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Colour Analysis of Rembrandt Pastels

Paul Centore

c June 2, 2015

AbstractMunsell specifications, calculated from spectrophotometric measurements, are pre-

sented for the complete Rembrandt line of 218 artists pastels, produced by Royal Talensin Holland. The pastels hue, value, and chroma distributions are delineated. Tablesand figures are presented, that painters can use in making practical decisions aboutpastel employment and purchases. A list of duplicates (pairs of pastels whose namesare different but whose colours agree to within a CIE DE 2000 value of 2 or less) isgiven.

1 Introduction

A single manufacturer typically produces a few hundred pastels, of various colours. Thiswide variety is necessary because, unlike paints, pastels cannot be readily mixed to producelarge quantities of new colours. The variety, however, requires a significant organizationaleffort from both artists and manufacturers. The current document helps to provide thatorganization for one pastel brand, Rembrandt (produced by Royal Talens), by analyzingtheir line of 218 colours. The analysis is performed largely in terms of the Munsell coloursystem, whose concepts of hue (red, yellow, etc.), value (light vs dark), and chroma (dullvs saturated) are basic to painting. Tables and figures are presented, that should helpartists select Rembrandt pastels intelligently, both when painting and when purchasing.Purchasing is especially an issue for online or catalog sales, where the artist cannot see thepastels directly. The Rembrandt gamut is delineated in terms of hue, value, and chroma.Several pairs of duplicates, in which two different pastels produce very similar colours, arealso identified.

This report first describes the Rembrandt pastel line, and then describes the Munsellsystem, which will provide rigorous terms of reference for the pastel analysis. The analysisprocedure and results are given next, including tables and figures for practical use. Finally,an approach is suggested, involving contributions from painters and manufacturers, to extendthis analysis to further pastel brands

2 Description of Rembrandt Pastels

Rembrandts complete line consists of 218 pastels. This total includes fifteen dark pastelsthat were added in the last year. Currently, they can be purchased online in the United

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PAUL CENTORE

States from Dakota Pastels. The two major American art supply retailers, Jerrys Artaramaand Dick Blick, do not yet stock the dark set, but Dick Blick at least has indicated that itwill be available soon. Rembrandt also offers a complete set of pastels containing 225 sticks.Over 20 of these 225, however, are duplicates. Furthermore, the complete set of 225 doesnot (as of September 2014) contain the 15 recently added dark pastels.

Rembrandt divides its pastels into 43 series, each of which consists of lighter and darkerversions of the same hue. A series is indicated by a three-digit number, such as 372, which is ared hue. After the three-digit number there is a comma, followed by another number. Whenthe second number is 5, the pastel contains a pure pigment. When the second number is lessthan 5, some amount of black (or perhaps another pigment) has been added to darken thecolour. When the second number is greater than 5, the colour has been lightened. The lowerthe second number, the darker the colour, and the higher the second number, the lighter thecolour. 372,2, for example is a dark reddish brown, while 372,10 is a pale orangish red.

3 The Munsell System

Albert Munsell devised his colour specification system for painters and visual artists. Itclassifies surface colours by three perceptual attributes that are basic to painting: hue, valueand chroma.

Hue is universally understood. It says whether a colour is red, yellow, purple, etc. Munselldesignates 10 basic hues: R (red), YR (yellow-red, or orange), Y (yellow), GY (green-yellow),G (green), BG (blue-green), B (blue), PB (purple-blue), P (purple), and RP (red-purple).Each basic hue is further subdivided into 4 steps, denoted with a numerical prefix. Forexample, the four greens are denoted 2.5G, 5G, 7.5G, and 10G. 2.5G is a yellower green,that is closer to GY than it is to BG. 10G is a bluer green, that is closer to BG than it is toGY. In all, then, the Munsell system specifies 40 hues (4 steps for each of the 10 basic hues).These 40 hues are equally spaced perceptually. One could interpolate any desired amountbetween two adjacent hues. For example, the hue 6GY is a yellowish green that is between5GY and 7.5GY, but closer to 5GY. White, black, and greys are not considered hues in theMunsell system. Rather, they are designated N, for neutral.

Many different colours can have the same hue. Figure 1, for example, shows the hueleaf for 6GY, a set of colours all of which have hue 6GY. The different colours within a hueleaf are specified further by value and chroma. The empty boxes indicate colours that arein the Munsell system, but that are beyond the gamut of the process used to produce thefigure. The hue leaf shades smoothly into the neutral axis, consisting of greys, shown on theleft.

