Scientific examination of art : modern techniques in conservation and analysis

R00534--00 Sackler- FRONT MATTEScientific Examination of Art
Washington, D.C. March 19–21,2003
THE NATIONAL ACADEMIES PRESS 500 Fifth Street, N.W. Washington, D.C. 20001
This work includes articles from the Arthur M. Sackler Colloquium on the Scientific Examination of Art: Modern Techniques in Conservation and Analysis held at the National Academy of Sciences Building in Washington, D.C., March 19-21, 2003. The articles appearing in these pages were contributed by speakers and attendees at the colloquium and were anonymously reviewed, but they have not been independently reviewed by the Academy. Any opinions, findings, conclusions, or recommendations expressed in this work are those of the authors and do not necessarily reflect the views of the National Academy of Sciences.
The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the U.S. Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters.
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Cover: “Corner of the Studio” by Antonio Ciocci. Courtesy of Catherine and Wayne Reynolds
The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Bruce M. Alberts is president of the National Academy of Sciences.
The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. Wm. A. Wulf is president of the National Academy of Engineering.
The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Harvey V. Fineberg is president of the Institute of Medicine.
The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy’s purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Bruce M. Alberts and Dr. Wm. A. Wulf are chair and vice chair, respectively, of the National Research Council.
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Born in Brooklyn, New York, Arthur M. Sackler was educated in the arts, sciences, and humanities at New York University. These interests remained the focus of his life, as he became widely known as a scientist, art collector, and philanthropist, endowing institutions of learning and culture throughout the world.
He felt that his fundamental role was as a doctor, a vocation he decided upon at the age of four. After completing his internship and service as house physi- cian at Lincoln Hospital in New York City, he became a resident in psychiatry at Creed-moor State Hospital. There, in the 1940s, he started research that resulted in more than 150 papers in neuroendocrinology, psychiatry, and experimental medicine. He considered his scientific research in the metabolic basis of schizophrenia his most significant contribution to science and served as editor of the Journal of Clinical and Experimental Psychobiology from 1950 to 1962. In 1960 he started publication of Medical Tribune, a weekly medical newspaper that reached over one million readers in 20 countries. He established the Laboratories for Thera- peutic Research in 1938, a facility in New York for basic research that he directed until 1983.
As a generous benefactor to the causes of medicine and basic science, Arthur Sackler built and contributed to a wide range of scientific institutions: the Sackler School of Medicine established in 1972 at Tel Aviv University, Tel Aviv, Israel; the Sackler Institute of Graduate Biomedical Science at New York University, founded in 1980; the Arthur M. Sackler Science Center dedicated in 1985 at Clark University, Worcester, Massachusetts; and the Sackler School of Graduate Biomedical Sciences, established in 1980, and the Arthur M. Sackler Center for Health Communications, established in 1986, both at Tufts University, Boston, Massachusetts.
His pre-eminence in the art world is already legendary. According to his wife Jillian, one of his favorite relaxations was to visit museums and art galleries and pick out great pieces others had overlooked. His interest in art is reflected in his
Arthur M. Sackler, M.D. 1913-1987
philanthropy; he endowed galleries at the Metropolitan Museum of Art and Princeton University, a museum at Harvard University, and the Arthur M. Sackler Gallery of Asian Art in Washington, D.C. True to his oft-stated determination to create bridges between peoples, he offered to build a teaching museum in China, which Jillian made possible after his death, and in 1993 opened the Arthur M. Sackler Museum of Art and Archaeology at Peking University in Beijing.
In a world that often sees science and art as two separate cultures, Arthur Sackler saw them as inextricably related. In a speech given at the State University of New York at Stony Brook, Some reflections on the arts, sciences and humanities, a year before his death, he observed: ‘‘Communication is, for me, the primum movens of all culture. In the arts. . . I find the emotional component most moving. In science, it is the intellectual content. Both are deeply interlinked in the humanities.’’ The Arthur M. Sackler Colloquia at the National Academy of Sciences pay tribute to this faith in communication as the prime mover of knowledge and culture.
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BARBARA BERRIE, Senior Conservation Scientist, National Gallery of Art, Washington, D.C.
E. RENÉ DE LA RIE, Head of Scientific Research, National Gallery of Art, Washington, D.C.
ROALD HOFFMANN (NAS) (Chair), Frank H. T. Rhodes Professor of Humane Letters, Cornell University
JANIS TOMLINSON (NAS), Director of University Museums at the University of Delaware
TORSTEN WIESEL (NAS) (Chair), President Emeritus, The Rockefeller University
JOHN WINTER, Conservation Scientist, Freer Gallery of Art and Arthur M. Sackler Gallery, Washington, D.C.
Staff
KENNETH R. FULTON, Executive Director ALYSSA CRUZ, Program Administrator (from October 2005) MIRIAM GLASER HESTON, Program Officer (until October 2005)
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Preface
The study of works of art using scientific methods dates back to the late 18th century but expanded exponentially in the late 20th century. The Sackler conference held March 19-21, 2003, assembled a group of leading conservators and conservation scientists to present and assess recent initiatives providing a unique overview of this important field. Six of the following fourteen papers begin with a key material for cultural artifacts (Venetian pigments, works of art on paper, photographs, stone sculpture, modern paints, and early Chinese jade) and enu- merate various means of identification and analysis. Four of the papers start with an advanced analytical method and discuss its applications: infrared reflectography, multi-spectral imaging, Raman microspectroscopy, and quantita- tive gas chromatography-mass spectrometry. Two papers focus on mechanisms of deterioration—biodeterioration of outdoor stone and disruptions in the sur- faces of aged paint films. The breadth of the discourse is well illustrated by the topics listed above and by three summary papers: an overview of the concept of conservation science, a brief history of the evolution of practical conservation techniques and attitudes in the 20th century, and a discussion of the impact of collaborative research among conservators, scientists, and art historians. These fourteen contributions exemplify the wide variety of art materials that challenge the investigative scientist and the increasing sophistication of an array of scientific tools that now aid in the decision making for the important task of the preservation of works of art and cultural heritage.
Dr. Joyce Hill Stoner, Professor and Paintings Conservator Winterthur/University of Delaware Program in Art Conservation
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Contents
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Overview 3 John Winter
Material Innovation and Artistic Invention: New Materials and New Colors in Renaissance Venetian Paintings 12
Barbara H. Berrie and Louisa C. Matthew
The Scientific Examination of Works of Art on Paper 27 Paul M. Whitmore
Changing Approaches in Art Conservation: 1925 to the Present 40 Joyce Hill Stoner
An Overview of Current Scientific Research on Stone Sculpture 58 Richard Newman
Biodeterioration of Stone 72 Thomas D. Perry IV, Christopher J. McNamara, and Ralph Mitchell
xii CONTENTS
Molly Faries
Color-Accurate Image Archives Using Spectral Imaging 105 Roy S. Berns
Multi-Spectral Imaging of Paintings in the Infrared to Detect and Map Blue Pigments 120
John K. Delaney, Elizabeth Walmsley, Barbara H. Berrie, and Colin F. Fletcher
Modern Paints 137 Tom Learner
Material and Method in Modern Art: A Collaborative Challenge 152 Carol Mancusi-Ungaro
Raman Microscopy in the Identification of Pigments on Manuscripts and Other Artwork 162
Robin J. H. Clark
Paint Media Analysis 186 Michael R. Schilling
A Review of Some Recent Research on Early Chinese Jades 206 Janet G. Douglas
APPENDIXES
The State of the Field
3
This paper introduced a colloquium whose theme was the study of works of art by scientific methods. To present a brief overview of a field where all kinds of works might be studied by any applicable kind of scientific technique is hardly a practical possibility. Rather, I would like to try to give a little more depth to all of this, in terms of both the history and the diversity to be found in studies of these types.
One basic problem lies in the conceptual magnitude and diversity of such a field. A “work of art” can mean a human artifact designated as such and made from an enormous variety of materials. Implicitly we are attempting to bring together objects made from rocks and minerals, metals of all kinds, ceramics, organic materials derived from plants and animals, or synthetically created—the list goes on. An artifact may be a complex, partially ordered system with components of diverse chemical nature, as is true of most paintings and many other things, or it may comprise only one type of component. The scale can vary from thumbnail size to that of architecture and monuments. Even the word “art” does not help much, since any familiarity with the field reveals people working with what is usually termed self-conscious art, with decorative art, or with functional objects regarded for the purpose as art. For the most part, scientists who choose to do this kind of research do not seem to trouble themselves overmuch with how artistic the art is. The field overlaps that of archaeological science, which studies archaeological, usually excavated artifacts, although much archaeological science is not concerned with artifacts at all. All these things might be examined using any method from any branch of science that holds the promise of yielding some kind of result. This colloquium will be covering large segments of this whole area,
Overview
John Winter Freer Gallery of Art and Arthur M. Sackler Gallery
Smithsonian Institution Washington, D.C.
