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A face lift for our cultural heritage

   
 
Picture
Using the Lama laser to clean one of the statues of the Hôtel de Ville, Brussels
Conveyed by optical fibre, a beam of light wipes away the imprint of time. Designed for cleaning buildings, in particular monuments to our cultural heritage, the LAMA laser allows the buildings to be freshened up without causing any harm to the material. This innovative tool, which is efficient, manageable, ergonomic and quiet, does not involve the use of water or sand and makes it possible to reach areas inaccessible by other methods.

 

Four minuscule dots of light act like a magic eraser, and the sculpture changes rapidly from a dark to a light colour. The persistent beam brings details to life. Certain parts remain greyish, revealing a history of earlier botched restorations. In Deinze (Belgium), in a workshop belonging to the small MRT company, all the statues of Brussels' Hôtel de Ville, which forms the heart of its illustrious Grand'Place, have been brought along to benefit from this soft facelift. In Italy, the restorers Trivella have tested the LAMA handheld laser and compared it with other methods - such as microscale sandblasting or chemical pulps - on a number of pilot sites including Milan Cathedral, the Strozzi Palace and Lucca Cathedral. In France, following the renovation of the Hôtel Godin (Tours), a demonstration was organised at the Galliera Museum (Paris) in the presence of various parties responsible for historic monuments, who will eventually decide, on a case-by-case basis, whether to make use of this technology.

Non-destructive renovation
The effect of light beams on stains has been known since the early 1970s, and various teams of researchers (in Italy, the United Kingdom and France) have carried out studies on the possibility of applying this technology to the restoration of the cultural heritage. Professionals from this sector, such as the specialists of the Jaulard company, a subsidiary of GTM Construction (France), are rapidly becoming aware of the attractions of this method. In their opinion, the "hard" techniques (pressurised water, sandblasting, hydro-sandblasting), frequently used because of their low cost (15 /m2), show scant regard for materials and the environment. They are also aware that the soft techniques (spraying, microscale sandblasting, chemical processes), while offering higher performance, nevertheless entail such high financial outlays (450 - 1500 /m2) that they are employed solely in the case of particularly valuable objects. But intermediate solutions , such as "erasing", which vary in cost from 30 /m2 to 75 /m2, remain abrasive.

None of these methods is completely satisfactory. As Jean Weiss, Technical Director of the GTM Group, points out: "To attack the stonework in the course of the cleaning process is frequently tantamount to attacking its protective layer, which is the epidermis of the stone and which keeps alive our memory of the original work down the ages. In reality, such an operation increases susceptibility to subsequent pollution and, consequently, shortens the life of the material."

Picture
Palazzo Ruffo (Rome). Graffiti, test and restoration.

Laser to measure
Under a project supported by the Brite-EuRam programme and implemented between 1993 and 1996, eight European partners got together to develop an effective, less expensive, soft technology.

The first stages of the research on the use of laser beams were devoted to an analysis of the "material-stains" pair. "These two elements are inseparable," Jean Weiss goes on, "since what we are trying to do is to eradicate the ravages of time without degrading the work itself. Each of these two materials is characterised by a light energy density "threshold" above which an ablation effect is produced." Fortunately, in the majority of cases, the threshold of the stain is less than the threshold of the material, and the density of the energy used will inevitably fall between these two values.

Altogether, 71 pilot sites across Europe, with 126 different "material-stain" pairs, were chosen. About a hundred samples of stone (limestone, marble, granite, etc.) stained by plants, urban pollution, traces of paint, etc. were meticulously analysed in terms of density, water absorption, type of stain, etc. They were then subjected to cleaning tests using various methods, ranging from the gentlest, such as atomised water and microscale dry sandblasting, to the most aggressive such as hydro-sandblasting and high-pressure washing, as well as laser beams. Very often the laser beams come out on top: no damage to the material, no abrasion effect, no excessive discoloration, no denaturing of the texture, etc. "A number of laser parameter combinations - wavelength, energy density, pulse duration, etc. - have been tried out, and it is on this basis that two prototypes were designed," explains Adolfo Pasetti of the Centro Studi Trivella.

Optical fibre, flexible beam
"The research effort was to be geared towards increasing the average power of the sources and looking into possibilities for standardising the distribution of energy over the impact surface," Jean Weiss explains. "This is why the research and development effort has focused on the possibility of using optical fibres as a means of conveying the radiation, rather than delivering the beams by means of mirrors. This option enhances quality - by avoiding the risk of excessive cleaning at the centre, at the expense of the periphery. It increases the rapidity of cleaning very significantly by increasing the utilisation rate of the projected energy, i.e. the useful energy. It also ensures a high degree of equipment manoeuvrability and better ergonomics."

The LAMA is available in the form of a particularly functional "kit". On the ground, there is the laser and its cooling unit (two small cube-shaped machines which can also be placed on scaffolding); the operator has a handheld instrument weighing about 1 kg, comparable to a light camera; positioned between the two there is a cable between 10 and 12 metres long to protect the optical fibres. (This has not prevented a demonstration, in the Lebanon, on stelae situated at the top of a cliff, where cleaning was carried out from a cage positioned at a height of 70 metres using suitably extended fibres).

Offering equivalent quality to that of the soft technologies, this promising tool, manufactured by Quantel, provides lower-cost high-efficiency performance (1m2 per hour, on average). Other applications employing this technology could be introduced in the industrial sector (e.g. mould cleaning in the glass and plastics industry) and - precisely because the radiation is conveyed by optical fibres - in the more specialised area of nuclear decontamination.

Meanwhile, in 1997 pending these potential developments, the ALTRAN Foundation for Innovation (France) awarded the LAMA its new technology prize "for services to the collective memory of man and the human heritage."

 

 

Project Title:  
Hand-portable laser for cleaning current building facades and historical monuments

Programmes:
Brite EuRam 2

Contract Reference: BRE20560

CORDIS databaseFor more information on this project,
go to the Cordis database Record

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