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For nearly 4000 years, a small gateway into a forgotten Canaanite city
lay hidden beneath the dry soil of Tel Dan in northern Israel, until a team
of archaeologists excavated the site in 1976. But their satisfaction in
discovering the gateway – one of the oldest arched structures in the world
– was diminished by the fact that it is rapidly turning to dust, the victim
of wind, rain, burrowing insects, and nesting barn owls. The story of the
Tel Dan gate is not unique. Every spring the swallows return to California’s
Mission San Juan Capistrano. The mission was founded by Catholic fathers
in the 18th century, at the time when the Spanish ruled ‘Alta California’.
While birds raise their young under the eaves of the 115-year-old settlement
today, the mission administrators struggle to protect the buildings from
the ravages of earthquakes, enthusiastic visitors, birds, bees, and the
weather.

The link between Tel Dan, San Juan Capistrano, and thousands of other
crumbling monuments is ‘adobe’ – a Spanish word for sun-dried earth. Calling
it adobe, pise, cob, jacal or any of a hundred other names, people have
used this simple building material for millennia to construct temples, churches,
palaces, and entire cities, as well as modest huts. The advantages of earthen
dwellings are obvious: they are well insulated against hot and cold weather,
simple to construct and repair, and the building material is, literally,
dirt cheap.

But there is a catch; adobe is inherently weak and highly vulnerable
to the elements. ‘Mother nature is the main culprit, especially moisture,’
said Michael Taylor, an archaeologist with the US National Park Service
in Santa Fe, New Mexico. If an archaeological site, for example, is left
uncovered, water hitting the tops of walls will break them down. Moisture
can also penetrate a wall from the ground; it is then drawn up inside the
wall by capillary action as ‘rising damp’. Wet walls are weak walls, and
weak walls fail, says Taylor.

Adobe’s other enemies include the wind, which picks up loose dirt and
sand that scour unprotected surfaces. Extreme day and night temperatures
make the adobe expand and contract, triggering cracks and failure at the
edges of walls. Insects can burrow into adobe, leaving behind conduits for
water to seep in. Earthquakes can distort walls and crack them. ‘And if
they collapse, they collapse catastrophically,’ says Charles Thiel, a seismic
engineer at Stanford University in California. And of course, people wreak
their own brand of damage by use and abuse.

The irony is that, if regularly maintained and properly repaired, an
earthen structure can survive nature’s worst for hundreds of years. People
who live in adobe buildings need to ensure that the walls and foundations
are protected from water by a good roof and efficient drainage. In many
areas, the inhabitants adopt an annual programme of maintainance consisting
of ‘remudding’ the walls. But as soon as a building is abandoned, the process
of decay begins.

Historians, archaeologists and governments who wish to preserve ancient
architectural treasures face a knotty problem, says Giacomo Chiari, a conservation
chemist at Turin University in Italy. The challenge, he believes, is not
to renovate or ‘freeze in time’ adobe structures that have been seriously
eroded. Rather, it is to strengthen and protect them, without diminishing
their scientific and historical value or altering their appearance in any
way.

The conservator’s task is complicated by the differing ages and conditions
of the structures that need tender loving care. These range from relatively
young buildings that are still in use, such as those at San Juan Capistrano,
through long-exposed ruins to sites such as Tel Dan which have been unearthed
by the archaeologist’s trowel after hundreds or thousands of years underground.

Surprisingly, the complexities of adobe conservation have only recently
begun to receive serious attention. Archaeologists have been content to
‘dig, document and desert’ earthen sites, and cultural agencies spent their
scarce conservation funds on high profile projects such as the Egyptian
pyramids. Adobe was generally dismissed as a poor cousin of stone or wood.

Today, scientific interest in adobe is on the rise, largely because
of its emergence as a fashionable building material in the American states
of New Mexico, Arizona and southern California and as a potential low-cost
solution to homelessness in poorer countries (See ‘Pied a Terre’, ¿ìè¶ÌÊÓƵ,
10 March 1988). Organisations such as the International Centre for the Study
of the Preservation and the Restoration of Cultural Property (ICCROM) based
in Rome, the University of Grenoble, and the Getty Conservation Institute
in California have established research and training programmes in adobe
conservation.

¿ìè¶ÌÊÓƵs such as Giacomo Chiari began the preservation battle in the
laboratory. There, they looked at the physical characteristics of adobe
– old and new – and its humble constituents: clay, sand, silt and, frequently,
organic material. Proportions vary, but Chiari says most historic adobe
is roughly 70 per cent sandy or gravelly soils, between 15 and 25 per cent
clay and sometimes, but not always, 5 per cent organic ‘fillers’. Modern
adobe contains additives such as asphalt emulsion, cement or acrylic polymers,
added to strengthen the bond between the constituents.

