James R. Druzik

Fading of alizarin and related artists' pigments by atmospheric ozone: reaction products and mechanisms

The colorants alizarin and Alizarin Crimson (a calcium- aluminum lake pigment) and their simple structural homologue anthraquinone were deposited on silica gel, cellulose, and Teflon substrates and exposed in the dark to ozone in purified air (~0.4 ppm O_3 for 95 days and ~10 ppm O_3 for 18-80 h). Exposed and control samples were analyzed by mass spectrometry. Alizarin Crimson reacted with ozone on all substrates, yielding phthalic acid {major), benzoic acid (minor), and other minor and unidentified products. Anthraquinone did not react with ozone irrespective of conditions. Alizarin did not react on Teflon or cellulose but reacted on silica gel to yield phthalic acid (major) and other products. A chemical mechanism responsible for the fading of these alizarin-related colorants by ozone is suggested that is consistent with the products distribution, the observed reactivity sequence, and the observed substrate-specific effects. The possible application of this work to predicting the ozone fastness of other alizarin-related pigments is discussed briefly.

The measurement and model predictions of indoor ozone concentrations in museums

Eleven museums, art galleries, historical houses and a museum library were monitored for 38 days during the summers of 1984 and 1985 to determine whether high outdoor ozone concentrations are transferred to the indoor atmosphere of museums. Museums having conventional air conditioning systems show peak indoor ozone concentrations about 30–40% of those outdoors. Buildings with no air conditioning but a high rate of air exchange with the outdoors show peak indoor ozone levels 69–84% of the outdoor concentrations, while other buildings where slow air infiltration provides the only means of air exchange have indoor ozone levels typically 10–20% of those outdoors. Indoor ozone concentrations as high as 0.143 ppm were found in one museum that lacks a chemically protected air conditioning system. For purposes of comparison, the recommended ozone level in places where works of art and historical documents are stored ranges from 0.013 to 0.001 ppm depending on the authority cited. A mathematical model was used to study the ozone concentrations within these buildings to confirm that the differences in indoor ozone levels between buildings can be explained in terms of building and ventilation system design.

THE OZONE FADING OF TRADITIONAL NATURAL ORGANIC COLORANTS ON PAPER

This report details the results of an ozone exposure experiment performed on a large number of traditional natural organic colorants applied to watercolor paper with no binder. These colorants were exposed in an environmental chamber to an atmosphere containing 0.397 ± .007 parts per million (ppm) ozone at 72°F and 50% RH in the absence of light for 12 weeks. This ozone concentration is typical of that found in the Los Angeles atmosphere during a heavy smog episode. The total ozone dose delivered to the samples is equivalent to about four years of exposure to outdoor air in Los Angeles or to about eight years inside a typical air conditioned building in Los Angeles. The ozone sensitivity of these colorant systems was evaluated by monitoring the color changes which occurred during the exposure. Almost all colorant systems tested showed some degree of fading after ozone exposure, and a few of these (curcumin, dragons blood, indigo, and madder lake) should be considered very ozone-fugitive.

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 USE OF DIGITAL IMAGE PROCESSING TO CLARIFY THE RADIOGRAPHY OF UNDERPAINTING

AbstractDigital Image Processing is a computer-assisted technique which can help the conservator interpret the visual information contained in the many photographic images produced when documenting a works condition. In this paper some of these techniques are utilized to assist in evaluating the radiography of a complex 17th century panel painting.

A Prism-Grating-Prism Spectral Imaging Approach

Abstract This paper describes the development and application of a spectral imaging system based on the prism.grating.prism (PGP) line-scan concept. The system is validated against an integrating sphere spectrometer using 22 pigments. Measurements were made on several Old Master drawings and one nineteenth-century watercolour. Results are reported for a watercolour by Eugene Louis Lami to illustrate application of the system. For a square object, 23970 individual reflection spectra are captured in a single scan resulting in file sizes of typically 115 MB. Software has been created that permits direct access, by on-screen selection, to the reflection spectrum of any given pixel in an image, reconstructed from the reflection data. PGP-type spectral scanning systems offer the advantage of potentially very high spectral resolutions of <0.5 nm, though the implementation here is for 5 nm resolution. Digital resolution across a square object is 512 × ∼50 for a PGP slit width of 80 μm. Details of the basic device construction, calibration and operation are provided. The system described here was designed for relatively low spatial resolution, although higher resolution can be achieved with modifications suggested in this publication.