4 SCIENTIFIC EXAMINATION OF ART
though it would be optimistic to suppose that all possible types of work and kinds of artifacts could possibly be covered in two days.
PEOPLE
A word should be entered concerning the scientists who choose to do this kind of work and where they do it. The field can scarcely be said to be overpopulated by practitioners, at least in relation to its overall conceptual scale. Tennent (1997) saw the organizational structure as being in four parts: laboratories in museums, those university departments that take an interest, research institutes (often national research institutes) that have departments established for this purpose, and to a lesser extent the private sector. Many people in the field nowadays are professional research scientists fully committed to this branch of research in the same sense that other scientists will consider themselves fully committed to a particular branch of science. These tend to be found working in the research institutes and in departments of the larger museums, occasionally in universities. The majority of them are scientists who started out in some branch of the mainstream sciences, typically a branch of chemistry or physics or materials science, before moving into the present field. There are now a few, though only a few, who were able to do graduate studies in the field itself. A smaller group of research scientists have their primary interests elsewhere but also take part in cultural properties studies. They tend to be in academic institutions and may work on projects of interest for a short or extended period and then move out of the field again. Then there is a less easily defined group of scholars and professionals who are trained in fields other than the sciences but who perform and apply research to problems in their own field: art historians, conservators, and archaeologists may fall into this category.
Most major branches of physical science have much higher numbers of researchers than is the case with us, and modern science has as a consequence a considerable social structure, for want of a better term. Leading scientists form groups and schools of research that interact with one another, perhaps in collabo- ration, perhaps in competition. This can be on a relatively large scale and may sometimes last for extended periods. It includes direct, informal contact as well as more formalized kinds. In our field this intensity of interaction, which depends on a kind of critical mass of people, is much less. The number of practicing researchers is too small in relation to the number of kinds of things that they might be doing, that is the number and variety of research topics that exist. Since it is at least arguable that the immense success of the twentieth-century scientific endeavor in general was to some extent a result of such social structuring, problems are implied for our own comparatively diluted areas for which it might be difficult to find answers.
OVERVIEW 5
TECHNIQUES AND TERMINOLOGY
The scientific methods that we use deserve some comment, though it is difficult to generalize. They have usually been methods of study—of analysis, imaging, accelerated testing, and so forth—taken quite directly from other areas of science and technology. They tend, as a result, to have been optimized for work within some other field. With a few important exceptions, such as one or two dating methods, most techniques were not developed specifically within our own field. This state of affairs, of a conceptually large research field populated by relatively small numbers of researchers using techniques borrowed from elsewhere, led one colleague, Irwin Scollar, (actually with reference to archaeological science) to suggest that this was equivalent to conducting guerrilla warfare using captured weapons (Olin, 1982, p. 102).
One of the consequences of the rather complex situation that I have just sketched is terminological: There is no general agreement on what to call this field of study, taken as a whole. There is not even total agreement on what general term to use for the objects of study. Since they may or may not be archaeological, may or may not be historical, and may or may not always be artistic (according to somebody’s definition), such phrases as “cultural heritage,” “cultural property,” and “cultural assets” have come into use but are clumsy when an extension of the terms into studies using scientific methods is required. For the field of study itself we have on the archaeological side, “archaeometry,” “archaeological science,” and “science in archaeology,” which have all been used, and sometimes criticized. These terms are not usually extended to research on works of artistic or historical importance unrelated to archaeology. Here “conservation science” has become prevalent, especially in the United States, though the work may or may not be related to efforts to conserve the objects concerned. “Technical studies of works of art” was in use in the 1930s but is seldom found now. “Technical art history” has appeared, and the parallel to archaeological science would appear to be “art historical science.” All these terms, however, seem to imply subsets of the field as a whole, which awaits its definitive title and therefore perhaps its precise definition.
HISTORY
It might help give some depth to the discussions to look briefly at the history of the field. Even an extended look would be partial, since to the best of my knowledge no definitive account is available: Much of the historical spadework remains to be done. We do know that scientific study of antiquities and works of art goes back to the late eighteenth century. Earle Caley (1951) located almost 100 publi- cations dated before 1875 (of which the earliest was late eighteenth century) mostly concerned with archaeological materials, and especially with the analysis of metals. Through the nineteenth century, work on this kind of material was
sporadic and mostly conducted by a few individuals concerned with identifying and analyzing archaeological and similar material on the side in laboratories primarily devoted to other purposes. Thus were the origins of one kind of research that continues to the present day: the study of artifacts that we consider archaeological, whether or not systematically excavated. It is now regarded as one segment of archaeological science, the segment concerned with artifacts. Much of this research seems to be done in academic institutions. The driving force is largely archaeological, and although the objects concerned may also be classified as fine art, this is largely coincidental. There is typically freedom to take samples necessary for analysis, and conservation of the objects has not usually been an issue. We might regard this as the archaeological tributary of the research efforts that developed during the twentieth century.
The examination of paintings and sculpture appears to go back over a similar time period. Since this paper was delivered, Nadolny (2003) has published a historical study of early analytical work on paintings, which appears to date from ca 1780. We know of analyses of pigments in mural painting by Haslam in 1800 and Humphrey Davy in 1815 (Rees-Jones, 1990), and of work in Munich on easel paintings from 1825 (Miller, 1998). It can be regarded as forming another line of development leading to where we are now. Two of the better-known practitioners were A. H. Church in the late nineteenth century and A. P. Laurie in the earlier part of the twentieth century; both served as professors at the Royal Academy of Arts in London. Much of the motivation for this type of work seems to have been historical interest, with reference being made also to various historical texts. Both connoisseurship and a desire to encourage contemporary artists to use appropriate and durable materials may also have played a part. This kind of research seems mostly to have taken place in the larger museums and in research institutes set up to work with them, occasionally in academic departments. Here conservation of the object is much more of an issue; the taking of samples is more restricted, especially in recent times, and may be forbidden outright. Consequently noninvasive methods have become important.
Scientific research devoted to making conservation itself more rational and effective came along a little later than the preceding two tributaries of development, though it can also be traced back to the nineteenth century. The National Gallery in London commissioned reports on the condition of its paintings in the 1850s (Brommelle, 1956), and the British Museum consulted outside scientists on conservation problems well before setting up its own facilities (Watkins, 1997). In 1888 Friedrich Rathgen’s laboratory was set up in the Königlichen Museen in Berlin (Plenderleith, 1998). The years following the First World War saw the founding of conservation departments in a number of places: the British Museum and the National Gallery in London, Le Louvre in Paris, the Fogg Museum at Harvard University, among others. This kind of research has come to overlap extensively the research in the preceding category, the historical investigation of
OVERVIEW 7
the fine arts. It tends to be done in similar places and often by the same people, and similar restrictions on methods of investigating an object usually apply.
There is much complexity in the ways that these historical streams have flowed down to contribute to the present state of affairs. There is overlap of major categories, both conceptually and in the sense that the same people may conduct kinds of research that might be looked upon as logically different. Different classes of cultural property also impose their own characteristics on any studies that are conducted on them. Research on large-scale entities (for example, buildings, monuments, and sites) is probably driven very largely by conservation needs, including protection and restoration, but its practitioners might see little in common with the conservation of museum objects.
AESTHETIC CONSIDERATIONS
Given this complexity in the study of anything held to be of cultural significance, using many techniques from the sciences, with a number of reasons and motiva- tions driving us, what are the common threads? What kind of conceptual framework is it possible to discern in all this? Before making any attempt to answer we must refer to yet another aspect of the situation. When we say we want to study works of art using the methods of science, we imply that these works have significance quite outside any scientific considerations, and that this significance is the reason for finding them important enough to study. This aspect cannot be ignored. Obviously the practicing conservator can never ignore it, but I suggest that the scientist doing research on works of art cannot ignore it either, even when the research appears to consist entirely of, say, solving problems of analysis and to be quite matter of fact in nature. The distinction to be seen here has been drawn before, perhaps many times.
