Firing
Faults
in
Ancient Chinese Porcelain
Just What Are Kiln Firing Faults?
I know many of you may not be quite clear about what
comprises a firing fault. Especially if you are relatively new to
collecting Chinese porcelain, or if your main interest is more with
European antique porcelain you may never have seen any.
They
seem to
be so rare in Western porcelain as kiln firing techniques probably were
already well
developed when porcelain production started in Europe. 
That is because probably because
European
antique porcelain really did not take off until the end of the 18th
century, after Kaolin clay was discovered and Europe's reliance on
imports from China decreased drastically.
Actually, the
Chinese kilns of that time also produced already fine porcelain with
almost
no faults as the
kiln firing techniques had matured at the time. Larger glaze fault Firing-related
problems seem to have been more
frequent during the Ming, Yuan and Song dynasties, but low-quality
products of
the Qing dynasty (1644-1911) and republic period also frequently show
firing faults 
 Firing faults on Zheng De (Ming
dynasty) dish.
Several
areas where glaze and/or decoration do not cover the ceramic body are
clearly visible.
Bottom
of Ming bowl. The bottom has cracked and the crack is quite broad and
going through to the other side. As it apparently was in use it must
have been mended, but the crack filling has now disappeared.
During the Qing
dynasty (1644-1911) such faults were much reduced and later was
limited mainly to glaze contractions.
Other
problems related to kiln firing were glaze cracks or hairlines
that developed during the cooling process after firing.
As
the ceramic body and glaze were cooling at a different rate, tensions
developed between ceramic body and glaze and lead to a crack or
hairline (a very fine split line) in the glaze.
Glaze contractions are the result of pre-firing processing. When some
oily or fatty substance adheres to the ceramic body,
before the glaze is applied, the glaze covers
these, but not the clay.
As soon as the
kiln temperature rises above a certain temperature these evaporate or
burn and their
is no glaze to cover the body in such areas, when it is
insufficient.
Tiny spots were sometimes
filled by the glaze partially or in full, but sometimes it
just didn't happen. As a result the unglazed body may show in some
antique Chinese porcelain, especially the older ones.
Ming
dynasty ceramics often show cracks in the bottom going through the
body, as shown in the picture above, and larger areas are
not covered by either decoration or glaze.
As these items often seem to have been used despite of this, they were
probably
mended with some low-fired clay.
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