Wednesday, January 23, 2008

Conservation of Tablets in the Persepolis Fortification Archive


Laura D'Alessandro, Alison Whyte, and Monica Hudak

[The following is a very slightly altered (addition of hyperlinks) version of the text of an article appearing in The Oriental Institute News & Notes, No. 196, Winter 2008, pp. 3-6. Page images of the article as it appeared in its original context follow below. It is republished here with the permisson of the editor and authors. News & Notes is available as a privilege of membership in the Oriental Institute]

As part of a science-based profession, conservators are always on the lookout for new technology and new equipment to help them perform their work. The conservation of artifacts must address not only the symptoms of a problem that are causing an object to be unstable, but also treat an underlying cause. The two very different types of equipment described here allow us to do both.


Last spring, the Conservation Laboratory, together with Matthew Stolper, approached the Women’s Board of the University of Chicago for funds to purchase a very special cleaning device — a Class 4 laser — for use on the Persepolis Fortification tablets. As long as these tablets remain in Chicago, the Institute’s priority is to record as much of the collection as possible. High-resolution digital imaging of the Aramaic tablets, and the seal impressions on them, using two technologies (high resolution, large-format scanning backs and polynomial texture mapping) has already begun. The Elamite texts will be imaged in the next phase of the process.

How the Laser Works

Using an infrared light source, the energy generated by the laser can effectively remove solid material, whether it is a lump of soil or an insoluble crust. When the Women’s Board generously awarded the Institute funds to purchase the equipment and train the conservation staff in its use, the excitement generated by the news was immense. Affectionately referred to by various parties within the Institute as a “death-ray” machine, this equipment allows the conservation staff to carry out meticulous and cutting-edge techniques (pun intended) on objects within the collections. Its initial, primary purpose is the cleaning of the Persepolis Fortification tablets, which are in very poor condition. The clay bodies are crumbly and fragile, and the text, inscribed on the surface of the tablets, is often obscured by dirt and soil. Removing the soil from the fragile surface without altering or damaging the underlying text can be tricky. In many instances, the mechanical means available to conservators for this process — scalpels and sharpened sticks — limit the level of cleanliness that is possible. Often, an obscuring crust remains on the surface. Preliminary tests indicate that the laser is unsurpassed at cleaning this last, critical layer of accretion on the surface or in the impressed strokes and incised areas of the clay tablets. The control permitted by these devices, both in the beam spot size and the depth of penetration of the laser beam, make them unique in the pantheon of conservation tools. By using a specific wavelength and pulse energy to convert the obscuring layer to plasma and dust, the soil layer is removed while leaving the clay body unaffected by the photon energy. The degree of clarity that can be achieved through laser cleaning cannot be duplicated by any other method now available. It allows exceptionally sharp, clear images of the texts and seals, critical for the short-term success of this project and the long-term study of the images once the tablets themselves are beyond the reach of scholars.

Figure 1. Alison Whyte, Assistant Conservator, tests the ability of the Class 4 laser to clean dark soil from the fragile surface of a textile excavated from Nubia

Safety Issues

The acquisition of a Class 4 laser will necessitate safety additions to the laboratory space and appropriate training for the conservation staff at the Conservation Centre, National Museums, Liverpool, England. Previous participants in this program include conservators from the Isabella Stewart Gardner Museum in Boston, and the Metropolitan Museum of Art in New York. The training will cover the science behind the laser-cleaning process and provide conservation staff with an in-depth understanding of the way the machine works so that it can be safely used on the various materials within the Oriental Institute collection. Working with the Radiation Safety Office at the University of Chicago, we will acquire the necessary enclosure and warning lights to allow for safe use of the laser within the confines of the conservation laboratory. Acquiring the laser now, to meet the needs of the Persepolis Fortification tablet project, will also have far-reaching longterm benefits. The laser device will become a valuable tool for Institute conservators, allowing for the treatment of objects in the Institute’s collections that currently cannot be treated by more traditional methods. Conservation staff have tested the laser device on a wide variety of materials — from Nubian textiles (fig.1) and plaster casts (figs. 2–4) to limestone reliefs and glazed ceramics. The ability to use the laser on the Oriental Institute’s collections will extend the useful life of the laser system well beyond the immediate crisis of the Persepolis Fortification Archive project. The laser cleaning device will be purchased in January 2008. With funding secure for the equipment needed to carry out the urgent treatment of the Persepolis tablets, the conservation staff can now begin looking to the future and ways in which it can better serve the Institute as a whole.

