Klaus Dickmann

Yellowing effect and discoloration of pigments: experimental and theoretical studies

Abstract Two issues of great interest in the field of lasers in artwork conservation are the so-called yellowing effect and the discoloration of pigments. We have viewed these issues from a comprehensive point of view, considering all our present experimental results as well as ongoing modeling and theoretical calculations. The first concern to be discussed is the yellowing effect in laser cleaning of marble or stone artifacts. Although, in most cases, a yellowish layer exists underneath the black encrustation, the so-called ‘patina’, it has become clear that there are situations where yellowing cannot be attributed to an existing layer. In the present study, a light scattering model that may account for the yellowing is presented. This model considers a thin absorbent layer and the surface roughness and/or created voids and accounts for the reflectance spectra measured by (i) hyper-spectral imaging and (ii) integrating sphere. Additional experimental data, such as the absence of yellowing when the third harmonic of a Q-swihed Nd:YAG laser is used, support this model. A thorough understanding of the quantitative characteristics of pigment discoloration, on the other hand, has been attempted by means of X-ray diffraction and theoretical studies. The model developed suggests a nucleation process for cinnabar resulting in a structural modification within the volume of a pigment’s crystal or particle close to the ‘ablation front’, which extends for a few nanometers from its surface.

Laser irradiation of medieval pigments at IR, VIS and UV wavelengths

The possibility to use laser radiation to clean historical objects has been established for several years. A complex case and widely met problem are polychromes. They react (chemically as also physically) very sensitively towards laser radiation. In this study, the reaction of pigments was investigated in dependency on the incident wavelength (Nd:YAG, λ = 1064, 532, 355, 266 nm) and energy density. The chemical and also the physical interactions were investigated. In this work, the following analytical methods were utilised: differential thermal analysis (DTA), colour measurements (CIE-L*a*b*), X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDX). It turned out that the colour change of the pigments can have different origins: they can for instance be induced by laser induced oxidation, reduction or phase changing. Most of the pigments show reactions at very low energy densities (H < 100 mJ cm –2 ). Overall the fundamental wavelength of the Nd:YAG-laser (λ = 1064 nm) proved to be most suitable, whereas λ = 355 nm shows most influence on the colour change.

Comparative study of different wavelengths from IR to UV applied to clean sandstone

Abstract The dependency of ablation rate and wavelength is studied by means of an exemplary chosen sort of sandstone in order to determine the cleaning regime and the feasibility of different wavelengths for practical cleaning. Excimer laser radiation (KrF, XeCl) and Nd:YAG laser radiation (ω, 3ω) were used for the ablation of the black crust and sandstone in two independent experiments with a constant number of pulses.

LIBS-spectroscopy for monitoring and control of the laser cleaning process of stone and medieval glass

Abstract On-line monitoring or even closed-loop control is necessary to avoid over-cleaning in case the ablation process is not self-limiting. Therefore, the laser-induced breakdown spectroscopy (LIBS) was used. Basic investigations were carried out on original sandstone samples ( Elbsandstein ) with strong encrustations as well as medieval stained glass samples (13th century from Cologne Cathedral). The spectroscopic study has shown that the plasma emission can be used for determination of the elemental composition of the ablated material. The plasma was initiated by 248-nm pulses of an KrF-excimer laser (30 ns FWHM). For the spectroscopic analysis, a grating spectrograph in combination with an optical multichannel analyser was used. For the glass and stone samples we obtained a continual alteration of the LIBS spectrum (vanishing of peaks and generating of new element peaks) during the removal process. Thus, certain element peaks can be used to distinguish between encrustation layer and valuable underlying material. To show the potential of LIBS we designed an experimental laser cleaning set-up including closed-loop LIBS control and demonstrated successful automatic cleaning of an original glass fragment.

Investigations on cleaning of black crusted sandstone using different UV-pulsed lasers

Abstract Laser cleaning of stone is mainly based on Nd:YAG laser radiation at λ=1 064 nm. Within an international co-operation, experimental studies concerning the application of various UV-wavelengths were carried out. The studies were performed using 355 nm (3rd harmonic of Nd:YAG), 308 nm (XeCl-Excimer) and 248 nm (KrF-Excimer) and in comparison 1 064 nm (Nd:YAG fundamental). Sandstone samples from the Dresden Zwinger (Germany), called Elbsandstein, covered by a superficial black crust were used for laser ablation rate studies. The ablation rates (mg per pulse) were measured with respect to the laser fluence.

