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.     

Laser cleaning as a part of the restoration process: removal of aged oil paints from a Renaissance sandstone portal in Dresden,Germany

Laser cleaning was applied to remove aged and soiled oil paints of the 19th century from weathered, brittle sandstone surfaces. In contrast to other tested cleaning methods, the laser technique allows the removal of the oil paint layers with a minimum of material loss in brittle zones. The cleaned test areas have been investigated by optical and scanning electron microscopy as well as by colour measurements. The investigations have shown that the dirt and the oil paint layers can be removed by laser cleaning without affecting the original sandstone surface and that the pores can be opened again. The brown shade after cleaning is not caused by laser interaction with the sandstone surface but by historic impregnation with linseed oil. Immediately after cleaning, a treatment with chemical consolidants is necessary to preserve the brittle sandstone surface. Sustainable consolidation could be problematic in some areas, because the laser cleaning also preserves heavily damaged, crumbling zones of the carvings. For economic reasons, laser cleaning should be applied only in the most endangered zones of carvings and combined with classical methods (organic solvents and scalpel).     

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

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.     

Laser and chemical cleaning tests for the conservation of the Porta del Paradiso by Lorenzo Ghiberti

In the framework of the preservation project of the Porta del Paradiso, carried on by the Opificio delle Pietre Dure of Florence, we investigated a cleaning methodology based on integrated chemical and laser techniques. This novel approach, which will replace the previous chemical protocol, was aimed at cleaning a number of gilded sculptural elements of the door frieze, without their being dismounted from the bronze framework. To such an end, optimised chemical and laser cleaning techniques were first singly investigated, and then integrated treatments were tested. The critical evaluation of each procedure was achieved through various diagnostic means to characterise the cleaned surfaces from chemical and physical points of view. The analysis demonstrated the various advantages of a laser-based treatment followed by light sodium potassium tartrate poultice application.     

Investigations regarding the behaviour of historic glass and its surface layers towards different wavelengths applied for laser cleaning

The application of lasers for the removal of superficial deposits from historic stained glass is a comparatively new field of scientific interest. Experimental studies concerning the behaviour of glass substrates and the corresponding superficial deposits towards different laser wavelengths were carried out. The experiments were performed using wavelengths of λ = 193 nm (ArF-Excimer), 308 nm (XeCl-Excimer), 355 nm (Nd:YAG third harmonic) and 1 064 nm (Nd:YAG fundamental) in comparison to λ = 248 nm (KrF-Excimer). This comparison is due to the fact that the present knowledge is based on the 248 nm wavelength. Specially prepared model glass samples representing the original fragments and samples of organic polymers (formerly used as a protective material for historic stained glasses) were used to study the effects of laser radiation and were subsequently characterised by optical microscopy.     

Application of a new laser cleaning procedure to the mausoleum of Theodoric

Within the conservation programme of the mausoleum of Theodoric, located in Ravenna, Italy, we applied a new cleaning procedure based on the use of a recently developed Nd:YAG laser system, suitably equipped in this case with long optical fibre cables for the transmission of laser radiation to work sites on the façade. The intervention of the laser technique was required to complete the cleaning of a decoration surrounding the monolithic dome, after conventional cleaning methods had been applied with unsatisfactory results. Preliminary laboratory analyses on stone samples collected from the dome and subjected to laser treatment indicated that this technique was effective and could fully preserve the lime–Ca oxalate film existing below the thick layer of black crusts. Before starting the operative phase on the mausoleum, it was necessary to solve the practical problem of keeping free access to the monument during cleaning operations, which hindered the location of the laser in close proximity to the sites to be cleaned. This was achieved by preparing long fibre cables (up to 50 m) which permitted the successful completion of the cleaning while leaving the laser body far away from the dome.     

