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Two laser techniques combined to probe artwork

17 Jun 2002

Scientists from the UK and Greece have combined two complementary analytical laser techniques to chracterise pigments in a Byzantine work of art layer by layer. Robin Clark, of University College, London, used laser induced breakdown spectroscopy (LIBS) and Raman microscopy to analyse a 19th century Russian icon of St Nicholas constructed on a wooden panel.

Using LIBS to reveal the elemental composition of the top layer of the artwork suggests the presence of certain pigments, which can then be positively identified with the Raman technique. LIBS and Raman microscopy are both non-destructive, so in situ analysis is possible.

Knowing the pigment characteristics is useful for restoration and conservation, because some chemicals used in restoration can mix with materials previously used for "retouching" the work, producing a black mess.

According to Clark, a major advantage of using LIBS is that the artwork can be depth-profiled, so that successive layers of material can be studied in detail. By carefully controlling the number of laser pulses directed at the icon, a range of depths can be accessed. If the work has been treated or restored during its history, this can be revealed.

Clark used a Q-switched Nd:YAG laser at the 355 nm third harmonic to perform the LIBS analysis to a depth of 50 to 100 micron. Because of the sensitivity and value of the artworks, he used lower than normal power densities of 10 to 20 GW/cm-2. For the Raman ananlysis, a HeNe emission of 0.1 to 2 mW was focused onto the sample through a microscope lens and the scattered light captured on a CCD detector.Scientists from the UK and Greece have combined two complementary analytical laser techniques to chracterise pigments in a Byzantine work of art layer by layer. Robin Clark, of University College, London, used laser induced breakdown spectroscopy (LIBS) and Raman microscopy to analyse a 19th century Russian icon of St Nicholas constructed on a wooden panel. Using LIBS to reveal the elemental composition of the top layer of the artwork suggests the presence of certain pigments, which can then be positively identified with the Raman technique. LIBS and Raman microscopy are both non-destructive, so in situ analysis is possible.

Knowing the pigment characteristics is useful for restoration and conservation, because some chemicals used in restoration can mix with materials previously used for "retouching" the work, producing a black mess.

According to Clark, a major advantage of using LIBS is that the artwork can be depth-profiled, so that successive layers of material can be studied in detail. By carefully controlling the number of laser pulses directed at the icon, a range of depths can be accessed. If the work has been treated or restored during its history, this can be revealed.

Clark used a Q-switched Nd:YAG laser at the 355 nm third harmonic to perform the LIBS analysis to a depth of 50 to 100 micron. Because of the sensitivity and value of the artworks, he used lower than normal power densities of 10 to 20 GW/cm-2. For the Raman ananlysis, a HeNe emission of 0.1 to 2 mW was focused onto the sample through a microscope lens and the scattered light captured on a CCD detector.

Clark found that a zinc white layer had been overpainted on the original lead white paint, with from 10 to 40 pulses needed before the zinc signal disappeared, depending on the area of the work analysed. Two red paints, one vermillion (mercury sulfide) and one of organic origin, were identified. An extremely thin layer of silver foil was also found.

Story courtesy of Opto and Laser Europe magazine.

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