Cleaning with laser light

In the course of technological progress in the field of laser technology in recent years - more compact design and greater reliability - lasers have now left the laboratory and opened up new interesting applications in trade and industry. An example of this is cleaning with pulsed Q-switched Nd:YAG lasers. The driving force behind the development of laser cleaning was the desire to restore valuable sculptures and monuments as gently as possible and without any loss of substance. The process is now well established in the preservation of monuments, and further applications from the field of technical cleaning have been added.

The decisive property that makes a laser interesting for cleaning tasks is the short duration of its light pulses. Therefore, pulsed Nd:YAG lasers with pulse durations of less than 10 ns are used. While continuous wave lasers heat up the irradiated objects, Q-switched lasers deposit their energy in such a short time that a plasma is formed on the irradiated surface, which spreads away from the surface in the direction of the laser beam at high speed. The transfer of heat to the underlying layers is minimal, since thermal processes are very slow in comparison. Due to the rapid expansion of the plasma, a pressure wave is created which causes the layers of dirt or cover to flake off. This effect is also audible. Ideally, the absorption properties of the layer to be removed (dark, high absorption) differ from those of the object (bright, metallic, reflective or transparent). Under these conditions, the cleaning process on the object itself stops as soon as the absorbent layer is removed. Examples are bright limestone and sandstone.

The first significant application examples of successful laser cleaning come from the Cathedral of Amiens in France. Several lasers have been in use here for years to clean the ornate portals. Other successful restorations have been carried out using lasers on the cathedrals of Rouen, Lille and Notre Dame in Paris.

In Dresden, a group of figures from the large garden was cleaned with the laser. Further laser cleaning work is in progress on marble figures in Potsdam Sancouci and on the citadel in Jülich. In addition to stone cleaning, the cleaning of wood, fabric, ivory, paper, gold and bronze has also proved very interesting.

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