Castle Hill, Knucklas, is a wild, remote place where England and Wales, ancient and modern, history and myth intermingle. Guinevere, daughter of the giant Gogyrfan Gawr, is said to have married King Arthur in the wooden castle which once stood on the hilltop here. The Norman knight Ralph Mortimer rebuilt the castle in stone in the 13th century. Owain Glyndwr, the charismatic rebel of part one of Shakespeare's Henry IV, besieged and overran it in 1402. Its grass-grown remains are now the haunt of ravens and red kites, rubble from its ramparts incorporated in the dizzying, 13-arched railway viaduct which spans the valley far below.

Castle Hill Farm is where potter Tony Hall and his partner Lois Hopwood live with their three-year-old twins Billy and Kate. The farm is reached by a steep, twisting track overhung with bracken. At the top is a full-size fishing boat.

"Very Noah!" laughs Lois.

The boat's position on a hilltop, 1,000ft above sea level, as if awaiting the Flood, is in keeping with the fairy-tale air of the place. For the pots Hall makes, as befits the former home of a Welsh giant, are seriously big pots. His Ali Babas are big enough for the Forty Thieves to hide in. His Giant Jar really is a giant, weighing in at 240lb.

Hall learnt the art of making big pots at the village of La Borne, near Bourges, in France, where families of potters have been making salt-glazed saloirs (very large vessels for salting meat) since the 17th century. "I'd messed around with clay before, but had never really been inspired," he says. "The fact that the people at La Borne made a living from wood, fire and clay, such simple materials, and the physicality of it ­ it¹s very hard work ­ is what appealed to me."

The process is at once incredibly simple and immensely difficult, dating back at least 5,000 years. "After 30 years, I¹m still learning," he says.

"And I"ve found no technical limitation to the size yet." He has gone on making bigger and bigger pieces, including one current project, a huge terracotta Hiroshima bell (modelled on the bronze bell installed in the Peace Park at Hiroshima, Japan), weighing 180lb. When complete, it will be installed on the hillside in his own garden. "It is not meant to be prescriptive in any way," he says, "but by its title, its position in a beautiful quiet space and (I hope) its sonorous tone, it will encourage contemplation of the nuclear issue." Hall also makes bespoke pots, each presenting a new technical challenge. His Mulberry Pot, for example, was originally designed to house a client's fully grown mulberry tree. Like all Hall¹s larger pots, it was made with a removable disk inside, instead of a fixed bottom, so that the rootball can be gently pushed out when the plant needs repotting.

The big pots are made in sections, each one consisting of up to 60lb of clay, thrown on a conventional potter¹s wheel. Hall starts with the base, which is thrown the right way up; subsequent sections are thrown upside-down. The upper ones are then inverted ­ a perilous job requiring two people ­ with the giant pot, momentarily suspended in midair, hanging from the 'bat', the circular wooden board on which it has been thrown.

The oversized pots are not cheap ­ the largest can cost up to £700 ­ but each one represents weeks of work. "And you always lose 10 per cent in production," says Hall. "Clay is a living thing ­ it moves as it dries, and it can crack in the firing."

It's part of the tenacity of potters, he says, to accept that there will be casualties.

"I never get excited until it's out of the kiln."

The clay Hall uses is a careful blend of ball clays, Staffordshire fire clays and Etruria marls, with the addition of 'grog' ­ ground-up fired clay of different sizes, used like grit in a potting compost ­ to keep the mixture 'open'. This gives the pots their characteristic strength and texture. The pots are allowed to dry for up to five weeks, and are then fired at 1160 degrees, a higher temperature than that at which most British flowerpots are fired, and much higher than many imported pots. It is the combination of this high firing with the special blend of clays which gives the pots the porosity which enables them to withstand anything a British winter can throw at them. All Hall's pots, from little herb pots to giant Ali Babas, come with a ten-year frost-proof guarantee.

Hall and Hopwood are making a garden on Castle Hill, terracing the hillside which falls steeply away in front of the house. The site is very exposed, and the growing season short ­ testing conditions for both plants and pots.

"If you are serious about your pots," says Hopwood, "come and see us." It is worth the trek.

For a catalogue and further information (including details of pottery courses), contact Tony Hall at Castle Hill Pottery, Castle Hill Farm, Knucklas, Powys LD7 1PS (01547 529670; hallaw@btinternet.com )

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