With an image processing program equipped with color channels, such as Photoshop, it is possible to obtain a “false color” rendering in infrared that simulates the results obtained with Kodak IR color film.
It is necessary to have a digital camera with the infrared shooting function, often referred to as night-vision, with which an image in light and dark values, ie in black and white is obtained. The image must be captured by placing a visible light barrier filter (87 or 87C by Kodak) in front of the lens in order to record only infrared radiation.
With the photo editing program the IR image in grayscale will be imported and its contrasts adjusted with the appropriate filter.
The dark spots are due to the different reflectance of the pigments used in the restoration compared to the original ones: different pigments of the same color may (not always) have a different infrared response, this can allow the identification and therefore the localization of the pictorial retouching.
It is also necessary to have a tripod on which to place the camera and capture the painting, from the exact same position, with a normal color photograph.
The color photograph will be split into three distinct images (blue, green, red) with the command “split channels”.
Then, with all the four images loaded into the photo editing program, we will use the “merge channels” command. In the dialog box you will select: RGB color, 3 channels. Then you will specify the channels so that the IR image will be attributed to the red channel, the red image to the green channel and the green image to the blue channel (the blue will be excluded).
With colour infrared photography it will be possible to make a first hypothesis on the pigments used by the artist during the execution of the work and also on those of the restorer who performed the retouching (1). The “fake colour” can in fact provide useful information on the identity of a pigment by comparison. However, it is risky to draw conclusions based only on the observation of the restitution in IR color also because pigments were often used mixed or superimposed in various layers.
Artificial ultramarine blue, cobalt blue, cerulean blue and smaltino are seen in false color as a red color, indigo as an intense red color, while lapis lazuli or natural ultramarine blue are seen as a burgundy color. Azurite and Prussian blue keep roughly the blue colors they have in visible light.
Cadmium red, vermilion and minium are seen in false colors with various shades of yellow; crimson lacquer and “robbia” lacquer (pink madder lacquer) tend to shift towards dirty orange tones. Burnt Sienna (like the natural one) and Venetian red turn out instead as a dirty green or brown color.
(1) Sullo studio per l’identificazione dei pigmenti cfr. Matteini A. Moles P. Tiano, L’infrarosso colore nell’indagine dei dipinti, in – Le Scienze – , giugno 1980, pp. 40-48
Si veda anche lo studio di Roberta Iannaccone, Tecniche di imaging innovative per la messa a punto di un protocollo integrato per la caratterizzazione dei pigmenti utilizzati nell’antichità, Tesi di Dottorato di Ricerca in Scienza per la Conservazione Beni Culturali, Università degli studi di Firenze 2014, p. 55
Sui principi di funzionamento nell’infrarosso delle fotocamere digitali cfr. Analisi ottica dei dipinti in falso colore, Università degli Studi di Milano, SEZIONE DI FISICA PER I BENI CULTURALI, L’AMBIENTE E IL CLIMA, Metodologie Fisiche per i Beni Culturali
Sulle applicazioni dell’esame ad alcune opere d’arte si veda Elena Toffoletto, Analisi non invasive per la caratterizzazione di dipinti ad olio su tela, conservati nel museo Ca’ Rezzonico di Venezia, lTesi di Laurea Corso di Laurea magistrale in Scienze Chimiche per la conservazione e il restauro 2014-2015, Università Ca Foscari, Venezia