From its earliest days, the Getty Conservation Institute has sought to advance the preservation of earthen architecture, including earthen archaeological remains, through research in the laboratory and in the field, and through education. These efforts have included work on adobe (sun-dried cast earthen bricks)—in particular, adobe consolidation and seismic strengthening of adobe structures. The GCI is also participating in Project Terra, a program for the study and conservation of earthen architecture that is a collaboration with the International Centre for Earth Construction-School of Architecture of Grenoble (CRATerre-EAG) and the International Centre for the Study of the Preservation and the Restoration of Cultural Property (ICCROM).

We recently asked several specialists in earthen architecture conservation to share with us their views on the state of the field. Anthony (Tony) Crosby—a conservation architect in Colorado—has for 30 years worked in the protection of heritage sites in the United States and internationally for the U.S. National Park Service and in private practice. Hugo Houben—cofounder of CRATerre and codirector of Project Terra—has specialized in earthen construction since 1972 and worked in over 50 countries. John Hurd—a private conservator in architectural and archaeological conservation—has been involved in the conservation of over 40 earthen structures in the United Kingdom and serves as a consultant to several projects in China and central Asia. All three are board members of the ICOMOS International Committee for the Study and the Conservation of Earthen Architecture.

They spoke with Neville Agnew, a GCI principal project specialist who has worked extensively on adobe research, and with Erica Avrami, a GCI project specialist who serves as the Institute's project manager for Project Terra.

Neville Agnew: I'd like to begin by discussing the historical continuity of earth as a material for human habitation.

Hugo Houben: From what we know, the most ancient urban settlements were built with earth. It is believed that at least one-third of the world's population still lives in earthen structures. For those reasons, earthen architecture should be acknowledged as the world's most ancient and most widespread existing architectural expression.

John Hurd: I've just returned from central Asia, where I visited the ancient city of Otrar in Kazakhstan, which was Tamerlane's westernmost capital. At that site are seven earthen cities built on top of one another—the earliest dating from the first century before the Christian era, the latest dating to 1400. Just being there brought that continuity home to me in an extraordinary way.

Neville Agnew: Based on your personal experience, what are some of the important earthen sites internationally -- including both archaeological or historic sites and inhabited sites?

Hugo Houben: Well, you have lots of sites, like the over 4,000-year-old Mari site in Syria, for example. There's the city of Shibam in Yemen, which has existed for about 2,500 years. The pueblos of the U.S. Southwest are another example, and of course there's the 6th-century site of Joya de Cerén in El Salvador. In Ecuador, there's the historic core of Quito. In Peru, there's the 9th- to 15th-century site of Chan Chan, as well as the center of Lima. In fact, there are a great many earthen sites.

John Hurd: What about the Great Wall of China? There are 2,000 kilometers of it in northwest China made from earth. As I mentioned, I've just come from Otrar, which occupies an area of 120 square kilometers. Other extraordinary sites in central Asia include Merv in Turkmenistan, which dates to the 6th century before the Christian era, and the 2,000-year-old city of Bam in Iran.

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Tony Crosby: We also need to think about earthen sites that have become important because of the conservation research that's going on there. Joya de Cerén is one of those. The Tel Dan gate in Israel, which dates from the Canaanite period—about 1800 B.C.E.—is another, to some extent.

Hugo Houben: I'd like to point out that last year 16 percent of the World Monument Watch's 100 most endangered sites were earthen sites. Ten percent of UNESCO's World Cultural Heritage List is earthen architecture. And 57 percent of the UNESCO World Heritage Convention list of world heritage sites in danger are earthen sites.

Neville Agnew: Has the attention that earthen architecture has received in the conservation field been less than that received by historic and archaeological structures made from other materials, such as stone and timber?

Tony Crosby: I think we have a lot of examples where, in fact, we can say that that's true. A more important question would be, "why is that true?" I remember when the U.S. National Bureau of Standards did its initial work on earthen architecture materials in the late 1970s. One of its publications began with the premise that in many places in the world, earth is used as a building material when more conventional materials are lacking. It struck me that in most places in the world, earth is the conventional building material.

John Hurd: I've never understood it, but wherever I go, people assume that earthen structures are unimportant or in some way more primitive than buildings constructed with other masonry materials. People also seem to assume that they're inevitably in decline and not conservable in some way—except possibly in the area of freshly excavated archaeology, where there seems to be a great deal of appreciation and respect for the material.

