Firing Faults
in
Ancient Chinese Porcelain

Just What Are Kiln Firing Faults?
 
Firing faults often but not exclusively refers to glaze problems that developed in the kiln.
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 likely because European porcelain production did not really take off until the end of the 18th century, when Kaolin clay was discovered in Europe and its reliance on imports from China decreased drastically.

Actually, the Chinese kilns of that time also were able to produce  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.

During the Qing dynasty (1644-1911) such faults were gradually reduced essentially 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 during the cooling in the kiln, which can lead to a crack or hairline (a very fine split line) in the glaze.

Another cause are glaze contractions which find there cause in the  pre-firing processing. If some oily or fatty substance adheres to the ceramic body, at the time when the glaze is applied, the glaze may covers this but the glaze does not adhere to the clay.

As soon as the kiln temperature rises above a certain temperature such adhering substance(s) evaporate or burn and, if their is not enough glaze to cover the area in such areas a little hole may result.
Tiny spots may sometimes have been 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, or a glaze indent shows.

Ceramic bodies of Ming dynasty ceramics often have cracks in the bottom as the one shown in the picture above, or have larger areas that are not covered by decoration and glaze. As like in the case above 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.

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