Munsell value designates how light or dark a colour is. The theoretically darkest blackhas a value of 0, and is denoted N0. The theoretically lightest white has a value of 10, andis denoted N10. Between N0 and N10 are 9 progressively lighter greys, denoted N1 throughN9. The spacing between the greys is perceptually equal. All colours have a Munsell value,not just the neutrals. For example, there are light blues and dark blues. A blue with value8.5 has the same lightness as N8.5.

Munsell chroma refers to how intense, or saturated, a colour is. For example, a lemon isan intense yellow, while masking tape is a dull yellow. A dull colour is closer to a neutral

c 2015 Paul Centore 2

COLOUR ANALYSIS OF REMBRANDT PASTELS

N2

N9

N8

N7

N6

N5

N4

N3

N1

2 4 6 8 10 12

Chroma

Hue: 6GY

Valu

e

Figure 1: The Hue Leaf for 6GY in the Munsell System

grey than an intense colour is. The Munsell system denotes chroma numerically. Greyshave chroma 0. A colour with a chroma of 10 or higher is generally perceived as saturated.Colours of low chroma, say 4 or less, are perceived as subdued, with a high grey content.

The Munsell notation for a colour takes the form H V/C, where H stands for hue, Vstands for value, and C stands for chroma. For example, the colour 10R 9/6 would be a verylight (V is 9), moderately intense (C is 6), orangish red (H is 10R). A colour with chroma 0is a neutral grey, which is denoted NV, where V stands for value. For example, N5 is a greythat is midway between white and black.

The Munsell system is important to painters for two reasons. First, the attributes itidentifies (hue, value, and chroma) are all vital to art. Second, the system provides a languageto communicate rigorously about colour. The 1943 Munsell renotation1 has standardizedthe calculation of hue, value, and chroma, from objective spectral measurements, so thatthese terms can be quantified. The standardization was based on a large number of colourjudgements made by about 40 human subjects, to insure that Munsell specifications areperceptually meaningful.

4 Analysis

4.1 General Procedure

The data set for this analysis consisted of 218 samples, comprising the complete line of pas-tels. Each sample measured about 1 inch square, and was applied by hand on white paper,in a layer that was opaque (or nearly so). The samples were measured three times with an X-Rite i1Pro2 spectrophotometer, using the M2 setting, and the final measurement was takento be the average of the initial three measurements. The output of a spectrophotometer

3 c 2015 Paul Centore

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measurement is a samples reflectances, at intervals of 10 nm, from 380 to 730 nm. CIE xyYvalues2 were calculated for each sample, under the assumption of Illuminant C, which is theilluminant on which the Munsell system is standardized.1 The 1943 Munsell renotation1 pro-vides a look-up table from Munsell colours to xyY coordinates. An inversion algorithm3 wasused to find a Munsell specification for each pastel, from its xyY coordinates. Tables 1 and 2list these Munsell specifications. For researchers who would like to continue these investiga-tions further, the reflectance spectra and the Munsell specifications are available in the com-puter files RembrandtPastelsReflectanceSpectra.txt and RembrandtPastelsMunsellSpecifica-tions.txt, respectively, at the website www.99main.com/centore/ColourAnalysisOfPastels.

Measuring pastels can be difficult because of their uneven, dusty texture. Analysis ofi1Pro2 repeatability, when measuring pastels, revealed that the mean colour difference fromthe mean (MCDM), assuming Illuminant C and the 1931 Standard Observer, was slightlyover 1. Printed colours, by contrast, tended to have MCDMs that were about 0.25. Whenproducing printed reproductions of pastels, then, the largest source of variability is likelythat the colour of the pastel target is not known very well.

Figures 2 through 11 are a sort of inverse to Tables 1 and 2. These figures share thesame chroma and value axes that appear in Figure 1. Rather than referring to a singlehue, however, each of Figures 2 through 11 refers to a range of hues, which share the sameMunsell letter designation. Figure 2, for example, shows all pastels whose hue is between 0Rand 10R, so a pastel of hue 3.2R could appear alongside a pastel of hue 9.6R. Each pastel ina figure is placed at coordinates given by its chroma and value, as in Figure 1. If multiplepastels with the same hue letter designation (but not necessarily the same numerical hueprefix) also hav

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