Encyclopaedia Britannica > Volume 15, PLA-RAM

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-pcrceUin. Jenfity and luftre of this porcelain proceeded only from
v~J » a fufible and vitreous quality ; but as they do not, and
as this porcelain is as fixed and as unfufible as the Ja-
panefe, its denfity, fo far from being a fault, is a valu¬
able quality : for we mull allow, that of porcelains equal
in other reipedls, thofe are bell which are moll firm and
jcompail. Hence the interior fubllance of the Japanefe
.porcelain is elleemed for its greater denfity, compadnels,
and Inlire, than our vitreous fand or fritt porcelains ;
becaufe thefe qualities indicate greater cohefion, and
more intimate incorporation of its parts. For the fame
reafon alfo the fuperior denfity of the Saxon porcelain
■ought to give it the preference to the Japanefe. Be-
fides, nothing would be eafier than to give the Saxon
porcelain the granulous texture of the Japanefe, by
-mixing with the palle a certain quantity of fand. But
the perfons who perfected that manufacture were cer¬
tainly fenfible that fuch a conformity to the Japanefe
porcelain would lelfen the merit of theirs : for we know,
that in general porcelains are better in 'proportion as
they contain a larger proportion of clay or earth, and
lefs of fand, flints, or other matters of that kind.
What we have faid concerning porcelain in general,
and the principal kinds of it, feems fulficient to give
jull notions of it, if not to perfons who without confi-
'dering the fubjedl are determined to prefer the moll
ancient, to thofe, at leall, who have made experiments
on this fubjedl, or who, having a fufficient knowledge
of chemiftry, are capable of lludying and examining it
thoroughly. We lhall finilh this article by giving a
fliort defcription of the method of manufacturing por-
gt celain as pra£lifed in Europe.
■Of making The balls, of the porcelains which we have called fu-
fufible or Jible, vitreous, or faljv porcelains, is called by artills a
vitreous yr/// . which is nothing elfe than a mixture of fand or
porce ains ^ p0Wdered flints, with falts capable of difpoiing them
to fuflon, and of giving them a great whitenefs by
.means of a fufficient heat. This fritt is to be then mixed
with as much, and no more, of a white tenacious earth
of an argillaceous or marly nature, than is fufficient to
make it capable of being worked upon the wheel. The
whole mixture is to be well ground together in a mill,
and made into a palle, which is to be formed, either
upon the wheel or in moulds, into pieces of fuch forms
as are required.
Each of thefe pieces, when dry, is to be put into a
cafe made of earthen ware (a) ; which cafes are to be
ranged in piles one upon another, in a furnace or kiln,
which is to be filled with thefe to the reof. The fur¬
naces are chambers or cavities of various forms and
fixes ; and are fo difpofed, that their fire-place is placed
on the outflde oppolite to one or more openings, which
communicate within the furnace. The flame of the fuel
is drawn within the furnace, the air of which rarefying,
determines a current of air from without inwards, as in
all furnaces. At firll a very little fire is made, that the
furnace may be heated gradually, and is to be increafed
more and more till the porcelain is baked, that is, till Porcelain.,
it has acquired its proper hardnefs and tranfparency ;
which is known by taking out of the furnace from time
to time, and examining, finall pieces of porcelain, placed
for that purpofe fin cafes which have lateral openings.
When thefc pieces (how that the porcelain is fufficiently
baked, the firll is no longer to be fupplied with fuel,
the furnace is to be cooled, and the porcelain taken out,
which in this Hate refembles white marble not having a
flrining furface, which is afterwards to be given by co-
-vering them with a vitreous coinpofition palled the
glazing. _
The porcelain when baked and not glazed is called Porcelain
hifcuit, which is more or lefs beautiful according to the ft;ulPturca’
nature of the porcelain. The manufacture of Sevres
excels all others in this refpeft, and it is therefore the
only one which can produce very fine pieces of fculpture ;
that is, in which all the finenefs of the workmanfhip is
preferved, and which are preferable in fmoothnefs and
whitenefs to the finell marble of Italy.
As no piece of fculpture of this kind can preferve all
the delicacy of its workmanlhip when covered with a
flazing, and as fculptors avoid polilhing their marble
gures, becaufe the lultre of the pofilh is difadvantageous;
therefore, in the manufadlures of Sevres, all figures or
little ftatues, and even fome ornamental vafes, are left in
the Hate of bifeuit. The other pieces of porcelain are
to be glazed in the following manner.
A glafs is firH to be compofed fuited to the nature Method of
of the porcelain to which it is to be applied; for every K^azin|, <)r ..
glafs is not fit for this purpofe. We frequently find^'^^
that a glafs which makes a fine glazing for one porce¬
lain fhall make a very bad glazing for another por¬
celain ; fhalFcrack in many places, ifhall have no luilrc,
or {hall contain bubbles. The glazing, then, muH be
appropriated to each porcelain, that is, to the hardnefs
and denfity of the ware, and to the ingredients of its
compofition, &c.
Thefe glazings are prepared by previoufly fufing to¬
gether all the fubfiances of which they confilt, fo as to
form vitreous mafles. Thefe mafles are to be ground
very finely in a mill. This vitreous powder is to be
mixed with a fufficient quantity of water, or other pro¬
per liquor, fo that the mixture ffiall have the confifience
of cream of milk. The pieces of porcelain are to be
covered with a thin firatum of this matter; and when
very dry, they are to be again put into the furnace in
the fame manner as before for the forming of the bifeuit,
and to be continued there till the glazing be well fufed.
The neceffary degree of fire for fufing the glazing is
much lefs than that for baking the pafie.
The pieces of porcelain which are intended to remain
white are now finiflied; but thofe which are to be
painted and gilded mull undergo further operations.
The colours to be applied are the fame as thofe ufed
for enamel painting. They all confiH of metallic calces
bruifed and incorporated with a very fufible glafs. Cro¬
cus
(a) The cafes are called by Englifh potters feggars. They are generally formed of coarfer clays, but which
snull be alfo capable of fuHaining the heat required without fufion. By means of thefe cafes the contained por¬
celain is preferved from the fmoke of the burning fuel. The whitenefs of the porcelain depends muck
x>n their compa&nefs of texture, by which the fmoke is excluded, and on the purity of the clay of which
they are made, 3