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.
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.
KEYWORDS: Digital image correlation, Humidity, 3D image processing, Climatology, Cameras, 3D modeling, 3D metrology, Climate change, 3D acquisition, Data modeling
A non-invasive and non-contact optical method for tracking overall and local deformations of canvas painting is
presented. The technique was tested on a model painting on canvas with inhomogeneities introduced by infilling gaps,
mending tears, and applying patches on the reverse of the canvas. The deformation of the sample was induced by
changes of relative humidity. The feasibility of 3D Digital Image Correlation technique for evaluation of conservation
methods is discussed.
In this contribution a proof of concept for the alternate way of twofold increasing the axial resolution of Optical
Coherence Tomography systems is shown. On the contrary to expanding the bandwidth of the light source, the number
of passes of light between sample and the Michelson interferometer is increased. In two simplified novel configurations
of Spectral OCT devices designed for this research, the interferometer is equipped with polarization controlling elements
in order to force light to pass the distance from the beam splitter to the sample four times: during the first pass the initial
linear polarization of the probing beam is converted to the perpendicular one and on return to the interferometer
deflected by the polarization sensitive beam splitter towards the additional mirror reflecting it back to the sample. After
the second pass the state of polarization is changed again and restored to the initial one in order to interfere with the
reference beam. As a result in both set-ups optical paths difference between both arms of the Michelson interferometer is
twofold longer comparing to the standard system. This results in two times smaller axial calibration coefficient and
finally twofold increase of an effective axial resolution for the same coherence length of the light source. In the paper the
experimental evidences are given and limitations of the method discussed.
In this contribution we describe an apparatus for precise laser ablation of delicate layers, like varnish on pictures. This
specific case is very demanding. First of all any changes in colour of remaining varnish layer as well as underneath paint
layers are unacceptable. This effect may be induced photochemically or thermically. In the first case strong absorption of
the radiation used will eliminate its influence on underlying strata. The thermal effect is limited to so called heat affected
zone (HAZ). In addition to colour change, a mechanical damage caused by overheating of the structure adjacent to
ablated region should be considered also. All kinds of treads must be carefully eliminated in order to make laser ablation
of varnish commonly accepted alternative to chemical and/or mechanical treatments [1].
Since the varnish ablation process is obviously irreversible its effective monitoring is very important to make it safe and
trusted. As we showed previously [2-6] optical coherence tomography (OCT) originated from medicine diagnostic method
for examination and imaging of cross-sections of weakly absorbing objects can be used for this task. OCT utilises infrared
light for non-invasive structure examination and has been under consideration for the examining of objects of art since 2004
[7-10]. In this case the in-depth (axial) resolution is obtained by means of interference of light of high spatial (to ensure
sensitivity) and very low temporal coherence (to ensure high axial resolution). In practice, IR sources of bandwidths from
25 to 150 nm are utilised. Resolutions obtained range from 15 down to 2 μm in the media of refracting index equal 1.5.
In this contribution we expand application of OCT to space resolved determination of ablation rates, separately for every
point of examined area. Such data help in better understanding of the ablation process, fine tuning the laser and finally
permit increase of the safety of the ablation process.
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.
Optical Coherence Tomography (OCT) is an interferometric method utilising light of low temporal coherence for noninvasive
structural imaging of objects weakly absorbing and scattering light. In this contribution, using various examples
of images of objects made of glass affected by the atmospheric corrosion and/or by crizzling, we demonstrate a software
developed in our laboratory specifically for 3D OCT imaging of samples with a fine structure. For this task we employed
the OpenGL platform (Open Graphics Library), an Application Programming Interface (API) for writing applications
dedicated to interactive 3D computer graphics. In our application we have utilized texture rendering with a modulation of
transparency and a colour as a function of elevation.
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.
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