BASICS OF AUTHENTICATION:
Glaze is what gives Ceramics their glossy appearance and smooth texture. The effect is created when a muddy-looking liquid mixture containing silica is applied onto the surface of the ceramic and then fired in an oven. At high temperature, the silica melts to form a glass cover over the ceramic.
In very early times, Chinese Earthenware Ceramics was glazed to make it water impermeable. Stoneware and Porcelain, heated at a higher temperature than Earthenware, is already water impervious but glaze was still used. In many eras, glazing was applied to give colour and strength to Ceramics (as in Celadon or Whiteware). In other times, it was used to protect intricate decoration underneath (as in Blue and White and multi-colour Qing Porcelain).
Compared to most art mediums, Ceramics is remarkably durable and glazing is a primary reason behind this. Ceramic artifacts can emerge in mint condition despite being buried for centuries underwater or underground. Given that the art tradition in China is probably 1,000 years older than that of the west, we would have lost much of our understanding of ancient Asian civilizations had they chosen less durable materials for their art.
New Ceramics such as those you see at department stores appear to gleam and are highly reflective. Ancient pieces should naturally look less lustrous and even a bit dull by comparison.
Ceramics excavated from shipwrecks can be recovered in mint condition but a lot depends on their position in the wreck. If pieces are buried deep in a bed of soft mud, they can be protected. If they were submerged in sand and exposed to shifting currents, they can lose a lot of their glaze and will have a coarse surface. In some cases, even the underglaze decoration will be partially removed.
Many ancient Whiteware pieces of the Sung dynasty period have survived because they were buried underground. These show a different type of deterioration (see Qingbai vase at bottom).
See more examples of glaze deterioration below from pieces of the Chalre Collection.
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of Asian Ceramic Art