Instrumental Colour Mixing to Guide Oil Paint Artists
Total Page:16
File Type:pdf, Size:1020Kb
Journal of the International Colour Association (2019): 24, 24-34 Kirchner Instrumental colour mixing to guide oil paint artists Eric Kirchner Color Technology Group, AkzoNobel Paints & Coatings, 2170 BA Sassenheim, the Netherlands Email: [email protected] Painters use a creative but time-consuming process to mix differently coloured pigment paints in order to obtain an intended colour. During the past decades it became technically possible to achieve this task also using instruments. However, their high prices have prevented painters from making this choice. Very recently, a new generation of colour instruments became available at much lower prices, making this an affordable approach for both professional and amateur painters. We selected one such instrument, and tested its ability to help painters in mixing pigment paints to achieve a desired colour. Our approach started by choosing a limited set of pigmented oil paints from an artist line of a commercial brand (Van Gogh line from Royal Talens). We applied these paints in a range of well-chosen mixtures to determine the spectral properties of each pigment paint. Using a spreadsheet program and an optical model we were then able to create any desired colour by calculating the optimum mixture of pigment paints. In this article, we test the accuracy of this method for use by artist painters. Received 12 May 2019; revised 27 June 2019; accepted 10 July 2019 Published online: 22 July 2019 Introduction A basic understanding of colour theory is fundamental to painting. But traditional colour theories offer little guidance on creating the intended colours [1], and the artist's colour wheel seems to hardly capture the complexities of paint colour mixing [2-3]. Therefore it is not surprising that artists and art students often find it difficult to use colour theory in practice when they need to mix paints [4]. Mixing colours on the palette often leads to surprises that are sometimes inspirational, and sometimes frustrating. Even some experienced painters need considerable time for mixing pigment paints and testing the result before they obtain the intended colours [5]. In this article, we will introduce a new digital tool that may help painters who have not yet fully mastered the complexities of colour mixing. This tool may become helpful in those cases where a painter has a clear idea about the colour he/she likes to accomplish on the canvas. In such cases, inspiration may come from a piece of material, a cloth or a flower, but creating exactly that colour by mixing pigment paints on the palette may require perseverance and substantial time. 24 http://www.aic-colour.org/ | http://www.aic-color.org/ ISSN 2227-1309 Journal of the International Colour Association (2019): 24, 24-34 Kirchner Since the 1960s it is possible to let computers calculate the optimum paint mixture. This approach needs a spectrophotometer that measures reflectance values as a function of wavelength, and requires colour formulation software to find the best paint mixture. Over the past decades, mixing machines in do-it-yourself shops have proven that this approach is technically feasible. However, for painters this approach is too costly, since the price of spectrophotometers is (far) beyond €5000. In the past decade, for the first time, colour instruments have become available on the market at prices below €300 [6]. In this article, we investigate the potential of these instruments to guide artists on mixing pigment paints in order to achieve a desired colour. For this we chose the Spectro1 instrument (Variable inc., Chattanooga - USA). This is the first low-cost colour instrument that produces spectral data similar to any other spectrophotometer. This paper is organised as follows. First, the Spectro1 instrument is described, along with the line of oil paints that we used as an example in this investigation. We present the process of determining spectral properties of the oil paints, and the optical model that we used to calculate the optimum mixture of pigment paints. These calculations use colour measurements as a starting point. Finally, we present and analyse the results of this approach for 36 different target colours, taken from reproductions of art works and common objects. The paper ends with conclusions. Instrument and paint samples Description of the instrument Since 2005, ten different colour instruments became commercially available with prices below €300 [6]. These low-cost colour devices directly measure values for three colour coordinates, and therefore they are colorimeters. This is in contrast to traditional instruments that measure reflectance for a series of wavelength values. These are the so-called spectrophotometers, which are much more expensive than colorimeters. Unfortunately, colorimeters are not