Iridescence of Chinese ceramics

Toxicity of some glazes

An iridescent glaze can be a sign that a glaze is toxic. Please be aware that antique porcelain should only be used for display and not for serving or storing food.

Iridescence in Porcelain of the Ming and Qing dynasties

The subject of iridescence of antique porcelain and its cause reappears at intervals, notably in view to its use for authentication of Chinese porcelain. The following is what I know regarding this issue.

Only some color glazes or on-glaze color decorations do become iridescent, notably "fencai" enamel used from the Kangxi reign of the Qing dynasty onwards. These glazes contain lead.
Iridescence appears on the surface only after a considerable time. It could appear after about a hundred years, but only on the surface of certain glaze types or enamels. The thin iridescent  layer on the surface can be washed off using hot water, but this is only a temporary remedy. Its source remains. It is best to consider antique porcelain as unsafe for drinking or eating.

Some Chinese like using antique porcelain for tea drinking, but I would not recommend that!
Once iridescence has appeared on the surface, it is likely seeping into the hot food or tea. All color glazes and on-glaze color decorations should be considered toxic.

Again - please note that iridescence normally appears on color glazes and decorations only. Blue and white porcelain where the blue decoration is under the glaze should not have it.

Use for age authentication
Iridescence can be considered an age sign, if genuine. Unfortunately, the fakers are up-to-date on this too and they can reproduce iridescence artificially. Ever seen iridescent plastic? Modern surface plating can add iridescence to porcelain glazes or almost anything else, even plastics; the fakers know this and use such methods.

Therefore, we cannot judge age based on the presence of iridescence alone. As always multiple factors need to be evaluated, and iridescence is only one of these.

Don't use antique porcelain for eating or drinking, or for storing food. When storing tea (leaves) in antique tea caddies made of porcelain, it is recommended to store the tea either in its original package or put some lining material inside the caddy. With antiques it is always best to make sure that food items are not in direct contact with them, in order to protect from possibly harmful materials used in ancient times.

Han dynasty ceramics and iridescence

Iridescence with another cause can be found too on green glazed pottery wares of the Han dynasty (BC206~AD220). The green glaze may be covered with a silvery layer if the piece is authentic.
The cause is another, even more toxic metal - mercury (aka quicksilver).

It is almost only found on the glazes of Han dynasty pottery. Of course, they didn't put it in there intentionally, it is part of the source material used at the time for the glaze. Over time the mercury works itself through the glaze to the surface. A long time after its firing the glaze then turns to a a silvery color that may or may not show iridescence.
Nowadays this too is being faked, but fake iridescence is easily detected in this specific case. On 'genuine' green glazes of the Han dynasty  the mercury has formed flakes on the surface. These will easily fall off during handling / touching. With fake glazes the silvery surface layer is firmly glued on and no flakes will fall off.

For your health it is recommended to always wash your hands thoroughly after handling Han dynasty ceramics.

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