Cristina Montagner

Library of UV-Vis-NIR reflectance spectra of modern organic dyes from historic pattern-card coloured papers.

An accurate characterisation of the organic dyes used in artworks, especially those made of paper, is an important factor in designing safe conservation treatments. In the case of synthetic organic dyes used in modern works of art, for example, one frequently encountered difficulty is that some of these dyes are not still commercially available. Recognizing this problem, the authors of this paper present the results of an analysis of UV-Vis-NIR fibre optic reflectance spectra of 82 samples of dyed paper prepared with 41 dyes. The samples come from a historic book, The Dyeing of Paper in the Pulp, which was published by Interessen-Gemeinschaft (I.G.) Farbenindustrie in 1925. The dyes used in the paper pulp belong to the azo compounds, acridine, anthraquinone, azine, diphenylmethane, indigoid, methine, nitro, quinoline, thiazine, triphenylmethane, sulphur and xanthene classes.

Ochres and earths: matrix and chromophores characterization of 19th and 20th century artist materials.

The present paper describes the main results obtained from the characterization of a wide range of natural and synthetic ochre samples used in Portugal from the 19th to the 20th century, including powder and oil painting samples. The powder ochre samples came from several commercial distributors and from the collection of Joaquim Rodrigo (1912-1997), a leading Portuguese artist, particularly active during the sixties and seventies. The micro-samples of oil painting tubes came from the Museu Nacional de Arte Contemporânea-Museu do Chiado (National Museum of Contemporary Art-Chiado Museum) in Lisbon and were used by Columbano Bordalo Pinheiro (1857-1929), one of the most prominent naturalist Portuguese painters. These tubes were produced by the main 19th century colourmen: Winsor & Newton, Morin et Janet, Maison Merlin, and Lefranc. The samples have been studied using μ-Fourier Transform Infrared Spectroscopy (μ-FTIR), Raman microscopy, μ-Energy Dispersive X-ray fluorescence (μ-EDXRF), and X-ray diffraction (XRD). The analyzed ochres were found to be a mixture of several components: iron oxides and hydroxides in matrixes with kaolinite, gypsum and chalk. The results obtained allowed to identify and characterize the ochres according to their matrix and chromophores. The main chromophores where identified by Raman microscopy as being hematite, goethite and magnetite. The infrared analysis of the ochre samples allowed to divide them into groups, according to the composition of the matrix. It was possible to separate ochres containing kaolinite matrix and/or sulfate matrix from ochres where only iron oxides and/or hydroxides were detected. μ-EDXRF and Raman were the best techniques to identify umber, since the presence of elements such as manganese is characteristic of these pigments. μ-EDXRF also revealed the presence of significant amounts of arsenic in all Sienna tube paints.

Statistics of colors in paintings and natural scenes

Painters reproduce some spatial statistical regularities of natural scenes. To what extent they replicate their color statistics is an open question. We investigated this question by analyzing the colors of 50 natural scenes of rural and urban environments and 44 paintings with abstract and figurative compositions. The analysis was carried out using hyperspectral imaging data from both sets and focused on the gamut and distribution of colors in the CIELAB space. The results showed that paintings, like natural scenes, have gamuts with elongated shapes in the yellow-blue direction but more tilted to the red direction. It was also found that the fraction of discernible colors, expressed as a function of the number of occurrences in the scene or painting, is well described by power laws. These have similar distribution of slopes in a log-log scale for paintings and natural scenes. These features are observed in both abstract and figurative compositions. These results suggest that the underlying chromatic structure of artistic compositions generally follows the main statistical features of the natural environment.

Features combination for art authentication studies: brushstroke and materials analysis of Amadeo de Souza-Cardoso

This work presents a tool to support authentication studies of paintings attributed to the modernist Portuguese artist Amadeo de Souza-Cardoso (1887-1918). The strategy adopted was to quantify and combine the information extracted from the analysis of the brushstroke with information on the pigments present in the paintings. The brushstroke analysis was performed combining Gabor filter and Scale Invariant Feature Transform. Hyperspectral imaging and elemental analysis were used to compare the materials in the painting with those present in a database of oil paint tubes used by the artist. The outputs of the tool are a quantitative indicator for authenticity, and a mapping image that indicates the areas where materials not coherent with Amadeo’s palette were detected, if any. This output is a simple and effective way of assessing the results of the system. The method was tested in twelve paintings obtaining promising results.

Estimating the Colors of Paintings

Observers can adjust the spectrum of illumination on paintings for optimal viewing experience. But can they adjust the colors of paintings for the best visual impression? In an experiment carried out on a calibrated color monitor images of four abstract paintings obtained from hyperspectral data were shown to observers that were unfamiliar with the paintings. The color volume of the images could be manipulated by rotating the volume around the axis through the average (a*, b*) point for each painting in CIELAB color space. The task of the observers was to adjust the angle of rotation to produce the best subjective impression from the paintings. It was found that the distribution of angles selected for data pooled across paintings and observers could be described by a Gaussian function centered at 10o, i.e. very close to the original colors of the paintings. This result suggest that painters are able to predict well what compositions of colors observers prefer.