One of most important parameters in defining laser cleaning in the field of stone conservation is in the identification type and condition of the stone, its surface as well as the presence of any historical paint layers. Characteristics of the patina layers including black crusts, dust, soot and salts to be removed with the laser are also important in determining the effect and time required for the cleaning program.
For example, each cleaning procedure should respect the historic stratigraphy as many assume that these layers represent the object’s past and the periods through present day. The task of laser cleaning is also connected with what may be identified as historically significant and other aesthetic values. For sandstone, calcareous sandstone, marble, red limestone as well as other stone materials, successful results can be obtained through laser treatments. For special cleaning problems e.g. white marble surfaces with sugaring, the laser system can gently clean and preserve the most delicate surface details.
A significant advantage is also the possibility to clean without pre-consolidation (e.g. ethyl silicate, acrylic resins, silicon resins and other mixtures). In this case, the application of laser cleaning can provide much more successful cleaning results than other cleaning techniques.
Examples of stone materials, where laser cleaning has been applied successfully include: Limestone, Marble, Sandstone and Concrete.
In the specific case of stone cleaning, the dark pollution layer strongly absorbs laser radiation, while the light-colored stone substrate does not. Absorption of high energy laser pulses by the black weathering crust results in vaporization of the crust material. As soon as the light-colored unaffected stone appears, photon absorption dramatically decreases and the interaction becomes ineffective. Clea-ning stoneworks with a laser can help restoring surfaces to their original color and texture. Laser cleaning is effective even when the polluting layer has penetrated deeply into the substrate.
Large monuments like the New York Public Library have been selectively cleaned with success by expert technicians of the Lasca team.
High efficiency, obtained by choosing a laser wavelength that is strongly absorbed by the surface, is important in practical applications to achieve faster throughput and to avoid substrate damage. Ultimately, the optimum exposure conditions depend on the media characteristics and on the contaminating layer to be removed.