The laser treatment cleans without damaging the underlying stone, as sand-blasting and some chemical cleaning methods have done. It works by heating the layer of dirt, making it expand. This breaks its bond with the surface of the stone, and the dirt is vaporised.
'Lasers are much more gentle and selective in what they remove from the surface of sculptures and other stonework than even the most sensitive mechanical techniques,' says John Larson, senior conservator at the National Museums and Galleries on Merseyside.
Mr Larson has spent three years researching laser-cleaning techniques in collaboration with the department of physics at Loughborough University. Different types of laser were tested on heavily polluted limestone sculptures from Lincoln Cathedral.
The cleaning system is now ready to be used commercially, according to Dr David Emmony, the head of the university's laser group. In August, it was demonstrated to engineers and contractors responsible for maintaining the Palace of Westminster. The department of works at the palace has recommended that contractors use lasers in future clean-ups.
On 1 November, Mr Larson is also due to demonstrate the laser's cleaning power on the Exhibition Road elevation of the Victoria and Albert Museum (where he spent 22 years as head of sculpture conservation). Later next month, it will be tested at Lincoln Cathedral.
According to Mr Larson, examination of treated sculptures by electron microscope shows the surface is not affected at all. This contrasts with highly sensitive mechanical processes - which blast the sculpture with very fine particles such as aluminium oxide - where some of the stone is always removed with the dirt.
Mr Larson says a great deal of damage has been done to Britain's historic buildings in the name of restoration. Cleaning limestone to remove black soot deposits also strips it of its protective layer of calcium sulphate. Without this skin, the stone is eaten away by acid rain which dissolves calcium carbonate, the main constituent of limestone.
Many buildings in Glasgow were cleaned chemically by applying acid to strip off the dirt, followed by an alkali to neutralise the acid. The chemicals have penetrated into the porous sandstone and are now causing salt deposits, staining and exfoliation. Some buildings also have green staining from algae growths. The Scott Monument in Edinburgh has similar damage.
The laser cleaners, which are based on lasers made by Spectron in Rugby, cost about pounds 25,000 each. This is more than for a comparable sand-blaster, at pounds 10,000 to pounds 15,000, but Dr Emmony says the laser has lower running costs because there are no materials to buy, such as sand, and no need to clean up afterwards.
The current laser cleaner is also slow (about two square inches per second) compared with mechanical techniques, which makes it unsuitable for vast expanses of wall. However, Dr Emmony says it is as fast as sand-blasting for delicate jobs such as cleaning a statue. 'I don't think we should be interested in cleaning stone as quickly as possible,' says Mr Larson. 'The method needs to be refined and precise.
'I hope we can persuade people to adopt the laser technique in this country,' he adds. 'Too much damage has already been done by other methods of stone cleaning.'
Mr Larson has tested the laser on different types of dirt and stains, and on different materials, including bronze. The Science Museum is interested in using it to strip paint off old industrial machinery, and Mr Larson says there is great commercial potential in the field of industrial archaeology.
The Leverhulme Trust has awarded the National Museums and Galleries pounds 120,000 for a study of the 'Conservation and Recording of Decayed Sculpture Using Laser Technology'. Mr Larson says an element of this will be to evalute how lasers of different wavelengths affected different types of deposit.
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