Magdalena Iwanicka

Structural Examination of Easel Paintings with Optical Coherence Tomography

Identification of the order, thickness, composition, and possibly the origin of the paint layers forming the structure of a painting, that is, its stratigraphy, is important in confirming its attribution and history as well as planning conservation treatments. The most common method of examination is analysis of a sample collected from the art object, both visually with a microscope and instrumentally through a variety of sophisticated, modern analytical tools. Because of its invasiveness, however, sampling is less than ideally compatible with conservation ethics; it is severely restricted with respect to the amount of material extirpated from the artwork. Sampling is also rather limited in that it provides only very local information. There is, therefore, a great need for a noninvasive method with sufficient in-depth resolution for resolving the stratigraphy of works of art. Optical coherence tomography (OCT) is a noninvasive, noncontact method of optical sectioning of partially transparent objects, with micrometer-level axial resolution. The method utilizes near-infrared light of low intensity (a few milliwatts) to obtain cross-sectional images of various objects; it has been mostly used in medical diagnostics. Through the serial collection of many such images, volume information may be extracted. The application of OCT to the examination of art objects has been in development since 2003. In this Account, we present a short introduction to the technique, briefly discuss the apparatus we use, and provide a paradigm for reading OCT tomograms. Unlike the majority of papers published previously, this Account focuses on one, very specific, use of OCT. We then consider two examples of successful, practical application of the technique. At the request of a conservation studio, the characteristics of inscriptions on two oil paintings, originating from the 18th and 19th centuries, were analyzed. In the first case, it was possible to resolve some questions concerning the history of the work. From an analysis of the positions of the paint layers involved in three inscriptions in relation to other strata of the painting, the order of events in its history was resolved. It was evident that the original text had been overpainted and that the other inscriptions were added later, thus providing convincing evidence as to the paintings true date of creation. In the second example, a painting was analyzed with the aim of confirming the possibility of forgery of the artists signature, and evidence strongly supporting this supposition is presented. These two specific examples of successful use of the technique on paintings further demonstrate how OCT may be readily adaptable to other similar tasks, such as in the fields of forensic or materials science. In a synergistic approach, in which information is obtained with a variety of noninvasive techniques, OCT is demonstrably effective and offers great potential for further development.

Depth-Resolved Multilayer Pigment Identification in Paintings: Combined Use of Laser-Induced Breakdown Spectroscopy (LIBS) and Optical Coherence Tomography (OCT)

A detailed feasibility study on the combined use of laser-induced breakdown spectroscopy with optical coherence tomography (LIBS/OCT), aiming at a realistic depth-resolved elemental analysis of multilayer stratigraphies in paintings, is presented. Merging a high spectral resolution LIBS system with a high spatial resolution spectral OCT instrument significantly enhances the quality and accuracy of stratigraphic analysis. First, OCT mapping is employed prior to LIBS analysis in order to assist the selection of specific areas of interest on the painting surface to be examined in detail. Then, intertwined with LIBS, the OCT instrument is used as a precise profilometer for the online determination of the depth of the ablation crater formed by individual laser pulses during LIBS depth-profile analysis. This approach is novel and enables (i) the precise in-depth scaling of elemental concentration profiles, and (ii) the recognition of layer boundaries by estimating the corresponding differences in material ablation rate. Additionally, the latter is supported, within the transparency of the object, by analysis of the OCT cross-sectional views. The potential of this method is illustrated by presenting results on the detailed analysis of the structure of an historic painting on canvas performed to aid planned restoration of the artwork.

Absolute LIBS stratigraphy with optical coherence tomography

In this contribution preliminary studies on the application of Optical Coherence Tomography (OCT) to absolute depth calibration of Laser Induced Breakdown Spectroscopy (LIBS) data in application to revealing stratigraphy of easel paintings are presented. The procedure of in-situ monitoring of LIBS by means of OCT is described. Numerical method developed for precise extraction of the depth of the LIBS ablation crater is explained. Results obtained with model paintings are discussed.

