Cellulose Acetate Based Chromogenic Reversal Films
Channel: AIC & FAIC
Date: 2016-08-19

By Joana Silva, A. M. Ramos, J.L. Ferreira, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa; B. Lavédrine, Muséum National d'Histoire Naturelle
From the Plastics Associated with Photographic Materials symposium

Abstract: Ângelo de Sousa (1938-2011) was a prestigious contemporary Portuguese artist, known for an experimentalism that enabled him to achieve exceptional originality. He worked with different materials and since the 1960s produced a very interesting work in photography.

Interested and informed about colour theories and perception, Ângelo de Sousa made many works in which he explored the primary colours as a way to achieve the maximum effect with
minimum resources. For a long period, he decided to privilege the primary additive colours, red, green and blue (RGB), using light as a source for image formation, as for instance in his work Slides de Cavalete (1978-1979), a diaporama composed of one hundred slides.

His photographic work is being studied in the context of a PhD thesis dedicated to his photographic and filmic collection, and specifically, to colour fading in the chromogenic reversal films. Prior to this research, no precise information about Ângelo de Sousa´s photographic and filmic collection had been collected and systematized.

After surveying the collection, which allowed to access the typology and global conservation condition based on visual examination, it is possible to say that the photographic collection is composed of circa 85600 images: 45% of the images are negatives, 40% are positive transparencies and 15% are prints. The most representative type of photographs are 35mm
black and white negatives on cellulose acetate support (43.5 % of the collection), and 35 mm chromogenic reversal films on cellulose acetate (31 % of the collection).

Although at least 84% of the collection is composed of cellulose acetate based photographs, no vinegar syndrome was detected. The most alarming degradation observed was the colour shift
in chromogenic materials, particularly in chromogenic reversal films.

Chromogenic films have multi-layer structure: a plastic support, three superimposed coloured gelatine based emulsions (cyan, magenta and yellow layers), among others. In the nonprocessed
film only silver salts and colour couplers are present in each layer. The dyes are produced during processing, where a chemical reaction between the colour couplers (colourless) and the oxidized colour developer (after development of the silver salts) occurs. After processing, the residual silver is removed and only dye clouds are left.

Dyes are susceptible to degradation, some more than others, induced by relative humidity, temperature and/or light, although producing different degradation products. Residual colour
couplers are also vulnerable to oxidation producing the yellowing of the image, and may interact with the dyes accelerating dye fading.

When the artistic intention relied unequivocally on the “plasticity” of colour, due to the abstract nature of the image, it is difficult to understand if dye fading and/or change in colour balance are occurring and, if so, to what extent. The loss of colour in this case may take particularly serious proportions, putting the artist intension at risk.

Therefore, in this study it is intended to develop a methodology to monitor the fading of chromogenic reversal films using non-invasive analytical techniques, such as spectrophotometry and colourimetry. UV/Visible Spectrophotometers which allow spectral and colorimetric analysis will be tested in order to assess the rate of degradation of the different dyes, by following the absorption spectra and CIELab coordinates.

Speaker bio: Joana Silva has completed the Master degree in Conservation and Restoration, with specialization in photography, in Faculdade de Ciências e Tecnologia, Universidade NOVA (Portugal) in 2009. She worked for Luís Pavão Lda. between 2009-2014 at the Art Library from Fundação Calouste Gulbenkian. Presently she is doing her Ph.D. in Conservation and Restoration of Cultural Heritage at the same University.