Thursday, February 20, 2014

The Treasury of Atreus - one of the world's greatest domed structures

I recently attended a ‘Friends of the British School in Athens’ lecture by William Cavanagh on the Mycenaean tomb commonly known as ‘The Treasury of Atreus’. I’ve visited the tomb several times, but until I heard Cavanagh’s lecture I didn’t quite appreciate what a feat of engineering the structure is. The diameter of the chamber of the tomb is 14.5m – a size best appreciated by standing within the structure, where standing under the soaring walls of carefully smoothed stone, one feels very much aware of the power and wealth that the builder of the tomb commanded.

The tomb was given the name the Treasury of Atreus (and sometimes the Tomb of Agamemnon) by the German archaeologist Heinrich Schliemann, who had a propensity for connecting his discoveries with mythical Mycenaean figures. The structure is most definitely a tomb and not a treasury, and we do not know the identity of the people who were buried within.  The Treasury of Atreus, however, is still capable of yielding a huge amount of information about Mycenaean society under close analysis.

Amazingly, if we consider all the known domed buildings, the Treasury of Atreus remains the largest free span dome for more than one thousand years, not only in Greece but the rest of the world, and was only surpassed with the introduction of concrete, except for a few exceptions in the Hellenistic period that were constructed with very large timbers and which have not survived.

We have to wait for the 1st century BC and so-called Temple of Mercury at Baiae to see the Treasury of Atreus ‘outspanned’. With the use of concrete the Temple’s dome achieved a span of 21.5cm, and then a century later the Pantheon in Rome achieved a span in excess of 40m. To this day, the Pantheon remains the largest concrete dome not reinforced by steel.

The date of the Treasury is traditionally placed between 1370 – 1250BC, though the origins of the type are earlier, dating to the Middle Bronze Age/beginning of Later Bronze Age (1750 – 1490BC). A variety of tomb types existed side by side from this early period including cist graves, pithos burials, tholos tombs (such as at Tragana, Messenia) and rock cut chambers (as at Epidavros Limera).

The rock-cut chamber tomb and the tholos built tomb both prevail in Mycenaean Greece and both types are similar in that they are made up of three distinct features comprising a chamber, an entrance and a passage. The origins of both types are still disputed by scholars, but there is evidence for both Minoan and local origins and influences.

The difference between the two types is that a chamber tomb is cut into rock and earth, but the tholos tomb is built out of stone, with the pieces laid in a circle on top of one another up to a tapered centre point. The dome would then be covered by an earth mound to make a tumulus. The difference between the two types seems to become largely due to wealth and display, with the tholos tomb becoming grander and more decorative.

The chamber part of the tombs is designed for multiple burials, probably a family or small grouping. Child burials and female burials are fewer – according to Cavanagh it seems that they may not generally have been allowed in the main chamber.

The ‘stomion’ or entrance way is carefully dressed and larger than it actually needs to be for its function, and the façade is sometimes decorated. It consists of a large rectangular opening often flanked by two stone columns and topped with a single stone.

The ‘dromos’ or passage that gave access to the tomb is made with great care – so much so that it must have served a specific part in the ceremonials around the burial.

Tholos tombs are known from the area of Mycenae itself, the Argolid, Messenia, Boeotia and elsewhere, and are being still being discovered. Very many of them have been robbed, but even those often show evidence of important grave goods. The famous gold cups from the tomb at Vaphio in Laconia, now in the National Museum in Athens, show how spectacular the grave goods could be.
Vaphio gold cups 
But even if the wonderful grave goods have often disappeared, the wealth and importance of the people for whom the tombs were built is reflected in the method of construction of the chamber - the corbelled vaulting. Corbelled vaulting is very widespread both geographically and chronologically. It is evidenced in the 5th millennium BC at Barnanez in Brittany, but can also be seen in the nuraghi of Sardinia, in Oman and even some sites in Latin America. It came into its own in the early Mycenaean period LHII: corbelled vaulting is seen in the so-called tomb of Aegisthus at Mycenae which boasts a diameter of 13m, at Kakavatos in Messinia which is 12m in diameter, Galatas in the Peloponnese 11m diameter, and the famous Vaphio tomb 10m.  Those who built the Treasury of Atreus, with its span of 14.5m, were pushing the technology of construction to its limits – as we shall see below.

The tomb was built at a place now known as Greater Mycenae, a residential area that was fairly densely populated, and the site of the Panagia House – it has in fact been argued that this residence may be connected with the tomb. Although largely residential, there were also some chamber tombs in the area. Excavations show that houses were demolished to make way for the construction of tomb, which implies that whoever commissioned it had some kind of control over the land and the people that lived in the houses that were demolished. The modern road follows more or less the same route as the ancient road, so those who have visited the tomb will appreciate that the Treasury is located close to the road and the tumulus would certainly have been visible from it. Does this mean that people travelling along the road to the Palace of Mycenae were meant to be aware of the tomb, and remember its occupant?