Hugo Houben: In 1957 the United Nations published the first book on earthen architecture. It mentioned that in places such as France, Germany, and central Europe, one could find earthen architecture, but that it was limited to applications in minor, rural buildings. Today we know that to be totally untrue. Earth has been used to build every kind of building, with no limitations on size or significance. Except for a few documents, you won't find in technical publications on art and architecture any reference to earthen architecture. The history of earthen architecture has never been looked at. As a consequence, earthen architecture does not have a history. It's perceived as unimportant.

Neville Agnew: Does that have something to do with the fact that earthen architectural structures are classified as vernacular, as distinct from the sort of high art of classical architecture and stone and wooden construction?

John Hurd: In a way, earthen construction is perceived as something anyone can do. And therefore it doesn't quite fall into the category of "decent" architecture.

Tony Crosby: We are talking about public awareness, not actuality. There are different levels of public awareness, and there is probably something of a disconnect between official public awareness, if you will, and local public awareness. As an example, a local community might have more interest in an earthen historical structure or site—its value being related to some local event or history—than someone evaluating it from the outside who doesn't have that local knowledge but does have some sophisticated understanding of architectural significance. The level of interest in a site is related to its perceived values.

Erica Avrami: Has there been some change over the last 30 years, within the conservation community, that has increased the recognition that this is important heritage that needs to be preserved?

John Hurd: We have a problem, and that is that over the last 30 years, we've had an awful lot of bad examples of conservation of earthen buildings. I see a lot of cement work that's been done to try to conserve an earthen structure. People are used to seeing failure in this area of conservation. There is a change inasmuch as we can now offer a new understanding and a new kind of analysis of the problems. And better conservation methods are being developed. Also, there's a lobby of people interested in sustainability that's become very important. That group has been an ally to us in conservation, because through our work, we have a better understanding of sustainable architecture than anyone else. So we have something to offer.

Tony Crosby:
I'm not sure we have a higher percentage of good examples of conservation intervention today then we had 30 years ago. We just have more of them. But I think we probably have more poor examples, too.

Hugo Houben: Historically, the attitude that we've seen toward earthen sites is negative compared to stone sites. Often the size of the resources for earthen conservation does not match the scale of the problem. The field of earthen architecture conservation is quite sick of small thinking, small action, small budgets, small know-how, small planning, and small research. Earthen architecture is big.

Neville Agnew: That's right. That's the point. Do you think some of these sites are capable of being saved? Chan Chan, for example? Or some of the central Asian sites that John was mentioning? Can they be saved for the future?

John Hurd: Yes, of course they can. A key thing is training and advocacy. The whole thing that I'm about in Otrar is to train local people and local institutions and to get indigenous conservation programs going. We don't want to lose the art of building with earth in countries where it's still practiced. I fear it disappearing all over the place, despite my confidence. I'd love to see professionals in the West disseminating the facts that this is a very useful form of architecture and that it may be the most appropriate building form in parts of the world.

Tony Crosby: The problems we face in conserving places like Chan Chan are immense. Perhaps with more knowledge of the deterioration process, a better understanding of interventions that are most appropriate, and more education—particularly regarding the importance of earthen monuments—the future will be brighter.

Neville Agnew: Let's discuss the nature of the threats to historic and archaeological earthen structures. What are the great challenges posed by these sites?

John Hurd: Earthen structures are inherently more easily damaged by environmental conditions then other forms of masonry. Unless the buildings are maintained and have a roof and so forth, they are easily destroyed. Once an archaeological site has been uncovered, there's a threat from salt and wind and from the enormous change of temperature that you get in the desert. A great challenge is figuring out how to minimize the effect of these harsh environmental factors.

Tony Crosby: In general, the major threats are environmental and then human. The human threats range from vandalism to a less-than-adequate response—the latter probably being the greater threat. And a less-than-adequate response is the product of a lack of a comprehensive understanding of the effects of those things John mentioned—such as thermal shock and salt. Another threat would simply be the need to answer age-old questions that haven't been answered yet. We still don't know enough.

Neville Agnew: That brings us to the next question—what are the research needs and priorities in earthen architecture conservation?

Hugo Houben: A number of research needs were identified during the Terra 2000 research meeting in May 2000 and outlined in the summary report. I'm very much an advocate for first understanding the fundamental things. What are the exact binding and unbinding mechanisms of earthen materials? What is the importance of the mineralogy of the materials? What is the importance of organic material? What is the importance of water and salts? As long as we don't answer those basic questions, we'll go another thousand years observing the material and trying to make out what is needed.

Neville Agnew: You're saying that research needs include acquiring a fundamental understanding of the mechanism of deterioration, the way in which earthen structures literally fall apart.

