Chinese ceramic glazes - a technical detour

I was going to begin with Tang ceramic glazes but like London public transport, some detours are inevitable on this journey.

Firstly, we can’t study glazes while ignoring the underlying body of the pot, as different pairings give notably different results. Traditionally, Western collectors have divided Chinese ceramic bodies into three categories: earthenware, stoneware and porcelain.

• Earthenware is the oldest, dating back to the early Neolithic age (10,000 – 4,500 BC), possibly earlier. The one below from north-east China is dated around 2,500 BC. Like all ceramics, earthenware is made of clay, whose main chemical components are silicon dioxide (SiO2, known as silica) and aluminium oxide (AL2O3 or alumina). Earthenware is fired in a kiln at around 1000˚ Celsius, which is low by pottery-making standards. Primitive earthenware is unglazed but later versions (like Tang dynasty sancai ware) were glazed.

Earthenware ‘hu’ jar. Majiayao culture, China, 2650-2350 BC. Met Museum NY

• Stoneware is also made of clay but with slightly different levels of silica and alumina, and is fired in a much hotter kiln, around 1,200˚ Celsius. It is a strong heavy ceramic, less porous than earthenware and usually glazed, making it more useful for household pots and jars.

Stoneware storage jar. China, 4th C BC. Minneapolis Institute of Art

• Porcelain, the finest type of ceramic, is made with clay that contains or has been mixed with the mineral kaolin. Oddly, kaolin itself is mostly silicon and aluminium too but it strengthens the clay by a miraculous process called hydrogen bonding, allowing pots to be made thin and light yet strong. Porcelain has a glassy (“vitreous”) translucent quality. It may be fired at very high temperatures, up to 1,400˚C. Chinese porcelain is nearly always glazed, though modern potters sometimes make unglazed pieces to show off the fine underlying quality of their work.

Inevitably, the boundaries between these categories may be blurred, with academics tying themselves in knots to distinguish one from the other. Chinese scholars simply divide ceramics into low-fired or high-fired, and then categorise by style and dynasty (which I suspect is becoming the preferred approach).

Glazed ceramics were invented well before the Tang period: in fact, glazed stoneware was known in the Shang dynasty (1600 - 1050 BC). The yellow-green or grey Shang glazes were made from burnt wood ash. Kerr and Wood believe that once technology allowed kilns to get above 1150˚C, calcium and potassium oxides in the ash reacted with silica and alumina to form an “accidental” glaze when bits of ash fell on to the pots. Eventually, potters collected the ash and mixed it with water to make glaze. But we’ll look at ash and lime glazes in later types of ceramic, so I think we can conveniently skip the Shang. 

We can’t skip the subject of glaze chemistry however, and here again, silicon and aluminium are key players. A glaze is a kind of melted glass overcoat for the pot, so silicon - the main ingredient of glass - is vital. Aluminium oxide makes the glaze viscous and gluey so that it clings to the pot. In addition, a good glaze needs a “flux”. Fluxes are chemicals that encourage the glaze to melt at lower temperatures and become more flexible (“expansive”) so that it fits the body nicely and doesn’t crack. Fluxes are usually something alkaline like potash or sodium oxide, but historically one of the commonest was lead oxide, which we now know is poisonous. As seen in the previous post, lead is still used in some rural areas because it is cheap and extremely efficient.

The fourth key element of glaze is colour, which comes from heating various chemicals in different ways. This too may have been an accidental discovery by potters whose local materials had different chemical compositions. The result is further governed by temperature and levels of oxygen inside the kiln (“kiln atmosphere”). This is where the potter’s art really gets going. Common types of chemical are copper (for red or green), cobalt (blue) and iron (red, brown, yellow, green and sometimes even blue). Titanium dioxide (TiO2) interacts with iron to give a yellowish colour (with high TiO2) or a cool green colour (low TiO2). This will come into focus when we discuss green ceramics or celadons.

Ewer with copper red underglaze. China, 14th C. Early Ming dynasty. Eskenazi Ltd.

In practice, of course, early potters didn’t go around chemical-testing their ingredients. Glazes were made by mixing water or liquid clay with crushed ore or soil from a particular place, according to a recipe handed down by predecessors. An experimental potter might try something different and if successful, would invent a new type of glaze.

But this has provided quite enough to chew on for now, so again like London public transport, this post (intended for the Tang dynasty) terminates here.