useful for calculating paint mixtures, since the three coordinate values that they measure are not sufficient to calculate concentrations for a larger set of pigments. Also, the calculated mixtures may not have colours that are stable with changing spectral light conditions (colour constancy and metamerism). Early 2019, the first low-cost colour instrument entered the market which does provide reflectance values for a range of wavelengths. Therefor this Spectro1 instrument is positioned by its vendor Variable as the first low cost spectrophotometer. It is based on sensor technology from AMS (Premstaetten, Austria) that provides spectral information for eight different optical wavelengths [7], combined with smart technology to produce reflectance data across the visual spectrum at 10 nm resolution. The design produces a measurement geometry that has similarities to traditional diffuse geometry (specular excluded) and 45/0 geometry. Because the instrument provides spectral data, we test if this instrument can be successfully used for calculating pigment mixtures of artist oil paints, as a digital and objective method to obtain any desired colour. Paint samples For this investigation, we used a selection of twelve pigmented oil paints from product line Van Gogh (Royal Talens, Apeldoorn - The Netherlands) [8]. In order to be able to formulate a large range of colours, the selection contained two different pigments for most colour categories (red, green, yellow and white), but three for the blue colour category and only one for black. The selection was partly 25 http://www.aic-colour.org/ | http://www.aic-color.org/ ISSN 2227-1309 Journal of the International Colour Association (2019): 24, 24-34 Kirchner inspired by the palette with thirteen pigmented oil paints that the artist Vincent Van Gogh used while staying in Arles, in Southern France (although Van Gogh used four different red pigments rather than two, and avoided using black pigment) [9]. Most of the artist's pigmented paints have a similarly named counterpart in the modern commercial paint line, although this obviously does not guarantee that the pigment constitution of the modern paint line is the same or even similar to the historic palette. Indeed, some pigments used by the artist have no counterparts in the modern palette at all. There is no counterpart for Vincent Van Gogh's Geranium (Eosin) Red pigment paint. This pigment was not replaced in the current investigation since we already had included two other red pigments. Vincent Van Gogh's Chrome Yellow Lemon and Chrome Yellow are no longer available because of their toxicity. Therefore we replaced them by modern Azo Yellow Lemon and Azo Yellow Medium pigments. For the blue and green ranges, we included the counterparts of all pigments that Vincent Van Gogh used. The mixing behaviour of pigmented oil paints is not determined only by their colour, but their spectral properties need to be taken into account as well [3]. In order to determine the colour mixing properties of each chromatic pigmented oil paint, these were mixed in standardised ratios with white in approximately 50:50 and 10:90 mixtures, as well as with black in 1:99 and 3:97 mixtures. For achromatic pigmented oil paints, an adapted scheme was used. These schemes originate from their use in the paint industry as described in an earlier publication [10]. While weighing and mixing, some small deviations from this scheme appeared. This was not problematic, since we carefully weighed and registered all concentrations that we actually used. These actual mixing ratios are presented in Tables 1 and 2. First Second First Ivory Second Pure Titanium Titanium Black Ivory Black White White Carmine Red (PR83/PR57:1) 100% 50.2% 9.95% 1) 99.01% 97.09% Vermilion Red (PO34/PR57:1) 100% 50.00% 9.85% 99.01% 97.10% Viridian Green (PG7/PY43) 100% 50.00% 11.47% 1) 99.01% 97.09% Emerald Green (PW6/PB15/PY3) 100% 48.75% 9.96% 99.01% 97.09% Ultramarine Blue /Synthetic (PB29) 100% 49.95% 9.73% 1) 99.01% 97.18% Cobalt Blue (PB28) 100% 49.90% 11.45% 99.01% 97.18% Prussian Blue (PB27) 100% 49.90% 9.91% 99.01% 2) Azo yellow Lemon (PY3/PW6) 100% 49.85% 10.01% 99.01% 96.90% 10.03% Azo Yellow Medium (PY74/PW6) 100% 50.10% 99.01% 97.09% 7.48% Table 1: Mixtures as prepared to characterise the colour properties of chromatic pigmented paints. 1)These mixtures also contribute to the characterisation of Titanium White. 2)Special mixture: 80.48% Titanium White + 10.06% Ivory Black + 9.46% Prussian Blue. Mixtures Ivory Cobalt Pure Carmine Red Viridian Green Black Blue 49.75%, 80.10%, Titanium White saffl. 100% 90.00%, 97.37%, 90.05% 3) 88.53% 3) 88.55% 3) oil (PW6/PW4) 99.01% 79.68%, 91.31%, Zinc White (PW4) 100% 90.93% 3) 89.69% 3) 90.70% 3) 99.01% Ivory Black (PBk9) 100% 0.99%3), 2.91% 3) 0.99%3), 2.91% 3) 0.99%3), 2.82% 3) Table 2: Similar as Table 1, for characterising achromatic pigmented paints.