By Neville Agnew and Jeffrey Levin
In a gently rolling savanna in northwestern Tanzania lies a piece of the great puzzle of human evolution. The 3.6-million-year-old hominid trackway at Laetoli—discovered and excavated by Mary Leakey and her team in the late 1970s—contains some 70 footprints preserved in hardened volcanic ash. Unmistakably human in appearance, the footprints predate the earliest known tools by nearly a million years, making them the strongest evidence yet that walking on two feet preceded brain development.
Unfortunately, in the years following their discovery, the Laetoli footprints were threatened with destruction. The site was reburied as a preservation measure by the Leakey team after they studied and recorded the trackway, but acacia trees subsequently grew in the reburial fill, and root growth endangered the footprints. To save this most ancient evidence of our ancestors, the latest in industrial technology was employed.
This past summer a Getty Conservation Institute-Tanzanian team completed the final campaign at Laetoli to preserve the extremely fragile trackway for future generations.
The recent campaigns reexcavated the site, removed tree roots, and restudied and recorded the footprints. Again the trackway was reburied—this time along with some carefully selected industrial materials: geosynthetics.
Developed in recent decades and now numerous in their variety, geosynthetics are widely utilized in construction where earth stabilization, drainage, or erosion control are involved. Relatively inexpensive, easy to install, efficient, and durable, these materials are more likely to be used in building roads, airport runways, or dams than in protecting archaeological sites. But even though using geosynthetics for conservation is somewhat novel, adapting technological materials and scientific instrumentation and techniques for conservation purposes is hardly a new practice. In fact, the conservation profession traditionally has exploited new products in the preservation of cultural heritage. An early example is Paraloid B-72, a protective coating developed for the paint and coatings industry and later widely adopted for the consolidation and stabilization of fragile objects and deteriorated stone. Its acceptance by conservators came after exhaustive testing and evaluation that established its chemical stability and other desirable properties.
Every year the U.S. Patent and Trademark Office (PTO) awards over 110,000 patents. Of the myriad new services, inventions, and products patented by the PTO, most never achieve the universal success of the ballpoint pen or the nonstick frying pan. Nonetheless, the sheer number of new and often ingenious products that come onto the market offers the conservation profession a bonanza. Conservation of cultural property touches upon so many disciplines within the sciences that it is well positioned to draw into its service the latest developments of science and technology.
However, knowing the extent of the new products available is a challenge. Then, too, the conservation profession is a small one, and few conservation scientists and conservators have the time and resources to screen products properly. Such is the pace of development that few new materials of the vast number generated undergo the rigorous screening necessary to ensure their appropriateness. The conservation profession is, by definition, cautious and conservative in its use of new materials and technologies—and for good reason. When priceless cultural property is at risk, the use of unproved products would be irresponsible. Disasters in the use of inappropriate materials in cultural heritage conservation are legion. Among these are epoxy resins that yellow and cannot be redissolved, reinforcing iron or steel bars that ultimately corrode and split architectural stone (as occurred at the Parthenon), and cement, a material that is incompatible with softer and more porous materials such as adobe and certain types of stones but which, unfortunately, is still widely used at cultural sites.
An important part of the GCI 's agenda is adapting the tools and materials of modern industry for use in the delicate task of preserving the past. The Institute has tested a number of tools and materials—among them, geosynthetics—adopting and refining some while rejecting others as unsuitable for various reasons.
At Laetoli the Institute reburied the precious footprints using several kinds of geotextiles, nonwoven polypropylene fabrics that transmit moisture and air, allowing the trackway to "breathe" and maintain a moisture equilibrium with reburial soil. One ingenious geotextile used in the reburial was BioBarrier, a product designed to exclude intrusion of roots. Originally developed to prevent tree roots from wedging apart city sidewalks and clogging outside drains, BioBarrier contains an herbicide that "bleeds" slowly over years from small nodules embedded in the fabric. The growth of root tips coming into contact with the geotextile is inhibited, while the plant or tree itself suffers no harm. BioBarrier will help prevent a future acacia root invasion that can damage the trackway surface.
The GCI has used geosynthetics for entirely different purposes in other field projects. At the Mogao grottoes in northwest China, sand dunes and wind-driven sand have degraded the site for centuries. In the Institute's Mogao project, sand movement was controlled by wind fences 3.7 kilometers (2.3 miles) long, constructed from a low-cost synthetic textile at a fraction of the cost of custom-designed windbreak materials. This textile, stabilized against ultraviolet light and developed as a shade cloth for the horticultural industry, also reduces wind speed by about 50 percent (as reported by the manufacturer who conducted wind-tunnel tests as a selling point for potential customers, because of wind damage to plants). The fabric proved effective in sand control because the quantity of wind-borne sand is dependent upon wind speed, and a reduction in wind speed results in a drop in the sand load
At Chaco Canyon in New Mexico, the Institute, with the U.S. National Park Service, is testing other types of geosynthetics in a project to develop technologies for the preservation of fragile Anasazi ruins. These geosynthetics include so-called geodrains, for subsurface drainage, and geomembranes, which exclude moisture ingress.
