Sebastian Blackie explores elusive answers to a simple question: who first used shells when firing their pots?

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When clay is heated to stoneware temperatures, in a kiln fired with wood, ash will combine with the clay to form a glaze. Fly ash contains the glass forming mineral silica, as well as ‘fluxes’, such as potassium, sodium, magnesium and calcium – oxides that drastically lower the melting point of a glaze to temperatures achievable in a potter’s kiln. When this layer of ash is thin it will react with the clay, at first wetting and darkening the surface, then producing a very fluid, glassy glaze that runs, sometimes right off a pot. As the ash layer builds there is less interaction with the clay body and the surface becomes crystalline and opaque. Sometimes this soft crust, like cooling lava, will move and fragment as it slips on the more fluid layer beneath.

Innumerable factors can affect the fire marks on a pot. As the ash travels directionally with the flame, from firebox to chimney, the build-up on the wares inside is uneven, so that several of these effects may be found on the same pot. Similarly unglazed areas can also be created by neighbouring pots ‘shadowing’ a piece from the ash.

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In the West, this variety of texture and colours is known simply as ‘flashing’. But in Japan, with its long history of sophisticated ceramic connoisseurship, there are several words to describe the markings that arise due to ash, flame and embers. Michio Furutani in his book on Anagama kilns lists eight: Koge, for example, where pots become buried in embers, darkening the clay body and turning its glaze lustrous. Even after years of experience with this type of firing, results can be unpredictable. As such, great care is needed to prevent pots from being glued to kiln shelves, or indeed one another, with errant glaze.

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Necessity being the mother of creativity, potters have long recognised that the marks left by ‘wads’, used to protect their work from sticking, have great decorative potential, and the use of seashells instead of refractory clay is a classic example. Many artists use clam and scallop shells to dramatic effect – among them Svend Bayer, Nic Collins, Anne Mette Hjortshøj, Ken Matsuzaki, and Randy Johnston. My research suggests that the practice was introduced to the USA and New Zealand by potters such as Johnston and Chester Nealie who studied in Japan in the 1970s, and that it has only become more widely used in the West in the last thirty years. But where, and when, did it start?

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The Victorian philosopher George Henry Lewes sternly warns us that, ‘We must never assume that which is incapable of proof.’ I admit that when Max Waterhouse asked me to write a piece on the use of seashells in wood-firing, I had assumed it was an old Japanese tradition, possibly associated with the tea ceremony. Many in the wood firing community believed, like me, that the use of shells was ancient – but I could find no one who had an example earlier than the 20th Century. In fact, those of us who had visited Japan could not remember encountering any historic example of shell use, even at coastal potteries like Tokoname, where one might most expect to see them.1 Nor could anthropologist Robin Wilson, who directs the Oxford University Anagama kiln project, recall finding any pot sherds with shells when helping to excavate the ruined Bizen kiln sites around the coastal Imbe valley.

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In Europe, where potters’ clays were mostly lower firing earthenware, they rarely encountered this problem of fusing, but in South China, Korea and Japan, kilns were fired to higher temperatures to mature their more refractory clays. Their fly ash glazes produced some aesthetically stunning results, but also the need to find methods to separate the pots after the firing. There are many examples from the medieval period of whole pots from Japan’s six ancient kilns showing the fragments of other pots attached to their surface. Yet within a few hundred years, innovative methods such as kiln stilts, wads and spurs in Mino and Seto, or bundles and wraps of high-silica rice straw in Bizen were in use.

