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Feldspar (AU O3 K, O 6Si O.) is the double
Ttldspar. silicate of aluminum and potassium. It is com-
posed of aluminum oxid, potassium oxid and silicon
oxid. The chemical difference between feldspar and kaolin is that feld-
spar contains potassium oxid, which kaolin does not, and kaolin contains
water of cr^-stalization, which feldspar does not. It composes about 82
per cent, of the basal mass of porcelain and adds translucency to it.
While feldspar is more fusible than either silicon or kaolin, it requires
more heat than can be produced in the electric furnace to fuse some
grades of it.
The materials commonly used as fluxes, to in-
TIUXCS. crease the fusibility of porcelain, are substances con-
taining sodium borate (Naa B^ 0-), sodium car-
bonate (Na^ C03), potassium carbonate (K, C03), or glass (which con-
tains the oxids of potassium or sodium). The more refractory ingre-
dients, the flux and the frit are ground together in the preparation of
the porcelain for the market. Then, when fused, a chemical change
takes place, forming an indefinite compound, from which none of the
original ingredients can be removed except by some process which
involves the destruction of other ingredients.
The pigments most commonly used in the
manufacture of dental porcelains are precipitated
Pigments.
gold, platinum, purple of Cassius (the oxid of gold
and tin), the oxids of gold, titanium, manganese, cobalt, iron, uranium
and silver. The colors produced by the use of these pigments in vary-
ing proportions are hues of red, yellow, blue, green, brown, and gray.
The basal ingredients of porcelain are used in
the formula for low fusing porcelains, but the fusi-
Qj„l^l
Porcclaln. bility is regulated by the proportion of flux added
to the formula. The process of fusing and grind-
ing is continued until the ingredients are thoroughly mixed. Both high
and low fusing porcelains may contain a small proportion of starch in
their formuke, w^hich imparts additional stability of form by agglutin-
ating the particles of the mass during the process of molding and
carving. It is not, however, a component ingredient of the porcelain,
for the heat reciuired to fuse the porcelain will volatilize the starch.
Dental porcelain is divided into high fusing and low fusing porce-
lain. The distinction has been accepted by the profession to mean that
a high fusing porcelain is one which fuses above the melting point of
pure gold, while a low fusing porcelain fuses at a temperature below this
point. Such a distinction is relative rather than accurate, because por-
celain lias no definite fusing point. Any enamel body or foundation