SPECTROSCOPIC ELLIPSOMETRY: A NON-DESTRUCTIVE TECHNIQUE FOR MEASURING THE OPTICAL PROPERTIES OF VARNISH FILMS ON PAINTINGS

• K. Polikreti, A. Othonos, C. Christofides (Photonics and Optoelectronics Laboratory, Dept. of Physics, University of Cyprus, P.O. Box 20537, Nicosia 1678, Cyprus)
• C. De Deyne

One of the most prominent problems in the diagnosis and conservation of paintings is varnish yellowing. Varnishes covering the painted surfaces gradually break down, resulting in a final haze, yellowish and obscured appearance (De la Rie 1988). Unfortunately, neither the exact composition of varnishes (natural resins) is known, nor the ageing process has been traced and well understood. Consequently no laboratory treatment has been proved to result in a reliable simulation of varnish ageing. All this problems have to be overcome in order to reveal the original appearance of the paintings, when the varnish layer was still fresh.

The specific aim of this presentation, at this initial stage of the project, is to measure the optical constants of varnish layers, together with their thickness, up to 10ìm. The technique used was UV-visible spectroscopic ellipsometry. The technique is based in the measurement of the change in the polarization level of light, a change induced by reflection (Azzam and Bashara 1987).

Samples of dammar (natural resin) and formo-phenolic (synthetic resin) varnishes, applied on acrylic paints were used. The real and imaginary parts of the refractive indexes of paints and varnishes were measured. Reproducibility of the measurements was ensured and the potential of the technique, for discriminating between different types of varnish was investigated. The errors included in the measurements were identified and estimated.

One of the errors included in the measurements, is generated from the fact that the surface of the paint is not smooth, even in minimal areas, due to the pigment particle aggregations, which protrude out of the dried media (Hunt et al. 1998). The error was minimized by representing the interface between paint and varnish by a homogenous medium with an effective refractive index, according to Bruggeman effective - medium approximation model (Spaeth et al. 1997). Another solutions for minimizing other sources of error will be discussed.

References

Azzam R. M. A. and Bashara N. M., Ellipsometry and Polarized Light, North Holland, Elsevier, Amsterdam, 1987.

De La Rie, R. E., Photochemical and thermal degradation of films of dammar resin, Studies in Conservation 33(2); 1988: 53-70.

Hunt, G. Y., Galler, M. A. and Martin, J. M., Microstructure of weathered paint and its relation to gloss loss: Computer simulation and modeling, Journal of coatings technology 70(880); 1998: 45-54.

Spaeth, K., Kraus, G. and Gauglitz, G., In-situ chracterisation of thin polymer films for applications in chemical sensing of volatile organic compounds by spectroscopic ellipsometry, Fresenius Journal of Analytical Chemistry 357; 1997: 292-296.

* The support of the EU under the RTD project (IST-1999-20163) - CRISATEL, is gratefully acknowledged.