Hugo Houben: That's my personal feeling. On the macro level, the general mechanisms don't seem to be that complicated. What is a bit more complicated is how it all functions on the micro level.

Neville Agnew: When we consider the materials in earthen construction—the clay, the silt, and the sand components, and then organic materials, such as straw—we've got a very complex composite.

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Hugo Houben: Yes. We have to think about the clay, but fundamentally we also have to think in terms of the compound. Earth must be considered as a composite material.

Erica Avrami: I think there needs to be a greater understanding of exactly how this material behaves. What are the critical points at which it begins to decay, and why? Of course, that sort of research takes a lot of time and resources. We still need to look at issues such as seismic threats, decorated surfaces, sheltering, reburial, and options that, at least for now, provide us with the best possibility for preventing further deterioration.

John Hurd: I agree with everything Hugo said, and I don't work in a research facility. I'm a conservator, and my research goes hand in hand with my conservation treatments. Obviously, I bring in all the technology that I can, but because I don't necessarily have the information that I need on a technical level, I have to work largely on an empirical level. As part of that, I go to the local people for the empirical understanding that they have. I'd love to see more recording and analysis of existing traditions while they are still alive, because they are fast disappearing. Yes, we need to know a great deal. And it will cost a great deal. And it will take a great deal of time. Meanwhile, monuments are falling apart, and I'm being asked to rescue them. Much more empirical understanding—an understanding of existing practices—would be very useful.

Neville Agnew: I'm hearing three things here: fundamental scientific studies, pragmatic testing and development of methods for conserving historic archaeological sites and structures, and, finally, preserving traditional folk knowledge that could inform current conservation practice.

Hugo Houben: While I'm not for putting one in front of the other, I am pleading to at least start work on the fundamentals, which has never been done. Like John and Tony, we are working in the field with the practical knowledge of local people and with empirical understanding. People argue that going into the fundamentals will take time and that, in the meantime, monuments are falling down. That is, unfortunately, true. But we have to acknowledge that in many cases, monuments are falling down because of empirical approaches and a lack of fundamental understanding.

Neville Agnew: How do we reconcile the use of traditional repair methods with the high-tech conservation solutions? How do we strive for an appropriate balance?

Tony Crosby: We have to evaluate equally all potential solutions for the decaying process. Whether it's a traditional solution or a high-tech one that's never been used before, it should be evaluated with the same parameters and the same guidelines. Too often we probably attribute more validity to traditional means then they may warrant. One example is the plant mucilage used in the consolidation of the walls at Joya de Cerén, a World Heritage Site. The material is a traditional local remedy. The result I've seen is that while it provides some resistance to abrasion, it is not "the solution" to the site's conservation problems. A great deal of research is going on there, which is leading to a better understanding of the cause-and-effect relationships of deterioration. This understanding will result in the utilization of different interventions, both traditional and nontraditional, based on their response to identified problems. We can learn much from traditional approaches, but we need to evaluate them with the same rigor that we apply to all potential interventions.

Neville Agnew: Sometimes high-tech solutions are criticized as being inappropriate or ecologically insensitive. Also, there's been a question of their cost.

Hugo Houben: Some high-tech solutions reflect a bit of arrogance—"we're going to solve the problem in a minimum of time, don't worry!" Some of those solutions are causing big problems today. But I hope that in the future we can come to high-tech solutions with modesty and really do the job. It's going to take patience and research to combine tradition with high tech. But I think we can come to satisfying solutions that combine traditional know-how, environmental sensitivity, and scientific knowledge. But, as John said, it's urgent that we go out and observe and register traditional knowledge before it's too late.

Tony Crosby: Something that was high-tech 500 years ago may now be traditional, and something we consider high-tech today may be traditional 500 years from now. Obviously, there are numerous tools that we need to take advantage of, and we are lucky today that we have more than someone did in the past. In the future, they'll have even more.

Erica Avrami: There seems to be a tension between what is considered "international conservation policy"—things dictated by conservation charters—and the ways in which we approach the conservation of earthen architecture. For example, using sacrificial renders and the replacement of deteriorated material. There are things we do in the field that wouldn't be acceptable by those international standards in the area of stone, for example, or timber in some cases. Perhaps we could discuss this in the context of the involvement of the local community and the maintained vernacular nature of the material and of the architecture.

