27 Kilns in 27 Days – vol 5.

The tea plantation potter.

We visit a potter in the Longquan region who specialises in the dark bodied celadon wares of antiquity, but not guan ware. However, he has been and still is a great fossicker and has the most amazing collection of authentic old Chinese shards that I have ever been lucky enough to sit and handle. It’s like being invited into the back room of a museum by the curator and allowed to see all the little gems that never make their way out in the glass cases.

A lot of these shards look to me to have been excavated from out of river beds or streams. Many of them appear to have been water ground on the edges. I see this exact quality on my pots after I have tumbled them in the ball mill to ‘age’ them.

It’s the white impregnated crackle of the crazing lines that give it away, and the soft matted surface, also the rounded edges on some pieces. It all fits. I saw pieces like this in the museum collection in Malaysia. They have a whole section of pots of Chinese origin that were recovered from ship wrecks off their coast, by archaeologists, in recent decades.

Pots that were packed inside other larger pots were left with their glazes largely intact, but pots that were spread out from the cargo hold and scattered around on the sea bed in the sand, were completely relieved of most of their glaze surface except in the crevices around the foot ring. I saw one pot that was only half exposed because it was retained inside something else. The exposed surface was cleared of its glaze, just as if it had been sand blasted.

This private Longquan collection also had a lot of shards that had split, shattered, and for want of a better word ‘delaminated’. That is, the pot had been split down the centre of the clay body, more or less evenly between both layers of glaze, when the thick coating of glaze on each side of the pot exerted massive pressure on the glassy body from each side. The glassy nature of this dense, vitreous, proto-porcelanous clay body was forced to seperate, or shatter, down the centre into two parts. I suffer this exact problem in my work occasionally.

It’s somehow reassuring that the Song Potters of ancient China experienced the same issues that I am grappling with. They managed to solve all these problems in the thousand years that they have been potting. I have just 50 years. I need to work harder.

The problem shows itself, if or when, I apply the rock glaze celadon glaze too thickly over the body. The problem isn’t apparent immediately, but only shows itself after firing, as the difference in the coefficients of expansion and contraction is expressed differently in body and glaze on cooling. At around 230 oC the exotic ‘cristobalite’ form of silica, that is often created in the clay body at high temperatures, undergoes a beta-alpha phase change and shrinks 2 or 3%. This sudden shrinkage at such a low temperature exerts such tremendous pressure on the body-glaze interface, that something has to give and sometimes the pot shatters into pieces. However, in this specific case, the glaze shrinks more than the body and the glaze tears the brittle body asunder.

There is a dramatic variation of this problem that occurs when the glaze is applied to the raw clay pot when it is bone dry. If the pot is glazed on both side simultaneously, the water based glaze entering the pores of the clay is drawn in by capillary attraction. This forces air in towards the centre of the body of the pot. There is such a build up of pressure that the pot can explode with a quite loud ‘POP’! A piece sometimes flies off, or at other times a coin shaped disc flings open, left hanging, with a slight hinge on one side, allowing the air pressure to escape. It really shocked me the first time this happened to me, when I was a student at art school, as I was still holding the pot in my hand, having just lifted it out of the glaze bucket. The solution is to glaze only one side of the pot, let it dry and then glaze the other side. Thus allowing the air to escape harmlessly each time, out from the other side of the surface.

Guan ware on the other hand, has more or less the same glaze, applied even thicker on occasions, but the shattering problem is avoided in this traditional ware because the body is porous and friable. The porosity of the clay allows the cracks that form due to the glaze cooling stresses to dissipate into the porous cavities and terminate quickly without passing straight through the body, as they do in vitrified and glassy porcelaneous clays.

A shard of guan-like crackle glaze.

There is such a lot of chemistry and physics to be learnt in making ceramics! I was at the Royal Society meeting last month and heard the speaker tell the audience that in science, there is only ‘physics’ and everything else is ‘stamp collecting’ or applied physics. A mathematician claimed that there is only ‘Pure’ mathematics and physics and chemistry is just applied maths. It seemed to me that there was some agreement that biology was stamp collecting — except from the biologists. Perhaps psychologists could claim that without ‘mind’, there can be no maths?

Anyway, what would I know, I’m just a stamp collector! A stamp collector who applies chemistry, maths and physics in a creative way to ceramics. I also apply stamps to my pots to identify them as mine.

The potter who had collected all the shards, had a big new concrete double story workshop. He was the leading student of the best student of a very important local potter in this region. Sort of 3rd generation celadon maker. I got the impression that the property that he workshop was built on was his parents land. Whoever’s land it was, it has a beautiful tea plantation covering it. The new workshop has a full wall of glass looking out onto the rows of tea.

A view to die for. So beautiful! I wouldn’t mind a view like that when I’m throwing pots.