Fire-clay is found in the greatest abundance and perfection for
manufacturing purposes in,
1. _Slate-clay._ (_Thon-schiefer_, Germ.) Its colour is gray or
grayish-yellow. Massive, dull, or glimmering from admixture of
particles of mica. Fracture slaty, approaching sometimes to earthy.
Fragments tabular. Soft, sectile, and easily broken. Sp. gr. = 2·6.
Adheres to the tongue, and breaks down in water. It occurs along with
_pit coal_; which see. Slate-clay is ground, and reduced into a paste
with water, for making fire-bricks; for which purpose it should be as
free as possible from lime and iron.
2. _Common clay or loam._--This is an impure coarse pottery clay, mixed
with iron ochre, and occasionally with mica. It has many of the external
characters of plastic clay. It is soft to the touch, and forms, with
water, a somewhat tenacious paste; but is in general less compact, more
friable, than the plastic clays, which are more readily diffusible in
water. It does not possess the property of acquiring in water that
commencement of translucency which the purer clays exhibit. Although
soft to the touch, the common clay wants unctuosity, properly so called.
The best example of this argillaceous substance is afforded in the
London clay formation, which consists chiefly of bluish or blackish
clay, mostly very tough. Those of its strata which effervesce with acids
partake of the nature of marl. This clay is fusible at a strong heat, in
consequence of the iron and lime which it contains. It is employed in
the manufacture of bricks, tiles, and coarse pottery ware.
3. _Potter’s clay, or Plastic clay._--This species is compact, soft, or
even unctuous to the touch, and polishes with the pressure of the
finger; it forms, with water, a tenacious, very ductile, and somewhat
translucent paste. It is infusible in a porcelain kiln, but assumes in
it a great degree of hardness. Werner calls it _pipe-clay_. Good plastic
clay remains white, or if gray before, becomes white in the porcelain
kiln.
The geological position of the plastic clay is beneath the London clay,
and above the sand which covers the chalk formation. The plastic clay of
the Paris basin is described as consisting of two beds separated by a
bed of sand. The lower bed is the proper plastic clay. The plastic clay
of _Abondant_, near the forest of Dreux, analysed by Vauquelin, gave--
Silica, 43·5; alumina, 33·2; lime, 0·35; iron, 1; water, 18.
This clay is employed as a fire clay for making the bungs or _seggars_,
or coarse earthenware cases, in which china ware is fired.
The plastic clay of Dorsetshire and Devonshire supplies the great
Staffordshire potteries. It is gray coloured, less unctuous than that of
Dreux, and consequently more friable. It becomes white in the pottery
kiln, and is infusible at that heat. It causes no effervescence with
nitric acid, but falls down quickly in it, and becomes higher coloured.
Its refractoriness allows of a harder glaze being applied to the ware
formed from it without risk of the heat requisite for making the glaze
flow, affecting the biscuit either in shape or colour. “Most of the
plastic clays of France,” says M. Brongniart, “employed for the same
ware, have the disadvantage of reddening a little in a somewhat strong
heat; and hence it becomes necessary to coat them with a soft glaze,
fusible by means of excess of lead at a low heat, in order to preserve
the white appearance of the biscuit. Such a glaze has a dull aspect, and
cracks readily into innumerable fissures by alternations of hot and cold
water.” Hence one reason of the vast inferiority of the French
stone-ware to the English.
4. _Porcelain clay or Kaolin earth._--The Kaolins possess very
characteristic properties. They are friable in the hand, meagre to the
touch, and difficultly form a paste with water. When freed from the
coarse and evidently foreign particles interspersed through them, they
are absolutely infusible in the porcelain kiln, and retain their white
colour unaltered. They harden with heat like other clays, and perhaps in
a greater degree; but they do not acquire an equal condensation or
solidity, at least when they are perfectly pure. The Kaolins in general
appear to consist of alumina and silica in nearly equal proportions.
Most of the Kaolin clays contain some spangles of mica which betray
their origin from disintegrated granite.
This origin may be regarded as one of their most distinctive features.
Almost all the porcelain clays are evidently derived from the
decomposition of the felspars, granites, and principally those rocks of
felspar and quartz, called graphic granite. Hence, they are to be found
only in primitive mountain districts, among banks or blocks of granite,
forming thin seams or partings between them. In the same partings,
quartz and mica occur, being relics of the granite; while some seams of
Kaolin retain the external form of felspar.