The best things I learned about ceramic glazes

The study of ceramic glazes was a challenging summer project, to say the least, but I came away even more impressed by the ingenuity of ancient Chinese potters. The project made me re-assess my views on some of their wares, one category in particular. And I only covered a few Tang and Song ceramics: the others like Yue, Yaozhou, Qingbai and all the post-13th century wares will have to wait, as my new academic term begins in a few days.

I wrote four detailed posts (not counting this one) and I'm sorry if they don’t make light reading: as I said in July, this was something I needed to get my head around. To make up for that, here is a "Best Of" list to give you an idea of the most interesting features. There are embedded hyperlinks in case you’re unexpectedly overcome by a thirst for knowledge!

1. Si, Al, Colour and Flux
Not the members of a Cantonese boy-band but the four things you need in a glaze. Silicon (chemical symbol Si) makes it glassy, aluminium (Al) makes it clingy, and various minerals like iron, copper or cobalt give it colour. But the key component is the flux (such as lead, potassium or soda ash): it is the go-between agent that encourages the other elements to melt easily and coat the pot smoothly without cracking. This is why glazes are often referred to by their fluxes: lead glaze, lime-ash glaze, lime-alkali glaze, etc. More about glaze chemistry here (about halfway down).

2. Iron makes the most amazing colours

From left: Tang sancai horse and rider, Ru dish (upper), Longquan dish (lower), Longquan vase with iron-spot decoration.

We think of iron as a heavy black metal, but all the above objects owe their colours to varying levels of iron in the glaze. The amber-brown shade of sancai ware, the blue-greens of Ru, Guan and Longquan, and of course the iron-spots of Longquan tobi seiji, are all derived from iron. The final colour depends on various things: the percentage content (as little as 2% iron can be enough), the level of titanium dioxide in the glaze (too much makes the iron go yellow), and the temperature and atmosphere of the kiln. Next time you admire a dreamy green celadon, remember that its colour comes from the same mineral later used to make railway engines.

3. Ru is rare, Guan is brilliant…

Ru ceramics are famously rare (fewer than 100 pieces remain), beautiful and staggeringly expensive. But it turns out that the glaze, while lovely beyond doubt, is not unusually complex from a technical POV. It’s a reworking of ancient Shang ash glazes but with a bit of iron and a lot of obsessively careful firing. You could argue that this simplicity is part of its charm but I suspect that the enduring popularity of Ru is more about its rarity and historic links to the tragically deposed Northern Song. More details in the middle section of this post.

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

National Palace Museum, Taipei.

Guan is partly inspired by Ru ware but it’s a very different creature. The abstract patterns of its crackled glaze are bold and modern, yet they also reflect the dynasty's sad past. More importantly, the potters’ idea of using low-silica materials to make this crackle (when the received wisdom was that silica prevented cracking) was a brilliant bit of experimentation, especially when working with unfamiliar southern clays. More info in this post.

Guan dish, D: 22cm. Southern Song, 12-13th C. Met Museum NY

4. But Jun is astounding

The glaze that most impressed me was Jun, a blue ceramic from the Northern Song period. Jun ware has a misty shimmering quality known as “opalescence”, sometimes enhanced with purple splashes of copper pigment. It is not especially rare, and I always thought it was a bit florid by Song standards.

Jun ware dish, Northern Song period, 11-12th C. British Museum (Percival David Collection)

But I hadn’t realised that Jun glaze is not really blue at all. Its colour comes primarily from its molecular structure which scatters blue light in the same way as the Earth’s atmosphere makes the sky look blue. The glaze on its own is straw-coloured but if applied thickly, it can make the underlying object look anything from milky blue (if under-fired) to purplish blue (high-fired). It was not until 1983 that two scientists worked out the chemistry behind this, introducing the phrase “liquid-liquid phase separation” to the world of art history. For more info, see the last section of this post.

While researching these posts, my most frequent question was “how did they work that out?”. How did potters know that adding crumbly white stone to the glaze would make it come out green? I tried asking a couple of modern ceramic artists about their glazes and wow, they clammed up fast, as if I’d asked for their PIN numbers or email passwords. Because of course it took years of work and repeated failures before the glaze finally came right: they weren’t going to just tell me all. Likewise with the potters of the imperial kilns (more to the point, the emperor would have been furious if they revealed the secrets of imperial wares). But I hope at least they would have been entertained by my efforts to study their work. 

Main sources used: Chinese Glazes (1999), Nigel Wood; Science & Civilisation in China, V5 Pt 12 (2004 edition), Rose Kerr and Nigel Wood; Song Dynasty Ceramics (2004), Rose Kerr.

Crackle and bubbles - ceramic glazes of the South

The future looked bleak for the Song dynasty after its defeat by Jurchen invaders in 1125-26. The court fled south to Hangzhou, where the Emperor Gaozong (a master of spin) promoted the use of art and literature to win allies amongst the powerful literati class. Out of this arose the two ceramics most commonly associated with the Southern Song: Guan ware and Ge ware.

Guan brush washer, D: 12cm. Southern Song, 12-13th C. Christies Ltd.

