With the purpose of obtaining objective, quantitative values on the visual consequences of dust deposition on archaeological mosaics, a study was undertaken based on image and spectral measurements of the mosaic at the House of Hippolytus. The surface of the mosaic is considerably large (7.50 × 3.80 m) so the study focused on the three Cupids scene (Fig. 2) which has a wide diversity of colors and is a representative area of the complex.
The measurements were taken after 6 weeks of exposure (from 23rd April to 5th June 2019) and before any type of cleaning (m0). The same procedure was repeated after a dry cleaning process by sweeping (m1), and a wet one by mopping (m2). In this case the measurements were undertaken after the surface was dry. Therefore the results of the last stage included both the dry and wet cleaning.
The methodology is based on the use of a spectrophotometer, as in previous research regarding quantification of visual changes on heritage materials, but this research has also made use of a LumiCam® 1300 camera, unprecedented for this purpose. This portable set of methods provided unambiguous information about color and reflectance of a complete sector of the mosaic and a particular group of tesserae selected by their color.
Colorimetric changes of the mosaic
The colorimetric study of the chosen part of the mosaic has permitted knowing the lightless, chroma and hue angle of all tesserae, and determining how these values have changed with the cleaning process, including the color difference after each cleaning stage.
The study was undertaken by means of a LumiCam® 1300 camera (Instrument Systems GmbH), which has a resolution of 1360 × 1010 pixels (Fig. 2). The method provided the luminance (L, in cd/m2), tri-stimuli (X, Y, Z) and RGB values of each pixel, data later processed by the camera software. As calibration references, a 75% reflectance white checker (Spectralon® by Labsphere) and an X-Rite Colorchecker® panel were placed on the ground next to the main scene (Fig. 3). This method incorporates the collection of areal rather than point data to evaluate the mosaic surface as a whole and without making contact with it.
The official color-difference formula is currently CIEDE2000, which is jointly recommended by the International Commission on Illumination and the International Organization for Standardization (ISO). However, many users continue being most familiar with the CIE 1976 L*a*b* (CIELAB) color-difference formula and coordinates [27, 28]. For this reason, both CIELAB and CIEDE2000 results have been provided.
The X, Y, Z values obtained from the LumiCam® were adjusted to the levels of illumination at the time of the measurement through the white reference from the X-Rite ColorChecker® panel. In addition, the reference illuminant (D100 CIE) has spectral characteristics very similar to the indirect natural light [29].
The L*, a*, b* coordinates were calculated from the tri-stimuli values (X, Y, Z) for each pixel of the image taken by the LumiCam® camera and for each cleaning stage (m0, m1 and m2), as recommended by the CIE1976 (L*a*b*) color space standard.
The data resulting from each pixel were: L*a*b*(m0) corresponding to a surface with a 6-week deposit layer; L*a*b′(m1) from the same surface after dry cleaning; and L*a*b*(m2) after the wet cleaning. The chroma (C*ab) was calculated from those coordinates as follows:
$${C}_{ab}^{*}=\sqrt{{a}^{*2}+{b}^{*2}}$$
(1)
And in the same way, the hue angle hab:
$${h}_{ab}={\text{tan}}^{-1}\left(\frac{{b}^{*}}{{a}^{*}}\right)$$
(2)
Both variables related to each cleaning process of the mosaic (m0, m1 and m2), resulting in C*ab(m0), C*ab(m1) and C*ab(m2) for the chroma values, and hab(m0), hab(m1) and hab(m2) for the hue angle values.
