Details on Glaze Characteristics
Basically, we should take into consideration that some glaze characteristics of very old Chinese porcelain do not exist in antique western porcelain. Europe's actual porcelain production only started in the late 18th century. Thus, we cannot compare Chinese porcelain older than the Qianlong period with European porcelain.
This said, the first thing to be mentioned are the glaze contractions. They can give some evidence of the age of porcelain, and yet they are not real age characteristics that developed over the years. This seems contradictory but can be understood in the light of the cause of these contractions. Glaze contractions can only be used as indirect age signs. Actually, they are indicative of environment conditions under which porcelain was produced in certain eras and/or certain kilns; glaze contractions were more prevalent in earlier times, but some early 20th century kilns also produced such glaze contractions, due to the presence of similar conditions. Sometimes even more so than in the 18th and 19th centuries.
Glaze contractions are basically a quality problem that became less prevalent during the middle of the Qing dynasty (18th century) because the production environment was better controlled by then. It deterioriated again in the 19th century, however, because of reduced control of these conditions.
More on glaze contractions.
The existence of contractions indicates a production environment that was not "clean" enough, a problem that many kilns experienced until conditions improved much later on.
This also clarifies why some porcelain items of the 20th century do have glaze contractions – the production environment in those kilns was not clean enough.
Crackles can develop naturally and can also be made to develop artificially during the firing, and making them visible by adding certain mineral substances to the glaze.
Age crackles that are naturally developing are often discolored by elements from the environment that seep into these very fine cracks, which then may attain a yellow or brownish tone on a base color that is white. With items that have been excavated the discoloration may contain elements of the soil. In tableware even in vintage ceramics we can see crackles discolored by sauces or other food ingredients that entered the crackled glaze. Sometimes the whole color of a bowl is darkened by these.
One thing that natural age crackles often have in common is that they are not uniform in size. Often they cover only part of the surface, or only a part of the crackles is discolored while the rest is barely visible in the transparent glaze.
The white glaze of Chinese porcelain seldom changes color over time, unless it was exposed for a long time to other substances. Even some porcelains of the Qing dynasty (1644-1911) are still white and clean today as if they were just made. For this reason, crackles sometimes are the only indication in the glaze that some changes took place over time. However, often no crackling is present. It really depends on the composition of the glaze. This is just another example how age signs may be different from item to item.
Shiny & mellow glazes
The transparent glass-like glaze characteristics that give some porcelains their shine was present throughout the Qing dynasty. Generally the glaze of porcelain of earlier times did have a mellow gloss. A bright light would appear as a bright area on the glaze, but such glazes are seldom reflective and shiny as the transparent glazes of the later Qing dynasty.
The thickness of the transparent glaze varied from kiln to kiln
and from period to period, thus the shiny or matte appearance also
differs according to the period and location (kiln) a porcelain item was
fired. The more lustrous and shiny a glaze is, the more likely it is as
a product of the late Qing dynasty. In the glazes of the late Qing dynasty the glaze can reflect not only the light, but even objects may be mirrored in the glaze.
Glaze characteristics of recent Chinese porcelain
Doubtless, when mentioning modern or recent glazes, the glazes used on fake antiques must also be mentioned. Sometimes the latter are different from the glaze of porcelain items we use in our daily lives today. The fakers will try to reproduce ancient manufacturing methods, but as the compositions of modern glazes are different, the result of the firing process is sometimes revealing a gloss that was unseen in glazes of the distant past. An important point when looking for traces of faking.
If a porcelain item of doubtful age has a very shiny and
reflective glaze, it often is attributed to the first half of the 20th
century. Later on modern glazes became a little more mellow, less
reflective than those from those during earlier decades.