The Delicate Balance of Conserving Earthen Archaeological Sites

Earth is a universal building material and earth buildings are among the most common structures in the world. 

However, the materials used, their design, and the conservation challenges they face can differ greatly depending on their scale, their natural environment, and how the buildings are used and maintained. Particularly vulnerable are earthen archaeological sites because they are structures that are no longer in use and often incomplete. 

How much or little intervention is appropriate for an earthen archaeological site? Should a site be uncovered at all? These are among the difficult conservation choices facing those who care for them. 

These topics and more are discussed in a roundtable of conservation professionals led by Jeanne Marie Teutonico, Getty Conservation Institute’s associate director for strategic initiatives and publications, and Jeffrey Levin, editor of Conservation Perspectives, The GCI Newsletter. This is an excerpt of the full interview in that publication, which can be read here.

Angelyn Bass is a research assistant professor in the Department of Anthropology at the University of New Mexico and an architectural conservator who has worked extensively with the US National Park Service in the US Southwest.

Annick Daneels, an archaeologist and full-time researcher at the Institute of Anthropological Research at the National Autonomous University of Mexico, is a specialist on earthen architecture of the Gulf Coast of Mexico.

Aqeel Aqeel is head of historic buildings conservation at the Department of Culture and Tourism of Abu Dhabi, where he is responsible for the planning, design, and implementation of conservation measures for Abu Dhabi’s Historic Environment.

Jeanne Marie Teutonico: Let’s start with the most general questions. What do you each see as the main issues in the conservation and management of earthen archaeological sites? And what do you consider the principal challenges to conserving them? 

Angelyn Bass: I often work on National Park Service (NPS) sites, some that were excavated over 100 years ago. Many of the sites under NPS management were constructed before 1450—prior to European contact—were partially excavated anywhere from 30 to 130 years ago, and are open to the public to some degree.

At each of these sites, earth is used in many different ways—as a structural material, either clay lump, adobe brick, or mortar for stone masonry; and as plaster for finishing walls, floors, and roofs. It’s also used to construct built-in features for weaving and food production. Earthen materials are often well preserved in alcoves where they have natural protection from the weather, which is why so many precious details survive. You see the handprints and fingerprints left in the plaster and the mortar, and you find little remnants like hair and bits of fiber left in the floors from weaving activities.

But these earthen materials are also highly ephemeral. They’re prone to loss from natural causes—weather, climate change, and earthquakes—as well as from anthropogenic causes, such as heavy visitation, inappropriate preservation treatments, and even vandalism. Unfortunately, these national parks are underfunded and understaffed, and maintenance sometimes has been deferred. With climate change and the increase in storm intensity, this situation is becoming worse.

One of my concerns is the overtreatment that’s occurred at many of the sites, especially those that have been excavated and exposed for decades. I think the biggest challenge is knowing how to respond in ways that preserve the legibility and the authenticity of these sites, and that respect the beliefs of the descendant communities, without foreclosing on values yet to be identified. 

Annick Daneels: I came to earthen architecture as an archaeologist. I’m not a conservator, architect, or engineer, though in my projects I’ve worked closely with these specialists. I’ve been thinking a lot about the issues, and I see two main challenges. One is raising public awareness of archaeological earthen architecture. People simply don’t know about earthen architecture—what it represents and how important it is. The other thing needed is training. Training at all levels. There’s very little professional training in the disciplines that should be collaborating in preserving earthen architecture—whether it’s archaeology, conservation, or engineering.  

Aqeel Aqeel: I’m an architectural engineer and have also worked in conservation for around 18 years, basically in the United Arab Emirates (UAE). The points made by Angelyn and Annick certainly are correct. These issues are essential. From my point of view, one of the main issues, especially for earthen archaeological sites, is the materials. In general, for living architecture, you have the flexibility to repair and to maintain easily and periodically. But for archaeological earthen architecture, it’s challenging because you have issues with authenticity, which we need to retain.