The formula CIEDE2000 is the official notation for colour-difference [30], and has been used to determine the lightness L′, chroma C′ and colour differences ∆E00 comparing m0 with m1 and m2:
$${\Delta E}_{00\left(n\right)}={{\left[{\left(\frac{{\Delta L}_{\left(n\right)}^{\prime}}{{k}_{L\left(n\right)}{S}_{L(n)}}\right)}^{2}+{\left(\frac{{\Delta C}_{\left(n\right)}^{\prime}}{{k}_{C\left(n\right)}{S}_{C(n)}}\right)}^{2}{\left(\frac{{\Delta H}_{\left(n\right)}^{\prime}}{{k}_{H\left(n\right)}{S}_{H(n)}}\right)}^{2}+{R}_{T(n)}\left(\frac{{\Delta C}_{\left(n\right)}^{\prime}}{{k}_{C\left(n\right)}{S}_{C(n)}}\right){\left(\frac{{\Delta H}_{\left(n\right)}^{\prime}}{{k}_{H\left(n\right)}{S}_{H(n)}}\right)}\right]}}^\frac{1}{2}$$
(3)
The differences in lightness (∆L′), chroma (∆C′) and hue (∆H′) have allowed calculating the chromatic shift between two areas (∆E00), on the grounds of the following combinations regarding the cleaning process:
The parametric weighting factors are kL, kc and kh, and for the reference conditions of this case, are considered equal to 1 according to the CIE 101–1993 standard [26]. The weighting functions SL, SC, SH and RT were acquired from the calculation of ∆E00 by the CIEDE2000 color-difference formula [30].
Colorimetric study of specific tesserae
A representative area of the ornamental floor (500 mm × 510 mm) was chosen to undertake the colorimetric study of specific tesserae, with particular chromatic characteristics. This has permitted a more detailed assessment on the color performance of the mosaic during the cleaning process.
Fifty pixels of tesserae of the same color were selected from the image of the mosaic obtained with the LumiCam® after wet cleaning (m2), when the colors could be better appreciated. The mean XYZ value was calculated from the XYZ values of the pixels measured by the LumiCam®. On the other hand, CIELAB L*a*b* values were obtained from the XYZ ones and the color difference (ΔEtessera) in relation to the mean was calculated for each tessera. The tesserae were grouped based on their color (white, ochre, brown, black and grey) by means of MatLab®, which was programmed to determine ΔEtessera CIELAB ≤ 3 [31]. The color groups were named after comparing the mean values with colors of the x-rite ColorChecker®. Figure 4 highlights the brown, white and ochre tesserae grouped by MatLab® from the same area of the mosaic shown in Fig. 3.
With the purpose of determining the chromatic changes on these groups of tesserae after each cleaning stage (m0, m1 and m2), the chroma C*ab, hue angle hab and lightness L* values were calculated based on the standard CIE1976, corresponding to the CIELAB color space, as was done previously for the whole section of the mosaic. To complete this analysis, the color difference ∆E00 was also defined by CIEDE2000 (∆E00(n) wh, ∆E00(n) oc, ∆E00(n) br, ∆E00(n) bk and ∆E00(n) gr), after comparing the cleaning stages (m0, m1 and m2) in the situations n = (1, 2).
Analysis of reflectance
The measurements for the analysis of reflectance were taken with a spectrophotometer (CM-2600d Konica Minolta®) (Fig. 5). This gives absolute reflectance measurements in the visible range (from 380 to 740 nm in 10 nm steps). The area of measurement is 8 mm in diameter. The light source is provided by the three xenon bulbs of the meter, which emit in the visible range. A complete calibration of the instrument was undertaken prior to the analysis by means of the white and black reference checkers provided with the meter.
Six tesserae were selected by their color (two per each): ochre (1R and 2R), white (1B and 2B) and black (1N and 2N) (Fig. 6). The reflectance measurements (ρ) were taken twice per tessera by placing the instrument directly onto them before cleaning (ρxV(m0)), after dry cleaning (ρxV(m1)) and after wet cleaning (ρxV(m2)), where x = (1, 2) represents the tesserae numbered with 1 and 2, and V = (R, N, B) the colors. The spectral difference Δρ(n) regarding each cleaning process (m0, m1, m2) was calculated by:
$$\Delta {\rho xV}_{\left(n\right)}={\rho xV}_{\left({m}_{1}{m}_{2}\right)}-{\rho xV}_{\left({m}_{0}\right)}$$
(6)
With n = (1, 2), previously described in the calculation for the color differences in relation to the cleaning process.
The reflectance values and differences in reflectance provided detailed information about the spectral characteristics of each color. It has also been useful for understanding how deposits affect specific tesserae spectrally, changing their color, lightness and hue.