CLAY

16¢

that heat melts some of the constituents, the fluxes, which,
on re-solidification, cement the rest into a compact material.
Clay may be thus burnt into brick, earthenware, or porcelain.
In ordinary brick clay the fluxes are usually alkali from a
felspar, carbonate of lime, and oxide of iron; they fuse
at a moderate temperature. Common brick is therefore
made with a low expenditure of fuel, but will not withstand
a high temperature. A good brick clay contains about
45 per cent. of silicate of alumina, 35 per cent. of silica,
3 to 6 per cent. of iron oxide, 3 to 8 per cent. of carbonate
of lime, 1 to 4 per cent. of magnesia, 3 to 6 per cent. of potash
and soda, and 4 to 6 per cent. of water. Bricks made with
more silicate of alumina shrink too much in burning, and
those with more silica are too brittle. The red or yellow
colour depends on the iron oxide present.
Clay with a low proportion of fluxes or a high proportion
of infusible material serves as fireclay; it requires greater
heat in burning, but its products withstand temperatures as
high as 3400° F. The temperature in the kilns is determined
by the use of *“ Seger cones ”’ of various clay mixtures, which
bend and melt at different temperatures.
The refractory nature of some fireclay, such as the underclay
 of a coal seam, is due to the alkalis having been withdrawn
 by the growth of plants; in others it is due to the
clay particles having been washed during their deposition,
and their soluble constituents thus leached out; or it may
be aided, as in the Glenboig fireclay {silica 62-5, alumina
34 per cent., Fe,Oy 2-7 per cent., alkalis and loss -8 per cent.),
by the large size of the quartz grains, which are only slightly
fretted at a high temperature.
Pottery clays and the better clays used in earthenware
usually consist of weathered felspathic material which is
white owing to the absence of iron, and highly refractory
owing to the poverty in lime and alkalis.
Cuina-CLay—PnEumMaTOLYTIC ORIGIN—The weathering
of felspar produces various amorphous silicates of alumina,
such as halloysite, and some products of which the particles
are so minute that they cannot be determined under the microscope,
 and are grouped as * clay substance.”
The action of hot acids on felspar produces a crystalline
hydrous silicate of alumina—kaolinite (Al,04, 28i0,, 2H,0),