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Wednesday, November 29, 2017

Mud brick and termites in Amarna – Amarna Anniversary

Mud brick and termites in Amarna

While famous for its decorated stonework talatat, Amarna was predominantly a mud-brick city. Richard Hughes outlines efforts to protect and stabilise its mud brick ruins.

The siting of Amarna was ideal for its speedy construction via a military-like mass-production campaign making, shifting around and placing down mud bricks (adobes). The riverbank and floodplain graded silty soils and mixed-in calcareous desert graded coarser sandy-silts were ideal for moulding into bricks, the sun then used for free quick drying and hardening of the soil mix. The simple prismatic modules, like 'lego' blocks, were ideal for rectangular and flat-faced buildings of all types. As with buildings today one was generating the right brick sizes and bonding configurations for the design wall widths, wall heights and corner turning – those of best quality were used in the wall faces and 'seconds' in the wall cores. The walls were constructed using brick-bedding mortars of the same muds but of a sloppy consistency; the house shells were then variously finished off with better quality internal and external soil renders. The city's major monuments were commonly finished off with the now famous dressed and often decorated stone talatat. Only a few buildings appear to have used lime/gypsum-based renders. The more temporary mud/rubble huts and small houses of the workers were usually left un-rendered.

Consolidation of brickwork at the North Palace in 2002, with new brick cappings and facings added to protect the ancient brickwork. Note the horizontal render-filled grooves in the wall faces where wooden beams were once located.

Generally, mud brick construction followed normal procedures found elsewhere in pharaonic times, not much different from as seen today with a traditional approach throughout Egypt – not a particularly skilled undertaking, the product valued for the semi-industrial/hand-mechanised speed of construction and for the resulting consistent form, size and surface finishes. Within the various archaeological excavations it has been possible to see precisely how the vast construction labour-force(s) worked to make and precisely place bricks and mortar, how they used their tools and how the many small teams interfaced. It is even possible to differentiate left and right handed workers and in which directions walls were built. Many workers left impressions of their feet in mud splattered around the base of walls they were constructing. The same mass-construction, in the same short time-frame, would not have been achievable using dressed or even rubble stone walls!

As some of our technical studies showed, the resulting soil walls also had a remarkably good thermal performance, not so much related to soil mineralogical composition but to wall orientation, surface shading and especially the low degree of mud compaction within the casting moulds, and by default, internal entrapment of air in the open soil pore structure as this replaced pore water during brick drying.

However, there were by design, some rather interesting tricks-of-the-trade – particularly the use of a tough coarse grass (possibly related to Imperator Cylindrica) for reinforcing the mortar beds around doors and windows. Another was the incorporation of imported and embedded Cedar of Lebanon timbers, for giving monument walls a higher level of robustness and in- and out-of-plane structural stability, via an improved wall tensile and shear strength capability.

However, in one respect the builders got it all wrong! Occasionally the mud mix before casting into bricks, and even more so that used for renders, was 'improved' with chopped straw. This is a technique noted in the Bible, used to good effect with the very plastic clayey soils of the Nile delta. The Amarna more granular and less clayey soils didn't need an artificial means to control any minor shrinkage and cracking.

This 'stabilisation' has to be seen in context of Amarna being mostly sited, then as now, laterally at the geomorphological interface zone where soil moisture reflects on the desert (very dry) conditions to the east and the (moderately damper) river induced regime to the west – a zone good for agriculture and brilliant, unfortunately for Amarna, for termites. Present-day Amarna villages are also located in this zone.

As we know Commander Kemp has waged a military campaign against termite multi-pronged / mass-attacks into the Amarna artefact storage magazine. Concrete floors and insecticides only give some effective resistance – in the villages chickens are the defensive troops. In the conservation of the Small Aten Temple putting back some of the timber reinforcement tie beams for the conservation programme, even with rather dangerous (to us) chemical treatment, hasn't halted the battalions of termites munching yet again their way through the wood. For the well-known grain storage bins found in many Amarna houses it was perhaps only elevated levels of CO2 that provided resistance to overwhelming insect infestation and territory capture!

But for many house walls the story is different, the chopped straw sometimes added to the mud bricks, and particularly their renders, was a good way of donating an in-built free food supply to the termite NGO armies – they must have had an instant set of biblical sized good harvests. Then and as now archaeologically excavated, these mud bricks, and their plaster coatings even more so, quickly became sponge like in appearance – full throughout of interconnected holes and tunnels. This also supported how the occupying armies moved around and were billeted internally, as walls (effectively classic termite air conditioned towers) heated up and cooled down during daily sun, shade and micro-wind cycles. Wooden fittings such as door and window frames, and columns, would have provided another source of sustenance. Perhaps to counteract this, many mud bricks used gravel rather than straw render, and these have often remained quite stable when protected from weathering, even if their plaster coatings are mostly lost. In a sense it was all rather fortunate that Akhenaton died not too long after Amarna was reasonably complete, this resulting in its abandonment! Otherwise, there was going to be one hefty payment to someone for carrying out repairs. The instant ruins then became rather self-buried in brick rubble mounds, also one assumes abandoned by the termites, marching on to richer pickings elsewhere!

SAT brick-making
Newly made bricks, laid out to dry in the sun.

Today in the conservation agenda the straw is not added, so there is perhaps some loss of replication authenticity. But, at least we hope they last longer and now we already know they are effectively protecting original in situ wall fabrics from on-going decay. The experimental programmes originally utilised smidgens of Ordinary Portland cement and building lime, and now just a small amount of the latter, for soil stabilisation in mud brick making. This is to replicate the effect of calcium carbonate deposition which slowly takes place over the centuries. High levels of engineered compaction is also applied, where we have needed an 'instant' improved unconfined compressive strength to match that of the original bricks found now after some 2500 years survival. Perhaps even more importantly we seek a far greater abrasion resistance, as occurring in desert sand storms, aiming to reduce the normal needs for maintenance.

For those soon heading off to Amarna for the next excavation season, termites don't feature in camp life dinner menus except via chickens, but they could be used for emergency rations. Vintage mud brick dust is freely available for toothpaste or salt/pepper substitute, whether one likes it or not!

1988 , 24 , 4
Adding mud mixture to the brick mould during early consolidation work in 1988.

Richard Hughes is a Structural Engineer and Historic Building Conservator. His experimental work in the 1980s helped establish current protocols for protecting Amarna's brickwork. To read more about the consolidation of monuments at the site, work supervised in the field by Michael Mallinson and later Suresh Dhargalkar, please visit the Amarna Project website.

Sent from my Linux system.

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