Digitally reconstructing Van Gogh's Field with Irises near Arles. Part 1: Varnish

Abstract

Varnish layers applied to paintings often discolor as they age, upsetting the original colour relationships intended by the artist. The removal of aged varnish layers using physical and chemical means is a highly skilled and often time-consuming operation, which is not lightly undertaken. There are many aspects under consideration before embarking upon such treatment, including the visual result inferred by spot cleaning tests. In this article, we develop a technique for digital removal of discolored varnish that can help to envisage how a painting will look following cleaning treatment. The digital technique was applied to Vincent van Gogh's painting Field with Irises near Arles (May 1888), in parallel to the painting actually being cleaned, which allowed direct validation of the method developed. In the new method, we utilized not only hyperspectral data from parts of the painting with and without the varnish, but also experts' identification of spots on the painting where unmixed white pigment has been applied. The physical model that we use is based on Kubelka-Munk twoconstant theory, commonly used to model the optical properties of paint. We show that with this model it is possible to determine the transmittance and reflectance of the varnish layer as function of wavelength. Results from previous studies confirm the calculated values. With the new method, we created a high-resolution digital image of the painting, as it would look after varnish removal, at a moment when the actual varnish was still present on the painting. The new method may help conservators and others involved in decisions made regarding issues of varnish removal from paintings, or may help to visualize the colors of a painting without discolored varnish in cases where its physical removal cannot be safely accomplished and so is not an option. K E Y W O R D S cultural heritage, Kubelka-Munk, varnish wileyonlinelibrary.com/journal/col Color Res Appl. 2018;43:150-157.

Digitally reconstructing Van Gogh'sField with Irises near Arlespart 3: Determining the original colors

“…Since the brightness of these types differs considerably, in Figure all data is normalized with respect to the maximum reflectance, also making it easier to recognize the color hue as indicated by the wavelength at which reflectance values increase. In this way it is clear that the reconstruction paint made from the “chrome yellow Otero” pigment, which has a rise in reflectance values at approximately 500 nm, is most similar to the pigment chrome yellow bright identified on the aged painting and also to the chrome yellow “type a” from Ref. .…”

“…. Similarly, the chrome yellow regular pigment could be associated with colorant chrome yellow dark on the aged painting, and to chrome yellow “type c” from Ref. .…”

“…When calculating the pigment concentration maps for the painting in our previous investigation, we had already found the need to distinguish 2 different types of chrome yellow pigment, which we called chrome yellow bright and chrome yellow dark . In Figure , we compare the spectral reflectance curves of these types of chrome yellow with each other and with reflectance data for 3 types of chrome yellow published by Kühn and Curran .…”

“…We will use them to describe the color mixing behavior of all pigments in Field with Irises near Arles , in the state these pigments presumably had originally, when Van Gogh created the painting. In our previous study, we determined the variation in concentrations of all pigments spread across the painting, resulting in pigment concentration maps . We will combine the pigment concentration maps with the values of the Kubelka‐Munk parameters derived from the reconstruction paints for the pigments, in order to predict the original colors of the painting.…”

Digital color reconstructions of cultural heritage using color‐managed imaging and small‐aperture spectrophotometry

“…This ideal approach was largely realized by Kirchner et al to reconstruct van Gogh's Field with Irises near Arles . Hyperspectral visible spectrum imaging, XRF imaging, various analytical techniques, and the general form of Kubelka‐Munk (K‐M) turbid media theory were used to identify, map, and estimate concentrations.…”