Large-scale assessment of light-induced color change in air and anoxic environments

Abstract

A wide-ranging sample set consisting of dry pigments, dyed textiles, organic and aniline-based dyes, gouaches and watercolors, fluorescent inks, and natural history specimens was exposed to light in air (20.9% oxygen) and near-anoxic environments. After a light dosage of approximately 17.5 Mlux-hours under controlled temperature and humidity conditions, 113 of 125 samples (90% of the sample set) were shown to exhibit less color change in a low-oxygen environment compared with its behavior in air. Thirty-nine percent of this subset displayed color change in anoxia that was between two and four times lower than that observed in air, whereas 47% showed color change in anoxia reduced by a factor of four or more. In contrast, six samples exhibited greater color change in anoxia than in air – these samples included Prussian blue watercolor (three samples), Antwerp blue watercolor, Verdigris dry pigment, and Fluorescent Yellow Winsor & Newton Gouache. Although the results from this small sample subset may cause concern when considering the use of anoxia in the conservation of cultural heritage, particularly for colorant systems whose behavior in anoxia has not yet been identified, this study demonstrates the overwhelming benefits of anoxic light exposure for the vast majority of samples investigated here.     

The materials of J.M.W. Turner: pigments

Abstract

Analyses of pigments from palettes used by J.M.W. Turner (active c.1792–c.1850) by means of microscopy, microchemical analysis, thin-layer chromatography, energy-dispersive X-ray analysis, X-ray diffraction and Fourier transform infrared spectroscopy are given. The occurrence of pigments in Turner's dated oil paintings and watercolours is also given, arranged by five-year periods. These findings are discussed in the context of published analyses of pigments from the first half of the nineteenth century. Turner used cobalt blue, emerald green, viridian, orange vermilion, barium chromate, chrome yellow, chrome orange and chrome scarlet within a few years of their known dates of discovery. It has become clear that Turner was using, or at least experimenting with, practically all the pigments known to be available at that time. In a few cases, in the light of these results, ideas on the availability of pigments to English artists have been revised backwards to the first known date of manufacture. Turner also possessed and used a wide range of red and yellow organic pigments, but few organic greens. The dyestuff extracted from Rubia tinctorum L. madder on an aluminiumcontaining substrate can be distinguished from the same madder on different substrates by its strong pink fluorescence in both ultraviolet and green light. The other red organic pigments (a second madder, brasilwood and cochineal dyestuffs on a range of substrates containing aluminium, copper, iron, aluminium/copper and clays) show negligible fluorescence. The red organic pigments were used in oil medium as well as watercolour, the yellows only in watercolour medium.     

Exposure of artists' colorants to airborne formaldehyde

Abstract

Thirty artists' colors have been exposed for 12weeks to 120ppb of formaldehyde in purified air as well as to purified air alone (control experiment). The exposure was carried out in the dark at ambient temperature (19 ± 2°C) and humidity (RH 44–52%). Color change (?E) was measured using a reflectance color analyzer after one, two, three, four, six, eight, 11 and 12 weeks of exposure. Color parameters (x, y, X, Y, Z, L*, a*, b* and ?E)"were also calculated from the 380–700nm spectra, recorded with a reflectance spectrophotometer, of unexposed colorants and of colorants exposed for 12 weeks to purified air and to formaldehyde. Regression analysis of the two data sets indicated that the color changes measured by these two methods were in excellent agreement (nearunity slopes, with correlation coefficients > 0.99). Exposure to either formaldehyde or pure air on watercolor paper resulted in little or no color change for all the colorants tested, including inorganic colorants, alizarin lakes, quinacridones, triphenyl methanes, indigo derivatives, arylamides and natural colorants such as curcumin. Formaldehyde, which is ubiquitous in indoor air, including museum air, does not appear to be a major threat to colorants in museum collections.     

Some technical notes on Whistler's ‘Peacock Room’

Abstract

The small areas of white metallic pigment seen on the south wall of J. M. Whistler's ‘Peacock Room’ in the Freer Gallery of Art are platinum; this has been used, with a yellow-brown ground, to overpaint a preceding layer of silver which, in one place only, is over a preceding layer of gold. The yellow metallic pigment is confirmed as gold and the blue colour of the south wall and elsewhere is based on Prussian blue. On a separate wood panel a green was tentatively identified as copper resinate, and iron-oxide-based pigments appear to be present. In all areas investigated a white, used as a ground and in admixture, was essentially lead white and calcium carbonate. The significance of these results is discussed briefly.     

Conservation of a Chinese gu at the Canadian Conservation Institute

Abstract

Conservation and technical issues pertaining to the investigation of six mediaeval Spanish ceramic lustre fragments are reported. Assessment of methods and materials to replicate the lustre glazes indicates that the spray application of nacreous (pearlescent) pigments in combination with conventional inorganic pigments and transparent colorants in an acrylic lacquer gives good results. The salient optical features oflustre glazes and nacreous pigments are discussed. Scanning electron microscopy in conjunction with electron diffraction studies are employed to characterize the glaze and ceramic body composition of the fragments. Transmission electron microscopy indicates that a uniform dispersion of copper crystallites, c. 2-4nm in diameter, is responsible for the lustre effect in one sherd.     

