The Art of Science
How analytical chemistry contributes to understanding and preserving our cultural heritage.
Science for Art’s Sake:Four analytical chemists who contributed to the February issue of The Analytical Scientist each describe a project aimed at understanding and preserving our cultural heritage.
Rebecca Stacey, British Museum, London: "My research remit is to apply analytical science methods to the full range of organic substances in the Museum’s collections, from prehistory to the present day. There is always something new to discover, it is endlessly fascinating."
Understanding Ancient Prescriptions. Few medicine containers from the Roman period survive. This bronze medicine container (BM Cat no. 1968,0626.37) is the only known example with separate sections that stack together.
Content of an ancient bronze medicine container. It contains well-preserved residues of the medicinal contents.
Picture of Roman surgical instrument set. The medicine container is part of this Roman surgical instrument set dating to the 1st century AD. (BM Cat nos 1968,0626.1-39)
Graphic of our protocol for analyzing the residues. A multi-analytical approach ensures maximum capture of compositional information from the sub-milligram samples of the medicine residues.
Graphic of Typical results. Medicine residues from different compartments of the container. The upper (a) made with beeswax, the lower (b) with Punic wax. The difference is the absence of the longer chain fatty acids (F22-F34) which are removed when beeswax is processed to make Punic wax.
One of the ointments appears to contain beeswax processed to produce a product comparable with that described by Pliny as ‘Punic wax’. The variable composition of the four residues in the multi-compartment container implies that each section contained a different treatment.
Koen Janssens, University of Antwerp, Belgium. My expertise is X-ray analysis, and since X-rays are non-destructive and non-invasive, it was natural to start developing instrumentation to look ‘inside’ oil paintings and related works of art.
Picture of Moreel Tryptych. Did it always look like it does today? If not, what was changed? And what does it tell us about the creative process that led to this masterpiece of medieval art?
My colleagues and I used X-ray fluorescence, a non-destructive method of element analysis.
A motorized assembly moves an X-ray tube and detectors in small steps over the surface of a painting, determining the local composition and generating element-specific distribution images of the painting.
Radiation from fairly deep below the surface can be detected, revealing overpainted representations that are no longer visible to the naked eye.
The faces of the additional seven daughters were painted later on top of a verdant background (Mrs Moreel had a total of 18 children). Thus, the process of creating this altarpiece went through at least two major stages.
My research focuses on the development and adaptation of remote sensing imaging techniques to study works of art in support of conservation and art history. The job is a dream come true.
This is an illumination from a choir book commissioned by the Camaldolese monks of Santa Maria degli Angeli in Florence at the end of the fourteenth century. Can the pigments and paint binders used be mapped in situ? Lorenzo Monaco, Praying Prophet, (1410/1413) Rosenwald Collection. Image courtesy of the National Gallery of Art, Washington DC.
We use visible and near infrared (NIR) reflectance imaging spectroscopy, which is the collection of contiguous calibrated spectral images to provide the reflectance spectrum for each pixel of the scene.
Image cubes, whose third dimension is spectral, are collected and calibrated. The hundreds of images are processed via an algorithms to find a minimal set of reflectance spectra that best represent the illumination.
Using the acquired set of spectra, a map is created to represent the artist’s materials. Assigning identity to materials is based on comparison with reflectance spectral databases and by noting characteristic vibrational features such as position and slope of electronic transitions.
The center map represents the pigments and their location. An organic yellow dye was mixed with azurite to obtain the green portion. Blue areas use two grades of ultramarine blue. Orange leaves were painted with red lead, pink leaves with an insect-derived red dye. The Prophet's red robe was painted using vermilion and a red dye-based glaze. The paint binder map unexpectedly shows the use of fat-containing binder, most likely egg yolk (represented in red), on the prophet's robe.
These new analytical methods allow for a more rapid and portable analysis that can be performed on site where collections are stored. The hope is such methods will reveal more about artists’ methods and workshop practices. ‘Field work’ with Conservation Scientist Dr Paola Ricciardi on site in Florence.
Picture of Marco Leona. "I supervise a team of eleven scientists. The group conducts research on artists’ materials and techniques and on art conservation."
The painted decoration of this remarkable bowl attributed to Hans Wertinger and dated to 1498 (MMA 2008.278) is applied to the underside of the bowl, in a reverse painting process. The pigments were identified using Raman microscopy, focusing the beam through the thick glass of the bowl: without Raman spectroscopy this type of analysis would be impossible.
Gas Chromatography Mass Spectrometry is the standard technique for the identification of organic materials in paints or varnishes. Here, Julie Arslanoglu (L) and Adriana Rizzo (R) install a pyrolyzer on the Met’s GCMS.
Fiber Optics Reflectance Spectroscopy is used to study the painted decoration on a Hellenistic tombstone (04.17.1). The pink area of the tombstone was painted with an organic dye mixed with a blue pigment. This detail points to the level of understanding of color science on the part of the Hellenistic artist. The pink colorant obtained from plant root extract has a yellow cast: adding a little blue pigment neutralizes the yellow tint for a cooler, more appealing tone.
