Measuring the color of granite rocks: A proposed procedure

Abstract

In spite of color being one of the physicochemical parameters most commonly used to characterize ornamental stone, there is yet no standardized protocol for measuring this parameter. Such a protocol is of particular importance for characterizing the color of heterogeneous surfaces, as in the case of granite. The aim of the present study was to determine the minimum area and the number of measurements required to characterize the color of granite rocks. A spectrophotometer and a tristimulus colorimeter, were used to measure the color of granite samples, and the measurements were expressed in CIE L*a*b* color system units. Three parameters were considered as variable factors: the type of rock (Labrador Claro, Grissal, Rosa Porrin˜o, and Blanco Cristal), surface finish (polished, honed, sawn, and flamed), and target area (circular apertures of diameter 5, 8, 10, and 50 mm). The results of the application of multivariate analysis of variance and of the classical CIELAB formula and CIE L*a*b*-based color-difference formulae (i.e., CIE94 and CIEDE2000) to the data revealed that, although all considered factors affected the minimal area and the number of measurements required, the different circular apertures of both the instruments can be disregarded if the number of measurements and area recommended in this study are used.

Evaluation of Accelerated Ageing Tests for Metallic and Non-Metallic Graffiti Paints Applied to Stone

“…Characterization of colour in ageing studies of paint is therefore a key technique. Reflectance colour measurements were made with a portable spectrophotometer (Konica Minolta CM-700d, Tokyo, Japan) equipped with CM-S100w (SpectraMagicTM NX) software (Professional Edition, Tokyo, Japan), following the method developed by Prieto et al [28] and carrying out the number of measurements corresponding to the target surface area (Section 2.1). Measurement conditions were as follows: Illuminant D65, observer 2 • and a 3 mm diameter target area.…”

Degradation of nitrocellulose-based paint by Desulfovibrio desulfuricans ATCC 13541

“…The visual colour difference threshold or just noticeable difference (JND), which constitutes the lower limit of perception in an individual with normal colour vision (Brown 1957;Macadam 1942) established as 1 CIELAB unit (Wyszecki and Stiles 1982), was not overcome by the partial difference of the rednessgreenness parameter (∆a*: 0.3 CIELAB units). The remaining colour differences exceeded this limit and also the value considered the general limit of perceptibility: 3 CIELAB units (Berns 2000;Prieto et al 2010;Völz 2001), which is the upper limit of rigorous colour tolerance (Figure 3b). Even taking into account a higher threshold of perception of 6 CIELAB units, considered an evident colour change (Hardeberg 1999), the overall change in colour or total colour difference (∆E* ab : 6.7 CIELAB units) overcame also this value, and therefore the colour change as a result of D. desulfuricans ATCC 13541 can be considered noticeable at first glance.…”

Color assessment of granitic rocks and implications for their ornamental utilization

“…We recommend, therefore, the use of the larger aperture, especially when the samples have large surfaces. The larger apertures make it possible to obtain the color parameters with a lower number of shots for a given standard deviation, as pointed out in a recent published work 11 . When the samples have a small size, it is more useful to use smaller apertures to create a more uniform distribution of the position of the shots.…”