Facilities

Non-destructive analysis of pigments with raman spectroscopy
UV Pulsed Laser
IR Pulsed Laser
Scientific photography
IR Relectometry

Non-destructive analysis of pigments with raman spectroscopy

Raman Spectroscopy is an analytical technique that allows the non-destructive molecular identification of the pictoric materials present in the work of art. This technique is mainly based on focusing a laser on the targeted area allowing the detection of the inelastic scattered light. Thus, the obtained spectrum (i.e. Raman spectrum) represents an unique spectrum key of the illuminated material with analogous behaviour as a fingerprint. The captured Raman spectrum is compared with the spectra stored in a database that belongs to pictorial material patterns previously analysed. This comparison allows the identification of the pictorial material corresponding to the obtained Raman spectrum. Figure 1 shows the Raman spectrum obtained in a real work and its corresponding spectrum pattern.

Figure 1. An unknown pigment identification by means of comparing the Raman spectrum with one stored in a database of pictorial material patterns.

Figure 2. Raman equipment of the UPC that is used in the analysis of works of art.

The used spectroscopic Raman equipment is an Induram model provided by JobinYvon (Horiba Group), Figure 2 shows the system block diagram that is been used in the UPC to analyse the works. A general description of how this equipment functions follows. The device is composed of three sources of exchangeable monochromatic light (He-Ne laser, 17mW 632.8nm; Ar laser, 40 mW 514,4 nm; and IR laser, 100 mW 785nm) in which the output is guided through the active optical fibre. The main objective of the optical head is to focus the light on the targeted area and gather the scattered light. Then, this light is guided to the collector fibre. This fibre carries the non-elastic scattered light (Raman signal) to the monochromer where the different frequency tones are spatially divided. Finally, the CCD detector converts the optical voltage into electrical current and sends the data to the computer. The computer monitors the Raman signal allowing a full control of the rest of the equipment.

UV Pulsed Laser

Pressed laser Polaris III

A pressed laser Polaris III made by New Wave Research Inc. is available in our laboratory. This laser is able to generate I.R. pulses (Nd:YAG, 1064 nm). By including non-linear active filters in the laser structure, the second armonic (VIS, 532 nm), third armonic (UV, 355 nm) and fourth armonic (UV, 266 nm) can also be generated. The active crystals are composed by a KTP and two BBO. The KTP device allows generation of the second armonic, and the BBO is utilised to generate the third and the forth armonic. Significally, the presence of a Haas diode which provide a very detailed information on the exact collision part of the pulses.

Polaris III laser stands out in its flexibility, portability and outstanding features.

 Figure 1: Pressed laser structure constituted by a non-linear active crystal, dichroic filter and the Haas diode at the output of the optical head.

Figure 2: General view of the New Wave Research Inc. Polaris III laser.

Pressed laser applications. UV Radiation

Most recent work in patimonial work is been made using UV pressed radiation. However, the alternative use of IR pressed radiation offers numerous advantages.Mentioned aforehard, while IR pressed radiation is widely used in the cleaning of stone. UV pressed radiation is mainly employed in the cleaning of polichromics, glass windows and textile materials. The effect caused by interaction between the type of radiation used and the material is completely different depending on the chosen technique. When the radiation is UV pressed, the interaction result is a non-thermal photoblation based on the photochemical effect.

One of its most important applications is the controlled elimination of varnish layers. These layers must be suppressed because build up with time and often result in discolouration. From an analytical point of view, micro-eliminations of the varnish may be very useful to ensure good detection of the pigments that compose the work. Detection is easier without the fluorescence typical of the varnish.

Figure 1: Pictorial work and a detail of a non-varnished area through non-thermal photoblation with UV pressed radiation.

Figure 2 is shown in the box

Figure 3: Non-thermal photoablation process of the varnish over a massicot layer (1,2,3,4,5 and 15 pulses).

IR Pulsed Laser

Pressed laser Polaris III

A pressed laser Polaris III made by New Wave Research Inc. is available in our laboratory. This laser is able to generate I.R. pulses (Nd:YAG, 1064 nm). By including non-linear active filters in the laser structure, the second armonic (VIS, 532 nm), third armonic (UV, 355 nm) and fourth armonic (UV, 266 nm) can also be generated. The active crystals are composed by a KTP and two BBO. The KTP device allows generation of the second armonic, and the BBO is utilised to generate the third and the forth armonic. Significally, the presence of a Haas diode which provide a very detailed information on the exact collision part of the pulses.

Polaris III laser stands out in its flexibility, portability and outstanding features.

Figure 1: Pressed laser structure constituted by a non-linear active crystal, dichroic filter and the Haas diode at the output of the optical head.

Figure 2: General view of the New Wave Research Inc. Polaris III laser.

Detail of a blackish marble area that has been cleaned using IR radiation

Another application of IR pressed radiation consists of elimitating the repaints that have been made subsequent to the original work. A photoblation process that is controlled layer by layer can be obtained through varying both the energy and frequency of pulse repetitions. That process allows the underlying pigment to be exposed layer by layer.

Photoblation red lead . The underlying white lead layer can be observed at the centre.

Scientific photography

Microscopy and analytic photography

A Leica stereomicroscopy MZ-12 is also available at the Raman spectroscopy lab at the UPC. This device is equipped with two interchangeable lenses (surgical and 1,6x) and eyeglasses of 25x, This combination allows one to obtain a maximum of 400x augments. A Sony SSc-C3700P detector CCD is connected to the stereomicroscopy allowing a signal to be sent from the MZ-12 to the monitor. Finally, a Sony Up-1200A EPM videoprint is connected to the monitor which allows the image that is shown on the monitor to be printed.

Analytical microphotography is for obtaining a very useful technique for the study of "craqueladuras", signature integration, for obtaining microsamples and even to obeserve if a pigment presents a mineral origin (mineral overseas front to synthetic overseas, for example). 

Fig.1: Full stereomicroscopy equipment formed by Leica MZ-12, Monitor Sony, Photo TV Printer and CCD Detector SSC-P350

Several images obtained by the stereomicroscopy-CCD detector-videoptint equipment are shown next.

Fig. 2: Study of different signatures: Modest Urgell (left) y David Teniers (right).

Fig. 3: Study of brushstokes of the work (left) and microsample obtained using the microscopy system (right).

IR Relectometry

IR reflectography is used in order to display the possible underlying drawing that contains the work. It is based on the capacity of IR light to get through pictoriallayers. If the work is illuminated with white light (ample set of wavelengths), only the infrared components reach the underlying sketch and the print out. The previous charcoal sketch absorbs this type of radiation while the imprimitation, normally prepared with white lead, scatters most of the IR component. Thus, through using a highly sensitive IR camera, such as the one that we have in our lab (1,2 mm), we can detect the IR light that is been scattered by the print-out phenomenon.

The underlying sketch can be observed through using IR recflectography. This sketch offers the possibility to identify some possible "regrets" of the author.