Figure 2. Laura D’Alessandro, Head of the Conservation Laboratory, cleans the surface of a heavily soiled plaster cast of a limestone relief (C205: plaster cast of limestone relief of Herihor before Amon. Egypt, Thebes; New Kingdom, Twentieth Dynasty)

Figure 3. Plaster cast C205 before laser-cleaning Test

Figure 4. Plaster cast C205 after laser-cleaning Tests

Figure 5. The hand-held XRF is used to analyze a glazed-brick lion from Babylon (OIM A7481: Neo-Babylonian period; 604–562 B.C.)


An important component of any conservation treatment is the knowledge and understanding of the material from which an object is made. This information is critical to the successful treatment of the object and impacts not only our understanding of an object’s historical context, but also its optimal storage and exhibit conditions. The conservation staff at the Oriental Institute have had access to the scanning electron microscope on campus for several years. With its amazing capabilities, our conservators have been able to identify raw materials and alteration products on a wide range of objects; however, a major limitation of the device is the fact that only very small objects can fit within its vacuum chamber. The majority of the objects we analyze must be sampled, and while the sample is very small, this is a serious drawback of the equipment. A device now in use in major conservation laboratories, both in this country and abroad, is the small but powerful handheld x-ray fluorescence (XRF) machine. Analysis by x-ray fluorescence has been around for many decades, but traditional equipment required special lead-lined rooms and other expensive retrofits and safety amendments. The new device on the market, designed specifically for the art conservation profession, is a small, lightweight and highly portable machine which uses low-energy x-rays and provides highly accurate information about material composition. The device can be carried to the artifacts in the museum (figs. 5–7) or the objects can be brought to the laboratory and analyzed there. The device employs a low vacuum that allows it to work in the ambient air, thus avoiding the need for a vacuum chamber which would limit the size of the object able to be analyzed. Xray fluorescence is a totally non-destructive technique. No sampling of any kind is required and the objects are neither altered nor marked in any way. The device works by bombarding a tiny area of the object’s surface with a mix of x-rays and gamma rays and then reading the energy levels emitted by the material. Each element has a specific energy level that is unique to it alone. This fact, coupled with knowledge of the physics behind how the detectors work, enable the user to very accurately identify the material under analysis. With such a tool, conservation staff will literally be able to instantly identify materials within the collection and, in some cases, make corrections to erroneous identifications made in the past when less sophisticated methods were used. This information is of critical importance to researchers, scholars, and students alike. The hand-held XRF will also help in solving some of the mysteries of earlier treatments of the collection that date to a time before conservators were around to leave conservation records. Finding traces of some of the chemicals used in early restoration treatments or elements present in corrosion layers will aid the conservators in preserving and caring for the collections. Another key activity for the hand-held XRF is the testing of the organic materials within the collection for the presence of dangerous heavy metals, such as arsenic, that may have been applied decades ago as part of a preservation activity. These products were often used on organic collections in an attempt to retard or discourage microbial and pest activity and it is only in recent times that the danger these chemicals pose to humans has been understood. The acquisition of the hand-held XRF will also allow the conservation staff to share data and establish standards with other laboratories with the same equipment. As the databases grow, almost exponentially because of the ability to capture more data quickly and non-destructively, the study of ancient materials will be enriched. The Conservation Laboratory hopes to pursue funding for this device in the next few months. The ability to take part in nondestructive testing, as permitted by the hand-held XRF, coupled with the lab’s current access to the scanning electron microscope on campus, will be a huge step forward in the laboratory’s ability to contribute not only to scholarship within the Institute, but to research in the broader international field as well.

Figure 6. XRF analysis is carried out on a limestone column capital from Persepolis (OIM A24066: Achaemenid period; 486–465 B.C.)

Figure 7a. The hand-held XRF is used to analyze the blue pigment present on the surface of one of the Room 7 Reliefs from the Palace of Sargon II, Khorsabad, Iraq (OIM A11255; Neo-Assyrian period; 721–705 B.C.)

Figure 7b. A spectrum of elements identified by the XRF machine as being present in the blue pigment on the Room 7 Relief from the Palace of Sargon II, Khorsabad, Iraq (OIM A11255: Neo-Assyrian period; 721–705 B.C.). Elements found include copper, calcium, iron, and sulphur.

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