Anything that we call a work of art is being seen by definition from at least two points of view. One point of view sees it as a physical object, the other looks at whatever properties the object has that lead us to say that it is a work of art, and to attach value to it on this basis. Joseph Margolis (1980) defined a work of art as a token embodied in a physical object. Referring to a work as an image conveys much the same idea. When we speak of such aspects of the work as expressiveness, style, symbolism, the meaning of the whole work or parts of it, any emotional feelings (positive or negative) that may be aroused, we are adopting the token or image point of view. Seeing the work as a physical object is, I believe, self-evident in meaning, and doing so is not confined to the research scientist or conservator; however, to study a work of art using scientific methods means scrutinizing it as a physical object to a greater depth and from more points of view than would be done with any other approach. The specification of what should be studied springs from other parts of human culture.
Traditional art history adopts the token or image point of view largely, though
not entirely. Around the late nineteenth to early twentieth century we see ex- amples of art historians, such as Konrad Fiedler, who saw the final form and the style of a work as the product of the interaction of artists with their materials, and Gottfried Semper, who appeared to see art as the byproduct of handicraft (Hauser, 1985). Although this kind of thing does not represent very much that has endured in art historical concepts, the physical object that embodies the art as a token has meant something in traditional art history. For example, art historians have from time to time taken an interest in workshop organization and procedures in the production of paintings (e.g., Phillips, 2000; Shimizu, 1981). For all this, the conceptual framework of art history has been established very largely in aesthetics and similar considerations. It is reasonable to ask how far this can affect our own interest in the same objects of study and how far there can be intersections in the frames of reference.
ART AND TIME
Apart from the fact that we study works of art rather intensively as physical objects, what other commonality can be discerned to make us think that the scientific study of this huge mass of disparate cultural assets can form a coherent subject? One way of looking at it is to say that we study those products of human- kind, defined as cultural assets—or art—along each object’s time axis. Such a concept can be divided into three phases. At one end of the time axis we look at the materials the creator (or creators) of an artifact used and how they used them. Then we can consider what changes have occurred in the product. Finally we assess what is the situation for the artifact in question now and how we can predict and influence its life into the future.
We start with the production of the work of art. Art historians talk about the inspiration of the artist, that artist’s vision, the influence of other artists or schools, and so on that results in the creation of the particular thing that we now admire and discuss. The fact remains that no painting or sculpture or anything else springs from somebody’s mind in the fashion of a “thinks” bubble in a cartoon strip. It has to be fashioned from whatever materials were available, using whatever techniques were in use, and these aspects are among the things we are trying to discover about that object. The identity of the artist may or may not be known, and commonly more than one person was involved. We could look on this as investigating the ethnology of the creation of a surviving work. We take account of the historical context and the cultural context in which this process occurred, both of which inevitably had their influences on what was created, which raw materials were used, and on how it all happened. We have a link with human beings who lived in the past—perhaps the recent past, perhaps a more remote past—not just in the sense of the aesthetic concepts or visions they possessed (important as these were) but also in the sense of how they got their hands dirty to make something; ultimately we are investigating not just interesting assemblies of
OVERVIEW 9
pigments, binders, stone, ceramic, wood, or whatever it may be but the real people who created things.
On to the second phase: What has happened to our cultural asset since it was made. Any artifact, whether artistic or not, starts to change from that moment. The kinetics of such changes vary rather a lot, but on some time scale changes are happening. We call these deterioration mechanisms, and to me as a chemist they are both extremely interesting and quite difficult to study. An understanding of deterioration mechanisms is important from at least two opposite-facing points of view. If we are concerned with the production of an artifact by bygone persons, we are presumably concerned with what they actually produced, which will have changed to a greater or lesser extent in the meantime. There are some areas where such changes are small enough to be ignored but a great many more where they are not. To project our understanding back to the start of the object’s time axis, we need to talk about what has happened to it. This is true even though many artists may have known well that their creations would change over time and they may have been perfectly content with that. The second reason for understanding deterioration mechanisms is conservation, which one may think of as facing forward rather than backward. Conservators are given the responsibility for stabiliz- ing, treating, and perhaps restoring something that has survived in better or worse condition, and trying to ensure its continued survival into the future. To deal with this rationally they need to know what has been happening chemically and physically to the assembly of materials constituting each object.
This links directly to the third phase of our time axis: how to extend it forward as far as possible. The conservator needs to know not only what is there in a material sense but also what is likely to happen with it chemically and physically, possibly after some treatment has been applied. Knowledge of such pro- cesses is also needed for any present-day materials that may be used for treatment in the context of the ways in which they are used. Investigations of these complex issues in conservation have become of primary interest in recent years.
IMPLICATIONS FOR THE SCIENTIST
To the researcher in this field who was brought up, as many of us were, in some branch of the mainstream sciences, the demands can be challenging. Typically, work to obtain a scientific research degree, possibly followed by a year or two of postdoctoral research, will lead to proficiency in some branch of science taught in universities, probably a subdiscipline of chemistry or physics. The science thus mastered may be applied to situations arising possibly over many types of works of art and cultural heritage generally. Committed professionals in our field may soon find themselves with some research specialty defined in terms of the works of art themselves; my own, for example, happens to be East Asian paintings. The professional researcher then finds that studying the works of art as physical objects within his chosen area, whether limited or broad, requires the application of
scientific knowledge and understanding from a number of scientific disciplines, which may be removed from his original area of proficiency. There has been a kind of orthogonal transposition of concepts; rather than specializing in a single scientific discipline in depth, the researcher needs to take a range of basic disciplines and apply them to a class of objects that will themselves be studied in depth. No doubt this happens in other fields of research too, and it is certainly intellectually stimulating. It can also be alarming. Most scientists, I think, are sensitive to the implications of specialization, to the probability of wandering into error when they venture into branches of science other than their own. The physicist John Ziman published a book (1987) some years ago dealing with questions of mobility and career change in the sciences, including the reasons why most scientists tend to be reluctant to change areas of research in which they work. The problem of how to apply selected, specialized areas of science to a further understanding of things that ultimately have to be understood on their own terms is also an intellectual challenge of the field.
CONCLUSION
I conclude with a few words about the colloquium that followed. For reasons that I mentioned earlier, describing all aspects—or all important aspects—of the scientific examination of art is impractical. We hope to have organized a fair sam- pling of what the field is about, in all its variety and complexity. This first day was intended to give fairly broad reviews of progress in at least some of the major areas of work. The second day saw accounts of significant progress in more spe- cific topics. This was intended to give us some realistic perspectives on what has been achieved and what has not been achieved in research, particularly that of the past few years. I think that most of the presentations will fit on the time axis of an object that I suggested as describing the kinds of work done. Some may look at questions of the materials and methods used by the creators of artifacts that we choose to call “art,” some at research on deterioration mechanisms, and others at questions of an object’s present status and the prognostications we may have for its future.
In this introductory paper, rather than discussing modern techniques or recent progress, which others will discuss later, I have tried to give some sugges- tion of depth, even (in a sketchy kind of way) historical depth to the subject. I would like to be able to give it some coherence, but I fear that would be claiming altogether too much. Do we really have just one field here, or several smaller fields that happen to overlap here and there? What are the connections between scientific studies and considerations of aesthetics, the original intent behind creating something, and the connections to questions of intended use? This colloquium was never intended to cast light on problems of this nature, but if we have a serious intellectual discipline underpinning what we do, the more fundamental questions implied by its pursuit should at least be recognized to exist.
OVERVIEW 11
REFERENCES
Brommelle, N. 1956. Studies in Conservation 2:176-187. Caley, E. R. 1951. Journal of Chemical Education 28:64-66. Hauser, A. 1985. The Philosophy of Art History. Evanston, Ill.: Northwestern University Press. English
version of Philosophie der Kunstgeschichte, Oscar Beck, Munich, 1958, pp. 216, 232-234. Margolis, J. 1980. Art and Philosophy: Conceptual Issues in Aesthetics. Brighton, Sussex: Harvester
Press. Miller, B. F. 1998. In Painting Techniques. History, Materials and Studio Practice. Contributions to the
Dublin Congress 7-11 September 1998, eds. A. Roy and P. Smith, pp. 246-248. London: Interna- tional Institute for Conservation of Historic and Artistic Works.