Cavanagh has provided fascinating data on the ‘man days’ that the construction would have required. These are estimated as below:
Quarrying the stone 3400
Carting to the site (yoke days) 680
Labour (from clearing the site to construction) 14100
Dressing 2100
By any reckoning, an impressive amount of work, and an indication of the power and status of the person who would have been buried there. The lintel block (over the entrance) alone is estimated at 120 tonnes, and scholars have shown that beasts of burden could not have been harnessed to put the lintel into place, so hundreds of men would have been used. There is no precise agreement on how it was done. So, all in all, we are looking at a huge exercise in manpower, and a very high level of organization, involved in the construction of this tomb. Whoever wanted it built could apparently marshal resources.

We get an idea from Linear B tablets about some of the logistical elements of construction. The tablets give details of rations for workers and the engagement of craftsmen, for example bronze workers.

The calculations of ‘man hours’ made by Cavanagh leave out those necessary for embellishment. We know that there was bronze decoration on the interior as hole for pins can be seen and also on the corners of the façade. Thus the structure was not only functional, it was decorative, and considerable resources could be spent on this too.

The stone used for the embellishment came from different quarries – the rosso antico from Profitis Ilias, Laconia, and the greyish green limestone from the Mani. This is a significant point because these quarries actually lie in the territories of different states – so this is evidence of international trade and indeed international relations. All of these factors are building a picture of power and influence – and the identity of the occupant of the tomb as a powerful person, not only locally, but perhaps further afield.

Cavanagh paints a picture of the Mycenaean centres controlling the natural resources of their hinterlands, and then trading or exchanging with the other Mycenaean centres. The discovery of a New Mycenaean palace at site of Aghios Vassilios west of Sparta is a possible contender for the centre controlling the green limestone quarries. The material would have been shipped round Cap Malia, up the Cape of Argos and transported via land to Mycenae, where it was worked to a high finish at site.

Cavanagh also noted that the hard green limestone in the so-called Treasury of Minyas from Orchomenos bears very similar designs to those in the Treasury of Atreus (both tombs also feature a side chamber) – so does this point to an exchange of workmen or craftsmen as well as materials?

Now on to the magic of the corbelled vaulting itself, for which Cavanagh went into an explanation of ‘plastic theory’. The theory provides that a structure adapts itself to small changes in its geometry, and relies on the ‘safe theorem’ – if there exists any line of thrust at equilibrium within the masonry the structure is safe.

The following assumptions must be made:
  • -       stone is not tensile
  • -       stone has great compressible strength
  • -       sliding doesn’t occur
  • -       the mass of the structure stays substantially the same

Cavanagh developed a mathematical model to analyse the ‘line of thrust’ by looking at ‘orange slice’ sections of the Treasury, which is that  r = αd 6 when α is a constant.
 This model also works with the tombs at Dimini and Thoriskos. The amazing thing is that, though Cavanagh has been able to apply this theory retrospectively, we don’t know how the Mycenaean engineers did this as they actually undertook the construction. They must have had a rule of thumb they used when they started the construction, but we don’t know what.

It will be difficult to find out more of how the builders managed their feat given the number of intact tombs discovered to date. So far, only 12 complete tholos tombs have been discovered, 10% of the known tombs.  The other 90% are in varying states of collapse or remains, so Cavanagh’s model can’t be applied to these.

Construction starts off as a drum of stone bricks in rows below ground level (or less commonly at it) and then at some point there is a ‘change point’ and the stone rows start to move in (i.e. decrease in diameter) to form the corbelling. The earth goes up in parallel as it is built. The stones didn’t require an internal structure of wood to hold them in place (as concrete does). Once in position, the inside of the stones were curved and smoothed.  Decoration and embellishments would have been added to the tomb, such as the ceiling of the side chamber, or flanking half columns to the entrance. Then earth was piled up further, and over the top of the structure so that the whole thing ends up underground, very like the Middle Bronze Age tumuli, which would have been visible in the landscape around the site.

Carved green stone half
column flanking
entrance of tomb 
The Treasury of Atreus is a remarkable monument to visit, and to me seems even more impressive now that I have listened to William Cavanagh’s lecture. The apparent simplicity of the structure belies the remarkable engineering involved in its construction.

The Treasury is also remarkable for the wealth of the information it yields about the society of the time. For example, the nature of power in terms of control over land, command of labour, patronage of skilled craftsmen, international exchanges, and the level of wealth of the elite. These things, to some extent, tie in with information known from Linear B.

It deserves to be considered as one of the great domes of the world – not least because, despite the passing millennia and the earthquakes in Greece since its construction, it hasn’t moved: the carefully carved out capstone is still exactly above the diametric centre of the tomb!