See also:metal or other See also:flux, are best made from See also:fine-grained plastic clays; whereas firebricks used in fire-mouths and other places which are subjected to rapid changes of temperature must be made from coarser-grained and consequently less plastic clays. In all cases care should be taken to obtain a texture and also, as far as possible, by selection and mixing, to obtain a chemical composition suitable for the purpose to which the goods are to be applied. The Coal Measure clays often contain nodules of siderite in addition to the carbonate of iron disseminated in fine particles throughout the See also:mass, and these nodules are carefully picked out as far as practicable before the clay is used. A firebrick suitable for ordinary purposes should be even and rather open in texture, fairly coarse in See also:grain, See also:free from cracks or warping, strong enough to withstand the pressure to which it may be subjected when in use, and sufficiently fired to ensure practically the full contraction of the material.

Very few fireclays meet all these requirements, and it is usual to mix a certain proportion of ground firebrick, ganister, sand or clay with the fireclay before making up. The fireclay or shale or other materials are ground either between rollers or on perforated pans, and then passed through See also:

sieves to ensure a certain See also:size and evenness of grain, after which the clay and other materials are mixed in suitable proportion in the dry See also:state, See also:water being generally added in the mixing See also:mill, and the bricks made up from plastic or semi-plastic clay in the ordinary way. The proportion of ground firebrick, &c., used depends on the nature of the clay and the purpose for which the material is required, but generally speaking the more plastic clays require a higher percentage of a plastic material than the less plastic clays, the See also:object being to produce a clay mixture which shall dry and fire without cracking, warping or excessive shrinkage, and which shall retain after firing a sufficiently open and even texture to withstand alternate heatings and coolings without cracking or flaking. For special purposes special mixtures are required and many expedients are used to obtain fireclay goods having certain specific qualities. In preparing clay for the manufacture of ordinary fire-See also:grate backs, &c., where the temperature is very variable but never very high, a certain percentage of sawdust is often mixed with the fireclay, which See also:burns out on firing and ensures a very open or porous texture. Such material is much less liable to splitting or flaking in use than one having a closer texture, but it is useless for furnace lining and similar See also:work, where strength and resistance to See also:wear and See also:tear are essential. For the construction of furnaces, fire-mouths, &c., the firebrick used must be sufficiently strong and rigid to withstand the crushing strain of the superimposed brickwork, &c., at the highest temperature to which they are subjected. The wearing out of a firebrick used in the construction of furnaces, &c., takes See also:place in various ways according to the character of the brick and the particular conditions to which it is subjected. The firebrick may See also:waste by crumbling—due to excessive porosity or openness of texture; it may waste by shattering, due to the presence of large pebbles, pieces of See also:limestone, &c.; it may gradually wear away by the See also:friction of the descending charge in the furnace, of the solid particles carried by the flue gases and of the flue gases them-selves ; it may waste by the See also:gradual vitrification of the See also:surface through contact with fluxing materials: in cases where it is subjected to very high temperature it will gradually vitrify and confract and so split and fall away from the setting. It is a well-recognized fact that successive firings to a temperature approaching the fusion point, or See also:long continued See also:heating near that temperature, will gradually produce vitrification, which brings about a very dense mass and See also:close texture, and entirely alters the properties of the brick. Where firebricks are in contact with the furnace charge it is necessary that the texture shall be fairly close, and that the chemical composition of the brick shall be such as to retard the formation of fusible See also:double silicates as much as possible. Where the furnace charge is basic the firebrick should, generally speaking, be basic or aluminous and not siliceous, i.e. it should be made from a fireclay containing little free silica, or from such a fireclay to which a high percentage of alumina, lime, magnesia, or iron oxide has been added.

For such purposes firebricks are often made from materials containing little or no clay, as for example mixtures of calcined and uncalcined See also:

magnesite; mixtures of lime and magnesia and their See also:carbonates; mixtures of bauxite and clay; mixtures of bauxite, clay and See also:plumbago; bauxite and oxide of iron, &c. In certain cases it is necessary to use an See also:acid brick, and for the manufacture of these a highly siliceous See also:mineral, such as chert or ;anister, is used, mixed if necessary with sufficient clay to bind the material together. Dinas fireclay, so-called, and the ganisters of the See also:south See also:Yorkshire coal-See also:fields are largely used for making these siliceous firebricks, which may be also used where the brickwork does not come in contact with basic material, as in the See also:arches, &c., of many furnaces. It is evident that no particular See also:kind of firebrick can be suitable for all purposes, and the manufacturer should endeavour to make his bricks of a definite composition, texture, &c., to meet certain definite requirements, recognizing that the materials at his disposal may be See also:ill-adapted or entirely unsuitable for making firebricks for other purposes. In setting firebricks in position, a thin See also:paste of fireclay and water or of material similar to that of which the brick is composed, must be used in place of ordinary See also:mortar, and the See also:joints should be as close as possible, only just sufficient of the paste being used to enable the bricks to " See also:bed ' on one another. It has long been the practice on certain See also:works to See also:wash the See also:face of firebrick work with a thin paste of some very refractory material—such as kaolin—in order to protect the firebricks from the See also:direct action of the flue gases, &c., and quite recently a thin paste of See also:carborundum and clay, or carborundum and silicate of soda has been more extensively used for the same purpose. So-called carborundum bricks have been put on the See also:market, which have a coating of carborundum and clay fired on to the firebrick, and which are said to have a greatly extended See also:life for certain purposes. It is probable that the carborundum gradually decomposes in the firing, leaving a thin coating of practically pure silica which forms a smooth, impervious and highly-refractory facing. (J. B.*; W.

End of Article: FIREBRICK

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