OUTSIDE EDGE / Robert Hanks on the French lasermen

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It's one of those problems every housewife dreads: how do you get rid of those difficult, hard-to- budge calcium sulphate stains without damaging your medieval stonework? New lasers, on the other hand, leave even delicate polychromes looking fresh as the day they were painted.

Last month Didier Groux and members of his team of conservators hopped over from northern France, where they are in the final stages of a 10-month cleaning programme at Amiens Cathedral, to do something similar. They spent a week touring English cathedrals to show off their lasers to assorted architects, archaeologists, art historians, conservators and administrators, proving that they can get limestone whiter than white.

Most medieval limestone buildings have a sooty crust of calcium sulphate, created when sulphur dioxide in the air reacts with the calcium carbonate of limestone. Usually it is blasted off using jets of water, or a stream of air containing tiny particles of a hard material, such as aluminium oxide. These work fine, but create mess - slurry or gritty dust - and aren't particularly delicate. Calcium sulphate is harder than the limestone it covers, so that blasting it off usually means losing some of the surface beneath - particularly annoying if there is medieval paintwork lurking under the soot.

The idea of using lasers came out of California 20 years ago; but it's only in the last few years that the French, in a government-sponsored effort, have achieved a practical technology. The theory is simple: the black layer of calcium sulphate absorbs the laser's intense heat - equivalent to a thousand million suns, give or take - while the stone underneath reflects it.

Some of the soot is instantaneously vaporised. The laser pulses 50 times a second for maximum efficiency, rather than generating a continuous beam; and the rapid heating and cooling the surface of the stone, causing abrupt expansion and contraction, sets up a tiny localised shockwave which shakes more soot off.

At the west front of Salisbury Cathedral, on a very chilly winter day, Didier Groux is demonstrating how it all happens. Out of a box about three feet long sprouts a long, ungainly arm, with several lumpy joints - these contain the mirrors that direct the laser (a rival company has tried optical fibre, apparently less successfully). On the far end is the gun.

When everything is in place, everybody puts on dark glasses, M Groux points the gun at the stone from about a foot's distance, and fires. With a flurry of clicks, a thin beam inches over the soot, and a paler colour gradually emerges from the black. To me, it looks painfully slow, but the assembled conservators are enthusiastic; most want to have a go.

According to Nicholas Durnan, cathedral architect at Salisbury, this is all very impressive: for detailed work, he reckons, it's at least as fast as air abrasives, far more gentle, and it doesn't leave any grit behind. His reservations concern the cost - around pounds 50,000 a machine. In France, cathedral conservation is sponsored by the government, fairly generously. On this side of the Channel, by contrast, it's a haphazard affair involving uncertain funding from a number of bodies; and it may be that for the present nobody here can afford Didier Groux's services. You may be prompted to reflect, as others have done before, that they order these things much better in France.