Tender porcelain, or soft china-ware, is made with a vitreous frit,
rendered less fusible and opaque by an addition of white marl or
bone-ash. The frit is, therefore, first prepared. This, at Sèvres, is a
composition, made with some nitre, a little sea salt, Alicant barilla,
alum, gypsum, and much siliceous sand or ground flints. That mixture is
subjected to an incipient pasty fusion in a furnace, where it is stirred
about to blend the materials well; and thus a very white spongy frit is
obtained. It is pulverized, and to every three parts of it, one of the
white marl of Argenteuil is added; and when the whole are well ground,
and intimately mixed, the paste of tender porcelain is formed.
As this paste has no tenacity, it cannot bear working till a mucilage of
gum or black soap be added, which gives it a kind of plasticity, though
even then it will not bear the lathe. Hence it must be fashioned in the
press, between two moulds of plaster. The pieces are left thicker than
they should be; and when dried, are finished on the lathe with iron
tools.
In this state they are baked, without any glaze being applied; but as
this porcelain softens far more during the baking than the hard
porcelain, it needs to be supported on every side. This is done by
baking on earthen moulds all such pieces as can be treated in this way,
namely plates, saucers, &c. The pieces are reversed on these moulds, and
undergo their shrinkage without losing their form. Beneath other
articles, supports of a like paste are laid, which suffer in baking the
same contraction as the articles, and of course can serve only once. In
this operation saggers are used, in which the pieces and their supports
are fired.
The kiln for the tender porcelain at Sèvres is absolutely similar to
that for the common stoneware; but it has two floors; and while the
biscuit is baked in the lower story, the glaze is fused in the upper
one; which causes considerable economy of fuel. The glaze of soft
porcelain is a species of glass or crystal prepared on purpose. It is
composed of flint, siliceous sand, a little potash or soda, and about
two-fifth parts of lead oxide. This mixture is melted in crucibles or
pots beneath the kiln. The resulting glass is ground fine, and diffused
through water mixed with a little vinegar to the consistence of cream.
All the pieces of biscuit are covered with this glazy matter, by pouring
this slip over them, since their substance is not absorbent enough to
take it on by immersion.
The pieces are encased once more each in a separate sagger, but without
any supports; for the heat of the upper floor of the kiln, though
adequate to melt the glaze, is not strong enough to soften the biscuit.
But as this first vitreous coat is not very equal, a second one is
applied, and the pieces are returned to the kiln for the third time. See
STONE, ARTIFICIAL, for a view of this kiln.
The manufacture of soft porcelain is longer and more difficult than that
of hard; its biscuit is dearer, although the raw materials may be found
every where; and it furnishes also more refuse. Many of the pieces split
asunder, receive fissures, or become deformed in the biscuit-kiln, in
spite of the supports; and this vitreous porcelain, moreover, is always
yellower, more transparent, and incapable of bearing rapid transitions
of temperature, so that even the heat of boiling water frequently cracks
it. It possesses some advantages as to painting, and may be made so
gaudy and brilliant in its decorations, as to captivate the vulgar eye.