POR 
FOR 
■species, viz. P. volubilis, a native of tlie 
East Indies. 
PORCELAIN, a fine sort of earthen- 
ware, chiefly manufactured in China, and 
thence called china-ware. 
The combination of silex and argil is the 
basis of porcelain ; and, with the addition 
of various proportions of other earths, and 
even of some metallic oxides, forms the dif- 
ferent varieties of pottery, from the finest 
porcelain to the coarsest earthenware. — 
Though siliceous earth is the ingredient 
which is present in largest proportion in 
these compounds, yet it is" the argillaceous 
which more particularly gives them tlieir 
character, as it communicates ductility to 
the mixture when soft, and renders it cap- 
able of being turned into any shape on the 
lathe, and of being baked. 
The clays are native mixtures of these 
earths ; but they are often rendered unfit 
for the manufacture of at least the finer 
kinds of porcelain, from other ingredients 
which they also contain. 
The perfection of porcelain will depend 
greatly on the purity of the earths of which 
it is composed ; and hence, the purest natu- 
ral clays, or those consisting of silex and argil 
alone, are selected. Two substances have 
been transmitted to Europe, as the materials 
from which the Chinese porcelain is form- 
ed ; which have been named Kaolin and 
Petunse, which see ; it was found difficult 
to procure, in Europe, natural clays equally 
pure, and hence, in part, the difficulty of 
imitating the porcelain of the east. Such 
clays, however, have now been discovered 
in different countries; and hence, the supe- 
riority to which the European porcelain 
has attained. The fine Dresden porcelain, 
that of Berlin, the French porcelain, and 
the finer kinds which are formed in this 
country, are manufactured of such clay, 
which, from the use to which it is applied, 
has received the name of porcelain earth, 
and which appears, in general, to be derived 
from the decomposition of the felspar of 
granite. It appears also that natural earths, 
containing magnesia, are used with advan- 
tage in the manufacture. The proportion 
of the earths to each other must likewise be 
of importance ; and from differences in this 
respect arise, in part, the differences in fhe 
porcelain of different countries, as well as 
the necessity frequently of employing mix- 
tures of natural clays. The argil communi- 
cates tenacity and ductility to the paste, so 
that it may be easily wrought: the silex 
gives hardness and infusibility ; and, on the 
proper proportion of these depends, in a 
• \ 
great measure, the perfection of the com- 
pound. The proporlion of silex in porce- 
lain of a good quality is, at least, two-thirds 
of the composition ; and of Wgil, from a 
fifth to a third. Magnesia is of utility, by 
lessening the tendency which the composi- 
tion of silex and argil alone has to contract in 
baking, and which is convenient in the ma- 
nufacture. In the manufacture of the finer 
kinds of porcelain, the ingredients are care- 
fully washed, dried, and ground by a mill to 
a very fine powder, which is passed through 
a sieve. This is made into a paste with water, 
which is well kneaded, so as to be uniform 
in cojiiposition. The vessels shaped from 
this paste are baked in earthen pols, to ren- 
der them tolerably hard and compact : they 
are then covered viith the materials for glaz- 
ing, which, in the better kinds of porcelain, 
consist of a mixture of earths, which form a 
compound more vitrifiable than the porce- 
lain itself. 
These materials are diffused in a very 
fine powder in water, into which the baked 
vessels are dipped : the surface is thus co- 
vered with a thin crust, the water being ab- 
sorbed. When dry, they are again placed 
in the earthen pots, and exposed to a very 
intense heat. The solid matter of the por- 
celain undergoes a semi- vitrification, whence 
it possesses all the hardness of glass, and 
has an additional value in being less brittle, 
and much uiore able to bear sudden altera- 
tions of temperature : it derives also much 
beauty from its semi transparency and 
white colour. The glazing on the surface 
is, from its greater fusibility, more com- 
pletely vitrified, and is, of course, more 
smooth and impervious. See Glazing, 
Enamelling, &c. 
PORCH, in architecture, a kind of ves- 
tibule supported by columns; much used at 
the entrance of the ancient temples, halls, 
churches, &c. See Architecture. Such 
is that before the do'or of St. Paul’s,' Co- 
vent Garden. When a porch had four 
columns in front, it was called a tetrastyle ; 
when six, hexastyle ; when eight, octostyle, 
&c. See Tetrastyle, &c. 
PORCUPINE. SeeHisTRix. 
PORE, in anatomy, a little interstice or 
space between the parts of the skin, serv- 
ing for perspiration. See Cutis and Phy- 
siology. 
Pores, are the small interstices between 
the particles of matter which compose bo- 
dies ; and are either empty, or filled with 
some insensible medium. 
Condensation and rarefaction are only- 
performed by closing and opening the pores. 
