Norbert S. Baer
BOOK REVIEWSROBERT L.FELLER, ACCELERATED AGING: PHOTOCHEMICAL AND THERMAL ASPECTS, Marina del Rey, Calif.: Getty Conservation Institute, 1994. xvi+279 pages, softcover, $30. Available from Getty Trust Publication Distribution Center, P.O. Box 2112, DPT GSNG, Santa Monica, Calif. 90407-2112.
Among the publications of the Getty Conservation Institute (GCI) are a series of volumes, published since 1988 under the rubric “Research in Conservation,” intended “to make available the findings of research conducted by the Getty Conservation Institute and its individual and institutional research partners, as well as state-of-the-art reviews of conservation literature.” The present volume is listed in the Library of Congress cataloging-in-publication data as the fourth of what are now eight volumes in print; the eighth, on current stone conservation research, was published in spring 1996. Other subjects included airborne particles in museums, protection of works of art from atmospheric ozone, cellulose nitrate and cellulose ethers in conservation, epoxy resins in stone consolidation, and statistical analysis.
The conservator proposing the use of a novel treatment method is faced with a burden of proof similar to that for the introduction of pharmaceuticals: Is it safe and is it effective? Implied in these criteria, when the application is to cultural property, is the constraint that this safety and effectiveness holds for the intended lifetime of the artifact or monument—in effect, for the foreseeable future. Even if the application is intended to be for only a fraction of that lifetime, and so is additionally required to be fully reversible, it is required that the removal process have no lasting adverse effects and that no degradation in the condition of the artifact or monument result from the treatment and its reversal. Thus, in principle, not only must a wax-resin lining applied to a painting be safe and effective, but, in the long term, no degradation of the original canvas, pigment, or binder can be tolerated, even after the lining has been reversed and some new lining approach has been adopted.
The prediction of long-term aging properties presents one of the most difficult tasks for the scientist engaged in conservation research. Though industry has invested huge sums in accelerated aging and weathering studies, very little of this work is directly applicable to the needs of the conservation scientist. Few museum scientists asked to evaluate materials have the resources, time, or inclination to make appropriate use of the industrial literature. Robert L. Feller, a past president of the International Institute for Conservation–American Group, the precursor of the American Institute for Conservation, and for many years the leading conservation scientist in the United States, dedicated more than four decades to the evaluation of materials of the artist and conservator, focusing initially on varnishes and other materials associated with easel paintings and then on issues associated with the chemistry of cellulose. As a foreign-born crystallographer once said to me, “It is peculiar in the English language that one becomes retired before one becomes tired.” We are indeed fortunate that since his retirement as director of the Research Center on the Materials of the Artist and Conservator at the Carnegie Mellon Research Institute, Dr. Feller has vigorously worked to commit to the written record much of his incomparable experience with the evaluation of conservation materials. We are also in debt to the Getty Conservation Institute for its support of this effort.
In a statement entitled, “Objectives of Accelerated Aging,” Feller lists the goals of a program of accelerated aging as a ranking of a series of comparable materials, the prediction of long-term serviceability, and the study of degradation mechanisms. The first is certainly the easiest in terms of execution but ultimately is of limited value. The last of the three is intellectually the most challenging and represents the principal reason chemists become chemists—that is, their curiosity about why chemical reactions occur and how we can apply that knowledge. The middle task is the most vexing, given the considerable uncertainty we face in finding a real accelerated aging regime where one has the same mechanisms, both chemical and physical, and degradation products of accelerated aging, as one has with natural aging. In recent studies, researchers have put great emphasis on this question. Given his interests, Feller emphasizes photochemical methods and museum artifacts and gives only limited attention to mechanical stresses. He totally omits issues of outdoor exposure and the role of air pollutants. Others have published widely in these areas, and so these omissions should be regarded as a reasonable delimiting of the scope of the discussion, though the volume title suggests a more universal approach.
Chapter 1 defines classes of stability, noting some of the pitfalls that one encounters if careful attention is not paid to the comparability of conditions to which the materials under study were subjected. Here as elsewhere in the text the author introduces a variety of energy terms including lux hours, kilojoules per square meter, footcandle hours, and kilocalories. A table, appendix, or footnote defining the equivalencies among these terms would be of assistance to the reader not completely comfortable with their interconversion. The second chapter deals with measured changes with age, considering deterioration mechanisms, degradation of mechanical properties, and the relationships between chemical change and critical physical properties. Chapter 3 provides an introduction to kinetics, while chapter 4 is devoted to the prediction of useful lifetime, considering empirical equations, statistical analysis of the probability of failure, and correlation of natural versus accelerated aging. In chapter 5 the theory of photochemical deterioration is presented. An instructive figure relates chemical bond dissociation energies to specific bonds and wavelengths. Other topics include the concept of reciprocity and the observation that photochemical deterioration is primarily a surface phenomenon. The role of wavelength in the deterioration process is examined in chapter 6, while valuable insights are afforded in the seventh chapter, “Practical Aspects of Testing,” where light sources, testing strategies, filter stability, and the choice of fresh versus aged samples in the test protocol are discussed.
Chapters 8 through 10 consider the role of moisture and oxygen in oxidative degradation, and, specifically, thermally induced oxidative deterioration. Included are experimental considerations as well as a review of Arrhenius-based extrapolations and their limits. A final chapter, entitled “Toward the Future,” sets a research agenda including the search for methods to monitor changes as artifacts age, consideration of what one might call the epidemiology of degradation, study of the role of peroxides, standards of durability, the role of cycling of light and dark exposure, the effectiveness of inert atmospheres and oxygen scavengers, and the development of simple standard procedures for screening new products and variations of old favorites: a full agenda indeed.
The main text is supplemented with appendices dealing with specialized subjects: (A) photochemical effects in the discoloration of wool, noting important experimental details associated with the selection of light sources, (B) the fading of dyes, citing and discussing seven principal factors (photochemistry of the dye molecule, physical state of the dye, chemi-stry of the substrate, fine structure of the substrate, the presence of foreign substances, atmosphere, and illumination), (C) a brief review of the auto-oxidation of organic compounds, (D) wavelength sensitivity as determined by using narrow band-pass filters, and (E) photo-initiation rate determination by measurement of nitroxide termination.
In his introductory remarks, the author addresses the issue of the intended audience. It is suggested that the conservator, researcher, and the student might benefit, though ultimately the work is written for the author himself. While Feller has always in his writing for the conservation audience, in distinction to his publications in the primary scientific literature, attempted to speak to the reader rather than simply to present results, in this volume the multiple purposes of literature review, text, and research summary give the work the flavor of a memoir without the personal history. The practicing conservator will find the exposition hard going; the few conservation scientists actively engaged in evaluating materials and treatments will certainly wish to turn to it for reference and guidance. Conservators eager for evaluation of the many new products and procedures offered them have at times called for the publication of the results of student research exercises or conservators' relatively informal test results. In the absence of an understanding of the principles of experimental design and the limits of the several protocols for accelerated aging, such results are of limited value. I would argue that any student or conservator proposing to undertake and publish the results of accelerated aging studies should attempt to gain at least a minimum understanding of the contents and message of this book.Norbert S.BaerConservation Center, Institute of Fine Arts New York University 14 East 78th St. New York, N.Y. 10021
In the article by L. A. Daffner, D. Kushel, and J. M. Messinger II, “Investigation of a Surface Tarnish Found on 19th-Century Daguerreo-types” (JAIC 35:9–21), the image printed for figure 6 is incorrect. The correct image is printed here.