CLAY. 
white; it has no lustre, no transparency. 
It occurs either IHable or compact; stains 
the fingers; adhere.s to the tongue; is soft, 
but meagre to the feel; is easily broken. 
Specific gravity about 2.3. It fells to pieces 
in water, and by kneading becomes ductile, 
though not in a very great degree. The 
Cornish porcelain clay certainly originates 
irom the decomposition of felspar, and con- 
tains particles of quartz, mica, and talc, 
fi-om which it is separated by eleutriation. 
The Chinese kaolin also contains mica, and 
is probably of the same origin as the Cornish. 
The same remark may be applied to the 
French, &c. It is, however, by no means 
certain, tliat all porcelain clay is derived 
from felspar, as it varies considerably in its 
composition and fusibility; all tlie kinds in- 
deed are infusible at any temperature less 
than a white heat ; but some, esjiecially the 
Japanese, are refractory in the most power- 
ful furnaces. The Cornish clay, according 
to Wedgewood, consists of 60 per cent, 
alumina, and 40 silex. 
2. Steatitic clay. Its colour is a light 
flesh red, passing into cream colour; its tex- 
ture is minutely foliated; it has a slight 
somewhat greasy lustre, and takes a polish 
from the nail. It stains tlie fingers, is very- 
friable, and has a smooth unctuous feel. 
JVhen laid on the tongue it dissolves into a 
smooth pulp, without any gritty particles. 
It is very plastic, and has a strong argilla- 
ceoos odour. It occurs in nodules, in a hard 
cellular horn-stone, that forms large moun- 
tainous masses near Conway, in North 
Wales, and originates from the decomposi- 
tion of indurated steatite. 
3. Clay from slate. Its colour is ash- 
grey, passing into ochre-yellow: its texture 
is foliated : it has a smooth unctuous feel, 
and its siliceous particles are so small, as to 
occasion scarcely any grittiness between the 
teeth. It occurs in thin beds on the tops 
of the softer kinds of slate-rock, and from 
its impervionsness to water is always found 
lining the bottoms of the peat-mosses with 
which this kind of mountains is generally 
covered, and in these situations it is of a 
white ash colour, being deprived of its iron 
and carbon by the acid of the peat. It also 
occurs in thicker beds at the foot of the 
mountains, but is of a darker colour, and 
less plastic. 
4. Clay from shale. Its colour varies 
from greyish blue to bluish black: its tex^ 
tore is foliated : it has a smooth unctuous 
feel, takes a polish from the nail, is exces- 
sively tenacious and ductile, and has but a 
slight degree of grittiness. It occurs abun- 
dantly in all collieries, and is produced by 
the spontaneous decomposition of the shale 
vvith which the beds of coal are covered. A 
sandy clay, of a greyer coloin-, and more re- 
fractory nature, is procured from the de. 
composition of the indurated clay that forms 
tlie floor of the coal, and is proviucially 
called clunch. The Stourbridge clay, front 
which crucibles, glass-house pots, &c. are 
made, is of this kind. 
5. Clay from trap. At the foot of the 
softer rocks of trap-formation, such as 
wakke, clay-porphyry, and some varieties 
of grunsteii) and hornblende rock, are found 
in beds of clay, evidently originating from 
the gradual disintegration of these by the 
weather. 
6. Marly clay. The colour of this is 
bluish, or brownish red: it occurs either 
compact or foliated : it has a soft unctuous 
feel, takes a polish by friction with the nail, 
is very plastic, more or less gritty, though 
not so nmcli so as the common alluvial clay. 
It burns to a brick of a bulf or deep cream 
colour, and at a high heat readily enters in- 
to fusion. It effervesces strongly with acids, 
and contains from a to ^ of carbonated 
lime. It originates sometimes from the 
decomposition of compact argillaceous lime- 
stone ; but more frequently fi om the softer 
slaty vmieties usually called stone marl. It 
is largely employed as a manure, and where 
the calcareous part does not exceed 10 or 
12 per cent, it is esteemed as a material for 
bricks. 
7. Clay from metallic veins. Its'colour 
is grey, verging into bluish, greenish, and 
yellowish, or red. It has a smooth unctu- 
ous feel ; is veiy tenacious ; often contains 
sulphuric acid, and certain metallic oxides, 
which are never observed in other clays, 
such as lead, silver, antimony, copper, and 
bismuth. Is found in metallic veins. 
8. Alluvial clay. The circumstances 
which characterize alluvial clay are the fol- 
lowing. It contains a larger proportion of 
quartz sand than the preceding; rounded 
pebbles of various kinds are also imbedded 
in it; thus showing it to have been carried 
from its native situation, and mingled in its 
progress with a variety of extraneous bodies- 
At least tliree kinds of it may be distin- 
guished; viz. pipeclay, potter’s clay, and 
chalky clay. Pipe clay is of a greyish or 
yellowish white colour, an earthy fracture, 
and a smooth greasy feel; it adheres pretty 
strongly to the tongue; is very plastic and 
tenacious; when burnt is of a milk-white 