The sticky clay binds the ingredients of adobe together making an easily
moulded, highly plastic medium. The reason is that microscopic grains of
clay – mostly platelets of aluminium and magnesium silicates – are each
surrounded by a thin layer of water. The more water, the more easily the
platelets slide past each other and the more slippery and pliable the clay.
But this handy arrangement makes clay unstable; it swells when wet and shrinks
when dry.

To stabilise the clay and to provide bulk, ancient builders added pebbles,
sand, and silt to the adobe mixture. Quartz and feldspar predominated. Often
they also used organic materials such as straw, camel hair, even blood and
honey, to toughen the material while it was being worked. In this respect,
adobe is the earliest analogue of modern composite materials. Successful
builders assessed the qualities of available raw materials by trial and
error and modified adobe recipes accordingly. A Getty Conservation Institute
(GCI) study of eight ancient adobe structures in China, Egypt, El Salvador,
Israel and the US concludes that, generally, the most durable adobe contains
relatively absorbent clay soils and organic fillers.

Armed with this basic information, researchers are attempting to evaluate
the hodgepodge of preservation techniques that have been tried by architects,
historians and conservation workers. The goal is to find out what really
works. Researchers can then put together a ‘bag of tricks’ that can be adapted
to the individual needs of each site, says chemist Neville Agnew, head of
the GCI Scientific Research Program in Marina del Rey, California.

To this end, Agnew and his colleagues in 1986 began testing various
chemical consolidants at the GCI laboratory. The job of a consolidant is
to improve the compressive strength of the material by replacing the natural
bonding agents destroyed by weathering. According to Agnew, the ultimate
consolidant is one that gives an adobe wall wet strength. The wall is not
waterproofed, but neither will it slump back to mud when soaked. The natural
porosity of the material is maintained, so that moisture can pass into and
out of the walls. ‘The wall breathes,’ says Agnew. At the same time, the
consolidant should form polymeric bonding within the wall to add strength.
Lastly, the ideal consolidant will not discolour a treated surface.

The team from the GCI has been evaluating three types of consolidants:
silanes, isocyanates and acrylics. They chose these polymers because they
have been developed into commercially available protective products for
everything from wooden floors to stucco walls. The researchers applied the
chemicals to adobe samples in various concentrations by spraying, brushing,
multiple coating and ‘bulk infiltration’, that is, injecting the consolidant
directly into the adobe through bore holes. The top performers are now being
field-tested in New Mexico by Michael Taylor and colleagues as part of the
world’s first scientifically controlled trial of adobe preservation techniques.

Taylor began the first field project in southern New Mexico in 1985
when he was employed by the Museum of New Mexico. The site he chose was
Fort Selden, a state monument near the little town of Las Cruces, abandoned
by the US army in 1890. Because he did not want to risk damaging the fort’s
crumbling walls, Taylor built 12 adobe test walls, each roughly 10 metres
long and 25 centimetres thick, each embedded with a soil moisture sensor.
The walls could have been built anywhere in adobe country, but at a state
monument they are protected from vandalism and urban development.

Taylor is experimenting with traditional Indian remedies such as the
juice of the prickly pear cactus as well as contemporary consolidants such
as acrylics. In addition, he is looking at ways to halt rising damp. Among
them are trenches filled with pebbles, called ‘French drains’, aprons of
concrete poured around the base of walls, and plastic ground covers that
slope away from the walls. Taylor is also assessing a variety of techniques
for adding caps of brick, cement and chemically modified adobe to the tops
of ruins to stop further decay. In this part of the project he allows all
the walls to weather naturally and monitors the results.

Adobe and artificial weather

In 1987 Agnew joined Taylor and added another 35 test walls to the site.
Instead of long walls, however, he built slabs like tombstones, roughly
125 centimetres long, 15 centimetres thick and 152 centimetres high. Half
of each wall is covered with a thin mud plaster of the same composition
as the adobe bricks. This type of sacrificial coating deteriorates when
it rains and has to be replaced periodically. The other half of each wall
is left bare, and sensors monitor temperature and moisture within the walls.

In order to speed up and control the weathering process, Agnew set up
a watering system that sprays a set volume of water on the walls at timed
intervals. Each of his walls is artificially weathered at the same pace.
Two of the walls were covered in earth except for their very tops to simulate
conditions at an archaeological site that has been reburied as a means of
protection.

Not only is Agnew using this outdoor laboratory to put the consolidants
and application techniques already tested in the laboratory through their
paces, he is also studying measures for the protection of archaeological
sites as a whole. One approach is the ‘hexashelter’, a lightweight covering
designed as a low-cost alternative to roofing. The six-sided, curved panels
are made from a knitted polyethylene fabric, stable in ultraviolet light,
supported by lightweight aluminium poles to limit its impact on sensitive
sites. Because each panel is hexagonal, the shelter can be easily extended
in any direction as a dig proceeds, and the resulting zig-zag roof has greater
wind stability than a flat covering.

