I HAVE noticed in the last few years, that ruined castles and abbeys have started to put on shows, as if they no longer trust their inherent beauty or historical interest to attract visitors.

Last summer, my friend and I were in fact visiting Rievaulx Abbey for its own sake. Only on arrival did we discover that something extra was going on. Between the car park and the actual abbey, an exhibition was laid out, which turned out to be connected with the iron industry centred on the abbey and recently studied by Bradford University.

Rievaulx was not just a centre of religion. It had been a hive of industry. Not only had it quarried local limestone to build the abbey, established big sheep farms and an international wool trade, it had also smelted iron, to make the tools for these activities, smelted it from locally-mined ore, with charcoal from its own woodlands.

Iron-working sites had been studied to the north of the abbey, in the Bilsdale valley, some of them much older than the monastery, primitive 'bloomeries' going back as far as prehistoric times.

Recently, the university, in partnership with English Heritage, now in charge of Rievaulx, was smelting iron, as nearly as possible by the method that would be used in monastic days.

The centrepiece of the show was the furnace, made of clay, around a framework of willow: cylindrical, open at the top, with very thick walls, about five feet tall and 30 inches across. There was a large hole near the base, where any slag that formed could be tapped out. Two small holes higher up allowed bellows to be inserted.

The furnace had been built two weeks before, to allow the clay to dry thoroughly before being subjected to heat, and had been burning some hours before we saw it. We could smell its heat. Two volunteers were working with a will at the bellows to keep it going. Small puffs of smoke escaped, but not as much as I would have expected. Charcoal, the fuel used by the abbey, and used here also, is a good fuel, giving little smoke. It is virtually pure carbon.

What they were using here was supplied by 'Yorkshire Charcoal' made from locally-grown oak and beech. The iron ore was Australian, but very similar to a type found in Bilsdale, we were told by a student standing by to feed the furnace and answer questions from the public. She had a cardboard box of samples of both to show us.

We stood by her for a time, then moved to another part of the field, where they had built an 'Earth Mound Charcoal Kiln', the type used by the abbey.

It didn't look at all like a kiln, more like a giant mole-hill. It was gently oozing smoke, and smelt like a charcoal brazier. Essentially, it was a pile of wood, built wigwam shape, layer upon layer, round a central pole, called a 'motty peg': covered over with a tamped-down leaf-mould, splashed with water. Over all came a thin layer of dust, made from burnt soil and powdered charcoal.

Then the motty peg was pulled out, leaving a central hole, into which a shovel of hot embers was tipped, to set the pile alight. The elaborate covering was to keep out the air. The wood must not burn, but be heated to a high temperature without oxygen.

Rings of small holes over the surface of the mound allowed smoke to come out. Its colour was carefully watched. White at first, because of water vapour driven off from the wood, it presently turned brownish, then to a blue haze, the sign that at that level the wood was turned to charcoal. That ring of holes was then blocked up, and more made lower down. It could take anything up to five days for ground level to be reached, during which the mound could not be left unattended. Charcoal burners used to live in huts near their mound.

When the whole interior was converted, the mound was 'quenched' by pouring water over it, and the cooled-off charcoal raked out.

A few hours were obviously not long enough to see all of this process. But it was fascinating to see - and smell - a charcoal kiln, working.

In another part of the field, they had on display, and also to try out if you wished, some of the apparatus their archaeologists had used to help them decide where to dig.

These included an 'Earth Resistance Meter' - rather like a zimmer walking frame, but attached to electrical apparatus. Walking along with it, an archaeologist could detect and record walls or buried ditches, or any solid remains under an apparently empty field. We had seen them in use on TV archaeological programmes, like Meet the Ancestors, but had been somewhat mystified.

Explanations and demonstrations here did not much enlighten us. Some visitors were daring to try it for themselves, but whether there was anything below this particular bit of turf we never discovered.

We did have a go on a machine that, with the help of one of the many satellites that now circulate above our heads, would tell you exactly where you are on the earth. Of course we already knew where we were today. But we could see it would be invaluable to explorers lost in a jungle.

There was so much to see and do and take in, that the day had passed more quickly than we knew. It was time to go home when we realised we had not yet been near the ruins themselves. But we had enjoyed looking at them, the old stone all sunlit, as we ate a leisurely lunch on one of the picnic benches Rievaulx provides. And after all, the abbey would still be there another day, whereas we should probably never again have a chance to see mediaeval iron smelting.

Updated: 13:35 Monday, December 29, 2003