Jens Hildenhagen

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.

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.

Laser interaction with polychromy: laboratory investigations and on-site observations

Abstract The impact of laser cleaning of soiled natural stone surfaces possibly with underlying polychromy was studied by means of laboratory samples and during the cleaning of a Gothic tympanum at St Martin’s Church in Brunswick, Germany. Paint layers with pigments (1) frequently encountered in historic polychromies and (2) applied with different binders to cardboard strips and stone tablets (and subsequently coated with an artificial black gypsum crust) were irradiated. Laser light at different wavelengths and fluences was used to (a) determine fluence threshold values for pigment alteration, and (b) to determine if the artificial gypsum crust can be ablated from the paint layers without discolouring them. The wavelength-dependent diffuse reflection and scattering of light were measured for the chosen pigment/binding medium combinations and turned into absorption coefficients according to the Kubelka–Munk theory. These data served as a basis for theoretical considerations about the laser–pigment interaction with respect to less critical wavelength regimes. For the pigments under investigation, the following sequence in order of sensitivity to laser irradiation from highest to lowest was found: vermilion, massicot, lead antimonate (Naples yellow), minium, malachite, red ochre, yellow ochre, azurite, smalt, green earth. In the case of malachite, azurite, minium, and vermilion, the chemical reaction which brings about the colour change was deduced from the X-ray diffraction curves of irradiated and altered pigment powders. Observations made during trial laser cleanings in the laboratory and on-site suggest that historic polychromies are more endangered by the loss of pigment flakes than by discoloration. The role that the binding medium and ageing effects play in the interaction of the laser with polychromies must be the subject of further investigations.

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.

Reaction of Historical Colours and their Components Irradiated at Different Nd:YAG Laser Wavelengths (ω, 2ω, 3ω, 4ω)

Cleaning of artworks with laser radiation may become a delicate situation in cases where a paint layer is directly underneath the undesired top layer. However, this is the situation in many applications of restoration, e.g. cleaning of paintings. It is known in general that historical colours may react very sensitive on illumination. Within an extensive study first we investigated separately the behaviour of 11 pigments and 3 binding materials under laser irradiation. Then in a next step we investigated the behaviour of 33 historical colours, consisting from a combination of pigments and bindings mentioned above. The experiments have been carried out using four different wavelengths from a Nd:YAG laser with optional frequency multiplier (1064 nm, 532 nm, 355 nm, 266 nm). At various laser fluences and wavelengths the corresponding colour change was analysed by CIE-L*a*b*-System. As a result of our study we have built up a data base containing threshold fluences for various historical pigments, bindings and their combinations.

Comparison of Cleaning Methods for Stained Glass Windows

Any cleaning process for stained glass windows has to consider the effectiveness of the treatment but also the potential damage for the art object. A variety of mechanical and chemical methods is currently used in restoration practice. The most effective ones are criticized because of their long-term risks. Therefore, an interdisciplinary research project, carried out in Germany and funded by the “Deutsche Bundesstiftung Umwelt (DBU)” had explored the possibilities and limits of lasers for cleaning glass windows. At previous LACONA conferences the Excimer Laser equipment and results from the research project have been presented. This contribution puts the cleaning experiments in a broader context, by comparing lasers with conventional techniques. Scientific and practical aspects will be discussed, focussing on the removal of crust and aged polymers.

Low-cost sensor system for online monitoring during laser cleaning

Abstract The self-limiting effect during laser cleaning only occurs in a limited amount of specific applications in restoration (e.g. removal of black crust from white marble). In all the other cases, a control of the removal process will be necessary either by the operator himself or by the employment of sensor equipment. Various methods, mainly spectroscopic (e.g. LIBS), have been investigated and proposed by others. Despite the fact that these have been shown to be promising, they all have in common rather high investment cost close to that of the cleaning equipment. Furthermore, this highly sophisticated control equipment is not easy to handle by conservators in practice. As an alternative low-cost method, we employed a simple photodiode to detect the scattered light from the irradiation area on the artwork surface. In many cases, this signal contains several pieces of information on the layer just being removed. The scattered radiation detected by the photodiode originates from the laser-induced plasma as well as reflected laser radiation. A separation, if necessary in order to separate the information, is possible by spectral filters. First applications during laser cleaning of corroded metal, encrusted glass and stone were promising. It has turned out that there is a distinct influence on the scattered light amplitude or even the pulse-bandwidth once the laser has removed the encrustation completely. The corresponding signal can be used in a closed loop control or for online monitoring.