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Finally, American scholarship came to the rescue in the form of world authority Louise Cort of the Smithsonian who confirmed that trafficked Korean potters introduced the use of shells to the kilns on Kyushu Island after the Imjin war, around 1600. Andrew Maske of the University of Kentucky wrote to tell me that shell use (both sea and fresh water) on glazed ware was only common in this period at the very beginning of the 17th century – sporadic, but not forgotten thereafter. Nor, say the experts, was the practice confined to Kyushu: Maske noted that shells were also used in nearby Hagi, while Cort added that they were employed by Chinese potters from Guangdong who had immigrated to the Vietnam province of Phu Yen. Many things ceramic in Japan originated in China via Korea, but in this strangest of cases no one has identified the use of shells in China. With one mystery solved, several others seem to emerge…

Why then were shells not adopted more widely? Australian wood fire potter Owen Rye has pointed out that though ancient potters were keen observers of their craft, none could have understood or intuited the science of shell use. The shells themselves are formed from calcium carbonate which, when heated, converts to calcium oxide or ‘quick lime’, a powerful flux. In isolation, calcium is one of the most refractory oxides, only melting at 2572˚C+. But combine that calcium with alumina and silica and you have what is known as a ‘eutectic’ – a combination of oxides that melt at a lower temperature than they would on their own (in this case, a much more manageable 1170˚C).

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All this means that on the interface between shell and pot there is a melt that leaves a shell-like scar on the surface of the clay. As the shells are stuffed with clay to prevent them from collapsing, the same happens on the inside. The core of the shell remains pure calcium, which is dry, friable, expands when wet and can be easily removed after the firing. An aesthetic bonus of using seashells is that they contain small quantities of salt. During the firing, this salt can volatilise, leaving a subtle halo of salmon pink or orange on lighter coloured clay bodies.

It remains unclear whether all use of shells in the 20th century was a conscious act or, in some locations, an inherited tradition. Perhaps even with chopsticks, which are more tolerant of rough surfaces than knives and forks, shell scars on domestic wares would prove too much for many people. Randy Johnston recalls his teacher, Tatsuzo Shimaoka, spending all day rubbing down the shell marks on one pot with a fistful of rice straw.

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Shimaoka’s own teacher, Shoji Hamada, seems to have introduced seashells to inland Mashiko when he returned from Britain to make salt glaze in the 1950s. As an accomplished chemist, he would have understood how this oriental technique could be combined with occidental salt firing. And so the use of shells outside Kyushu and Hagi became part of the wider breaking of boundaries, and a reappraisal of Japanese culture following their defeat and occupation.

Japan claims to be the first culture in the world to have made pottery, with the earliest Jomon pots are dated 14,500 BC. Nicole Coolidge Rousmaniere, formally of the British Museum and now Research Director of the Sainsbury Institute for the Study of Japanese Arts and Culture (UEA), tells me shells were in use then both as wads and to temper the clay.

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The beginnings of pottery are closely linked to the settled nature of agriculture. Because the Jomon lived from coastal fishing, the making and use of heavy, fragile pots was possible when it would have been a burden to other types of hunter gathering. Perhaps they included their food as an offering to the kiln gods, in a gesture similar to sprinkling the kiln with salt and sake as some potters still do today? Salt fumes can also produce very attractive markings on clay fired at low temperatures: mixing salty shells in a pack of pots, deliberately or by accident, would have left a pleasing decorative effect. One of the features of Jomon pottery, compared to other international cultures at similar stages of development, is their passion for decoration and something of an obsession with shells and creatures of the sea.

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What should we make of this technique in the hands of contemporary potters, some of them living far from the ocean shore? The shell is a powerful signifier of the natural world. It protects life, and is evidence of one its earliest forms. As a fossil, it is part of the pallet of minerals, produced over ungraspable eons of time, with which present day potters form their work. The mark of a shell on the foot of a yunomi is like a trace, a signature that the pot is as much a product of nature as it is of man. Seashells can produce beautiful, sensual markings on the surface of pots, but their imprint can mean much more than ‘eye candy’. For me, when I see a Svend Bayer jar with tell-tale marks on its side, I reflect on the drama of his firings. A pot’s future precariously balanced on three fragile shells; a metaphor for our present-day relationship with nature. They are the scars of a survivor.

Sebastian Blackie is a ceramic artist, author of the acclaimed Dear Mr Leach (A&C Black), and professor of Ceramics at the University of Derby, where he runs a Masters in Art and Design.

1 Furutani Michio briefly suggests the use of red clam shells was common in coastal areas like Tokoname, but he does not offer a date or on what type of wares.