Nadolny, J. 2003. Reviews in Conservation 4:39-51. Olin, J. S., ed. 1982. Future Directions in Archaeometry. A Round Table. Washington, D.C.:
Smithsonian Institution. Phillips, Q. E. 2000. The Practices of Painting in Japan, 1475-1500. Stanford, Calif.: Stanford Univer-
sity Press. Plenderleith, H. J. 1998. Studies in Conservation 43:129-143. Rees-Jones, S. 1990. Studies in Conservation 35:93-101. Shimizu, Y. 1981. Archives of Asian Art 34:20-47. Tennent, N. 1997. In British Museum Occasional Papers, 116: The Interface between Science and Con-
servation, ed. S. Bradley, pp. 15-23. London: The British Museum. Watkins, S. C. 1997. In British Museum Occasional Papers, 116: The Interface between Science and
Conservation, ed. S. Bradley, pp. 221-226. London: The British Museum. Ziman, J. 1987. Knowing Everything about Nothing. Specialization and Change in Scientific Careers.
Cambridge: Cambridge University Press.
12
Material Innovation and Artistic Invention: New Materials and New Colors in
Renaissance Venetian Paintings
and Louisa C. Matthew
Department of Visual Arts, Union College, Schenectady, N.Y
Sixteenth-century Venetian painters have been regarded as “colorists” since their own time. The phrase “Venetian palette” is used today by art historians to describe the colors used by Renaissance painters of Venice, among whom Titian, Giovanni Bellini, and Tintoretto are the most famous. There is in fact little written consensus about how to define this so-called Venetian palette, but our knowledge is continually expanding thanks to scientific research on these artists’ paintings. One color has always been mentioned as being particularly Venetian: a rich deep orange, used generously by Venetian painters from about 1490. These artists used the arsenical sulfides yellow orpiment (As2S3) and orange realgar (As4S4) to achieve this color. Until the end of the fifteenth century this pair of minerals had been largely confined to the miniaturists’ palette, but they became so popular in sixteenth century Venetian painting that G. P. Lomazzo remarked in his 1584 treatise “burnt orpiment is the color of gold and it is the alchemy of the Venetian painters” [1]. Artists such as Giovanni Bellini used it abundantly in their paintings; for example, Bellini used it for Silenus’ robe in The Feast of the Gods (1514; reworked by Titian, 1524) (Figure 1). The analytical data we discuss here, while still fragmentary, points to a richness of materials and their innovative use by Venetian artists that is greater than imagined heretofore, and much more than simply the addition of the arsenical minerals.
Recently discovered evidence has established that professional color-sellers plied their trade in Venice from the end of the fifteenth century. It appears that they existed here as much as a century earlier than in any other Italian city. These color-sellers were neither apothecaries (“speziali”) nor general grocers from whom artists had purchased their painting supplies throughout the middle ages and
MATERIAL INNOVATION AND ARTISTIC INVENTION 13
FIGURE 1 The Feast of the Gods, Giovanni Bellini and Titian, 1514/1529, oil on canvas, (National Gallery of Art, Washington, D.C. 1942.9.1).
early Renaissance. They were sources who specialized in materials used in the arts and trades that dealt with color and color manufacturing. Some of the most interesting and useful evidence for the existence of professional color-sellers takes the form of inventories of the contents of their shops. The earliest found so far dates to 1534 [2]. Another, longer inventory of a color-seller’s shop dated 1596 has been found and published [3]. Examination of the materials in the 1534 inventory and investigation of their uses, particularly in glass-making and ceramics, coupled with our new analyses, reveal relationships that encompass both tradition and innovation. There is evidence for more cross-fertilization of tech- nological know-how and taste among artisan industries than previously sup- posed. In this paper we will show how the information from the inventories combined with new analytical data has been used to expand our knowledge and understanding of the materials used by painters in Venice and add to the complexity of the definition of the Venetian Renaissance palette.
The 1534 inventory lists 102 items; weights or amounts are given but no monetary values. Many of the materials on the inventory have an established connection with the easel painters’ art, including, for example, the pigments azurite, vermilion, lead white, and orpiment. Kermes and brazilwood, organic extracts which were used to make red dyes as well as red paints, are listed. Other items in the “vendecolore” shop that relate to the dyers’ craft include alum for mordanting dyes, galls (for making black dyes), and various resins.
The first printed book on dyeing on a commercial scale was published in Venice in 1548, titled The Plichto of Gioanventura Rosetti [4]. It was written not by a dyer but by a technologist, Gioanventura Rosetti, whose intention was to provide information on what might be termed “best practices” to benefit the Venetian Republic. The recipes in the Plichto contain many of the items on both the 1534 and the 1596 inventories, including some usually considered by historians as pigments, including orpiment, vermilion and azurite, which are described in one recipe as mineral dyes (Figure 2). The overlap between painters’ and dyers’ colorants continues to become more apparent.
FIGURE 2 Extract from “The Plictho of Gioanventura Rossetti” first published in Venice in 1548. Translated by Sidney M. Edelstein and Hector C. Borghetty, The MIT Press (1969).
The Venetian glass industry, centered on Murano, one of the islands in the Venetian lagoon, was burgeoning in the late fifteenth century. By this time the glassmakers had produced a clear glass called “cristallo” after the rock crystal that had inspired its invention. Large quantities of clear and colored glass were produced for making a wide variety of objects, including tableware, goblets, glasses, and mosaic tesserae. Recipes for richly colored glass, both single-toned and multi- colored to imitate opal and chalcedony, were developed. Special, deeply-colored glass was produced for making false rubies, sapphires, and emeralds that were as intensely and beautifully colored as the real gems. In the first decades of the sixteenth century recipes for glassmaking were being compiled [5]. The Darduin manuscript provides important information on Renaissance glassmaking, and the work of the Florentine, Antonio Neri (died 1614), who wrote L’Arte Vetraria (1612), a compilation of recipes including many of sixteenth-century origin, is an invaluable source. [This recipe book was translated into English by Christopher Merrett in 1662.] For our knowledge of the Venetian glassmaking industry we also owe much to the work of the Muranese, Luigi Zecchin [6].
Materials necessary for glassmaking are found on the 1534 inventory. Recipes for glass indicate that tin and lead were required in large quantities; both of these are on the inventory. Other ingredients include tartar, mercuric chloride, borax, alum, salt, and “tuzia” (zinc oxide), as well as orpiment. These materials are also used by dyers and some by painters.
The wide range of materials available at the color-seller’s shop suggests that artisans from many trades that used color went there to obtain their raw materials. The variety available in this one place prompted us to consider whether there was more cross-fertilization among artisans than previously assumed and if we might find some evidence for this in the painting practice of the Venetian artists.
We reanalyzed samples from paintings in this light, looking for materials not previously recognized. Samples from several paintings by Venetian Renaissance artists were available from prior studies. They are preserved as cross-sections of the paintings mounted in bioplastic polyester/acrylate resin. For optical microscopy, a Leica DMRX polarizing light (PL) microscope was used with PL fluotar objectives. For fluorescence microscopy the light source was a mercury lamp (100W) and the D and I3 filter packs. Scanning electron microscopy (SEM) was undertaken using a JEOL 6300 equipped with an Oxford Instruments Tetra backscatter detector. For energy dispersive spectrometry (EDS) the SEM was fitted with an Oxford Si(Li) ATW detector (capable of detecting low-energy x-rays) with a resolution at the Mn kα line greater than 130 eV. The cross-sections were usually carbon-coated, but sometimes gold-palladium coatings were used. X-ray powder diffraction patterns were obtained using Philips XRG 3100 x-ray genera- tor with a copper tube. Data were collected on photographic film in a Gandolfi camera (radius 57.3 mm). Line spacings were measured against a calibrated rule and relative intensities estimated by eye.
Samples from paintings by the Venetians Lorenzo Lotto (1480-1556) and Jacopo Tintoretto (1519-1594) were among the first to be re-examined. Although the samples are limited in number they already show that the range of materials used to make paint is wider than previously known.
Lotto was “rediscovered” in the late nineteenth century, but it took most of the twentieth century for him to become acknowledged as a Venetian painter. Recent research on his painting technique and color palette has helped define his place in the Renaissance [7, 8]. There is little documentary information on Lotto’s early career as an artist, but it is believed that he trained in Venice and spent his first years as an independent artist there. Later, he painted in Bergamo and the Marches. He traveled a good deal, usually within the economic and political orbit of the Venetian Republic, and he returned to the city itself for several periods. Our knowledge of Lotto’s working methods is augmented by the survival of one of his account books in which he documented commissions and expenditures during the years 1538 to 1556 [9]. One particularly valuable section of the account books is an appendix of spese per l’arte (expenditures for art), where he recorded the purchase of painting supplies, among which are notes on pigments he purchased in Venice.
