By Francesca Piqué
Historically, there has been a widespread misconception among art historians and restorers that most wall paintings are frescoes. The term fresco has a specific and precisely defined meaning: pigment particles (substances that add color) mixed in water and bound to the surface of the wall by the carbonation of fresh lime plaster. This durable painting technique enjoyed brief periods of popularity among the ancient Romans and Italians, but in its purest form it was generally rejected by artists as too limiting. Instead, painters from all periods, across the globe, have relied on a wide range of organic materials (e.g., egg, glue, resins) as binders (materials that hold pigments together and bond paint to a surface) for works of art on walls. This lack of understanding of the nature and presence of the organic materials used in wall paintings has resulted in considerable damage from misguided restoration interventions.
While identifying inorganic components in wall paintings today is a relatively straightforward process, doing the same for organic materials remains a challenge. The challenge arises principally from four factors: low binder-to-pigment ratio (as low as one-tenth of a percent of binder); the unstable chemico-physical nature of organic materials, which may result in rapid and dramatic decay; the large, often vast, heterogeneous, open and porous wall painting systems, which are highly susceptible to degradation and contamination; and the complex interactions among the painting materials (typically pigments and binders but also other materials as well), which can limit and alter the capability of scientific instruments to identify materials.
Appropriate conservation treatment should be minimal, compatible with the original material, and stable in the long term. To achieve this, interventions—both preventive and remedial—must be developed according to a methodology that begins with the characterization and identification of the problems through rigorous diagnostic investigations. Conservation science, by virtue of its multidisciplinary nature, has a crucial role in this process. Environmental science, analytical chemistry, and materials science, among the many disciplines that make up conservation science, are key to answering specific questions regarding the causes and mechanisms of deterioration in works of art. Conservation science may also provide effective solutions to remedy the situation.
To enhance the way that conservation science supports the conservation of wall paintings, the Getty Conservation Institute has partnered with a number of scientific laboratories to create the Organic Materials in Wall Painting (OMWP) project. The goal of the project is to develop a set of guidelines to facilitate the study of organic materials in wall paintings. The project has two parts: first, to evaluate various investigation techniques and to develop a series of guidelines for organic materials identification; second, to apply these guidelines to wall painting conservation case studies to illustrate the guidelines and their practical benefits.
Developing a Methodology
There is not a single area of expertise or a single investigative technique that can be used to efficiently detect organic materials in wall paintings. Different types of investigations and techniques must be used and integrated to obtain significant results. Characterizing organic materials through scientific investigation in a way that is both resource effective and that minimizes the amount of sampling required is a principal aim of the OMWP project.
The methodology advocated by the project is based on a sequence of investigations and, during the case study phase, on the role of the conservator in the formulation of questions and in the interpretation of results. The investigations include:
Noninvasive investigations (no sampling required)
- Imaging and surface mapping techniques—providing topographic information on areas of the paintings
- Point analysis—explores a tiny area on the surface of a painting
Invasive investigations (sampling required)
- Nondestructive—carried out directly on an unmounted sample without altering it
- Paradestructive—carried out on a prepared sample (e.g., mounted as a cross section or thin section showing the painting stratigraphy). Preparation procedures may induce physical and chemical alterations and prohibit the recovery of the sample in its original form; however, the same sample may be used for other analytical procedures, such as:
- Imaging and surface mapping—providing topographic information on an area of the prepared sample (cross section)
- Noninvasive point analysis—explores a point-like surface of the prepared sample
- Destructive—completely consumes the sample analyzed
Proper sampling procedures are crucial for invasive techniques. To ensure representative results, sampling locations should be carefully selected based on the results of noninvasive investigations. It is important to select a representative area of the phenomenon under study and to ensure that sampling is minimal and confined. Samples are then studied, with preference given to nondestructive investigations that do not preclude further testing; destructive methods are left for last.
The process is iterative—each new result is interpreted and its significance is reviewed in relation to previous results; it will either serve to confirm or reject previously held hypotheses. Even after an exhaustive study of a sample, it is not uncommon for questions to remain or for new questions to arise, requiring additional sampling.
Evaluating Investigative Techniques
The OMWP project's first phase evaluated a number of techniques by examining a group of lime-based wall painting replica samples of known composition made by Leonetto Tintori between 1993 and 2000 and provided to the project by the Laboratorio per Affresco Elena e Leonetto Tintori in Italy (see Conservation, vol. 18, no. 3). These reference standards are characterized by the use of different types of binders and pigments and by the timing of the application according to the level of carbonation of the plaster—fresh, partially carbonated, or fully carbonated (i.e., plaster that is wet, partially dry, or fully dry).
Each of the partner laboratories investigated the analytical potential of one or more techniques on the Tintori samples. For newly developed techniques, testing these samples has been useful not only for evaluating each technique but also for creating a database of reference information on the particular pigment and binder combinations studied.
