Multi-analytical approach for the evaluation of the stability of painting materials for conservation purposes.

The paint layers of art objects are daily subject to aggression by atmospheric agents that, with different mechanisms of degradation, can damage them irreparably. The main causes of alteration in artistic materials are climate, weather and biological agents. The degradation patterns occur also in commercial materials used for conservation and restoration, sometimes with unknown and unpredictable mechanisms. The evaluation of stability of commercial products, used in conservation, is so fundamental in assessing their possible use and for this several techniques can be applied. A lot of techniques, requiring micro-samples, cannot be used in conservation especially do to the difficulty or impossibility to repeat the measurements in the same points and during time. The aim of this contribution is to investigate the stability of a selected number of painting materials by different spectroscopic techniques and under different ageing conditions. The aim is also to evaluate the sensitivity of the chosen techniques in identifying the variations on paint layers, induced by photo-degradation, before they could be observed by eye; furthermore, the aim is also to use, by multivariate approach, the supervised classification methods for monitoring the painted surface changes. As painting materials, starting from previous studies, some commercial water colours, supplied by Winsor&Newton, and a conspicuous group of natural ochre and earths (whose origin is known and applied by Arabic gum as binder in order to have the same binder of commercial water colours) were selected in order to compare their stability. All pigments were characterized by different analytical techniques such as: X-ray fluorescence spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, in order to know the composition of the chosen materials. These painting materials were applied on setting layers in order to create colour check tables with 54 different pigments, both organic and inorganic, for simulating the high variability on real artwork. One table was used as blank. The other two were aged under controlled UV irradiation. In particular one table was aged in Solar Box chamber under the following conditions: temperature 55 °C, irradiation power 550 W/m2, UV filter at 280 nm, relative humidity 50%. The other one was aged at ambient temperature under UV lamps Philips TUV36 operating at 36W, emission at 250 nm. Measurements were taken at 0, 196, 268 and 504 hours. Colour and chemical changes were investigated by reflectance spectrophotometry in the visible range through a X-Rite CA22 spectrocolorimeter and hyperspectral imaging (HSI), in particular in the SWIR range by Sisuchema XL (Specim®). The choice of these techniques allowed non-invasive and non-destructive control, exactly in the same points, of the variations in the painting samples before the changes could be observed by naked eyes. Data from colour measurements and HSI when compared at the same times of ageing.