Development of Contact Portable Microfade Tester to Assess Light Sensitivity of Collection Items

The microfade tester is used to assess fading rates of fugitive colors of collection items. The paper presents the research considerations to design a simple, less expensive and portable contact microfade tester that could serve as a screening tool for conservators. Hardware design for such an instrument is presented that includes variations in light source (xenon or LED) and measuring head (ball lens or angled holder fiber), and does not need refocusing between measurements. Performance of the portable microfade tester versions was tested on lab samples and a paper based collection item and based on the ability to rank light sensitivity relative to that of ISO Blue Wool Standards 1, 2 or 3. The results are compared to the ranking obtained with the bench instrument when testing sensitivity of the same items for the same duration. All versions of the portable hardware were found suitable for use as a screening tool to discriminate light-sensitive collection items, with performance of the portable microfade testers using an LED optimized for samples more fugitive than Blue Wool Standard 3. These portable microfade testers need not replace the bench microfade tester as the presented portable microfade tester versions are only suitable to test collection items tolerating surface contact with the instrument.

Aqueous Light Bleaching of Paper: Comparison of Calcium Hydroxide and Magnesium Bicarbonate Bathing Solutions

AbstractThe immediate and long-term effects of aqueously light bleaching both unsized and gelatin-sized cotton cellulose papers in either calcium hydroxide, Ca(OH)2, or magnesium bicarbonate, Mg(HCO3)2, solutions were investigated. After samples were treated by washing, bleaching, or control bathing in the dark, one-half of each was artificially aged, and the condition of all samples was assessed. Appearance was analyzed by reflectance spectrometry, tensile behavior was observed using stress-strain measurements, and pH and gelatin content in the papers were measured with modified TAPPI standard measurement procedures. The results showed that exposure to light per se during the bleaching process did not adversely affect the properties of the papers. It appeared that aqueous light bleaching of the unsized paper was equally effective in either solution. The visible changes in appearance, as well as alterations in other properties of the gelatin-sized paper, were influenced by the partial removal of sizing up...

A REVIEW OF COLOR-PRODUCING MECHANISMS IN FEATHERS AND THEIR INFLUENCE ON PREVENTIVE CONSERVATION STRATEGIES

Abstract Lighting policies as part of a preventive strategy for feather collections in museums often do not consider the different colorant systems found in the feathers. Ornithological research about plumage coloration, while targeting an understanding of signaling in bird behavior, provides valuable information about different colorant systems and their response to light exposure. Because photosensitivity varies for different colorants, it is important for conservators to be able to identify colorants as a first step in designing a preventive strategy for feathers. Visual assessment methods are described as a means for helping conservators distinguish colorant mechanisms. Color measurement methods, especially as used to monitor color change, must be able to reproducibly measure feather colors despite their complex geometries. Measurement methods are referenced that permit reproducibility by accounting for the variations in measurement angles and feather position. Studies reviewed indicate that pigment-based color and structural color do not have the same light sensitivity, with pigment color being more sensitive. This light sensitivity will depend on the nature, concentration, and chemical environment of the pigment. Recent work provides evidence that the insensitivity of structural colors has been over-simplified and photochemical effects beyond fading are necessary and important directions for further study.

Aqueous Light Bleaching of Modern Rag Paper: An Effective Tool for Stain Removal

Summary When conservators of art on paper in the USA find it necessary to carry out a bleaching procedure the method most frequently chosen is aqueous light bleaching. Experiments and practical experience with this technique have shown that when applied to modern rag papers it is effective and the results are aesthetically pleasing. The long term efficacy of overall aqueous light bleaching for removal of stains in gelatine sized papers resulting from contact with acidic ligneous materials has now been investigated. Unstained, stained, and treatment control samples were included. Bleaching was camed out under daylight fluorescent tubes; following treatment, and subsequent ageing, the bleached samples were the least discolored of all. Tensile properties of maximum strength, elongation at break, and stiffness were not adversely affected by exposure to light during treatment, and pH was similarly unaffected. Some gelatin sizing was removed from the papers during immersion, but the aqueously light bleached sam...