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.