Just What Are Kiln Firing Faults?
Many collectors of antique porcelain may not be quite clear about what this term refers to.
Especially if someone is relatively new to collecting Chinese porcelain, or if your experience is limited to European antique porcelain, you may never have seen any.
Firing faults seem to be rare in Western porcelain as kiln firing techniques probably were already well developed when porcelain production started in Europe. That is probably because European antique porcelain really did not take off until the end of the 18th century, after Kaolin clay was discovered in Europe and its 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.
Firing-related problems seem to have been more frequent
during the Song, Yuan and Ming dynasties, but low-quality
products of the late Qing dynasty (1644-1911) and republic
period also frequently show firing faults.
Firing faults on a Zheng De (Ming dynasty)
Bottom of Ming bowl. This bottom cracked during the firing
process and the crack is quite broad, going through to the
other side. As it apparently was in use it must have been
filled, but the filling has now all but disappeared.
During the Qing dynasty (1644-1911) such faults were gradually
reduced and became 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 are cooling at a different rate, tensions develop between ceramic body and glaze and can lead to a crack or hairline (a very fine split line) in the glaze.
The cause of glaze contractions often lies with pre-firing processing. When some oily or fatty substance adheres to the ceramic body, before the glaze is applied, the glaze covers these, but the glaze does not adhere to the clay.
Introduction to Chinese Porcelain
As soon as the kiln temperature rises above a certain
temperature these substance(s) evaporate or burn and if their
is not enough glaze to cover the body in such areas a little
hole may result.
Tiny spots are 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 older antique Chinese porcelain.
Ming dynasty ceramics often have cracks in the bottom as the one shown in the picture above, and larger areas are not covered by decoration and glaze. As these items apparently were used despite of this, they must have been mended with some low-fired clay. Porcelain was a valuable commodity in those times.