We have investigated the visual texture properties of effect coatings. The key question addressed in this work is how the visual texture properties of effect coatings can be visually assessed in a reproducible way.We show that it is possible to define two important visual texture parameters: Diffuse Coarseness and Glint Impression. These two parameters describe the visual texture under two different illumination conditions: diffuse and directional lighting, respectively. Strict definitions of these illumination conditions were found to be crucial when discussing visual texture of effect coatings. For each of these two visual texture parameters, experiments were set up, and observation procedures were designed that standardize the observation conditions. Visual tests were performed on both visual texture parameters. We found good results in terms of observer-repeatability and observer-accuracy. Our results show that if the visual texture of effect coatings needs to be visually characterized under different illumination conditions, such as different light booths or changing weather conditions, both diffuse and directional illumination should be included separately.

We have developed new parameters that help to interpret multi‐angle reflection data. By expressing measurement geometries in terms of so‐called flake‐based parameters, a physical explanation is given for several well‐known colorimetric characteristics of effect coatings. The new concepts are based on expressing angles with respect to the normal vector of flakes, instead of the conventional usage of the coating normal as a reference.  We discuss and give physical explanations for the usefulness and limitations of previously defined concepts such as aspecular angle, cis‐ versus trans‐geometries, interference lines and aspecular lines.  For example, it is known that the trans‐15° geometry has added value next to the cis+15° geometry when samples are analyzed that contain colored effect pigments with large color travel, such as Colorstream, Chromaflair, or chromatic Xirallic pigments. Our results demonstrate and explain why there is much less added value in the case of samples that contain less striking chromatic effect pigments, such as metallic or mica‐based effect pigments, or “white” Xirallic pigments. Further, interference lines are shown to be suitable for characterizing effect pigments because they selectively sample the reflectance values of flakes as a function of the angle of incidence with respect to the flake normal.  In another example, the similarity between an off‐plane measurement geometry and several in‐plane geometries is investigated. The results are clearly explained when referring to the corresponding values of the flake‐based parameters. © 2011 Wiley Periodicals, Inc. Col Res Appl, 2012

We have investigated how texture and color combine when assessing the appearance of special effect coatings. In a previous study, the most important aspects of the texture of special effect coatings were identified as diffuse coarseness and glint impression. In the present study, objective measurements of these parameters are carried out using a recently developed instrument, and these data are combined with spectrophotometer data under six geometries. The instrumental data and visual data are analyzed using statistical techniques. From these visual data we were able to create a total appearance score which strongly correlates with the measurements of color and texture differences using this instrument. In this way a calculation procedure has been developed yielding predictions on acceptability that correlate well with visual judgments. Also, the added value of reflection data close to the gloss angle is demonstrated.

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.

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.