The use of LASER (Light Amplification Stimulated Emission of Radiation) technology in the conservation of artefacts, sculpture and architectural detail began in the 1960’s. An idea of selectively removing an optically absorbing substance (in this case black pigment) from a reflective substrate (white paper) was developed.

In 1973, in Venice, scientists and conservators applied a similar principle for the removal of black encrustations from white Carrara marble sculpture. It was understood then that Laser radiation would lead to major advancement in conservation techniques as it was developed into a tool that could selectively remove layers of environmental soiling without causing harm to the historic surface beneath.

Since then, the technology has been developed and refined resulting in a highly efficient, sensitive, non-invasive conservation tool. The Phoenix Conservation Laser system is the product of this development and has been working successfully in the field since 1995.

An oscillating shutter within the laser stores and releases energy as a pulse of light. The pulses of light, which can be adjusted in intensity and frequency, are delivered to the soiled surface by one of two systems, optical fibres or a mirrored arm. When the pulsed light is directed onto the soiled surface a photomechanical effect is produced where energy is absorbed by the extraneous soiling material and reflected by the underlying surface. A pulse of short duration ensures a photomechanical effect; without heat, to break the bond between the soiling and the underlying surface. This results in ablation where the soiling is ejected from the surface.

In many cases the removal of soiling using The Phoenix Conservation Laser is described as being ‘self-limiting’. This is where ablation of material from an historic surface stops immediately the soiling has been removed, automatically. This occurs because many pollutants strongly absorb laser energy at a specific wavelength where many artefact substrates do not.

The Conservation Laser is a Q switched Nd Yag laser; a reliable, rugged system that emits laser energy at a wavelength of 1064 nanometres (nm) which is selectively absorbed by a variety of extraneous soiling types including the following pollution products; gypsum encrustation, organic growths, previous surface treatments, salt efflorescence, applied graffiti and corrosion layers found on a variety of historic materials; marble, limestone, sandstone, granite, slate, scagliola, stucco, plaster, gesso, gilt work, aluminium, lead, terracotta, ivory, velum, basketry, paper, parchment, stained glass, timber and textiles.

Recording is a key aspect of any conservation treatment. The Phoenix Conservation Laser enables the Conservator to record in detail the levels of energy that are required to achieve a satisfactory effect. By calculating the energy per pulse (read from the energy monitor) divided by the spot size (the area of light on the subject’s surface) one comes to the energy density or fluence. Fluence is defined in units of J/cm2. By cleaning at a prescribed fluence throughout the subject, a homogenous effect is produced and a detailed record kept for future reference.