In offices the world over, heaps of printouts and photocopies from laser printers get used once before being discarded, or tossed on shelves to collect dust indefinitely. But what if they could be wiped clean and used again?

An engineering team at the University of Cambridge in the UK has figured out how to erase pages by vaporising common toners using a laser-based technique that doesn't damage the underlying paper.

Toshiba of Japan already sells a special laser printer/copier (video) that uses a blue toner which can be almost completely erased with heat treatment. David Leal-Ayala and his colleagues at Cambridge have taken the idea a step further, though, with a method that can recover the paper from any laser printed or photocopied document. After testing and ruling out toner removal processes that use mechanical abrasion and chemical solvents, they focused on the most promising method: laser pulses which vaporise toner particles in thin layers until they are no more.

"The key idea was to find a laser energy level that is high enough to ablate - or vaporise - the toner that at the same time is lower than the destruction threshold of the paper substrate. It turns out the best wavelength is 532 nanometres - that's green visible light - with a pulse length of 4 nanoseconds, which is quite long," Leal-Ayala told New Scientist.

"We have repeated the printing/unprinting process three times on the same piece of paper with good results. The more you do it, though, the more likely it is for the laser to damage the paper, perhaps yellowing it," he says. The team have found toner-paper combinations in which almost no appreciable traces of toner can be seen after lasing and in which the paper suffers "no significant mechanical damage".

"Toshiba have been selling the 'e-blue' toner for a while - which, like old thermal fax paper, fades under the right type of light. However that - of course - applies only if you buy their magic toner," says the Cambridge project's supervisor Julian Allwood.

"Our ambition was to develop a method that would remove conventional toner from conventional paper in order to allow re-use of the paper. Toshiba's is a different approach to the same problem."