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Police forces have been rushing to buy fingerprint technology developed by a University of Wales professor, that has been on sale for the past three weeks. The new method produces much clearer fingerprints on rough and porous surfaces, such as polythene, wood and paint.

Traditionally, fingerprints are revealed by taking a delicate animal hair or glass fibre brush and painting a fine powder over them. The powder sticks to the ridges of the print, which are lines of sweat produced by the pores of the ridges of the finger, but does not stick to the furrows.

But the brush can often damage the print by smearing the sweat lines. "About 10 per cent of prints are damaged," says Brian Wiltshire, of the department of materials engineering at the University of Wales in Swansea.

He has developed a method in which iron filings are ground as fine as the traditional flaked powder and sprinkled over the fingerprint. The iron is magnetic. The scene-of-crime officer then takes a device called a magnabrush, which contains a magnetised steel rod, and holds it over the print. The device sweeps up all the particles except for those which are stuck fast to the sweat of the fingerprint ridges.

The iron particles form a powder that produces much less of a cloud than the old powder. This may also allay fears that scene-of-crime officers are exposing themselves to respiratory problems by inhaling powder every day.

Professor Wiltshire says that the technique can also be used to reveal impressions made by footwear on window ledges, floor tiles and other polished surfaces. It is being sold to the FBI.