Digitally reconstructing Van Gogh'sField with Irises near Arles.Part 2: Pigment concentration maps

Abstract

Colors in many paintings of great art historical value have changed over time, due to the combined effects of natural ageing, accumulated surface grime, and materials added during later conservation treatments. The physical restoration of the colors in such paintings is not possible. This article describes one part of work done to digitally restore the colors of Van Gogh's painting Field with Irises near Arles, dating from May 1888. We have used multispectral reflectance data to estimate absorption K and backscattering S parameters of Kubelka-Munk 2-constant theory. This was done for all 13 pigments known to have been used by Van Gogh in this painting, and based on this the concentration maps for each of these pigments were calculated. We validated the calculated concentration maps in several ways. For some pigments, we were able to predict spots on the painting where the pigment is expected to occur in unmixed form based on visual examination. For several other pigments, the concentration maps could be shown to agree with XRF data. Finally, for some other pigments the concentration maps were supported by additional evidence from microscopic examinations, remarks in Van Gogh's letters and from early color reproductions. For the 1.7 million pixels for which multispectral data is available, the average color difference between the calculated and measured spectral reflectance curves is CIEDE2000 5 1.05. This further confirms that the Kubelka-Munk calculations are well suited to describe the variety of spectral reflectance on the painting. K E Y W O R D S Color formulation, cultural heritage, pigments wileyonlinelibrary.com/journal/col Color Res Appl. 2018;43:158-176.

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

“… Reflectance curves for 2 types of chrome yellow, as found theoretically for the reconstruction pigments chrome yellow regular and Otero (markers). These are compared to spectral reflectance (solid lines) of 3 types of chrome yellow as defined by Kühn and Curran, and also to reflectance curves for the 2 types of chrome yellow identified in the painting…”

“…In section 3, we showed how we determined the Kubelka Munk K and S values of the reconstruction paints for all pigments, thereby representing the best possible estimation of their coloristic properties at the time Van Gogh created the painting. In order to visualize the original colors of Field with Irises near Arles as well as possible, we combined these K and S values with the pigment concentration maps that we had determined earlier . In this way, we were able to calculate the original reflectance values at each pixel of the multispectral image.…”

“…These are the pigments for which the largest contribution to discoloration is expected. For all other pigments, the reconstruction in Figure uses the optical data of the aged pigments instead of the corresponding reconstruction paints. Therefore, Figure visualizes how the models predict that the colors of the painting would be if only pigments chrome yellow, eosin, and cochineal would change color.…”

“…This article investigates the digital reconstruction of a painting by Vincent Van Gogh, Field with Irises near Arles , dating from May 1888. In previous articles, we described how we were able to compensate for the non‐original yellowed varnish, and how we calculated the spatial distribution and concentrations of all the pigments used by Van Gogh in this painting …”

“…In section 3, we describe how we determined optical properties of the reconstructed pigments. Using the non‐hiding Kubelka‐Munk model, section 4 shows how we reconstructed the original colors of Field with Irises near Arles , as calculated using the pigment concentration maps that we had already determined . In section 5, we discuss the implications of the reconstruction for describing the color composition and choices made by the painter.…”