Enhanced radiation of a dipole placed between a metallic surface and a nanoparticle

Enhanced dipole radiation in the presence of a flat metallic surface and a metal nanoparticle is considered on the basis of Maxwells equations. For the case of axi-symmetrical illumination the initial problem is reduced to a system of boundary integral equations for the angular component of the magnetic field and its normal derivative. A boundary element method is used to solve the system of integral equations. The scattering of convergent cylindrical electromagnetic waves from a nanoparticle placed near a surface is calculated. The dipole placed between the nanoparticle and the surface is excited by the enhanced field in the gap and re-radiates electromagnetic waves of the same frequency into space. This dipole radiation in turn is enhanced by the nanoparticle/surface system. Two intensity enhancement factors are calculated: (1) the enhancement of the local electric field at the dipole position by the nanoparticle/surface system; and (2) the increase in dipole radiation due to the presence of a metallic nano-object. For very small gaps (1 nm) between the surface and nanoparticle, these factors reach very large values. At some frequencies the enhancement factors exhibit large resonance peaks which can be explained as plasmon resonances in the nanoparticle/surface system. For various shapes of metal nanoparticles and for different distances in the particle/dipole/surface configuration, the total intensity enhancement factor (the product of the two factors described above) is calculated using the developed model. The very large enhancement factors obtained in our calculations can be considered as a theoretical basis for single molecule Raman spectroscopy.

On the Field Enhancement at Laser‐illuminated Scanning Probe Tips

First results of a new dynamic approach to calculate the field enhancement at a metal ellipsoid close to a metal surface are presented. The theoretical approach is based on the solution of Maxwells equations. The numerical calculation is influenced by the dielectric constants of both metals, the angle of incidence of the monochromatic electromagnetic wave, the distance of the ellipsoid to the metal surface and the semi axis ratio of the ellipsoid.

UV-laser radiation: basic research of the potential for cleaning stained glass

Abstract A KrF-excimer laser operating at λ = 248 nm was used for basic studies in cleaning ancient glass surfaces. For irradiation a mask projection technique was applied. A modified optical set-up was designed using cylindrical lenses for large area removal by line scanning. Previous investigations revealed insufficient knowledge about interaction processes between UV-laser radiation and individual surface layers of encrusted historical glass. Thus, first detailed studies were carried out on special model glass samples simulating the behaviour of historical glass. The potential of removing crusts, bio layers as well as layers of different conservation materials (due to former conservation work) were examined. The avoidance of damaging material such as gel layers, paint layers or the bulk glass was of great importance. Removal rates and thresholds for the materials mentioned above were carried out. The collected data enables a comparison and evaluation of the feasibility for the removal of superficial layers from historical glass artefacts. The results indicate that in some cases a closed loop process control will be necessary to avoid over-cleaning.

Archaeological Ironwork: Removal of Corrosion Layers by Nd:YAG-Laser

Often archaeological ironwork is covered by a thick corrosion layer. In many cases the corrosion crust exceeds the volume of the original ironwork several times. In order to expose the original topography, containing several information about the former manufacturing process, recently the laser gained increasing interest. Investigations were carried out by a novel Nd:YAG-Laser with frequency multiplying (1064 nm, 532 nm, 355 nm, 266 nm). It has turned out that the combination of conventional cleaning methods (for coarse removal of corrosion crusts) with Nd:YAG-Laser technique (for fine removal) opens new possibilities in restoration of strongly corroded archaeological iron work.

CONTROLLED LASER CLEANING OF ARTWORKS WITH LOW RESOLUTION LIBS AND LINEAR CORRELATION ANALYSIS

An image communication apparatus comprises a connection unit for connecting an external terminal, an image communication unit for conducting image communication, a switching unit for selectively connecting a line to said connection unit and said image communication unit, a first detection unit for detecting a calling signal from said line, a second detection unit for detecting a signal relating to the image communication, a third detection unit for detecting a signal relating to data communication, and a control unit for controlling said switching unit in response to the detection by said first, second and third detection units.