When Len Castle was 10 a cousin took him to Wiri Mountain in Manukau when it was still largely intact. "I was taken down and into - it was a tight squeeze - a lava tube, a wonderful experience that stayed with me and I think it may have triggered off my interest in volcanic phenomena," says the 84-year-old potter.

The extreme combinations of heat and earth are in evidence at Lopdell House, where the travelling show Mountain to the Sea brings together stone and earthenware works from the 1980s to 2008 along with Castle's photos of geothermal activity and poems commissioned for the show by curator Tanya Wilkinson of the Hawkes Bay Museum and Art Gallery.

Castle discovered clay growing up at Westmere. He remembers sitting on the sand under the pohutukawa trees "modelling things with my hands while looking out at the black reef."

That memory came back when the late James Mack invited him to be among the select group of craftspeople showing work at the New Zealand Pavilion at the Seville Expo in 1991.

"He gave each of us a motivational statement. Mine was, 'The magma cools on its way to the ocean,"' Castle says.

What's striking about many of the works in the Titirangi gallery is their sculptural nature. Many New Zealand potters have made the transition from pottery to sculpture, but for Castle it just meant getting better.

"Back in the 1960s when I started potting professionally, I needed to make tablewares and I quite enjoyed that, but if I was making 12 casseroles, they would all be different. I was more interested in getting variations of form.

"As soon as I made my quota I started modelling objects that hinted at the organic world, and I found a few people interested whenever I had an opening of the kiln, and it built up from there."

Even before then, Castle had been working out not just how to make pots but how to be a potter. He remembers seeing his first Shoji Hamada pot in the early 1950s in the Upper Hutt home of fellow schoolteacher Ray Chapman-Taylor, who had sought out the master ceramicist while stationed with J Force in Japan after World War II.

"When I first saw my Hamada piece I kind of shuddered but I was drawn to it. I thought about it that night. What was it that has drawn me to this rugose [wrinkled] piece? It took me a while to sort it out. Here is a man who had probably thought about things carefully, but then he had acted, and that intuition played an important role in what he was doing.

"At that stage I didn't know very much about materials but I could sense he obviously understood his materials, and he allowed the kiln to have its say, and so on. This struck me as something valid, and so a truth to material approach became a key thing in my work.

"I was also in the process of learning about form, and this has been with me throughout my potting career; learning the language of form, and it's only now I think I am getting a handle on it."

On a walk around the show with Castle, he reveals some techniques built over a lifetime of throwing and firing, brushing on slips and glazes, knowing what will crack and what will spread, learning the chemistry to generate gases which bubble up through the glass in the heat of the kiln.

"I tend to get expressiveness out of materials and processes, and I regard myself as the orchestrator of these things, but it has taken years of observations, watching the materials, seeing what they will do, before I can add them in my increasing palette of effects."

He fossicks in his bag and brings out a small, conical fossil. It's a piece of coral, formed 400 million years ago in shallow seas off the coast of Pangaea and found on a subarctic beach on the Swedish island of Gotland.

Castle says it set him off making a series of what he calls marine works and fossils, chopping through blocks of clay with wire and manipulating it.

"What appears to be a hard outline can be softened by stretching it, getting it to slump a little, introducing this softness or undulating character. This is what I have always enjoyed in throwing, trying to make the right combination of a straight line that has a slight element of softness or

curvature in it, or a curved line that suggests there is the firmness of a straight line in it."

He picks up a blue bowl that looks "just belted out" into the shape of a rock pool.

"There was one thing I did spend time on. After it was made I applied a white clay slip. In the centre, where it was thicker, it tended to shrink and crack, so I flicked some of it away knowing that a clear glaze will tend to run away from any edges, so this area would be enhanced."

That brings him to the last piece made for the show, one of a long-running series of crucibles.

"I have made pieces like that in the past where you start off with a ball of clay and use a plaster of Paris pestle and just beat the clay. You need a cradle mould to support the energy you are putting in to getting the clay to spread to escape from the pressures you are applying.

"In this case it started off and didn't look too good, so I rolled the clay together again into a ball and put it back into the mould. It was coated with a lot of dry clay. When you coat a

piece of plastic clay with dry powdered clay and work on it and increase the surface area, cracking must occur at the surface.

"As I belted it, I approached it in much the same way as that rock pool piece. I worked it and I saw things happening. I thought, 'These are good, don't go any further. The form's not crisp but it has another quality that is really good. Save it'. Then in the firing the bottom blew off, but I thought, 'It doesn't matter, it goes with it'."

What: Mountain to the Sea, by Len Castle

Where and when: Lopdell House, Titirangi, to April 13