Agnew is also experimenting with a permeable poly-propylene ‘geotextile’
– a woven material used by civil engineers on construction sites. He has
wrapped it around one of the two buried walls. The idea is to find out if
this extra layer will help prevent damage from roots, insects, and bacteria.
If it does, archaeologists could cover their sites between field seasons,
or wrap and rebury sensitive structures until suitable methods of preservation
have been developed.

Another fabric – a tightly matted, non-woven polypropylene textile usually
used to shade food crops – has been wrapped around plastic drainage tubes
pierced with holes. The geotextile should keep the tubes from becoming clogged
with sediment and roots. Agnew predicts that these ‘buried open drains’
will draw water away from walls more effectively and cheaply than French
drains, and with less physical disturbance to the surrounding archaeological
site.

Time will be the true test of the Fort Selden experiments. Nonetheless,
some winners and losers are already emerging. The big losers are the modern
acrylic-based consolidants. ‘Acrylics do not penetrate the walls very well,’
Agnew noted. ‘They create a boundary layer, and this means that the very
surface that you want to preserve – say a decorated historic adobe – will
eventually fall off because water gets trapped behind the coating and changes
the porosity of the outer skin.’ In other words, rigid acrylics are incompatible
with the natural changes that characterise adobe.

Effective remedies

The isocyanates and silanes, however, show ‘excellent outdoor weathering
characteristics’, says Agnew. They react to moisture inherently contained
in the adobe to form a polymer known as a polyalkylurea. A particular advantage
of silanes is that they do not alter the characteristics of the adobe, because
they add only silicon and oxygen to the constituents of a wall.

Conversely, ‘home remedies’ such as cactus juice have proved ineffective
so far, says Taylor. He suspects they may not have been applied in the proper
concentrations or may need more time to cure and plans additional tests
to find out why they have not worked.

Taylor’s experiments with capping adobe have had one unexpected result:
all the caps protect the tops of the walls, but water runs around the caps
and gouges out the adobe immediately beneath. ‘What you get is a very narrowed
and weakened wall at the top,’ says Taylor. He hopes to solve the problem
by conducting more trials aimed at avoiding drips using overhangs and drip
edges like those on window sills, in conjunction with various methods of
keeping the dripping water from reaching the base of the walls.

In the drainage experiments, both French drains and buried open drains
reduce rising damp. In contrast, plastic sheeting and concrete aprons cause
more damage than they prevent. In both cases, water seeps into the ground
at the edge of the barrier and then migrates under it and up the inside
of the wall. Even worse, the concrete apron blocks any evaporation from
the surface that would otherwise help.

Agnew found that using textiles and reburial are promising techniques,
especially when used together. He points out that the section of wall that
he left exposed to the elements, and undraped, has totally disintegrated,
while the buried section is largely intact. And the wall swathed in the
polypropylene matting is in close to perfect condition, above and below
ground. The hexashelter concept is also proving itself. It is so effective
that the GCI has used one to cover a Roman mosaic in Cyprus.

These and other measures being tested at Fort Selden are comparatively
cheap and easy to use. They have to be. As a rule, conservation pro-jects
are seriously under-funded. The GCI is one of the few organisations anywhere
with sufficient money to undertake major research and conservation programmes.
Further, many important adobe monuments are in Third World countries. If
preservation techniques are not ‘quick and dirty’, little, if any, conservation
will happen. And it must happen soon, because, like the gateway at Tel Dan,
many historically valuable adobe structures have very little time left.

* * *

Building with adobe

Adobe is a wonderfully personal building material. It can be shaped,
built, and decorated to suit the taste and requirements of each and every
builder. Nonetheless, three basic techniques have been used throughout human
history, from the prehistoric past to the hi-tech present.

Adobe ‘bricks’ are made by pouring wet adobe into a mould. After baking
for one to two weeks in the sun, the cured bricks are ready for building.
California’s Spanish missions exemplify adobe brick construction.

The English countryside is still dotted with ‘wattle and daub’ cottages.
Here, adobe is used to fill the gaps between upright wooden polls or lathes,
much like a mortar. Similarly, adobe has been used in place of mortar to
set stone masonry.

Finally, pise de terre, or ‘rammed earth’, has been used to build large,
impressive dwellings. Big wooden moulds are set up on a foundation and filled
with wet adobe. The adobe is pounded to compact it and is then allowed to
dry. Once the adobe has ‘set’, the moulds are removed and repositioned on
the top of the new wall. The process is repeated until the desired height
is achieved.