Minimalist cool - ceramic glazes of the Northern Song

After the bold colours of the Tang dynasty, the Song (as seen previously) preferred a more refined aesthetic. Some of China’s finest ceramics were developed in this period: collectors talk about the “Five Great Wares of the Song” - Ding, Ru, Jun, Guan and Ge, to which many would add the beautiful Longquan. The collage below shows examples of all six.

From top left clockwise: Ding, Ru, Jun, Longquan, Ge, Guan

This is a lot to cover, so this post looks only at Ding, Ru and Jun – ceramics that flourished in the earlier part of the dynasty (the Northern Song, 960-1125 AD).

Ding ware, a white ceramic from Hebei, northern China, stands out amidst the typical Song blues and greens. The thin clear glaze is so subtle that it is almost a varnish. The main ingredient is magnesium oxide (MgO), an efficient flux which gives a snug glossy finish and has low expansivity so the glaze can withstand high firing temperatures without cracking.

Ding ware is all about the body, which I was surprised to learn is classed as porcelain because it contains kaolin and is fired up to 1300˚C. I’d always thought of porcelain as a later discovery but according to Nigel Wood, some northern clays are kaolinitic and unusually pure, so early potters could make porcelain as long as the kiln was hot enough. Wood describes early northern Chinese porcelain (around 7th C AD) as “the first true porcelains in the world”.

Ding ware basin. China, N Song dynasty, 11-12th C. Percival David Collection. 

Ding potters worked the clay so thin that it couldn’t sit unsupported in the kiln, so the pots were fired upside-down on a support called a “sagger”. This gave the rim a rough finish, so it was enclosed in a strip of copper which fires into a lovely dark chocolate colour.

Xing ware vessels. Northern China, Tang dynasty. JJ Lally, New York.

Ding ware illustrates the effect of kiln atmosphere on glazes. Its creamy tone results from an “oxidising” atmosphere: where there is enough oxygen in the kiln to oxidise the glaze chemicals. Earlier white wares (like Xing ware, below) are a cool ice-white because they were fired in a “reducing” atmosphere, where the kiln is tightly enclosed and low on oxygen. Early potters used wood-fired kilns which tend to give a reducing atmosphere. Song potters more often used coal-fired kilns which made oxidising atmosphere easier to achieve.

 Ru ware cup holder with finely crackled glaze. China, Northern Song Dynasty, 12th C. British Museum.

Ru ware narcissus bulb planter with uncrackled glaze. China, Northern Song Dynasty, 12th C. National Palace Museum, Taipei.

Ru ware is known for its lustrous blue-green glaze, made for the Emperor Huizong around 1100 AD and wildly desired by collectors ever since. Unlike Ding ware, Ru ware is all about the glaze: the underlying stoneware body is kept simple. The glaze is actually a descendant of ancient Shang lime/ash glazes, using high levels of calcium oxide (derived from wood-ash or limestone) as a flux. But the Song potters added a bit of iron (Fe2O3) which gave that beautiful blue-green when fired to about 1200˚C in a reducing atmosphere. 

It’s almost disappointing to learn that the beauty of Ru comes from something as mundane as iron, albeit in the hands of genius potters. We’re lucky to know this though, as there are fewer than 100 pieces in existence, each worth a king’s ransom (the one below sold for US$37mn in 2017), so even broken shards are hard to obtain for chemical analysis!

Ru brush washer with crackled glaze, D: 13cm. China, Northern Song, 12th C. Sothebys Ltd. 

Ru ware introduces the riddle of the crackled glaze. The technical term for cracks in glaze is "crazing", a fault which usually results in the piece being discarded, but it seems both crackled and non-crackled Ru pieces were equally prized by the imperial court.

Crackle occurs on ceramics if the glaze and body cool at different speeds after firing and the glaze no longer “fits” the body. An effective flux and proper kiln control helps prevent this. Yet most existing Ru pieces are finely crackled, some in ways so delicate that collectors have named the patterns: “cicada’s wing”, “fish scale”, “crab claw”, etc. It is believed that these were defects that later became desirable, the way that Japanese art values imperfections (and wouldn’t it be typical of the Song intellectuals to take that view).

The colour of Ru is compared to “the sky after rain”, which is ironic because it is Jun ware that is truly sky-blue. The colour of the sky is a trick of light through atmosphere: similarly, Jun glaze only appears blue because the glaze bends the light in that way.

Jun dish, Percival David Collection (above left). Jun bowl, V&A Museum (bottom left). Jun vase with copper splashes, Christies Ltd (right). China, Northern Song period, 11-12th C.

The luscious shades of Jun, a type of stoneware, have a cloudy tinge known as “opalescence”. Many pieces also have bold splashes of purple for decoration, derived from copper oxide. My art history class loved Jun ware, because it was so easy to identify in exams!

Scholars love Jun too because the glaze is so complex. Under high magnification, its structure is like caviar made of tiny globules of glass. This is due to something called “liquid-liquid phase separation” (Google if you’re interested) which allows it to diffuse light by a process called Rayleigh scattering: the same thing which occurs when light hits the Earth’s atmosphere. And because blue light-waves are the easiest to scatter, Jun glaze and the sky both look blue. Apparently, when a chip of Jun glaze is held up to the light, it is straw-coloured: there is no blue at all. How is that not genius?

The opalescence was once attributed to phosphorus (which certainly plays a part) but scientists have now found that a high ratio of silica to alumina (around 7:1), plus the right quantity of calcium and potassium oxides, will cause liquid-liquid phase separation and hence the scattering effect. This only works within a narrow range of combinations, called the “zone of opalescence”, but the Jun potters seem to have nailed it.

(Left) Milky blue Jun bowl, D:17.7cm, Sotheby's Ltd. (Right) Darker blue Jun bowl, H: 8.9cm, Asian Art Museum San Francisco

The varied shades of blue do not come from specific chemicals but from different kiln temperatures. Jun is fired in reduction to about 1200˚C. If the piece is underfired (slightly below 1200˚C), the globules of glass are big enough to scatter both blue and white light, giving a milky blue colour like the bowl on the left. At higher temperatures, the globules get smaller and the colour goes to sky blue, lavender blue and even a purplish blue (the AAM bowl above). If the glaze is too thin, it reverts to greenish brown like the rim of the V&A bowl earlier (presumably because it’s not thick enough to scatter blue light).