John Hurd: I don't agree that we behave differently between earth and stone and wood in inhabited structures. I'd probably cut out a rotten bit of wood, replace it with a new piece that takes up its load, and do it in such a way that my patch is reversible. And I would record it. Likewise in a stone building, stones have been turned or replaced. In an earthen building—a standing structure or a weak archaeological structure—the same things would apply. The point about community involvement is an interesting one. I find that local communities are surprised that an international expert is interested in what they are doing. I'm fortunate enough to go to areas where earthen architecture remains a living tradition and where there's lots of community involvement. Everyone wants to give their two cents on how you should do it. I don't find that to be true when I'm dealing with timber monuments. There is something special about earth. It connects to the vernacular spirit in ordinary people, rather than just to professionals.

Hugo Houben: We are quite often on sites where tradition is still alive and where the buildings are still in use. Even if the buildings are historic and only used once a week for ceremonies or other things, community involvement remains strong. Sometimes this leads to better maintenance, but in other cases it may lead to destruction.

Neville Agnew: There is a valid, fundamental difference in the treatment of inhabited buildings and that of excavated or uninhabited historic structures. Should the conservation profession respond differently to historic, uninhabited earthen structures than to inhabited ones?

Tony Crosby: In the broadest sense, we are dealing with a more complex system. The addition of modern conveniences introduces factors that may generate moisture or heating—basically things added for human comfort. For the conservator, it's a matter of compromises. You treat things based on their value. And for inhabited structures, one of the values is that of a shelter.

John Hurd: It has a lot to do with the sacrificiality of the system that you use in conservation. If I were working on an inhabited building, although I would want my introduced material to be sacrificial to the wall—in other words, for it to decay rather than for it to decay the wall—it would depend on its hydrophilic nature, hardness or softness, and so forth. If I were working on an ancient site, I'd be more cautious about the sacrificiality of my system and would make it more sacrificial, softer, and more hydrophilic, possibly. So that would be a major difference.

Neville Agnew: Does reburial of excavated archaeological earthen constructions—ones that are particularly difficult to save—represent the ultimate solution?

Tony Crosby: Reburial is a limited application in terms of the values that you are protecting and consequently presenting. With reburial, you're saying that the research potential of a site is its most important value. Obviously, that potential can be protected best that way. The other limitation of reburial is physical. It seems to me that it is pretty limited in the practical sense of fairly small walls and small objects.

Neville Agnew: I wasn't thinking about unexcavated sites but, rather, excavated sites that had been preserved for centuries underground. Places like Tel Dan, for example, that are particularly difficult to conserve.

Tony Crosby: In a site that's been excavated—that's suffered through that process of rapid drying and excavation shock—there may be an intrinsic failure in the building systems that we'd probably have a difficult time reversing.

John Hurd: When you're confronted with a site of 120 square kilometers, the recommendation for 90 percent of the site will be reburial, because the environmental factors are so severe that after a year or two the structures will be gone. What remains visible in those sorts of environments would nearly always need some shelter structure protecting it.

Tony Crosby: With respect to shelters, we need to look at more ways to construct low-cost, temporary shelters with local material and local techniques, rather than simply construct permanent sheltering. And I mean something as simple as protecting a small archaeological site overnight. There are basic principles that are important and easy to teach. The goal is to promote that approach rather than to rely on long-term permanent solutions in these traumatic deterioration situations.

Neville Agnew: What is the future of new construction in earthen architecture?

Hugo Houben: The future is bright. All through Africa, Latin America, and Asia, there are thousands of small enterprises that have been set up that are working with it. They are asking for more training, documentation, and testing standards so they can become involved in public building programs. In industrialized countries, earthen architecture activities stopped in the 1950s and 1960s. Today they're going on again. But it's still insignificant compared to general building activities. In Germany, for example, you have a total of something like 200,000 new buildings constructed each year. If you have 1,000 constructed with earth, that would be a lot.

Erica Avrami: Tony, is this mirrored in the U.S. Southwest in new construction in earth?

Tony Crosby: Absolutely. A lot of fairly prominent international architects have worked in the Southwest, and a lot of local architects continue to work with that material. Of course, some of the appreciation is not for the material itself but, rather, for the traditional forms. So you also see in the Southwest an awful lot of pseudo-earthen architecture made from completely different materials, such as concrete.

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John Hurd: In the United Kingdom, there's an upsurge. It's not a huge figure, maybe 50 to 100 structures a year, built by owner-builders looking for sustainability. But there is a renaissance here. In a way, it was always a cottage industry. In other parts of the world, it's very healthy. In Afghanistan, if you want a house built, they will assume you want to build in earthen block. And likewise, all through central Asia.

Tony Crosby: An opposite example would be if you traveled along the upper Nile today. Ten years ago, you didn't see any small structures of reinforced concrete. Today you see a lot of them. And what are they replacing? Earthen structures.