A note on Prussian blue in nineteenth-century Canton

Abstract

The use of Prussian blue in nineteenth-century Japan has been extensively researched, particularly in relation to the ‘blue revolution’ in ukiyo-e prints but its use by Chinese artists has not received the same degree of attention. A commodity traded by the East India Company, this pigment was used to ‘improve’ the colour of tea, but in or about 1825 the trade abruptly ceased. It now seems fairly clear that the cessation of the Prussian blue trade coincided with the setting up of a Prussian blue factory at the northern gate of Canton, and that knowledge of the industrial process was possibly acquired covertly from a London manufacturer. The pigment has been identified chemically among paint fragments collected during the dis-binding of an album of Chinese botanical watercolours sent from Canton to the Horticultural Society of London by tea inspector John Reeves between 1817 and 1830. This finding suggests that conservators should not exclude the possibility of finding Prussian blue on Chinese work dating from at least the early part of the nineteenth century.     

Rentokil Bubble in nitrogen anoxia treatment of museum pests

Abstract

The Rentokil Bubble, a commercially available, portable, fumigation enclosure which was designed for use with methyl bromide (bromomethane), phosphine or carbon dioxide, was tested for its applicability for nitrogen fumigations. The unit would be a valuable alternative for museums where a costly fumigation chamber cannot be afforded or a permanent space cannot be allocated for fumigations. The performance of two bubbles, 35m³ and 6m³, was investigated for both the oxygen transmission characteristics of the materials and the gas-tightness of the enclosure. The tests confirmed the suitability of the bubble for anoxia treatment, although some practical limitations were detected, especially the size of the unit and possible variations in quality from unit to unit. The article describes the set-up and safety considerations of a large-scale nitrogen anoxia treatment using the bubble.     

I. Azurite and Blue Verditer

Abstract

Samples of pigments from excavated wall paintings of Vergina’s second tomb were analyzed by the non-destructive methods of X-ray fluorescence, X-ray diffraction and mineralogical microscopic examinations. The results showed that one type of blue pigment was used, the well-known Egyptian blue. The red pigments, except one which is haematite, are cinnabar (HgS), and the grey pigments are carbon. None of the black pigments contained manganese compounds.     

A Monument of Byzantine Wall Painting—the Method of Construction

Abstract

Samples of pigments from excavated wall-paintings at the great civilization center of the Greek Bronze Age, Thera, have been analysed by the non-destructive methods of X-ray fluorescence, X-ray diffraction and mineralogical microscopic examination. The results showed that three types of blue pigment were used, namely the well-known Egyptian blue, glaucophane, which is a sodium magnesium (or iron) aluminium hydroxide silicate which occurs as a natural mineral on Santorini, and a mixture of Egyptian blue and glaucophane. Black pigments were rather puzzling in that some of them are carbon as expected, and some are manganese compounds. The rest of the pigments are very similar to those previously examined from Mycenae and Knossos.     

Restoration of a 15th-Century Romanian Court Garment

Abstract

Samples of pigments from a wall-painting of a house of the first Pompeian style (400 Bc–168 BC) found in Pella, Greece, were analyzed by the non-destructive methods of X-ray fluorescence and X-ray diffraction. Red pigments were identified as cinnabar and iron oxide pigments (haematite and goethite). Blue was the well-known Egyptian blue and yellow was identified as goethite. A characteristic feature of the wall-painting was the presenceof green earth, i.e. celadonite and glauconite, encountered for the first time in this series of investigations.     

Characterization of organic azo-pigments by pyrolysis–gas chromatography

Abstract

Pyrolysis–gas chromatography was applied to the characterization of synthetic organic pigments. On pyrolysis, azo pigments and phthalocyanine blue yield characteristic fragments. These pyrolysis products were identified, and the mechanism of decomposition was studied. In the case of paint samples, the main pyrolysis products derive from the medium, although the presence of additional peaks enables azo pigments and phthalocyanine blue to be characterized. Some examples of the analysis of artists' paints are given and the peaks are identified. The main limitation of this technique is that it is impossible to detect chemically and thermally stable pigments, or inorganic components.     

Modeling of Light-Induced Changes in Reflectance Spectra: A Predictive Tool for the Effect of an Arbitrary Spectral Irradiance

Optical radiation is capable of altering the spectral reflectance of layers of colorants. Light damage is of particular importance in respect to unique historical objects in exhibitions. The major parameter governing this process is the spectral irradiance applied to the surface. As this may vary significantly for different light sources, an experimental investigation of the damage potential of the numerous illuminants available is a cumbersome task. In this work a numerical algorithm for the prediction of modifications of the spectral reflectance of colorants subject to a given arbitrary spectral irradiance over a given time is presented. It uses the temporal evolution of the spectral reflectance of the dye or pigment at issue when irradiated by a set of narrowband light sources as input data. The predicted reflectance spectra and the resulting color changes are compared to those measured for samples irradiated with common polychromatic light sources. The concept described in this work aims at the implementation of a standardized procedure and database for the prediction of radiative damage for a comprehensive range of pigments and dyes.     