The application of macro-X-ray fluorescence and optical coherence tomography for examination of parchment manuscripts

Abstract Macro-X-ray fluorescence (XRF) is a newly commercially available research tool very useful in the examination of artwork. Its novelty lies in its ability to create maps of the distribution of chemical elements on scales of a few milimetres. In this contribution, its use together with optical coherence tomography (OCT) in the inspection of an illuminated manuscript is reported for the first time. The former technique is used both for mapping the elemental distribution over large parts of the folios – including illuminated initials – and for quantitative analysis of the composition of the smalt pigment, as well as of changes in the composition of iron–gall ink at different pages. The latter, by providing cross-sectional images of painted details, helps in interpreting the XRF results. All of the results shown relate to the examination of a late sixteenth-century-illuminated parchment manuscript (a gradual) originating from the Convent of the Benedictine Sisters in Lviv in the Ukraine.

OCT structural examination of 'Madonna dei Fusi' by Leonardo da Vinci

Madonna dei Fusi (‘Madonna of the Yarnwider’) is a spectacular example of Italian Renaissance painting, attributed to Leonardo da Vinci. The aim of this study is to give an account of past restoration procedures. The evidence of a former retouching campaign will be presented with cross-sectional images obtained non-invasively with Optical Coherence Tomography (OCT). Specifically, the locations of overpaintings/retouchings with respect to the original paint layer and secondary varnishes will be given. Additionally, the evidence of a former transfer of the pictorial layer to the new canvas support by detecting the presence of its structure incised into paint layer will be shown.

Application of optical coherence tomography (OCT) for real time monitoring of consolidation of the paint layer in Hinterglasmalerei objects

Optical coherence tomography (OCT) is a fast non-contact and non-invasive technique for examination of objects consisting of transparent or semitransparent layers. Since it is a useful tool for inspection of Hinterglasmalerei paintings, the aim of the experiment was to explore its feasibility for monitoring of the consolidation process, which plays the most important role in the conservation treatment of such artefacts.

The potential of OCT for assessing laser assisted removal of deposits from ceramic tiles

Optical Coherence Tomography (OCT), the technique of non-invasive imaging of transparent and semi-transparent sub-surface layers was employed to monitor the state of glazes of historic tiles before and after laser ablation trials. The laser used was Nd:YAG Thunder Art system from LightForArt (El. En. S.p.A.). In this paper, OCT cross-sectional images of both fragments and preserved historic tiles before, during and after laser ablation are shown to examine the process of the removal of surface deposits as well as mid-twentieth century conservation coatings. In case of some of the ceramic fragments (donated to science) the destruction of the glaze layer after a few steps of laser cleaning was induced intentionally and then visualized with OCT. The experiments performed confirmed that OCT is a potential diagnostic tool for the assessment and monitoring of laser removal of deposits from semi-transparent glazes on historic tiles, as well as enabled to define and identify specific risk factors for these objects, such as pre-existing cracks or delaminations within the glaze layer.

Optical Coherence Tomography for non-destructive investigations of structure of easel paintings

In this contribution the application of Optical Coherence Tomography (OCT) for non-invasive structural imaging of easel paintings will be presented. Since the technique permits imaging semi-transparent layers accessible for infrared light, the varnish and glaze layers are usually under investigation. The major emphasis will be laid on application of OCT to resolving specific conservation problems, arising during the restoration process. The examples of imaging multilayer varnishes and subsequent alterations will be given and the application of these images for authentication of inscriptions will be discussed. Since the thickness of imaged layers may be directly measured with OCT in completely non-destructive, quick and convenient way as many times as necessary, the application of the technique to generation of varnish thickness maps will be presented.

Using Optical Coherence Tomography to Reveal the Hidden History of The Landsdowne Virgin of the Yarnwinder by Leonardo da Vinci and Studio

Optical coherence tomography (OCT) was used for non-invasive examination of a well-known, yet complex, painting from the studio of Leonardo da Vinci in combination with routine imaging in various bands of electromagnetic radiation. In contrast with these techniques, OCT provides depth-resolved information. Three post-processing modalities were explored: cross-sectional views, maps of scattering from given depths, and their 3D models. Some hidden alterations of the painting owing to past restorations were traced: retouching and overpainting with their positioning within varnish layers as well as indications of a former transfer to canvas.