Among Lotto’s paintings at the National Gallery of Art in Washington, D.C. is St. Catherine, signed and dated 1522 (Figure 3). St. Catherine’s dress is a glori- ous red, perhaps reminiscent of the color of expensive red cloth worn by some Venetian brides at this time. A cross-section from the sleeve (Figure 4) shows the complicated layering Lotto used to create this color. In the cross-section, we see, from the bottom, the preparatory layer of gesso (CaSO4.2H2O in glue), used to provide a smooth surface for painting, over which many layers of paint were applied. The first layers of paint are pinks prepared from a mixture of vermilion and lead white. Lying over these are layers of transparent red paint. From fluorescence microscopy (Figure 5) it can be discerned that what appears to be a thick homogeneous paint film is in fact many layers of thin glazes of paint; there appear to be at least six layers. The same painting technique was found in two versions of another composition painted by Lotto in the same year, The Virgin and Child with Saints Jerome and Nicholas of Tolentino [8]. It was shown, using high-perfor- mance liquid chromatography, that for the version at the National Gallery, Lon- don, Lotto used both madder and insect lakes. The fluorescence of the lakes in St. Catherine’s dress implies that he used two different lakes here also.
Digital dot maps of the distribution of the elements in a sample from St. Catherine obtained using SEM-EDS are shown in Figure 6. The lowest layer of paint contains mercury, confirming that Lotto used vermilion for mixing the light red underpaint. Aluminum is present throughout most of the upper layers of transparent paint glazes. This strongly suggests that the pigment is a dye laked on alumina, the traditional way to prepare insoluble pigments from dyes made from lakes. Unexpectedly, several of the layers of transparent paint contain small, rounded particles, ca. 4-8 microns in diameter. These particles appear to be very pure silica. It is difficult to obtain information on individual particles embedded
FIGURE 3 St. Catherine, Lorenzo Lotto, oil on panel, Samuel H. Kress Collection, 1939.1.117.
in paint, owing to the comparatively large interaction volume (the volume being analyzed) in a low-density matrix such as paint made using lake pigments. EDS spectra were obtained at 20 kV and 15 kV accelerating voltage; lowering the voltage was designed to decrease the analysis volume. The spectra (Figure 7) indicate a (rather) pure form of silica; only aluminum is present, and its origin is likely the surrounding particles of red lake. Only silicon and oxygen are significant elements in line scans through the particles. Elements that would indicate this material is a glass, for example, the fluxes sodium and potassium or the stabilizers, calcium and lead, are below detectable limits. Venetian glassmaking required pure silica, which was, in this period, provided by quartzite pebbles from the Ticino River.
FIGURE 4 Cross section from a dark fold in the sleeve of St. Catherine (Figure 3) near the bottom edge, photographed in reflected light.
FIGURE 5 The cross-section illustrated in Figure 4, observed using fluorescence microscopy (filter cube: Leitz I3).
Fifteenth and sixteenth century treatises suggested using crushed marble or crushed travertine as additives to give body to paints [10]. Glass has been described as a drier for paint in Renaissance treatises and has been found in some artists’ red lake paint [11]. However, the presence of silica is unexpected, and this occurrence appears to be the first finding of this material used by Italian Renais- sance painters as an extender or an agent to give body in red lake paints. The major ingredient in Antonio Neri’s recipe for “cristallo” is pebbles “pounded small, serced as fine as flower” [12] (serce is probably a variant of sarce, to sieve through a cloth). This description corresponds to the material in Lotto’s red paint, which was a ground silica.
FIGURE 6 Digital dot maps of the cross-section shown in Figures 4 and 5.
FIGURE 7 Energy dispersive spectrum of small rounded particles in the translucent red paint; obtained at 20 kV.
The artist Jacopo Robusti, called Tintoretto, worked in Venice a few decades later than Lorenzo Lotto. Tintoretto was born in that city in 1519; his father was a member of the “cittadini” class, involved in the dyeing profession. Tintoretto lived and worked in the city throughout his career, and rarely traveled. He established a family workshop that outlived him, and he worked for a wide variety of Venetian patrons. Arguably his most famous surviving work is a series of paintings executed for the Scuola Grande di San Rocco over several decades [13].
The painting Christ at the Sea of Galilee (Figure 8) is attributed to Tintoretto and dated to 1575/80. This picture presents complicated issues in understanding its structure and the artist’s painting technique since the canvas support was assembled from several pieces of fabric that had been used for painting images different from the one we see now. The infrared reflectogram of the painting
reveals that at some point the largest, central piece of canvas had been used to begin a portrait. The portrait had been sketched out using a wash of dark paint, clearly imaged in the infrared. The x-radiograph reveals that the canvas had also been used for a landscape that is of a different scale from both the portrait and the current image.
Tintoretto’s painting techniques have been well studied [14, 15]. An investigation into the materials used for the Gonzaga cycle (1577-1578) showed that the artist employed a diverse palette [16].
Here we restrict the discussion to two pigments found in Christ at the Sea of Galilee that have special relevance to the use of glassy materials for pigments. A cross-section obtained from the sea at the right-hand side of the boat is shown in Figure 9. The bottom layer of the section appears to relate to the landscape observable in the x-radiograph. The pigment is a green, transparent, glassy- appearing pigment. The particle shape and size is similar to that of the blue glass pigment smalt (a potassium silicate colored by small amounts of cobalt). Al- though the term “smalt” is used in English today to describe only a blue glass pigment, reading the contemporary documents shows that artists of the sixteenth century used this term to describe not only blue but also numerous other
FIGURE 8 Christ at the Sea of Galilee, Jacopo Tintoretto 1575/1580, oil on canvas, Samuel H. Kress Collection,1952.5.27.
FIGURE 9 Cross section from the sea near the right hand side edge of the boat. The bottom layer contains a green glassy pigment.
colored glasses, including yellow, white, and green, at least some of which may have been used by painters [17, 18].
The backscatter image of this section is shown in Figure 10. The greenish pigment in the bottom layer appears dark gray, and therefore we can infer it is of low atomic weight. The EDS spectrum of the pigment shows that it has a composition very similar to blue smalt (Figure 11). An anonymous Venetian glassmaker’s recipe book dating to early-mid sixteenth century has recipes for green glass that have the same general composition as blue smalts: “Per fare smalto verde bellissimo. Prendi della zaffera e un po’ di manganese, pestati sottili e ben lavati e di questi prendi 2 libbre, aggiungi 3,5 libbre di pani cristallini e fa fondere in forno.” [To make a beautiful green glass. Take some zaffre (an impure cobalt ore), grind it fine and wash well and of this take 2 lbs, add 3.5 lbs of crystal frit (a potash glass) and melt in the furnace” [5].
This green smalt in Christ at the Sea of Galilee contains an impurity of bismuth. Bismuth has been found in late-fifteenth and early-sixteenth Venetian enamels and in fifteenth century cobalt blue enamels and smalt in a south Ger- man painting [19]. Bismuth is an impurity in the cobalt ore from Germany, and its presence in this pigment suggests that the source of the raw cobalt-containing material, “zaffera,” used for making this glass was from north of the Alps. The spectrum shows that the glass contains iron. Iron can give rise to a yellow glass. Therefore the green color of this pigment might arise from a mixture at the microscopic level of blue and yellow glasses.
FIGURE 10 Backscatter electron image of the sample in Figure 9.
FIGURE 11 Energy dispersive spectrum of the green pigment in the bottom layer of the section illustrated in Figure 9.
A yellow pigment is used widely in Christ at the Sea of Galilee. In a cross- section from Christ’s drapery it can be seen mixed with green earth for the sea painted under Christ’s red robe and as an intense yellow layer under the greenish paint of the sea. It was also used, well mixed with green earth and azurite, for the hills in the background. At first glance the pigment appears to be lead tin yellow type II (Pb(Sn,Si)O3). SEM-EDS clearly indicates that the colorant is an opaque yellow glass composed of particles of lead tin oxide suspended in a glassy matrix. X-ray powder diffraction (XRD) reveals that the yellow opacifier is similar but not identical to the material usually characterized in paintings. The XRD pattern of the pigment is given in Table 1. Although the pattern is very close to that published for PbSnO3, there are some subtle differences and additional lines not attributable to expected impurities. The compendia of recipes for making glass give several variations for the yellow colorant, which likely cause different hues. It would be interesting to compare the XRD pattern of the colorant and the composition of the glassy matrix of the pigment in this painting with those of enamels on metals and glazes on majolica and relate the results to the contemporary recipes. By comparing the details of these materials we may be able to shed further light on the variety of yellows that was available for the ceramic decorators and used by easel painters to increase the range of their palette. A recent paper differentiates between the production of lead tin yellow pigment and the “raw” material for the production of yellow glass [20]. This difference might be found among the materials used by Venetian artists and craftsmen. Thus the glassy matrix might be important, and this and other differences between glasses and pigments might be the source for the variety of materials and colors that painters used.