The impact of chloride desalination on the corrosion rate of archaeological iron

Abstract

Although desalination of archaeological iron reduces its chloride concentration and enhances object stability, the reduction in corrosion rate that this produces has never been quantified. This study measures post-treatment corrosion rates in accelerated corrosion environments to identify the impact of removing chloride ions on corrosion rate. Thirty-five archaeological iron nails, treated individually in either alkaline sulphite or nitrogen-deoxygenated sodium hydroxide, were exposed to 75°C and 75% relative humidity together with 31 untreated objects from the same archaeological sites. Object weight change and visual examination of physical change before and after the test period were used to monitor corrosion. 77% of treated objects showed no weight gain and no visible signs of corrosion, while 90% of untreated objects did corrode. The impact of chloride on corrosion of untreated objects was clearly established by a significant linear correlation between chloride content and weight gain. Treated objects with <400 ppm chloride content showed no corrosion behaviour. Corrosion of treated objects was attributed to incomplete treatment: 93% of objects treated to <5 mg/l Cl^? in the final solution bath displayed no corrosion behaviour. Based on these results, desalination of iron objects to enhance their stability offers a valuable option for reducing corrosion rates of archaeological iron, which should increase object lifespan. The results also raise the question of whether low levels of post-treatment residual chloride produce corrosion of any significance. Answering this will be an important step forward for managing the preservation of archaeological iron.     

Unusual pigments on a Greek marble basin

Abstract

The examination of the original polychromy of a fourth century B.C. classical Greek marble basin revealed a number of unusual pigments. In addition to natural cinnabar, and Egyptian blue, analysis has shown the presence of wild madder (Rubia peregrina) as the colorant in a purple pigment. This is one of the very few instances in which the use of madder as a colorant in classical painting has been established analytically. The white pigment was found to be neutral lead carbonate (cerussite). The use of this carbonate in painting is extremely rare. The yellow pigment was identified as misy (jarosite). The use of jarosites, iron sulphates, as pigments in painting has not been reported previously.     

Evaluating Single- and Two-Constant Kubelka-Munk Turbid Media Theory for Instrumental-Based Inpainting

Abstract

Instrumental-based color matching using non-destructive reflection spectrophotometry can be used as an aid in pigment selection for inpainting. This is useful when metamerism is of concern, particularly in modern art where the number of colorants available to artists and conservators is quite large. Simplified methods have been developed for art conservation where a single tint of each pigment mixed with white is required to define a pigment's optical properties based on the single-constant form of Kubelka-Munk (K-M) turbid media theory. Theoretically, this simplification can lead to errors in pigment selection for dark colors and colors not containing a white pigment. Instead, the two-constant form of K-M theory can be used where the model assumptions more closely match the optics of inpainting materials. The two methods were compared using four acrylic emulsion paints with a range of pigment absorption and scattering properties. It was found that the simplified method was inadequate for paints with appreciable and spectrally selective scattering, and as a consequence, pigment selection may result in a greater number of pigments than actually required. It was also found that only two samples per pigment were required to implement the two-constant technique: the mass tone and a tint.     

The effects of low oxygen atmospheres on the powderpost beetle,lyctus brunneus (stephens)

Abstract

The effect of low oxygen atmospheres on all life stages of the powderpost beetle, Lyctus brunneus (Stephens), was investigated. Artificial diet blocks infested with immature stages of Lyctus brunneus were exposed to a low oxygen atmosphere (0·4% oxygen, balance nitrogen) at 30°C and 70% RH for varying lengths of time. Mortality counts for immature stages were based on the relative number of adult beetles emerging from the treated and untreated diet blocks. One hundred percent mortality was observed for eggs, larvae and pupae exposed to low oxygen atmospheres for six, eight and 12 days, respectively. Adult beetles similarly exposed to low oxygen atmospheres all died within three days.     

Colorimetric Evaluation of Pictorial Coatings in Conservation of Plasterworks from the Islamic Tradition

ABSTRACT

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.     

THE POLYCHROME DECORATION OF GREEK TERRACOTTAS

Abstract

This paper describes the composition of the pigments used for the decoration of Greek terracotta figurines, in the light of previous scientific analyses, and of supplementary analyses recently made by the Research Laboratory of the British Museum.     

TOXICITY OF BENZOTRIAZOLE

Abstract

Blue and white Chinese porcelain specimens from Dehua and Zhangzhou kiln sites from the Ming dynasty (AD 1368–1644), and Jingdezhen kiln sites from the Yuan (AD 1280–1368) to Ming dynasties were collected. The chemical compositions of these valuable samples were determined using energy dispersive X-ray, fluorescence (EDXRF) spectrometry, a non-destructive method, and some Jingdezhen samples were analyzed by scanning electron microscopy and transmission electron microscopy. The EDXRF results were subjected to correspondence analysis. Differences in the composition of the specimen bodies were found for specimens from different production sites. The blue and white Jingdezhen imperial, porcelain from the Yuan to Ming dynasties can be divided into two periods based on differences in the chemical composztion and microstructure of the pigments. The reasons for these variations are discussed.