New Insights into the Cleaning of Paintings

Extended Abstract—Noncontact and
Noninvasive Monitoring of Overpaint
Removal with Optical Coherence Tomography
Magdalena Iwanicka, Dagmara Kończalska, Piotr Targowski, and
Bogumiła J. Rouba
INTRODUCTION
Magdalena Iwanicka, Dagmara Kończalska,
and Bogumiła J. Rouba, Institute for the
Study, Restoration and Conservation of Cultural
Heritage, Nicolaus Copernicus University,
Gagarina 7, 87- 100 Toruń, Poland. Piotr
Targowski, Institute of Physics, Nicolaus
Coper nicus University, Grudziądzka 5, 87-
100 Toruń, Poland. Correspondence: Magdalena
Iwanicka, magiwani@gmail.com. Manuscript
received 19 November 2010; accepted
24 August 2012.
Optical coherence tomography (OCT) is a noncontact and noninvasive technique
of depth- resolved structural imaging within media that moderately scatter and/or absorb
near- infrared light. It originates from diagnostic medicine and has been under consideration
for examining art objects since 2003. A fairly complete list of papers on application
of OCT to examination of artworks may be found on the Web (http://www.oct4art.eu).
The technique uses low- coherence interferometry; therefore, the light source must be
characterized by low temporal (to ensure high axial resolution) and high spatial coherence.
The instrument used in the study had a superluminescent diode as a light source,
with an 845 nm central wavelength and a 107 nm bandwidth, resulting in a 4.5 µm
axial resolution in air (3 µm in varnish). The spectral domain modality of the technique
was chosen to ensure high sensitivity (over 100 dB) and speed of data acquisition. The
modular construction based on fiber optics makes it fairly portable, and thus, it does not
require optical table mounting. The system’s portability together with fast and straightforward
data acquisition makes OCT particularly well suited for quick evaluation of
conservation treatment (Liang et al., 2008).
The results obtained by means of this method are presented in a convenient manner
of cross- sectional images (called B- scans in analogy to ultrasonography) that are easy to
compare with conventional microscopic stratigraphy analysis. The advantage of OCT lies
not only in its noninvasiveness but also in its ability to examine a relatively wide area.
The described instrument provides B- scans of 15 mm width and 2 mm depth. Additional
scanning in the perpendicular direction gives 3- D information about the spatial structure
of the sample. Because of the high speed of data acquisition, the tomograph is capable of
3- D (volume) data collection within seconds. The obvious disadvantage of the method is
that the imaging is limited to media at least partially transparent to infrared light (Szkulmowska
et al., 2007). Therefore, OCT is mostly used for examination of transparent and
semitransparent structures, such as varnishes and glazes on easel paintings. It is, however,
possible to resolve the thickness and structure of these layers noninvasively in as many
places as desired. If the axial resolution permits, the superimposed layers of different
varnishes, retouches, and primary layers can be differentiated. The method may also be
used for identification of the location of certain pigmented layers within the varnish- glaze
structure. In this contribution, the applicability of OCT to monitor the removal of secondary
strata is shown, using an eighteenth- century painting as an example.