GPR survey to study the restoration of a Roman monument

This paper describes the ground-penetrating radar (GPR) survey carried out on the Roman theatre of Sagunto (Valencia, Spain) following recent restoration work in 1991. The structure has been substantially altered a number of times: it was remodelled during the Roman and Moorish periods, partially destroyed during the Peninsular War (1808–1814) and the Carlist War (1833–1839), and extensively modified during the 1930s and 1950s. Major reconstruction work was carried out in 1991 to convert the building into a working theatre. The GPR survey was performed to detect the possible effects of the 1991 remodelling on the historical remains and to identify the point of contact between the present-day modifications and the Roman remains. We took several on-site measurements of the wave velocity in the different materials in known areas to determine as accurately as possible the depth of the contact point. The velocity was calculated by measuring the depth in these areas and the two-way travel time of the wave. The measurements were taken from the walls and the tiers as the thicknesses of the materials were known in these areas. The recorded values were compared with the velocities reported by other authors and with the information from diffraction hyperbolas recorded in the radar data, which are caused by small objects inserted in the medium. We used these velocities to determine the exact point of contact between the Roman remains and the materials used in later restorations. The contact points cause the reflections that can be seen in the GPR data. The results also reveal considerable variations in the surface of the Roman tiers, which can be attributed to partial erosion in these areas caused mainly by water.     

Assessment of the state of conservation of stone artworks after laser cleaning: comparison with conventional cleaning results on a two-decade follow up

The utilization of laser light to produce fine and selective cleaning of superficial deposits and encrustation from stones was originally proposed more than 20 years ago by John Asmus who performed pioneering trials of laser cleaning on stone façades in Venice, Cremona and Padua. The opportunity to collect stone samples from these sites allowed us to carry out an original study aimed at evaluating the conservation state of stone surfaces cleaned 10–20 years ago by means of three different techniques: Nd:YAG laser radiation, micron sandblasting and chemical pads. Samples of various stone types, as Istria limestone, Botticino limestone and Nanto sandstone were collected and examined by means of optical and SEM microscopy, FT/IR, and EDAX analyses. The results provided preliminary indications that the state of conservation of stone surfaces previously subjected to laser cleaning is generally better in comparison with those which concurrently underwent mechanical and chemical cleaning.     

An investigation into the effect of wavelength in the laser cleaning of parchment

An investigation has been undertaken into the effect of wavelength in the laser cleaning of parchment. Tests have been carried out using the fundamental (1 064 nm), second harmonic (532 nm) and fourth harmonic (266 nm) outputs from a Q-switched Nd:YAG laser (pulse length 10 ns). Initial testing was carried out on new parchment to characterise the damage caused by overcleaning. The fluence and wavelength of the laser beam were then optimised so that any damage to the parchment or loss of ink during dirt removal from a late eighteenth century parchment document were minimised. The cleaned surfaces were examined by optical and scanning electron microscopy and analysed by measurement of the shrinkage temperature of cleaned collagen fibres. The laser-cleaned areas were compared with uncleaned areas and areas cleaned by traditional techniques.     

Key block theory application for rock slope stability analysis in the foundations of medieval castles in Slovakia

This research investigates the stability of rock slopes in the foundations of selected medieval castles in Slovakia. In the first phase, static analysis of the 45 selected medieval castle rock slopes was performed, where more than 12,000 potentially unstable blocks were analyzed and the factor of safety in static condition was calculated using the key block theory implemented in the Kbslope module of PTworkshop software. Based on results of the static stability analysis, a pseudo-static analysis was performed adopting the seismic acceleration in accordance with Slovak Technical Standards – Seismic actions on structures. This was implemented by calculating the vectors of horizontal force acting upon shear failure in the direction of the slope face with a zero vertical component. When non-finite and tapered blocks were ignored, the results proved that 14% of the 12,217 blocks investigated under static conditions could be considered unstable. This number increased to 23% under pseudo-static conditions, when seismic acceleration was implemented in the stability calculations. A detailed stability assessment of the Gymes Castle located in western Slovakia was carried out with delineation of blocks prone to rock sliding and proper stabilization methods, based on joint sets orientation measurements performed on the 3D point cloud generated by laser scanner.     

Experimental study on the effect of wavelength in the laser cleaning of silver threads

Laser removal of surface contaminants on silver threads was carried out using Nd:YAG laser radiations from near infrared (1064 nm) through visible (532 nm) to ultraviolet (266 nm) produced by frequency harmonic generation. The thread in the museum textile was made of silver and silk in which the silver ribbon wraps the bunch of silk fibres. The goal of this work is to find a feasibility to clean the tarnished silver without any damage of the underlying silk since the conventional chemical treatment is problematic to apply in this specific specimen. From the results, it was found that the laser wavelength of 266 nm is most appropriate to clean the silver surface without causing any damage either to the silver or the silk surfaces while 1064 nm wavelength easily causes damages such as melting and burning to the silver as well as the silk inside.     