Sometimes research on a product is useful in ruling out conservation applicability. For example, Vikane (sulfuryl fluoride) is widely used in the United States for insect infestations (particularly dry-wood termites) in domestic and commercial buildings. Because of the chemical inertness of Vikane, the GCI, in collaboration with the manufacturer and other North American conservation institutions, evaluated its potential for use in museums. It was found, though, that the very small amounts of acids in the product preclude its use for this purpose.
One of the early research projects undertaken by the Institute was the investigation of a polymer called Parylene, which was developed as an extremely thin, conformal coating for the electronics industry. Parylene is deposited from the vapor in a vacuum chamber and can invisibly coat an object as delicate as a spiderweb, greatly increasing its strength. Parylene has found some use in the coating of fragile ethnographic artifacts and natural history specimens, though the requirement of vacuum deposition and an observed temperature increase during the process have proved to be limitations on wider use. Other polymeric materials tested by the GCI include, among others, aliphatic isocyanates—typically used to make high-quality automotive paints—for consolidating adobe, and silanes and epoxies for stone preservation.
Another Institute research project evaluated and tested passive monitors designed to measure the presence of certain carbonyl pollutants in the indoor environment. The monitors were originally developed for use in ensuring occupational safety by detecting dangerous levels of pollutants in workplaces. Research identified commercially available monitors that could also be used to detect levels of pollutants harmful to museum collections. These monitors offer a relatively inexpensive way to determine the degree of risk collections face from certain pollutants.
To create oxygen-free display cases for organic materials, a product from the food packaging industry was adopted. In the late 1980s, the Institute developed a storage case prototype for the pharaonic mummies in the Egyptian Museum. Mummies, being susceptible to oxidation and microbial deterioration, require an oxygen-free environment, so the GCI cases were filled with nitrogen, an inexpensive, totally inert gas that makes up 78 percent of the air we breathe.
Among the many possible tools for conservation, two others that have been explored show promise: thermography, which maps radiant heat from objects, was shown in tests at the GCI to be a feasible technique for conservation, while laser cleaning of surfaces, applied to conservation in the late 1970s, continues to be an active area of evaluation and testing.
A relatively recent evolution of the scanning electron microscope (SEM)—the so-called environmental SEM—has proved a powerful tool in the Institute's arsenal. The instrument does not require an ultrahigh vacuum, nor do samples need to be coated with a metallic film (to prevent charge buildup). With a large specimen chamber and the ability to raise and lower temperature, it is easy to study dynamic processes of change and deterioration. Thus, real-time observation of corrosion of metals, salt crystallization in stone, and swelling-shrinking cycles of adobe can be made, allowing new insights into processes previously observable only as "snapshots" under the SEM.
However, with the engineering challenge of building an essentially leak-free case solved, the problem arose of scavenging residual oxygen from the internal nitrogen atmosphere of the case. To eliminate oxygen from the case, the Institute tested a product called Ageless, developed to remove traces of oxygen in inert-gas packaged food, thereby keeping the flavor fresh. This special form of finely divided iron oxide, enclosed in a small sachet, absorbs residual oxygen in a sealed container. Ageless is now routinely used in the Institute's nitrogen cases. The cases have been replicated by the Egyptian authorities for the royal mummies (now on display in Cairo), for a mummy in the collection of the Biblioteca Museu Víctor Balaguer in Spain, and for the documents of the Constitution of India in New Delhi.
To collect environmental data at historic sites in order to guide the site's conservation, Institute staff combined existing hardware used in environmental science, agriculture, and engineering to create autonomous, low-maintenance environmental monitoring stations. The monitoring stations use traditional devices for measuring climatic conditions—temperature, rainfall, humidity, and wind—with other technology originally developed for use in agriculture and industry, such as photoelectric, wetness, carbon dioxide, and infrared sensors.
In another example of technology transfer, medical technology was put to use in identifying binding media, substances that hold pigment particles together and adhere paint to surfaces. Historically, binding media used by artists are extremely varied in nature and comprise such things as various kinds of carbohydrates and proteins. The GCI surveyed a number of medical diagnostic kits on the market designed to detect such substances and conducted experiments to determine which kits, or combination of kits, might be applicable for use in binding media analysis. The binding media identification kit subsequently developed is a simple alternative to expensive laboratory analysis and is particularly useful in the conservation of ethnographic objects.
Developments in science and technology continually offer a rich lode of new products, materials, and instrumentation that have promise for conservation. Mining that lode is a task that the GCI and others have embraced. If the challenge is enormous, so is the potential. Applying the kind of imagination and ingenuity displayed by the creators of our cultural heritage, those engaged in conserving that heritage can turn the industrial achievements of the present into guardians of the past.
Neville Agnew is associate director for programs at the Getty
Jeffrey Levin is editor of Conservation, The GCI Newsletter.