This work presents a spectral color-imaging procedure for the detailed colorimetric study of real artworks under arbitrary illuminants. The results demonstrate this approach to be a powerful tool for art and heritage professionals when deciding which illumination to use in museums, or which conservation or restoration techniques best maintain the color appearance of the original piece under any illuminant. Spectral imaging technology overcomes the limitations of common area-based point-measurement devices such as spectrophotometers, allowing a local study either pixelwise or by selected areas. To our knowledge, this is the first study available that uses the proposed CIE (Commission Internationale de l’Éclairage) light-emitting diode (LED) illuminants in the context of art and heritage science, comparing them with the three main CIE illuminants A, D50, and D65. For this, the corresponding colors under D65 have been calculated using a chromatic adaptation transform analogous to the one in CIECAM02. For the sample studied, the CIE LED illuminants with the lowest average CIEDE2000 color differences from the standard CIE illuminants are LED-V1 for A and LED-V2 for D50 and D65, with 1.23, 1.07, and 1.57 units, respectively. The work studied is a Moorish epigraphic frieze of plasterwork with a tiled skirting from the Nasrid period (12th–15th centuries) exhibited in the Museum of the Alhambra (Granada, Spain).

Understanding the use of colorant materials throughout history is now the objective of multiple studies centered on the identification and characterization of these materials. The main objective of this study is to increase knowledge of the degradation pathways of indigo blue dye (Indigofera tinctoria) and the detection of degradation products. This will enable the identification of the dye on historical objects, even when it is present in a degraded state. To this end, different analytical techniques were used for the identification of indigo blue and its degradation products. Capillary electrophoresis with diode array detection (CE-DAD) and fluorescence spectroscopy were used for characterization and detection of the colorant and its degradation products caused by the aging process. Nuclear magnetic resonance (NMR) spectroscopy was used to confirm the presence of degradation products. The evaluation of color variations (lightness, chroma, and tone) in indigo blue on paper samples was made with colorimetric study, while the changes in indigo solutions were evaluated with UV-visible spectrophotometry. Additionally, changes in the alkalinity and acidity of the colorant were made with pH measurements. Lastly, the main agents responsible for the modifications that resulted in the production of degradation products of indigo blue were also identified. These methods were applied to historic samples from the Collection of Drawings, Maps, and Plans (sixteenth to nineteenth centuries) and to the thread used in the binding of a notarial record book (Registro Notarial de Torres) (fifteenth century) held by the Royal Chancery Archive in Granada (Spain). This paper provides deeper insights into the materials used in artworks and helps in the identification of indigo blue despite the degradation caused by external agents or by its natural aging process.

ResumenSe han realizado medidas colorimétricas y goniocolorimétricas de 187 azulejos de la galería baja del Patio de las Doncellas (S. xiv), que pueden agruparse en torno a 5 tonos principales: blanco, negro, azul, verde y melado. Las medidas goniocolorimétricas se muestran necesarias para caracterizar correctamente la mayoría de estas muestras, siendo insuficientes las medidas colorimétricas convencionales. Los principales cambios de color se dan al movernos desde el ángulo de –15° (próximo a la dirección de reflexión especular) hasta el de +45° (dirección perpendicular a la muestra), consistiendo en un descenso de claridad (L*) y un aumento de croma (C*ab). Para las muestras negras se observa además un acusado descenso del ángulo de tono (hab), así como un valor alto de 16,0 para el índice de flop en claridad. Las muestras negras y meladas tienen apariencia metalizada, con valores del índice de metalizado superiores a 5,0. Las diferencias de color entre los ángulos extremos de –15° y +110° son bastante elevadas, oscilando sus valores medios entre 4,1 (blancos) y 31,2 (melados) unidades CIELAB. La presente caracterización goniocolorimétrica de los azulejos del Patio de las Doncellas aporta una metodología y resultados que pueden ser útiles para los trabajos de conservadores y restauradores.AbstractColorimetric and goniocolorimetric measurements of 187 tiles in the lower gallery of Patio de las Doncellas (14th Century) have been performed. They can be grouped around 5 main colors: white, black, blue, green and orange. Conventional color measurements proved to be insufficient for most of the samples whose goniochromatic nature requires the use of goniocolorimeters for proper characterization. The main color changes appeared from –15° (close to the specular direction) to +45° (direction perpendicular to the sample) and consisted of a lightness (L*) decrease and a chroma (C*ab) increase. For the black samples, there was also a pronounced decrease of hue-angle (hab), as well as a high value of 16.0 for the lightness-flop index. The black and orange samples had a metallic appearance, as indicated by average metallic index values greater than 5.0. Considering a displacement between the two extreme angles of –15° and +110°, the average CIELAB color differences were considerably high, ranging from 4.1 (white group) to 31.2 (orange group). Current methods and results for the tiles in Patio de las Doncellas may be useful for future work of conservators

