Mud bricks are among the least glorified of archaeological discoveries. Less durable and imposing than massive stone walls, not as captivating as a cryptic inscription or a work of art, their friable remains are rarely the star attraction at ancient sites. In antiquity and mainly in the Near East, construction with bricks made of mud and straw was common. The rub is that most of the structures built with them have since crumbled away, leaving only stone foundations for us to ooh and aah at.
Now, however, some Israeli archaeologists suggest a new way to date structures at multi-layered sites, ancient settlements that have been built, destroyed and rebuilt time after time, based on the humble mud brick.
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A warning to those of a delicate disposition: the method has a bit of an ick factor. It involves analyzing the chemical composition of the bricks based on the assumption that over the centuries the same raw materials were recycled to make new buildings, and with each generation larger amounts of human waste, as well as garbage and other organic residue, got mixed into the mud bricks.
The idea is the brainchild of a spry, 72-year-old archaeology student, Jacob Schreibman, who tested his method on the ruins of Azekah, a site in southern Israel that has more than 3,500 years of history.
Throwing out the past
Schreibman, an industrial engineer with a career as a hospital administrator, was always interested in the early history of the Levant and decided to study archaeology after retiring seven years ago. The idea to focus his research on this oft-overlooked construction material came to him as he observed a bucket line, which is what archaeologists form to clear buckets filled with sediment from the day’s digging and unceremoniously dump it on a pile.
Most of the dust and soil that is thrown out at a site like Azekah consists of the pulverized remains of mud bricks that were once part of the city’s architecture, Schreibman says.
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“We throw away about 90 percent of what we dig up and focus only on the pottery, stone walls, bones and other finds,” he tells Haaretz. “It’s not possible that 90 percent of what we find has nothing to say to us.”
Azekah is a tell, an artificial mound formed by the accumulated layers of human habitation over centuries or millennia. There are dozens of tells in Israel alone and many more across the world, mainly concentrated in the Near East, the region with the longest record of sedentary occupation.
The city lies atop a ridge overlooking the Elah Valley and the coastal plain, and is perhaps best known as the purported setting for the biblical duel between David and Goliath. But its strategic location means the city’s history stretches way back before and beyond the biblical period. In fact, it reached its greatest extension in the Middle Bronze Age, centuries before David supposedly lived, when the Canaanites built the first city on the site around 1,700 B.C.E.
Their large settlement was built mostly on bedrock, meaning there was no natural accumulation of soil atop the ridge that could be mixed with water to make mud bricks, says Oded Lipschits, a professor of archaeology from Tel Aviv University who directs the expedition at Azekah.
Yet archaeologists found remains of massive mud brick walls from that time, the Middle Bronze Age, particularly on the town’s fortifications. So where did the Canaanites get the raw material to build their bricks?
Lipschits’ theory is that those first builders of Azekah hauled approximately 30,000 metric tons of mud (the equivalent of about 160 Boeing 747s) from a stream in the valley below and up the hill in order to build a city that large.
This feat inadvertently benefited the later inhabitants. The multiple settlements that were subsequently built on each other in the Israelite, Persian, Hellenistic, Roman and Byzantine periods simply recycled the eroding remains of the preceding city to mold new bricks, the archaeologist postulates.
Under Lipschits’ guidance, Schreibman set out to test this theory by comparing the physical and chemical composition of mud brick walls already securely dated, by other methods, to different eras.
The mud in mud bricks is generally composed of sand, silt and clay in varying proportions. And surely enough, walls from the Middle Bronze Age contained those materials in amounts that were similar to samples taken from the surrounding valley, Schreibman told Haaretz during a recent interview.
“A city is built and people live in it for quite some time. Then something happens: war, an earthquake, climate change, and the place is abandoned and the bricks crumble due to erosion by water and wind,” he says. “Then new people come, maybe centuries later, and build a new settlement, creating new bricks from the same material – but meanwhile something has happened to this material.”
During the intervening years, the rain has washed away part of the clay in the sediment, meaning that the “recycled” mud bricks have a higher content of sand as time goes on, Schreibman explains. As expected, lab tests have shown that in more recent ruins from the Late Bronze and the Iron Age the clay content is progressively reduced, which also makes the bricks more fragile and less durable, he says.
Conversely, the analysis shows an increase over time of chemicals such as phosphorus that are associated with the decay of human waste, animal bones and other organic materials that would have accumulated in the settlement over time. For example, in later periods, the amount of phosphorus in the bricks made of the local mud is almost double that found in normal farmland, Schreibman says.
To put it bluntly, it seems that in each new generation the Azekahites were building their homes by recycling their ancestors’, ehm, leavings.
This slightly revolting factoid is not only a confirmation of the theory about the origin and reuse of the sediments on Azekah: it could potentially turn into a tool to help date newly discovered structures at this site and at other multi-layered tells just like it, Lipschits told Haaretz during a recent tour of the site.
A tricorder for archaeologists
Schreibman based his MA thesis at Tel Aviv University on the analysis of the mud bricks from Azekah, under the supervision of Lipschits and fellow archaeology professor Yuval Gadot. Now he’s working on a PhD that tries to prove this dating method can be successfully applied to other sites as well.
Of course, archaeologists have long been able to date ancient ruins by sampling organic materials and then radiocarbon dating them. The advantage of the new method is that it is cheaper and quicker than carbon 14 testing, Schreibman says.
The archaeologist is also working on a way to use X-RF (X-ray fluorescence) analyzers for this purpose. This technology uses X-rays shot from a handheld device to determine the chemical composition of a material, sort of like a tricorder in Star Trek. This would mean that the dating of ancient mud brick structures could be conducted on the spot in the field and less destructively than by sending samples to a lab.
On the downside, Schreibman acknowledges, his method is less precise than radiocarbon dating, and requires building a different calibration scale for each tell. In other words, archaeologists first need to build a baseline of each specific site using remains of brick walls that have been dated through other methods, and only then can then compare those results to newly discovered structures to quickly figure out their age.
Still, Lipschits maintains, mud bricks are the most common material found in digs, rivaled perhaps only by pottery, so the new method shows huge potential to help archaeologists investigate the mysteries of ancient cities.