AVS2014 Session CS-ThM: Conservation Studies of Heritage Materials

Heritage conservation is an ever-growing discipline, fuelled by factors such as environmental pollution, and the issues surrounding conservation are not always clear. Defining the conservator’s needs and the information required to address them can stimulate scientists to apply analytical techniques to new and unconventional challenges.

In this presentation, we will review some of the key issues faced in heritage conservation and the boundaries imposed by the ethics of conservation. Understanding these ethical considerations can guide the dialogue between conservator and analytical scientist to form a research approach that can satisfy both the scientific questions and the ethics of conservation. The application of electron and ion beam techniques will be considered in terms of conservation studies, and case studies will be used to illustrate the advantages and limitations of these techniques.

The conservation of art and cultural heritage objects requires advances in non-invasive, non-destructive analytical techniques to characterize cultural heritage materials, including substrates (paper, parchment) and media (inks, pigments, colorants). Spectral imaging systems developed for astronomical imaging and remote sensing have been adapted and customized for libraries and museums. T he Library of Congress LC) is using hyperspectral imaging to support preservation of cultural heritage materials with a range of capabilities. With an integrated 39 MegaPixel camera and LED illumination panels to capture high-resolution images in ultraviolet, visible and near infrared spectrum, researchers can create a spectral map of a manuscript or object that can be linked with other non-invasive analyses. Hyperspectral imaging captures non-visible and visible information in registered high resolution digital images, with further capabilities including identification of materials through spectral response, and monitoring of degradation or changes due to environmental conditions and conservation treatments.

The Library utilizes this system to address challenges associated with characterizing manuscript materials, including: early Portolan (nautical) Charts, L’Enfant Plan of Washington D.C., Jefferson’s handwritten draft of the Declaration of Independence, and Herblock political cartoons. It has been used to illustrate non-invasive characterization of materials, deterioration, and detection of non-visible changes due to exhibition and storage. Assessing the long-term effects of treatments on collection materials is a growing area of research at LC. The conservation of a 1513 hand-colored Ptolemy Geographia posed interesting challenges in terms of the treatment of select maps in poor condition, due to the presence of verdigris, and a later restoration treatment. Analysis for treatment to stabilize these seven maps included a combination of quantitative X-ray fluorescence, spectral imaging, and Raman spectroscopy. Development of reference databases and integration of data from other analytical techniques allows a more complete mapping of collection materials. Linking this mapping data with other spectroscopic techniques allows for more data from single-point analyses, and provides a greater depth of information. Scriptospatial mapping of data enables direct sharing and visualization of data, with capture of standardized instrumentation parameters and object metadata. The scriptospatial interface enhances interaction between a range of professions, allowing multidisciplinary collaboration for integration of preservation, scientific and cultural information.

The Encasement also features a custom designed sensor suite to monitor the status of the internal environment. The NIST design monitors and wirelessly transmits differential pressure, barometric pressure, temperature, humidity, oxygen content, and GPS location. All of the sensors were designed to be vacuum compatible with metal seals to ensure integrity of the encasement. The talk will feature discussions on oxygen permeation rates and measurement along with monitoring sensor performance.

Parylene, the generic name for a class of polymers with the base monomer para-xylylene, has been used to strengthen papers via chemical vapor deposition. The deposited monomers polymerize in situ, forming a thin conformal coating that adds strength. Compared to other paper strengthening techniques, such as lining and paper splitting, which are mostly based on individual sheet treatment, parylene coating has the unique ability to treat all the pages of a book simultaneously. Parylene as a paper strengthening technique, however, has had limited recognition within the conservation community since the pioneering research was completed in the 1990s. The major conservation concerns centered at that time on how well Parylene coatings improve the durability of brittle papers and how well the treatment would enable future conservation intervention. One of the earliest criticisms revolved around reversibility of the treatment. No one would reasonably argue that it is desirable to return an embrittled book back to a state of embrittlement, which prevents its being used and/or its ability to accept other traditional repair techniques. Given that we cannot readily correct depolymerization of cellulose at this stage, the real value in parylene is the extent to which it will impart adequate material strength and be receptive to the range of traditional repair techniques as would be used on a non-embrittled book, and all other things being equal, how long the strength will last.

To answer these concerns and highlight the potential of this paper strengthening technique, we present the results obtained from mechanical testing and the behavior of parylene coated paper in standard paper conservation treatments. These results show that Parylene-treated groundwood pulp book papers from 1951 reveal many of the characteristics of a new wood pulp paper, in term of rattle, turn radius, and general tactile experience. The treated paper has over 30% improvement in tear resistance and more than three times higher folding endurance (based on a log scale of the number of double folds via an MIT folding endurance tester). Additionally, parylene treated paper is receptive to conventional paper conservation treatments such as traditional wheat starch paste and Japanese paper tear mending, guarding, washing and resizing. We will also discuss moisture content measurements to clarify concerns about the vacuum treatment involved in the Parylene coating process and its effect upon the treated paper.

Iron gall inks were the major writing medium from the middle ages through the 19th Century in the Middle East and Europe, and are present in hundreds of thousands of important cultural heritage objects worldwide, including books, manuscripts and artistic drawings. Iron gall ink depending on its preparation is well known for its potentially corrosive effect on paper or other writing medium, over time, changes in temperature and humidity can accelerate this degradation resulting in the worst case, complete loss of documents. The major ingredients of ion gall ink are iron salts, most often iron sulfate, tannic acids derived from vegetable sources such as gall nuts and gum arabic used as a binder. In this study, X- Ray Photoelectron Spectroscopy (XPS), is used to probe the chemistry of iron gall ink and to study the effect of common preservation techniques such as deacidification and phytate treatment. Despite much research in this area the chemistry of iron gall ink is still poorly understood, including the structure of the complex itself, resulting from the reaction between iron sulfate and gallic acid. The two most widely accepted structures in literature have been proposed by Krekel and Wunderlich, these structures will be discussed and compared to XPS, Raman, FTIR and X-ray crystallography data obtained from model compounds.