Neville Agnew: Hugo, given that earth construction is the weakest unreinforced masonry, to what extent is seismic strengthening incorporated into new construction in seismic zones? In California, the use of concrete to seismically strengthen historic adobes has been tremendously invasive. That's one of the reasons we undertook our seismic adobe project at the GCI.

Hugo Houben: People have started to realize that for small structures—one to two levels—reinforcement should be rather simple. At one time it was thought that reinforcement should be concrete, which made it much more expensive. Today we know that when you are making nonhomogeneous structures, you have more problems and failure than when you work with homogeneous masonry structures with a ring beam in the right place and slight reinforcement around openings. Simple systems seem to work—and that brings people back to earthen structures. There's a great effort to inform the public of the research results that have been obtained in this field.

John Hurd: Traditional wooden ring beam structures—structures with continuous wooden beams that encircle the walls, which you see from Yugoslavia through Nepal and even in China—behave extraordinarily well in earthquakes. I remember that in Sarajevo there were three- and four-story structures that were undamaged when everything else wasn't. By showing people the houses that didn't collapse, people responded, "well, we've been building Western-style structures for the last 40 years, and these ring beams were expensive, but we didn't know what their function was." So going back to tradition is very useful. These traditional structures behave very well seismically, especially if the building is a regular form—square—as you tend to find in seismic regions.

Neville Agnew: Is research and development in new earthen construction informing the conservation field with respect to preservation of traditional historic earthen architecture?

Hugo Houben: We are extraordinarily equipped, from an intellectual point of view, to solve problems for contemporary construction. A lot of research has been done on material characteristics, stabilization, and construction systems. But when you come to the conservation of earthen architecture, very little or nothing of that research can be transferred through to conservation. With new construction, you control everything. With conservation, you go the other way around. You already have the building, and you have to work with the materials that were used.

John Hurd: Currently, the earthen architecture conservation field is teaching more to the new construction field then the other way around. As new construction finds its feet, more information will flow in the opposite direction. But it's flowing the other way at the moment.

Neville Agnew: In recent decades, the environmental movement has become powerful globally. Do you think that it's had any influence on the acceptance of earth as a building material?

Hugo Houben: Yes, there is a connection. The idea of biodiversity was brought about by the environmental movement. That seems to have initiated a move toward technodiversity. People are realizing you shouldn't get rid of all kinds of traditional technologies. As soon as you have an economic crisis or whatever, you've lost that traditional knowledge, and you can't produce it anymore. Today we think more in terms of sustainable development. And that's put local material, local resources, local technologies, and constructive cultures into the spotlight.

Tony Crosby:
Green architecture is certainly a response to the environmental movement, emphasizing low energy consumption in the production of building materials and low energy requirements for transforming structures into inhabitable environments. And that's where earthen architecture provides a valid solution. Although the impact of the environmental movement on earthen construction may be minimal, any appreciation of the material ultimately helps the conservation field.

John Hurd: Certainly in Europe, earth building technology and self-building are popular among environmentally aware people. You can find your material by fairly low-tech means and build by fairly low-tech means. It gives people an opportunity to build "buildings" rather than "architecture." Although there are architects doing excellent work, there are also many self-builders doing equally excellent things. They are liberated by the material—in its simplicity of use—to do things they normally couldn't do.

Neville Agnew: What are the two or three most pressing needs in earthen architecture conservation?

Tony Crosby: I think a greater understanding of how earthen architecture material responds to environmental conditions that result in decay. And related to that, a better understanding of the compatibility of interventions with the existing fabric and building systems. We need more collaboration among all participants in the conservation community: archaeologists, conservation architects, conservators, material scientists, traditional practitioners, and engineers. We have to take greater advantage of what each group can bring to the process. There is also a pressing need for conservation programs that will lead to a better understanding of the issues and of the significance and the historical values of earthen architecture. Hopefully, one result of that would be a standardization of conservation practices.

Hugo Houben: Collaboration between institutions and individuals working in the field: networking. Then there is training, training, training; education, education, education. The other thing we need very much is planning. I used to say that if you're working for the conservation of earthen architecture without planning, you're working against it. So planning is extremely important—and strategic thinking.

John Hurd: Planning, yes. And strategy, yes. Very important. On networking, I'm a private practitioner, so maybe I have a chip on my shoulder, but I'm convinced that there is better communication between institutions than there is between institutions and individual professionals. I'd like to see those barriers broken down. I'd like to see more internationalism. We've all done research in our regions, and we've read about each other's regions. But the amount of cooperation from region to region is not as good as it could be. We've got to break down those barriers—the ones between institutions and practitioners and the ones between regions.