Light levels in the galleries, and the balance between daylight and artificial lights are carefully set by lighting designers. Museum scientists work with designers, curators, and conservators to ensure the best environment for art in the galleries.
Behind the scenes at many museums, scientists provide essential support to archaeologists, art historians and conservators. Their work may or may not be immediately visible to museum visitors, but it is fair to say that no decision on authenticity, provenance, conservation, or even lighting and environmental conditions is made in a modern museum without scientific support. And analytical chemistry is at the foundation of scientific research in cultural heritage.
Recent initiatives devoted to advances in the field, including a workshop at the US National Science Foundation (1), a Gordon Research Conference (2) and a full issue of Accounts of Chemical Research (3), highlight the importance of materials analysis and structural characterization in the study of works of art and in their preservation.
Traditionally, the main techniques employed in museum laboratories have been polarized light microscopy (PLM), X-ray diffraction (XRD) and fluorescence (XRF), scanning electron microscopy and microanalysis, Fourier transform infrared microspectrometry (FTIR) and gas chromatography–mass spectrometry (GC-MS). In the last decade, Raman microscopy has become a common tool thanks to its ability to non-destructively characterize pigments, minerals, and a variety of polymers. A striking trend of recent years is the rise of portable instrumentation, mostly for XRF but also for FTIR and Raman. Handheld XRF analyzers with performance similar to much larger instruments are now being used not only by scientists, but also by conservators.
One notable emerging trend is the application of proteomics techniques, such as matrix-assisted laser desorption/ionization (MALDI), LC-MS, and high throughput capillary electrophoresis techniques for the study of protein-based materials, such as egg-protein binding media in paintings, collagen in parchments, and silk and wool in textiles. Another trend is the development of immunoassays for the spatially-resolved identification of protein on cross-sections from paintings, a task complicated by changes in the proteins structure introduced by aging and by degradation catalyzed by pigments.
The potential of spectral mapping techniques has been illustrated in the examination of documents, prints, drawings and paintings, and will probably become commonplace as commercial instrumentation is developed. John Delaney’s work at the National Gallery of Art in Washington, DC, is a key example of what can be done with hyperspectral imaging in the visible and near IR ranges. The extension of this approach to the mid-IR range is a logical progression of this technique.
One of the most exciting recent developments in the field is progress in fast XRF mapping instrumentation. The work of Koen Janssens in Antwerp and Joris Dik in Delft illustrates the utility of macroscopic elemental mapping in the study of paintings. Originally performed at synchrotrons, this type of analysis is soon going to be possible using commercial instrumentation, potentially extending to every museum the ability to identify changes in paint composition and detect images hidden under the present surface of a painting.
While imaging techniques are increasingly important in the field, microanalysis remains a key component of investigations into works of art. Surface-enhanced Raman scattering (SERS), the huge enhancement of Raman scattering experienced by molecules adsorbed on appropriate plasmonic substrates such as silver nanoparticles, has found one of its main areas of application in the identification of natural and synthetic compounds used as pigments and dyes in works of art. I have applied the technique to well over one hundred objects, ranging in dates from 2000 BC to the present.
Cultural heritage material is invariably heterogeneous and complex. In the case of archaeological findings, analysis is further complicated by aging and changes due to burial. Rebecca Stacey at the British Museum successfully used a multianalytical approach, combining Raman, XRF, and GCMS techniques to identify the content of a Roman medicine container. In this case, however, chemical analysis was only the first of many steps. To better understand the function of the substances identified, Stacey and her coworker went back to the laboratory, combining the analytical results with the study of contemporary accounts, to reproduce some of the pharmaceuticals.
Scientific research in the field of cultural heritage encompasses a large number of disciplines. It deals with the material and structural characterization of works of art and archaeological objects, the study of their changes over time, including aging, restoration and degradation, and the development of new treatment methods and materials. Analytical chemistry, however, remains the foundational science in the field: the questions that art professionals and the public want scientists to answer most often are ‘what is it?’ and ‘how did it get there?’.
Please read the other articles in this series:
Casting Light on Renaissance Illuminations
Deconvoluting the Creative Process
The Stories That Colours Tell
Understanding Ancient Prescriptions
- Chemistry and Materials Research at the Interface between Science and Art (http://mac.mellon.org/NSF-MellonWorkshop)
- Scientific Methods in Cultural Heritage Research (http://grc.org/programs.aspx?year=2012&program=heritage)
- Accounts of Chemical Research special issue on Advanced Techniques in Art Conservation, Vol. 43, Issue 6, 2010.
Marco Leona supervises a team of eleven scientists at the Metropolitan Museum of Art, where he is the David H. Koch Scientist in Charge of the Department of Scientific Research. The group conduct research on artists’ materials and techniques and on art conservation. Marco is also lecturer in analytical chemistry at the Conservation Center of New York University’s Institute of Fine Art. His route to these fascinating positions? “I studied at Universita’ degli Studi di Pavia in Italy, for both a Laurea in Chimica (MSc, Chemistry) and a PhD in Crystallography and Mineralogy. Prior to joining the Metropolitan Museum of Art, I worked at the Freer Gallery of Art in Washington DC, and at the Los Angeles County Museum Art LACMA.”