Imaging technologies commonly identify surface behavior associated with different material compositions—for example, UV-visible fluorescence associated with the presence of fluorescing organic materials. Maps of UV-visible fluorescence on a painting's surface, integrated with visual examination by conservators, permit the identification of areas and points of a wall painting's surface likely to be rich in organic materials. Point analysis is then used to further characterize the visible emission fluorescence of these materials. The research conducted by the OMWP project shows that the fluorescence emission of the organic material is strongly affected by the type of pigments used, as well as by the timing of the application (i.e., to fresh, partially carbonated, or fully carbonated plaster). Some pigments, such as red and yellow ocher, quench the fluorescence of organic materials. Therefore, in the presence of these pigments, lack of fluorescence does not imply the absence of organic materials.
Noninvasive techniques have an intrinsic limitation: they provide information about a painting's surface but not about its stratigraphy. However, the value of noninvasive imaging and point analysis resides in the techniques' mapping capacity. With large painted surfaces, this is a significant advantage because it allows characterization and mapping of similar surface behavior. As a consequence, only limited sampling is required to answer specific questions associated with the behavior observed; in fact, the most appropriate sample location can be selected from the information provided by the noninvasive investigations and by the conservator. Appropriate sample handling and preparation are also extremely important in order to prevent sample contamination, and for reliable identification of the limited amounts of organic materials in the samples.
The evaluation of the investigative techniques is ongoing and is based on various factors: the type of information provided by a specific technique, the sensitivity of the technique, the amount of sample required, the cost of the analysis, the length of time required, and the general availability and difficulty of the technique.
The development of a set of methodological guidelines will provide a useful tool in streamlining the identification of organic materials in wall paintings through both simple in situ examination and more sophisticated analytical procedures. It is important to illustrate the effectiveness of this methodology with case studies and to highlight the importance of planning diagnostic investigations based on information previously collected and on a conservator's observations prior to the conservation intervention. This is the aim of the second phase of the project.
Applying the methodology to wall paintings requiring conservation will provide an opportunity to verify and illustrate the validity of the methodology and its direct implications for wall paintings conservation. The case studies will focus primarily on lime-based wall paintings. Noninvasive research work began this spring on the wall paintings cycle of the lives of Saint John and Saint Stephen painted in the fifteenth century by Filippo Lippi in Saint Stephen's Cathedral in Prato, in conjunction with a conservation project being carried out under the supervision of the Italian Ministry of Cultural Heritage. The conservation work—under the direction of Mark Gittins of the firm Conservazione Beni Culturali—has been ongoing since spring 2001. These paintings have a complicated conservation history—the current conservation effort is the third in the past century. The Opificio delle Pietre Dure in Florence, an OMWP partner, has been responsible for the project's diagnostic investigations. The OMWP project team's work on the Lippi cycle complements research already conducted at the site during conservation. The scientific work at this site, along with the conservation work, will be completed by the end of 2005.
Not all of the investigative techniques tested in the first phase will be applied to the case studies; only those deemed appropriate, based on ongoing study and the needs of the wall paintings, will be employed. The case studies aim to demonstrate the iterative nature of scientific investigation and the essential role the conservator plays in the practical application of conservation science.
The work completed so far—the first phase of the project—has shown the advantages and the limitations of currently used investigative techniques. Evaluation of these techniques on wall paintings samples of known composition has proven very useful in illustrating the type of results that the various binder and pigment combinations provide. Research has shown that information about the inorganic materials present in the paintings is important in order to aid interpretation of the findings regarding organic materials. These results, to be compiled by the OMWP project, will be available as a reference for further wall painting studies.
On the one hand, the continuous advancement and availability of techniques are beneficial, as they provides conservation science with more tools to examine heritage. On the other hand, research that utilizes different techniques may produce an accumulation of data and may use significant resources without contributing substantially to the work of conservators. It is essential that scientific research focus on actual conservation problems, that it be conducted with the most efficient techniques, and that its results be interpreted and evaluated in order to provide the information needed by a wall paintings conservation program.
Francesca Piqué is the coordinator of the Organic Materials in Wall Paintings project and a freelance conservator based in Italy. She is a former project specialist with GCI Field Projects.
Courtauld Institute of Art, Wall Painting Conservation Department, University of London
The Getty Conservation Institute, Los Angeles
Forth Photonics, Athens
Istituto di Chimica Inorganica e delle SuperficiConsiglio Nazionale Ricerche CNR, Padua
Istituto Centrale per il Restauro, Rome
Istituto per la Conservazione e la Valorizzazione dei Beni Culturali CNR, Milan
Istituto di Fisica Applicata Nello Carrara CNR, Florence
Istituto di Scienze e Tecnologie Molecolari CNR, Perugia
Opificio delle Pietre Dure, Florence
Politecnico di Milano, Dipartimento di Fisica, Milan
University of Delaware: Winterthur Museum and Country Estate, Delaware
University of Perugia: Dipartimento di Chimica, Sezione di Chimica-Fisica, Perugia
University of Pisa: Dipartimento di Chimica e Chimica Industriale, Pisa
University of Parma: Dipartimento di Chimica Generale ed Inorganica, Parma
Laboratorio Scientifico dei Musei Vaticani, Vatican City