Many of the materials we find on the 1534 (and the 1596) inventory are materials used by dyers, glassmakers, and glass and maiolica painters. Some of these, including vermilion, kermes, brazilwood, orpiment, and lead white, are expected in paintings by Bellini, Giorgione, and Titian. The re-analysis of samples from pictures by these and other Venetian artists has begun to indicate that the palette they used was enriched by materials that until then had only been used by artisans and artists working in other media. Venetian painters (and others influ- enced by them) boldly incorporated into their work, to vivid effect, colorants not specifically designed for use in oil paint. We see that artists were using glassy materials and/or “smalti” more often and in greater diversity than we previously thought. Among these materials there appear to be frits and colorants designed for glass-painters and majolica decorators, in addition to the powdered glass, blue smalt and lead tin yellow type II, which have been identified previously.
The presence of the professional color-seller in Venice might have been the catalyst and the conduit for the transfer of materials among the arts and contributed to the emergence of the Venetian palette, a palette that cannot be precisely defined, but is characterized by its complexity and diversity of colorants.
TABLE 1 d-Spacings and Estimated Intensities of Lines in the Diffraction Pattern of the Glassy Yellow Pigment in Tintoretto’s Christ at the Sea of Galilee and patterns for PbSnO3 and SnO.
Yellow PbSnO3 SnO Pigment ICDD 17-607 ICDD 24-1342
d I/Imax d I/Imax d I/Imax Angstroms
6.17 18
4.65 4.5* 4.32 4.20* 3.93 3.63* 3.50 w 3.30* 3.25 3.22 12 3.10 100 3.09 100 2.98 20 2.9 80 2.85 2.77 20 2.78 80 2.69 80 2.61* 50 2.63 100 2.46 2.45 2.45 12 2.30* 2.21 10 2.24 10 2.10 5 2.12 10 2.05 2.06 6 1.95 10 1.95 30 1.90 80 1.89 75 1.864 65 1.83 25 1.61 80 1.61 80 1.61 20 1.54 1.52 16 1.23 1.227 29 1.195 1.196 16
*These lines can be attributed to lead white (International Committee for Diffraction Data 13-131).
ACKNOWLEDGEMENTS
We are grateful to the Center for Advanced Study in the Visual Arts (National Gallery of Art, Washington, D.C.) where we held a Samuel H. Kress Paired Fel- lowship. We benefited from discussions with members of the scientific research department of The National Gallery, London, and particularly acknowledge stimulating discussions with Jo Kirby-Atkinson.
REFERENCES
1. Lomazzo, G.P., Trattato dell’arte della pittura. 1590, Milan: Paolo Gottardo Ponto. 2. Matthew, L.C., ‘Vendecolori a Venezia’: the reconstruction of a profession. The Burlington Maga-
zine, 2002. CXLIV (1196): pp. 680-686. 3. Krischel, R., Zur Geschichte des Venezianischen Pigmenthandels - Das Sortiment des Jacobus de
Benedictus a Coloribus, in Sonderuch aus dem Wallraf - Richartz - Jahrbuch Band LXIII 2002. 2002, Cologne: Dumont Literatur und Kunst Verlag. pp. 93-158.
4. Rosetti, G., Plictho de l’arte de tentori. 1548. Translated by Sidney M. Edelstein and Hector C. Borghetty, 1969. Cambridge, Massachusetts: The M.I.T. Press.
5. Moretti, C. and T. Toninato, Ricette vetrarie del Rinascimento: Trascrizione da un manoscritto anonimo veneziano. 2001, Venice: Marsilio.
6. Zecchin, L., Vetro e Vetrai di Murano. Vol. 1-3. 1987-1989, Venice: Arsenale. 7. Lazzarini, L., et al., Pittura veneziana: materiali, techniche, restauri. Bollettino d’Arte, 1983. 5:
pp. 133-166. 8. Dunkerton, J., N. Penny, and A. Roy, Two paintings by Lorenzo Lotto at the National Gallery.
National Gallery Technical Bulletin, 1998. 19: pp. 52-63. 9. Lotto, L., (Libro di spese diverse [1538-1556] con aggiunta di lettere e d’altri documenti.), P.
Zampetti, editor. 1969, Venice, Rome. See also Bensi, P., Studi di storia dell’arte, 5 1983-1985, 63. 10. Merrifield, M.P., Medieval and Renaissance Treatises on the Arts of Painting. 1999, Mineola,
NY: Dover. p. clii. 11. The Painting Technique of Pietro Vanucci, Called Il Perugino Editors. B. G. Brunetti, C.
Seccaroni, A. Sgamellotti, Nardini Editore, 2003. Papers from the conference, 14-15 April, 2003. 12. Merrett, C., The World’s Most Famous Book on Glassmaking ‘The Art of Glass’ by Antonio Neri,
M. Cable, editor. 1662, Sheffield: The Society of Glass Technology reprint 2003. (Neri’s book had been first published in Italian in 1612.)
13. Krischel, R., Jacopo Tintoretto. 2000, Cologne: Könemann. 14. Plesters, J. and L. Lazzarini. Preliminary Observations of the Technique and Materials of
Tintoretto in Conservation of Paintings and the Graphic Arts. 1972, Lisbon Congress: International Institute for Conservation.
15. Plesters, J. and L. Lazzarini. I materiali e la tecnica dei Tintoretto della scuola di San Rocco, in Jacopo Tintoretto nel quarto centenario della morte. 1994, Venice: Il Polygrafo.
16. Burmester, A. and C. Krekel, “Azurri oltramarini, lacche et altri colori fini”: the quest for the lost colours, in Tintoretto: The Gonzaga Cycle, C. Syre, editor. 2000, Munich: Hatje Cantz Publishers. pp. 193-211.
17. Venturi, A., I due Dossi documenti - prima serie. Archivio Storico dell’Arte Nuovi Documenti, 1892. Anno 5 (Fase VI): pp. 440-443.
18. S. Pezzella, Il trattato di Antonio da Pisa sulla fabricazione delle vetrate artitiche, 1976. Perguia: Umbria Editrice.
19. Darrah, J.A. Connections and Coincidences: Three Pigments. in Historical Painting Techniques, Materials, and Studio Practice. 1995, University of Leiden, the Netherlands: The Getty Conservation Institute.
20. Heck, M., T. Rehren, and P. Hoffmann, The Production of Lead-Tin Yellow at Merovingian Schleitheim (Switzerland). Archaeometry, 2003. 45(1): pp. 33-44.
27
The Scientific Examination of Works of Art on Paper
Paul M. Whitmore Research Center on the Materials of the Artist and Conservator
Carnegie Mellon University Pittsburgh, Pennsylvania
ABSTRACT
The scientific examination of works of art on paper utilizes tools from the very simple to state-of-the-art analytical instrumentation, depending in large part on the question that is the objective of the investigation. Identify- ing pigments or paper fibers is straightforward, constrained only by the size of the samples that can be removed for destructive analysis. Inks are more difficult because of the lack of pronounced chemical differentiation between the ink types and because of possible interferences in the analyses from the paper substrate. Paper can be characterized easily to an extent, in identifying a watermark or the risk of deterioration from a high acid content, but the monitoring of the condition and degradation of paper remains an extremely difficult challenge. The assessment of light sensitivity, which is not easy to determine by merely identifying material composition, has been made straightforward by the development of a device that allows rapid, essentially nondestructive fading tests. Those tests are now being exploited to survey groups of objects to determine whether one may make generalizations about their exhibition needs. The further adaptation of nondestructive or micro-scale destructive analytical tools in the study of works of art on paper promises to allow even more extensive investigations of the creation and preservation of these objects.
INTRODUCTION
The scientific study of works of art on paper shares common objectives with the technical studies of any work of art. Artifacts are examined in order to answer art historical questions about the origin of a work, namely, where, when, and by whom a work was created. The scientific examinations seeking to answer these questions generally require identification of the materials and working methods used to craft the object. Other studies seek to answer basic questions about the care of the artifact: its physical and chemical condition, causes for deterioration, and vulnerability to storage or exhibition conditions.
Technical studies of paper-based artifacts tend to resemble the study of paintings, because many paper objects actually are paintings that just happen to be executed on a paper support. Manuscript illuminations, watercolors, litho- graphic prints—these objects could easily be viewed as paintings, amenable to analyses of the colorants, paint media, or layer structure of paints observable in cross-sections. Apart from the occasional thinness of the paint layer itself, as in watercolor paintings, or binder-poor paint layers, such as in pastels, these paper- based paintings can often be studied as one would study any other painting.