An Icelandic medieval stave church made of drift timber: the implications of the wood identification

This paper aims to view the implications of an identification of wood species found during an archaeological excavation on a medieval church building and a surrounding graveyard at Thorarinsstadir in Seydisfjordur, east Iceland. The excavation in Seydisfjördur is a part of a project entitled ‘The transition from paganism to Christianity in Iceland’ sponsored by The East Iceland Heritage Museum, the Research Council of Iceland and The European Commission. It started in 1997 and from 1998 it has been a part of the PARABOW project, in the frame of the Raphael project. The project involves archaeological researches on pagan and Christian remains from the early medieval times in Iceland. The aim is to examine the origin and development of the Christian religion in Iceland. Furthermore, it is the intention to investigate how Christianity evolved alongside paganism assuming the conversion to Christianity was a long process, climaxing in the year 1000 when Christianity was adopted as a national religion. The wood identification uncovered evidence concerning church architecture, limiting actors for building traditions, Icelandersˈ ritual practices as well as their contacts with the foreign countries during the Viking Age and early medieval times. It did show that the native species were mainly used as fuel but drift timber was probably mainly used for construction purposes and for making coffins. According to the results from the wood identification, for instance, the excavation at Thorarinsstadir in Seydisfjördur revealed a wooden church made of drift timber.     

The role of the fracture plane's inclination in the restoration of marble epistyles

The influence of the inclination of the fracture plane (with respect to the axis of the bar used for restoration) on the stress and strain fields developed in the restored structural member is explored numerically by employing the Finite Element Method. Attention is focused to marble epistyles fractured into two pieces joined together with the aid of a single threaded titanium bar and suitable cement paste interposed between marble and titanium according to the technique introduced by the scientists working for the restoration of the Acropolis of Athens monuments. The numerical model is calibrated and validated according to experimental results obtained by submitting an accurate copy of a fractured epistyle of the Parthenon Temple (under an 1:3 scale) to bending. The analysis indicated that the inclination between the fracture plane and the axis of the reinforcing bar drastically influences the intensity of the stress field developed even for relatively small inclination angles.     

Study on some sorption properties of treated bentonites for their potential use as a moisture regulating system for the preservation of historical wooden elements

We report the first results of a research study aimed at developing a new strategy for the conservation of wooden structural elements present in historical buildings, based on moisture regulating systems. As has been happening for artefact preservation in museums, the idea is to develop systems based on the ability of some highly hygroscopic materials to moderate variations in relative humidity. These materials could adsorb and release moisture to reduce the extreme values of humidity in the micro-climate, for example between wooden beams and masonry. In order to experimentally verify this possibility using current, low cost and easy handling building materials, 5 bentonite samples were laboratory processed to improve their adsorbing properties by means of treatment with sodium carbonate at 3 concentrations: 2, 3 and 4% by weight. The effectiveness of ion exchange between sodium carbonate and bentonite was controlled by measuring the swelling volume of the bentonites. All the samples (n = 15) were tested for their hygroscopic properties. Adsorption isotherms were measured at 25 °C, using desiccators with silica gel, saturated salt solutions and bi-distilled water. A comparison between isotherms of one of the lower hygroscopic treated sample of bentonite and of a sample of wood and of a sample of brick and some numerical analyses with the Delphin code were made in order to evaluate the potential use of this bentonite as a moisture regulating system for the preservation of historical wooden elements. Results show that it seems to be possible to use bentonites as a moisture buffering material in order to reduce moisture content in wooden beams at least during their adsorption phase. It remains to investigate their desorption phase and their behaviour if they be in a saturation condition. Further studies are currently under way.     

Innovative uses of 3D digital technologies to assist the restoration of a fragmented terracotta statue