Guan stoneware bears some resemblance to Ru ware, which was closely associated with Gaozong’s father Huizong.  In a way therefore, it acted as a reminder of the legitimacy of Gaozong’s rule. But at a deeper level, Guan (which means "official") is a very different creature, not least because the Hangzhou potters had to learn to work with local materials. For the glaze recipe, they followed more or less the Ru approach, using lime (calcium oxide) as a flux and iron for a bluish tone. But they experimented widely with production techniques, so Guan ware may vary between pale blue, green or tan, and the glaze may look opaque or glassy, depending on kiln temperatures (in the 1200-1250°C range) and atmospheres. The underlying clay bodies are dark and iron-rich, often showing through at the rim and base, a feature known as “purple mouth, iron foot”.

Guan eight-petal lobed dish, D: 17cm. Southern Song, 12-13th C. British Museum

While Ru ware is very subtly crackled, the Guan potters had no such inhibitions. A strikingly crackled glaze is the key feature of Guan, which often looks worn and ancient. The method used to produce this was revolutionary. The southern potters experimented with levels of silica (SiO2) which normally prevents crazing. They discovered that if both body and glaze were low in silica (around 65% instead of 70%), a conspicuous crackle could be achieved under the right firing conditions. Kerr and Wood, not usually given to hyperbole, describe this as “one of the greatest artistic achievements in Chinese ceramic history”.

Foliate dish, described as Ge ware. D: 14cm. Southern Song, 12-13th C. Christies Ltd.

Ge ware (ge as in 哥 or “older brother”) is a famously tricky subject. Many experts believe that what is commonly described as Ge ware is just a paler off-shoot of Guan ware. Ge is not mentioned in any known Song dynasty texts (it's first mentioned in Yuan texts) and no evidence has been found of a separate kiln. The key attributes of Ge (lighter-coloured bodies, more layers of glaze, deeper crackle) can all be explained by variances in production technique – and as we know, the Guan potters were an adventurous lot. The Palace Museum, Beijing, staged an exhibition dedicated to Ge ware in 2017 but even they were careful not to date everything to the Song period. Major auction houses sometimes offer pieces as “Ge ware” (including the ones pictured here) with appropriate caveats. But tellingly, neither of these lovely pieces found buyers at auction, so collectors are obviously cautious too.

Brush washer, described as Ge ware. H: 7.3cm, Southern Song, 12-13th C. Sotheby’s Ltd.

This poses an existential problem for glaze analysis: how do you select a piece for testing if you cannot be sure that it is Ge, because does Ge exist as a separate category in the first place? We know that these pieces also exhibit low silica levels in body and glaze, like Guan. It is worth noting however that the prettiest examples of Ge ware (like the ones shown here) have a distinctive double-pattern of crackle: an inter-woven network of dark and light lines which have been stained for effect, poetically named “gold thread, iron wire”. These intricate pieces argue for Ge’s place among the Five Great Wares. Except of course that they may be Guan. 

Conversely, there is plenty of evidence that Longquan existed in the Song period but it is not listed among the Great Wares, probably for two reasons. First, it was not made in an “imperial” kiln but was widely produced in Zhejiang (the coastal province south of Shanghai) for domestic and export markets. And although well-known in the Song period, it was in the early Yuan dynasty that its reputation was at its height and production boomed.

(Left) Longquan mallet vase, H: 17cm. V&A Museum. (Right) Pair of Longquan funerary urns, H: 25cm. British Museum. Southern Song, 12-13th C.

Longquan (a type of stoneware) is one of the most beautiful and influential of the Chinese ceramics. It is the quintessential celadon, with elegant colouring ranging from duck-egg blue to sea green. It was exported as far as Istanbul, where the Topkapi Palace still has a fine collection; while in Japan every tea-master wanted a Longquan vase to accent his exquisitely minimalist tea-house. Celadons are still made in South-east Asia today, in Chiangmai for example. You can find classic designs like the “twin fish” motif - compare my cheap and cheerful modern replica with a 900-year-old version, below.

(Left) Longquan dish, D: 14cm. Southern Song, 12-13th C. British Museum. (Right) Longquan-type dish, D: 14cm. Probably Thailand, 20-21st C. 

Like most celadons, Longquan gets its colour from iron fired in a reducing atmosphere but analysis also shows a high level of potassium oxide (K2O) as a flux, hence it is referred to as a "lime-alkali" glaze. This thick unctuous glaze was applied in several thin layers between firings, and if the potters got it right, microscopic bubbles would form and become trapped in the lower layers. As a result, the best Longquan glazes have an unusual depth and complexity. Kerr and Wood point out how difficult it must have been to fire each glaze-layer consecutively without them all melting into one. 

Longquan vase with iron-spot decoration, H: 27cm. Yuan dynasty, 14th C. British Museum.

The Longquan potters became more daring with their designs in the Yuan dynasty. For the piece shown above, specks of raw iron were placed on the surface before firing, which resulted in dark brown decorative spots (also seen in wares like Qingbai but the Longquan kilns did it best). These were so popular in Japan that they gave it a name – tobi seiji (“flying spot green-ware”). The trumpet-necked vase shown here from the BM’s Percival David Collection was bought in Japan, where its almost-identical counterpart still resides at the Bridgestone Museum, Tokyo.  

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).