During the firing of a porcelain bedy, the feldspar
melts first anc combines with the clay in a more-or-
less liquid form. This solution then gradually
dissolves the silica. It is crucial to understand
this as a sequence, for it implies the Time-Relative-
to-Particle-Size factor which determines the quality
of any porcelain body. A well-matured porcelain body
will contain the maximum SILICA GLASS (for vitrifica-
tion and translucency) and just enough FREE SILICA

to control the thermal expansion. To whatever
extent quartz crystals remain undissolved in the body,
that body is less translucent.

It 1s generally accepted and understood that SAND
(coarse quartz crystals) added to a stoneware body
should not be "counted" as part of the silica content
of that body, because the sand does not melt into the
body-glass. What is less generally understood is that
the ground silica (quartz or flint) is also, in relative
terms, a coarse material which resists dissolving,

and is more like fine sand in size than it is like the
extremely fine, easily dissolved silica in the clay
itself. Thus, quartz or flint added to a porcelain
body should be thought of more as fine sand than as
silica and will augment the silica glass content of
the body very little. This is true, unless of course,
the firing is long enough to accomplish the slow
dissolve of quartz. (Slow firing below 1100°C helps
very little in this respect. It is only as the body
nears maturity that slow firing or "soaking" is a
desirable and necessary ingredient. |

The size of the quartz particles is of great importance
because all materials dissolve only from their surfaces.
It will only partially dissolve unless the quartz has
greater surface (i.e. smaller size) or greater time to
dissolve, or both. Otherwise this would result in
diminished vitrification and translucency and increased
possiblity of the ills to be derived from too much free
silica.

Unless or until we understand the time/particle-

size factor in porcelain making, a recipe like this:

56 CHINA CLAY 14 QUARTZ
28 FELDSPAR 2 BENTONITE

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