HIGH RESOLUTION-MAGIC ANGLE SPINNING (HR-MAS) NMR ANALYSIS OF
SOLID PAINT VARNISHES
Kartsonaki Eypraxia1, Spinella Alberto2, Knuutinen Ulla3, Caponetti Eugenio2, Anglos
Demetrios1,4 and Spyros Apostolos1
1
Chemistry Department, University of Crete, Heraklion Crete, Greece, aspyros@chemistry.uoc.gr;
2
Centro Grandi Apparecchiature, University of Palermo, Palermo, Italy, caponett@unipa.it;
3
Helsinki Metropolia University of Applied Sciences, Helsinki, Finland,
ulla.knuutinen@metropolia.fi; 4 Institute of Electronic Structure & Laser, FORTH, Heraklion
Crete, Greece
Keywords: varnishes, NMR spectroscopy, solid state
Objectives. The word “varnish” - sometimes the word “lacquer”- is used to describe any type of
surface coating. Varnishes are made of resins which contain mainly terpenes. Resins are produced
by plants or they can have fossilized origin. Historically, there are three distinct types of historic
varnishes: spirit, essential oil, and fixed-oil varnishes based on their use and application. Varnishes
are important in cultural heritage materials research because of their role in protecting the outer
surface of paintings and other artifacts [1]. The identification and compositional analysis of
varnishes can assist the restoration process during conservation treatments and provide
information about the state of a painting [2], and previous restoration attempts. In this study we
examine the potential of HR-MAS NMR spectroscopy for the direct identification and analysis of
natural terpenoid resins in model paint varnishes directly in the solid state and without any sample
treatment.
Experimental. Resin samples (copal, dammar, colophony, mastic, sandarac) were used to prepare
model aged varnishes as follows: resins were dissolved in acetone, mixed with linseed oil in 50-50
ratio per weight, cast on glass plates and aged in the laboratory under ambient conditions for two
years. HR-MAS 1D and 2D NMR spectra were obtained with a Bruker Avance II 400 MHz
spectrometer.
Results. The 1H HR-MAS NMR spectra of solid
model varnishes were found to be dominated by
their linseed oil component, and thus to appear
practically identical. However, it was found that
the 2D HR-MAS NMR correlation spectra (either
1
H-1H gCOSY or 1H-13C gHSQC) of all varnishes
contain cross peaks that can be used to identify
the presence of a specific resin in the varnish.
These peaks were assigned by comparison with
2D NMR spectra of pure resins in solution. As an
example, Fig. 1 depicts the 2D proton correlation
NMR spectrum of aged solid copal varnish. The
two peaks indicated by arrows were assigned to
the side chain of copalic acid, a constituent of
natural copal. Using either gCOSY or gHSQC
HR-MAS 2D spectra, it was possible to
successfully identify all five resins in aged model
varnishes prepared with linseed oil.
Fig. 1 HR-MAS 1H-1H gCOSY NMR
Conclusions. The examination of several model
varnishes based on drying oil via HR-MAS NMR
spectrum of a solid copal varnish
spectroscopy shows that 2D NMR spectra can be
containing linseed oil that was aged for
used for the identification of the natural resin
two years. Arrows denote peaks originating
used in the varnish, even in the case that the
from copalic acid, a copal component.
varnish has been considerably aged. Further
studies using varnishes sampled from works of
art are in progress.
References.
1. JS Mills, R. White, The organic chemistry of museum objects, 2nd edn., Butterworth
Heinemann, Oxford, 1994.
2. A. Spyros and D. Anglos, Study of Aging in Oil Paintings by 1D and 2D NMR Spectroscopy,
Analytical Chemistry, 76 (2004), 4929-4936.