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MODERN GASWORKS PRACTICE
Firebricks under load seem to break down in one of two ways. In one type of collapse (fusion collapse) the firebricks gradually give way as the temperature rises, until finally the piece collapses. This beliaviour is typical of aluminous and zirconia bricks. In the second type of collapse (mechanical collapse) the firebrick may show symptoms of a gradual deformation.; but the characteristic feature is a more or less abrupt collapse. This is typical of silica and siliceous bricks, and, to a less degree, of magnesite and chromite bricks, altliough a sufficient number of varieties of the latter have not been determined to give the same confidence in making general Statements.
The efiects produced are exceedingly complex ; and they are affected by many factors other th.an composition, texture being one of the most important. In all cases the experiments were conduoted under a load of 50 1b. per square inch.
APPARENT POROSITY
The test for apparent porosity really indicates the density of the material and gives th.e percentage of the total volume which is occupied by air spaces.
. Volume of air spaces Percentage porosity = J Total
volume of test; piece X
Fig. 110.—Mellor’s Porosometbr.
Porosity is one of the most important properties of present-day fireclay articles and is determined by means of some forms of “ voluminometer ” or “ porosometer,” as shown in Fig. 110. The piece to be tested should be of the size of half a brick, having a volume of about 700 c.c. The principle of the test is based on the absorption by the brick of a suitable liquid (usually paraffin) until the air spaces and cavities are completely filled. The volume of liquid required. for this purpose can be measured in the burette, this being iden-tical with. the volume of the air spaces, hence the porosity may be determined. The test piece, dried at 100° C., is placed in the glass jar ; the paraffin from the latter having been previously sucked up into the burette by means of a water pump. Another water pump is then connected to the lidof the jar containing the test piece, and as rn uch air as possible is sucked out. With the pump still working, the paraffin is then allowed to enter the jar slowly until the brick is