Despite this similarity, many works of art on paper present special circum- stances that constrain analyses or warrant unusual examination techniques. Paper artifacts tend to be small: The sheets were traditionally made in molds that could be manipulated by people, and these sheets were then cut down for use. Thus, books, prints, watercolors, and other paper-based art are relatively small, meant for close-up viewing within an arm’s length. For this reason, analytical methods that require removal of paint samples are often not feasible, for the damage to the artifact can sometimes be visible upon close inspection. Nondestructive tools, particularly optical spectroscopic or imaging techniques, are more widely used to study these objects.
Another distinction between paper-based objects and traditional paintings is the use of the paper substrate as part of the image itself. Particularly with such graphic art as drawings and prints but also with printed text or even thinly painted watercolors, the paper substrate is exposed and is part of the image. Thus, the color of the paper and its surface texture are important contributors to the appearance and visual appeal of the object, and study of the paper and its preservation is of great importance. (Occasionally in historical times and more frequently in the twentieth century, paintings too have been created with unpainted canvas as part of the image. For these objects the concern about the appearance and stability of the canvas is of course shared.) A complication in studying objects in which the paper is so intimately associated with the drawing media is the dis- crimination between the two, so that many analyses must have very small spatial or depth resolution, or contributions to the detected signal from the paper must be subtracted.
Paper-based collections in museums are known to pose some of the most
THE SCIENTIFIC EXAMINATION OF WORKS OF ART ON PAPER 29
common preservation problems because many of the artifacts that are now prized were not created as lasting works of art but as more utilitarian objects. Because paper was inexpensive and widely available through much of history, it has seen use for many purposes, a primary one being for communication and recording of information. Some of these artifacts, such as books, were meant to last for a long time, but others, such as newspapers, announcements, or letters, were often not created with posterity in mind. Thus, it is not uncommon for museums and archives to have paper artifacts that are delicate or deteriorating because of their creation with impermanent materials or techniques. Preservation problems are common, particularly with those objects that were not made as art objects.
This review will survey the examination techniques of paper-based objects that are used both for art historical investigations as well as for preservation studies. Some of those techniques are routine and can be found in many well- equipped museum laboratories; others are less widely available and have not found widespread use. This survey will conclude with a description of a relatively new tool developed to detect a particular vulnerability, the susceptibility of colored materials to fade from light exposure, and illustrate its use for the study of Japanese woodblock prints.
SURVEY OF EXAMINATION AND MATERIAL IDENTIFICATION TECHNIQUES
The most common technical investigation for paintings or colored prints on paper involves identification of the pigments in the paint. For this, the routine analytical tools of polarizing-light microscopy, X-ray diffraction, and elemental analyses by X-ray fluorescence are commonly employed, usually on samples of the paint that have been removed from the artifact. Descriptions of these tools can be found in accounts of painting examinations, or in reference books devoted to pigment identification (Feller, 1986; Roy, 1993; FitzHugh, 1997). Nondestructive techniques can also sometimes be used to identify pigments on paper objects. Open-air X-ray fluorescence is used for elemental analyses of pigments, and Raman spectroscopy and Raman microscopy have been found useful for examin- ing both pigments in paints and dyes in colored paper (Bell et al., 2000; Best et al., 1995). Some pigments have distinctive features in the visible spectrum (Schweppe and Roosen-Runge, 1986; Leona and Winter, 2001), while others, like Indian yellow, can be detected by their peculiar fluorescence observable under ultraviolet light illumination (Baer et al., 1986).
Drawing materials can also be studied, although they present some difficul- ties. Early drawings were created using metal tools or wires as drawing imple- ments (thus the name “metalpoints” for these drawings), and they can be analyzed by measuring the elemental composition of the metals in the lines (by X-ray fluorescence, typically). Inks are more problematic, with the exception of iron gall inks, which can be distinguished by the presence of iron in X-ray fluorescence or
in more unusual techniques such as Mössbauer spectroscopy (Rusanov et al., 2002) or PIXE (Budnar et al., 2001). Inks can also be analyzed for the trace elements they contain, introduced in the ink ingredients or as residues from the printing process. Inks in early books (such as a Gutenberg Bible) have been examined for these trace elements by synchrotron-excited X-ray fluorescence in the hope of distinguishing books produced in the early German printing shops (Mommsem et al., 1996). Other organic inks, such as sepia (cuttlefish ink), bistre (from soot), or such black drawing media as charcoal, bone black, lamp black, ivory black, or graphite cannot usually be distinguished by their elemental composition (although bone black is often detected by the presence of phosphorus), nor do the infrared spectra of these inks usually present characteristic features useful for their identification. Polarizing-light microscopy remains a common tool to discriminate between inks on the basis of their particle morphologies. The media used as pigment binders for drawing and painting materials can be identified by analyzing the organic composition of micro-samples. Of the various methods available the most useful are the gas chromatography/mass spectroscopy analyses that have been developed for oils and resins used as paint binders (Mills and White, 2000; Schilling and Khanjian, 1996) and more recently adapted for the study of gums used in watercolors or gouaches (Vallance et al., 1998).
In addition to the study of the image-forming materials, the examination of the paper itself is also often a clue to the artifact’s origin. Paper is usually differen- tiated by its fiber composition, its physical characteristics, and its manufacturing method. The fibers can be studied with optical microscopy, and the plant origin of the component fibers can be determined by appearance or by reaction to certain stains, such as Hertzberg or Graff C stains. The fiber type, length, and heterogeneity can all be distinctive, as can such physical dimensions as sheet thickness. The evidence of manufacture is most easily detected in the pattern left by the papermaking mold, typically a pattern of lines called chain and laid lines in so-called “laid” paper. Watermarks, the decorative patterns often woven into the wire molds or embossed on the cast sheets, are also the most obvious characteristic patterns of the paper manufacture. The evidence of chain and laid lines and watermarks can be captured in any of a number of ways, with transmitted light photography or with beta or soft X-ray radiography, and various image process- ing tools have been applied to enhance such records (erasing interferences from the printing, for example) and making them more useful for indexing and re- trieval for comparison in a reference database (Brown and Mulholland, 2002). The presence of sizing (a water-resistant finish on the surface of the paper) can be determined by infrared spectroscopy or colorimetric methods (Barrett and Mosier, 1995), and fillers (typically finely ground minerals or clays added for increased opacity of the sheet) can be identified by optical or electron microscopies (Browning, 1969). The fiber, finish, or watermark, along with other historical evidence, can assist in tracing a paper’s origin (Hunter, 1978), yet many chal- lenges remain (Slavin et al., 2001).
EXAMINATIONS TO ASSESS CONDITION AND PRESERVATION PROBLEMS
As with all works of art, preserving works of art on paper focuses on main- taining both the physical integrity of the artifact and its appearance. The physical integrity is derived mainly from the paper sheet itself, and preservation of the sheet’s cohesive strength is of paramount importance. (For books and archival materials that are handled by users, other sheet properties, such as flexibility, are also important, but for works of art that are usually mounted in a frame, the physical stresses are usually merely the tensile stresses from the paper’s weight and from its reaction to temperature and humidity.)
The cohesive strength of a sheet of paper is derived from the strength of its constituent fibers and of the bonds between the fibers. Aging tends to reduce the fiber strength, and old weak papers are usually seen to fail from broken fibers rather than by unraveling from weakened interfiber bonds. The reduction of fiber strength is in turn a result of the breakdown of cellulose, the natural polymer of glucose that composes plant fibers. Chemical degradation breaks cellulose chains, which reduces the average molecular weight but more importantly also breaks the connections between the highly crystalline cellulose zones. This progressive rup- ture of the tie chains, the amorphous cellulose chains connecting the crystallites and imparting the cohesive strength to the fiber, is the underlying aging chemistry leading to physical failure of the paper sheet.
Unfortunately, there are no analytical tools that can allow detection of such deterioration in a paper artifact without destructive analysis of unacceptably large portions of paper. Typically for degrading polymers, nondestructive tools such as infrared spectroscopy do not have the sensitivity to detect the production of the very small concentrations of new chain ends in the degrading cellulose. Recent studies suggest that production of glucose or xylose residues (Erhardt and Mecklenberg, 1995) or low molecular weight acids (Shahani and Harrison, 2002) may be easier to track as some measure of cellulose reaction, but these techniques have not yet been applied to artifacts. Other efforts to develop micro-scale molecular weight analyses for cellulose have reduced the amount of paper needed (Rohrling et al., 2002), but a recent molecular weight analysis of cellulose in a single paper fiber, while successful, also suggests that such small sample sizes may not be typical of the other fibers or representative of the average molecular weight of larger samples (Stol et al., 2002). Thus, even if the analytical procedure can be adapted, the slow deterioration of the cellulose may not be easily tracked by successive measurements of individual fibers over time.