But at the same time, we need to preserve it for a longer time. Many archaeological excavations were done in the last 50 or 60 years. These excavations expose the sites to weathering factors that lead to deterioration over time. We lacked proper management planning during and after the excavations, and the result was the deterioration of the earthen material—and you can’t intervene as easily as you might with other materials, such as stone. 

Teutonico: Earthen materials have real vulnerability, especially if they are unprotected. But this perception of vulnerability sometimes leads to over-intervention. How do you assess relative vulnerability so you can avoid reactive interventions driven by a fear that something is going to disappear?

Daneels: The best way to keep earthen archaeological sites safe is to not touch them—or, if you do excavation, rebury them. Reburial is probably the best approach and least invasive. But this is a paradox because it defeats the purpose of raising the awareness of these sites.

People will only see mounds—mounds of grass and mounds of sand—so they won’t realize what’s there. To assess the importance of the architecture, you have to excavate, and the only way to raise awareness is to try to expose the architecture for what it is. But then you get into that whole spiral of problems of conserving the architecture, and you’re back to square one. 

Jeffrey Levin: Do you see any way to address this paradox? 

Daneels: I think by doing what the GCI has been doing—trying to select specific sites that can become showcases and provide resources for those sites. That includes good diagnostics, good definition of causes, good intervention, and a long-term management plan that includes attracting tourism so it raises awareness and there is money enough to maintain it. But then don’t touch the other sites. It’s a subtle balance. 

Levin: Angelyn, this question that Jeanne Marie raised with regard to assessing risks so you don’t overtreat—what’s been your experience in doing that kind of risk assessment? 

Bass: Measure, measure, measure, measure, measure! And how. We do it by assembling a team with skills appropriate for the conditions. With these alcove sites, there’s always an archaeologist—preferably an Indigenous archaeologist. We frequently include a structural engineer, a documentation specialist, and a conservator, but the team depends on the specific questions we’re trying to address.

We have a new project at Canyon de Chelly where we’re including a geotechnical engineer to measure movement of ancient middens that are the foundations for overlying structures, and we’re bringing in an environmental engineer to look at climate change impacts. We have a chemist looking at the organic pigments that color the walls. But in most instances, the greatest threats are structural. Most of these buildings no longer have roof frames, and they fracture into tall, slender wall segments. So we bring a team together to study the severity of threats and measure them.  

Teutonico: Even if you understand what the risks are, do you have all the conservation strategies you need to deal with those risks, or is there still research that needs to be done? 

Daneels: I would suggest that archaeological work be more connected with interdisciplinary analysis. In the project I’ve been working on in La Joya in Veracruz, the historically most recent construction had eroded, but the several earlier stages of construction beneath it were well preserved, which means they withstood the weathering. Clearly, the builders addressed this problem, but we hadn’t yet understood how. We started doing standard analysis, looking at mechanical properties and elemental composition, and then the chemistry to find organic additives. Finally, we focused on bitumen, and asphalt emulsions, and proved that asphalt had been used as an additive in the adobe and in the plasters.

So we started an experimental program, and it worked. An archaeological finding of pre-Columbian technology showed that asphalt was used and could be used again. We should learn more from the solutions that they invented thousands of years ago. Therefore, a thorough understanding of ancient technology will probably add to the conservation tool kit. 

Bass: In terms of research needs, we need more information on earthen grouts. How do we create them so they’re lightweight, flow properly, and are strong? Do they provide the structural benefits that we want? Are they needed as often as recommended? Another research area is in reburial. How do we monitor the reburied environment, and how do we communicate to managers and archaeologists that while reburial is reversible, its success relies on maintaining an environmental equilibrium? Re-excavation is not a viable monitoring strategy.   

Teutonico: Regarding reburial, monitoring is certainly one issue. But methodologically, when someone says they want to rebury something, do they necessarily know how best to do it? Obviously, there are many different ways to rebury a site or a specific part of it. 