The main aim of our current investigation is the colorimetric evaluation of protective treatments (consolidants) applied to traditional Islamic plasterworks, under natural ageing conditions. From analyses of the original pictorial plaster remains in the Courtyard of the Maidens of the Real Alcázar in Seville, Spain (a World Heritage Site) we prepared test specimens, using materials and techniques similar to the original ones. We analysed 56 test specimens painted with four pigments (yellow, green, blue, and red), using two different binders (animal glue and gum arabic), onto which five representative consolidants were applied: barium hydroxide, acrylic copolymer, polyvinyl butyral, ethyl silicate, and bacterial carbonatogenesis. The test specimens were subjected to natural ageing for one year (indoors and outdoors), enabling a colorimetric assessment to be made of the changes of the polychrome surfaces. The colorimetric heterogeneity of the 56 specimens after ageing registered an average value of 2.7 CIELAB units, assessed using the mean colour difference with respect to the mean. In the aged specimens, the addition of consolidants resulted in average colour differences (mainly lightness differences) of 10.7 and 6.7 CIELAB units, considering as a reference the specimens without consolidants aged indoors and outdoors, respectively. These colour differences were very similar for both binders but not for the four pigments, higher values being found for the blue and red pigments. Considering as reference the samples without consolidants aged outdoors, we found no statistically significant colour differences, either among the five consolidants (p = .094) nor the two binders (p = .674) used. In addition to the magnitude of colour differences, the choice of the most appropriate consolidants must also consider aspects related to performance and effectiveness. Overall, for the type of paints tested, the polyvinyl butyral consolidant appeared to perform the best, followed by the ethyl silicate.

Color reintegration is a restoration treatment that involves applying paint or colored plaster to an object of cultural heritage to facilitate its perception and understanding. This study examines the impact of lighting on the visual appearance of one such restored piece: a tiled skirting panel from the Nasrid period (1238–1492), permanently on display at the Museum of the Alhambra (Spain). Spectral images in the range of 380–1080 nm were obtained using a hyperspectral image scanner. CIELAB and CIEDE2000 color coordinates at each pixel were computed assuming the CIE 1931 standard colorimetric observer and considering ten relevant illuminants proposed by the International Commission on Illumination (CIE): D65 plus nine white LEDs. Four main hues (blue, green, yellow, and black) can be distinguished in the original and reintegrated areas. For each hue, mean color difference from the mean (MCDM), CIEDE2000 average distances, volumes, and overlapping volumes were computed in the CIELAB space by comparing the original and the reintegrated zones. The study reveals noticeable average color differences between the original and reintegrated areas within tiles: 6.0 and 4.7 CIEDE2000 units for the yellow and blue tiles (with MCDM values of 3.7 and 4.5 and 5.8 and 7.2, respectively), and 16.6 and 17.8 CIEDE2000 units for the black and green tiles (with MCDM values of 13.2 and 12.2 and 10.9 and 11.3, respectively). The overlapping volume of CIELAB clouds of points corresponding to the original and reintegrated areas ranges from 35% to 50%, indicating that these areas would be perceived as different by observers with normal color vision for all four tiles. However, average color differences between the original and reintegrated areas changed with the tested illuminants by less than 2.6 CIEDE2000 units. Our current methodology provides useful quantitative results for evaluation of the color appearance of a reintegrated area under different light sources, helping curators and museum professionals to choose optimal lighting.

Consolidation is a treatment of great importance for archaeological wall paintings due to their state of preservation, in which the lack of cohesion of the mortar and the pulverulence of the pictorial layer are common. The objective of this work has been to evaluate consolidation tests that have been carried out on decontextualized fragments belonging to five different wall paintings (from different periods and with differences in their state of conservation and in their painting techniques). For this, two of the most used treatments in consolidation of archaeological coatings, such as an acrylic resin and ethyl silicate, have been compared with two treatments that use nanoparticles, such as nanolimes and nano ethyl silicate, together with a bioconsolidation treatment used until present mainly for the consolidation of stone material: bacterial carbonatogenesis.