While the deterioration of paper artifacts may be difficult to detect directly, many years of investigation of cellulose degradation have clearly indicated that there are other materials that may be reliable indicators of instability in the paper. Acidity is well established as a catalyst for the hydrolytic breakdown of cellulose, the most important of the known degradation chemistries. Lignin is primarily
responsible for the discoloration of groundwood papers, and iron and copper impurities can also act both as acid catalysts for hydrolysis and as catalysts for oxidative breakdown of cellulose. It is much easier to determine the presence of these sensitizing agents in paper than to track the slow deterioration of the cellulose, so the study of artifact materials often does not go beyond a pH measurement, the detection of lignin with phloroglucinol stain or an infrared spectrum, or the analysis for iron or copper impurities by a technique such as electron spin resonance spectroscopy (Attanasio et al., 1995). Iron present not as a paper impurity but as an ink component is also a well-known and easily identifiable risk factor for the preservation of manuscript and print materials.
The maintenance of the image-forming materials, particularly the colored paints and inks used to create the image, is another objective of preservation strategies. Light exposure is the most common hazard to the pigments and dyes used on art objects. In contrast to preserving the paper support, in which the deterioration is difficult to monitor and easier to predict by detecting the presence of destabilizing components, the loss of color is easy to monitor by periodic color measurements but difficult to predict. The light stability of a pigment depends not only on the material but also on its preparation, particle size, and prior fading history. None of these is easy to determine from study of the pigments, and until recently the only means to detect light sensitivity was to monitor the damage inflicted by light exposure.
Recently a new device has been developed to determine the risk of future fading from light exposure (Whitmore et al., 1999). That device operates as a reflectance spectrophotometer using a very intense focused beam from a xenon lamp as the illumination for the measurement (see Figure 1). By making rapid repeated spectral measurements while the material is illuminated by the intense light, very slight degrees of fading can be detected in light-sensitive materials in only a few minutes (see Figure 2). Because of the high precision of the spectrum acquisition, extremely small amounts of fading are easily recorded, and the test can be stopped before perceptible changes to the art object have been produced. All of the different color areas on a work of art can be tested, and the overall sensitivity of the object can be judged by the fading rate of the most light-sensitive color (see Figure 3). These tests can be used to develop exhibition requirements that are tailored to the needs of the object, with the very light-sensitive objects receiving greater care (less frequent exhibition at lower light levels) so that they do not suffer from fading damage caused by inappropriate display. These same tests, done with filtered illumination, can also be used to test the effectiveness of different lighting in reducing fading rates. By performing the tests in air or under an inert gas, the efficacy of oxygen-free housings for slowing the fading of works of art can also be assessed (see Figure 4).
It has been found that this fading test can also be used to identify pigments, not by their elemental or chemical constitution but rather on the basis of their photochemical reaction. Prussian blue, a ferric ferrocyanide complex used in art
FIGURE 1 Schematic of fading tester. Reprinted from the Journal of the American Institute for Conservation, vol. 38, no. 3, with the permission of the American Institute for Conser- vation of Historic and Artistic Works, 1717 K St., NW, Suite 200, Washington, D.C. 20006.
FIGURE 2 Fading test results for selected Winsor & Newton gouache paints. “Blue Wool 1” designates fading test results for the ISO Blue Wool no. 1, the most light-sensitive of the standard cloths. Reprinted from the Journal of the American Institute for Conservation, vol. 38, no. 3, with the permission of the American Institute for Conservation of Historic and Artistic Works, 1717 K St., NW, Suite 200, Washington, D.C. 20006.
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since the early eighteenth century, is known to fade reversibly during light exposure, with the blue color being recovered in a subsequent dark reaction (Ware, 1999). The fading tests of Prussian blue using the tester described above demon- strated this peculiar reversible fading behavior on a cyanotype, an early photographic process used to create blueprints (see Figure 5) (Whitmore et al., 2000).
In addition to these fading tests designed to evaluate individual artifacts, current studies are measuring the fading rates of particular colorants in Japanese woodblock prints from different eras, printed at different depths of color, and of varying degrees of prior fading. Results of such a population study will reveal whether there is general consistency or a wide variation in light sensitivity for particular materials. If there is widely varying behavior, fading tests must be performed on each object in order to determine the sensitivity and light exhibition needs. If there are very similar fading rates among the different applications of a pigment, one need not test every object and can instead safely use a rule of thumb to make such judgments of the object’s required care. This formulation of new rules of thumb, based on actual fading sensitivities observed in a large population of objects, will bring a new level of intuition about how to preserve objects. The results of tests on a large number of Japanese woodblock prints indicate that the fading of the colorant dayflower blue (aobana) is very regular, and its sensitivity can probably be safely estimated without individual fading tests (see Figure 6a). By contrast, yellow passages on the Japanese prints vary greatly in their light
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FIGURE 3 Fading test results for all the different color areas on a Japanese woodblock print (Yoshitoshi, Carnegie Museum of Art No. 89.28.1516). “BW2” and “BW3” desig- nate the degree of color difference produced after five minutes in fading tests of ISO Blue Wool fading standards nos. 2 and 3.
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FIGURE 4 Fading test results in air (solid lines) and under nitrogen (dashed lines). (a) Results for a gouache paint (Winsor & Newton Rose Bengal), showing slower fading in anoxic environment. (b) Results for ISO Blue Wool cloth no. 1, showing no difference in fading rate in anoxic environment. Reprinted from the Journal of the American Institute for Conservation, vol. 38, no. 3, with the permission of the American Institute for Conser- vation of Historic and Artistic Works, 1717 K St., NW, Suite 200, Washington, D.C. 20006.
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FIGURE 5 Reversible fading of Prussian blue under exposure in fading tester. Solid lines are fading measured during light exposure; dashed lines represent period of recovery, and return of blue color (smaller color difference) in the dark. Reprinted from Tradition and Innovation: Advances in Conservation, eds. A. Roy and P. Smith, with the permission of the International Institute for Conservation of Historic and Artistic Works, 6 Buckingham St., London WC2N 6BA, UK.
sensitivity, probably because many different kinds of natural colorants were used in the printing (see Figure 6b). These materials will require individual testing in order to assess their fading risks.
CONCLUSION
The scientific examination of works of art on paper utilizes tools from the very simple to state-of-the-art analytical instrumentation, depending in large part on the question that is the objective of the investigation. Identifying pigments or paper fibers is relatively easy, while inks are more challenging because of the lack of pronounced chemical differentiation between the ink types and because of possible interferences in the analyses from the paper substrate. Paper can be characterized easily to an extent, in identifying a watermark or the risk of deterioration from a high acid content, but the monitoring of the condition and degradation of paper—or for that matter, any polymeric material—remains an ex-
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FIGURE 6 Fading test results for (a) 36 dayflower blue (aobana) passages on 25 different Japanese woodblock prints in the collection of the Carnegie Museum of Art; and (b) 55 yellow passages on 48 prints from that collection. “BW2” and “BW3” denote the color change produced in ISO Blue Wool fading standards nos. 2 and 3, respectively, after a five-minute exposure in the fading tester.
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tremely difficult challenge. The assessment of light sensitivity, which is not easy to determine by merely identifying material composition, has been made straightforward by the development of a device that allows rapid, essentially nondestructive fading tests. Those tests are now being exploited to survey groups of objects to determine whether one may make generalizations about their exhibition needs. The further adaptation of nondestructive or micro-scale destructive analytical tools in the study of works of art on paper promises to allow even more extensive investigations of the creation and preservation of these objects.
REFERENCES
Attanasio, D., D. Capitani, C. Federici, and A. L. Segre. 1995. Archaeometry 37:377-384. Baer, N. S., A. Joel, R. L. Feller, and N. Indictor. 1986. In Artists’ Pigments, vol. 1, ed. R. L. Feller, pp.
17-36. Cambridge, U.K.: Cambridge University Press. Barrett, T., and C. Mosier. 1995. Journal of the American Institute for Conservation 34:173-186. Bell, S. E. J., E. S. O

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Scientific examination of art : modern techniques in conservation and analysis

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