Bass: I intentionally used the word “rebury” instead of backfill because backfill implies simply throwing dirt from the site back into the trench. Often reburial designs and materials need to be engineered to address the environmental threats inherent in that site. In the US Southwest, we’ve had the benefit of the GCI’s research on reburial at Chaco and Laetoli that’s helped to create a framework to follow.  

Teutonico: And with climate change, reburial methodologies that work now may have to be altered. Monitoring of the reburial environment becomes increasingly important to ensure that new or changing conditions are addressed. 

Levin: One thing I’m hearing here is that there is no one-size-fits-all with respect to the materials. You need to research the specifics of the site to understand what materials will be compatible with any intervention you engage in.

Bass: The one-size-fits-all approach to site stabilization has been an issue at many archaeological sites in the US Southwest. In the past, there was often widespread, and perhaps overly optimistic, use of repair products like Portland cement amendments and spray-on water-repellent coatings of adobe walls, for example, which 10, 20, 40 years later have caused irreparable damage and have had to be removed. Many of these treatments complicate the study and interpretation of architecture.  

Aqeel: In the UAE, the issue in the past was the lack of any treatment for 30 years after the first excavations. Later on, there were some attempts to protect or conserve the remains based on personal judgments but not on a methodical scientific approach. In 2006 we started using a more systematic methodology for preservation. Before that, most things were done based on personal opinions, or nothing was done at all.  

Teutonico: Do you think it is sufficiently understood that the conservators who work on wall paintings are not necessarily the same people who work on ceramics or, indeed, on earthen walls and building materials? Could the various specialties in conservation be better understood? 

Bass: When we started the (San Bartolo) Guatemala project, there was an assumption the wall painting conservators could do it all. But it quickly became clear that a broader range of skills was needed on site and in the lab. There’s a learning curve in every large project, and we need to become familiar with other areas of expertise in order to engage in the work together. So much more can be accomplished when we do. But it takes time, and it’s not as simple as it sounds.

Daneels: I completely agree. These are the ideal projects where communication exists and there is respect for the expertise of each team member and the project goals. My experience, at least in Mexico, has been that once the archaeologist has made the assessment of the site as being important and that it should be preserved and open to the public, the conservators, architects, museographers, and management officers come in and say, “Okay, you’ve done your work, you may go.” And the archaeologist isn’t consulted anymore. Most of the time feedback between the different persons within a project is rarely achieved. Archaeologists are the first ones to go because it’s assumed they don’t know about conservation, or architecture, or structural refitting, or museography. That’s common.  

Bass: I’ll offer an example of the importance of an archaeologist on a project. There are many important elements on the surface of a wall painting—the residues left from ritual events, for example. If conservators don’t know these residues exist, they might remove them to clean the surface. We need archaeologists to help guide us about what’s important and what needs to be preserved.  

Levin: One thing we haven’t talked about is community engagement and consensus-building in the management of these earthen archaeological sites. I’m interested in what each of you has to say on that.  

Daneels: In Mexico there is a federal agency, the Instituto Nacional de Antropología e Historia (INAH), which has full power over everything archaeological. Projects must be approved by them, and they decide if a site is to be preserved or not. Most of the time they say no to earthen architecture because it’s too complicated and there’s enough tourism for sites with nice stone architecture. Now it’s different in northern Mexico where we have World Heritage Sites like Paquimé and Camino Real de Tierra Adentro. Preservation of those particular sites is inspired by what’s happening in the US Southwest, and there’s been collaboration with the National Park Service.

But throughout Mesoamerica, we have important earthen sites that are not restored. They’re just mounds, like La Venta, Izapa, and Tamtoc. Those are important sites, and they’re open to the public, but what is shown is the sculpture, not the mounds. The mounds are just mounds. Community input or sponsor input is completely subordinated to what INAH decides. The only way to legally participate is as a nonprofit organization. If a community wants to independently promote its site for public view and for economic development, it will clash with INAH because they’re not allowed to make money out of it. All archaeology is the property of the nation, and so INAH decides.