This paper describes how some innovative methodologies have been designed and employed to support the restoration of the Madonna of Pietranico, a terracotta statue severely damaged in the 2009 earthquake. The statue, fragmented in many pieces, has undergone a complex restoration performed by a multidisciplinary working group. The contribution of digital technologies was planned from the very beginning, since the complexity of this restoration originated the design of innovative procedures for managing the reassembly and restoration process. The Madonna test bed was therefore an example of how technology innovation could be pushed by clear application needs. A first important contribution was the study of the recombination hypothesis of the fragments. This initial phase was performed on digitized 3D models of the statue fragments, with the aim of reducing fragments manipulation, preventing further damages and increasing the capabilities to rehearse and evaluate different reassembly options. The accuracy of the 3D scanned models and the new recombination procedure introduced in this paper allowed to manage this phase in the digital domain with successful results. The digital 3D models were also used to design and produce an innovative supporting structure, constructed with a rapid prototyping device. Another important contribution concerned the study and virtual restoration of the polychrome decoration of the statue; our aim was to reproduce and restore in the virtual 3D domain the very complex original polychrome decoration, on the base of the remaining traces. Consequently, new virtual painting functionalities have been designed on the MeshLab platform (an open-source tool for 3D models visualization and manipulation) for reproducing pictorial decorations over digital 3D models and have been assessed on this specific test bed. This allowed us also to investigate the complexity of the virtual repainting process and to identify further technology enhancements. Finally, computer graphics technologies have been also used to produce a video that tells the story of the restoration.     

Virtual restoration of faces appearing in byzantine icons

Virtual restoration of cultural heritage (CH) artefacts is an important task that aims to digitally recreate the original appearance of damaged items. In this paper, a method that can be used for virtual restoration of faces appearing in damaged Byzantine icons is presented. Given a damaged face, the complete three-dimensional (3D) geometry of the face is reconstructed using data from the non-damaged facial parts and the texture of the damaged areas is restored. A key aspect of the proposed method is the use of a customized 3D deformable face model suitable for representing the geometry of Byzantine faces, the so-called Byzantine Style Specific Model (BSSM). A BSSM is generated by enforcing rule-based constraints on a deformable model trained using 3D scans of human faces. The use of a BSSM ensures that the Byzantine style is preserved during the process of shape restoration.     

An analysis of building methods: chemical-physical and archaeological analyses of micro-layer coatings on medieval facades in the centre of Genoa

The present research is an attempt to create a link among different disciplines and to confirm stratigraphic observations through chemical analyses. It involved detailed colorimetric examination and in-depth chemical-physical study of the samples taken from the historical centre of Genoa, which enabled connections to be made among many data. This led to the creation of an analytical method that is applicable to various geographical sites. Examinations involved several experimental techniques: X-ray diffractometry, scanning electron microscopy, optical microscopy and infrared spectroscopy; the data were compared with those from the stratigraphic examination. A detailed investigation of the Ligurian climate, including data kindly supplied by the Environment Department of the Provincia di Genova, allowed us to explain the presence of particular chemical substances and suggested how they might have been formed.     

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

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.     

A study of the effect of the wavelength in the Q-switched Nd:YAG laser cleaning of gilded wood

The removal of a brass-based paint (purpurin) used in painting gilded wood to cover losses of gold leaf, represents today a difficult task to conservators, who may have to resort to toxic chemical solvents in order to clean the painted surface. This action, due to its nature, is unsuitable for both the conservator and the artwork itself. In this study, a Q-switched Nd:YAG laser system outputting 1 064 nm (infrared), 532 nm (visible, green) and 266 nm (ultra-violet) radiation was used to study the interaction of these wavelengths in a gilded sample surface painted with purpurin. All tested wavelengths interacted differently with the tested surface and, also, the several layers that form the gilded surface (purpurin, varnish and gold) showed different interactions to each wavelength. The ultra-violet radiation (266 nm), in a multi-pulse mode at low fluences, was found to be the most efficient wavelength in cleaning the painted gilded surface.     

Diagnostic of the conservation state of antique Italian paintings on panel carried out at the Laboratorio di Restauro dell'Opificio delle Pietre Dure in Florence,Italy with ESPI-based portable instrumentation

The aim of this work was to use non-destructive optical measurement techniques to assess the conservation state of ancient Italian paintings and to experiment outside of the laboratory with the most recent ESPI (electronic speckle pattern interferometry) portable instruments developed within the Photonic Technologies and Diagnostic Laboratory of the European Commission Joint Research Centre of Ispra, Italy. The measurements described here took place at the Laboratori di Restauro dei Dipinti dell'Opificio delle Pietre Dure in Florence, Italy. The technique detects hardly visible and invisible defects on paintings on panel during the restoration phase and allowed the production of both qualitative and quantitative data, owing to its high resolution and sensitivity to thermal deformation. The system used allowed the inspection of a larger area (400×300 mm) in comparison to that reported in literature concerning continuous wave portable ESPI systems applied in the conservation field.