Just the treatment for a dirty gargoyle: Lasers are being turned to tasks ranging from cleaning sculptures to designing clothes, says Ruth McKernan
Monday 17 August 1992
The type of laser that reads bar codes is now being used by British scientists to make three-dimensional measurements of the human body, and to copy delicate works of art without touching them. Other, more powerful, lasers are burning off blackened deposits of lichen, bird droppings and pollution from the surface of Lincoln Cathedral's figures.
The laser used in the supermarket is a low-energy helium-neon (He-Ne) laser, which generates a continuous beam of light, but little heat. Conservators at the National Museums and Galleries on Merseyside are using this type of laser to record the shapes of pieces of sculpture. The laser can scan in increments of less than a millimetre, and by using strategically placed mirrors it can measure both positive and negative space.
Most museums still measure sculptures with a measuring tape, like a model's vital statistics. 'Several people will measure a bust and come out with different numbers. We can get an exact three-dimensional measurement on an electronic turntable in about a minute,' says John Larson, head of sculpture and inorganics conservation.
Researchers at the Department of Human Sciences at Loughborough University have measured busts, bellies and whole bodies using three-dimensional optical techniques. Using ribbons of light, they have measured people of all shapes and sizes, clad only in skin-tight body-suits, and have constructed a computer database of the information. High-street retailers are using this information to design clothes to fit the many varieties of the British figure. Tailors in Savile Row could use the same technology to make a perfectly fitting suit without having to ask the embarrassing question, 'does sir dress to the left or the right?' - the computer would know.
An automatic lathe can take the three-dimensional measurements of a sculpture stored on computer disk as a template to make a replica of the original, without touching it - an important consideration for fragile works of art. This technique could open the way for replicas of famous sculptures in a variety of media. Interior decorators might like a copy of the Manneken-Pis from Brussels, in marble for the bathroom, or a couple of caryatids from the Acropolis, in plaster, to support the back porch.
For museum curators and conservators, the technology has other uses. Exact measurements of a work of art can be used to make exact-fitting packaging to transport it more safely. The same information could be sent on disk in advance to aid exhibition design. 'We are laying the foundations for the way museums will operate in the 21st century. Within the next few years laser technology will become widely used,' Mr Larson predicts.
While some scientists have used lasers that generate a lot of light but only a little heat to make three-dimensional measurements of sculptures, others have used lasers with the opposite properties to clean sculptures. Lasers can produce light of many colours of the spectrum, depending on the element used. Krypton fluoride, for example, produces deep ultraviolet light while, at the other end of the spectrum, carbon dioxide emits in the infra-red range.
Researchers from the Department of Physics at Loughborough University, led by David Emmony, have tested many types of laser for their ability to clean marble and limestone.
'Blackening of stone buildings and sculptures is caused by many things: acid rain chemically changes the surface of marble or limestone, making it more porous so that it retains water. This encourages growth of microbes and absorption of carbon fragments. Energy, as light from a laser, is absorbed by the black surfaces and burns off or vaporises the dirt,' he says.
Since an object is black because it absorbs light of all colours, lasers producing light from any part of the spectrum could, theoretically, be used for cleaning. At Loughborough, they have found that a yttrium-aluminium-garnet crystal containing traces of the element neodymium is the best for this purpose. It generates infra-red light just outside the visible range, and is energy efficient without being expensive.
The principles governing the cleaning of stone are the same as those demonstrated 30 years ago by Arthur Schawlaw, inventor of the 'laser eraser'. Energy from the light of his laser beam, focused on a carbon character from an old-fashioned typewriter, was absorbed by the black letter but reflected by the white paper. The rapid absorption of heat burnt off the letter, leaving the paper undamaged. When the Loughborough physicists activate their neodymium laser there is a chattering sound, a small flash of light and a puff of smoke that leaves a pristine spot of marble. The laser will keep removing carbon until it hits stone, light is then reflected from the clean surface and the process stops of its own accord because the energy is no longer absorbed.
The National Museums and Galleries on Merseyside and Loughborough University have collaborated to develop the technology for cleaning small sculptures from some of Britain's most treasured churches and cathedrals. Gargoyles from Lincoln Cathedral have been the first to benefit from laser treatment.
'The advantage of using lasers for removing pollution is that they are precise, easily controlled and do not affect the chemistry of the stone,' says Mr Larson. Encouraged by success in their preliminary studies, conservators at the National Museums on Merseyside have commissioned a neodymium laser at a cost of pounds 10,500, to be used initially for cleaning museum pieces. The laser is to be paid for by a donation from the Henry Moore Foundation. That artist's large garden sculptures, when ravaged by weather and pollution in years to come, may themselves benefit from the technology.
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