The Corbel arches over the Trabeate

In the earlier piece, we had tried to trace the evolution of the post and lintel or beam design, known as the trabeate – something that has acquired the status of almost a universal architectural device for building, doors and windows.

We had also referred to the mayhem that was constantly caused by collapsing lintels or beams that carried roofs and walls with them. It is this tendency of the beam to crack and shatter that eventually led to the evolution of the technique of corbelling.

A corbelled arch was much like a trabeate, except that in order to prevent the load-bearing beam or lintel from cracking under the overhead weight, it was supported with two, three or four tiers of large stone slabs, inserted between the posts and the beam in such a way that one end was buried inside the wall in which the door was to be constructed, while the other end projected towards the opposite post, an inverted staircase kind of shape was thus created with each succeeding tier reducing the distance between the two posts and the top most tier became almost like a second beam, with just a small gap separating the two slabs. More than words the images of both the trabeate and the corbelled arch will help understand the form better.

The evolution of the corbelled arch was to greatly influence architecture all over the ancient world. It is difficult to say which civilisation was the first to use the form of the trabeate or that of the corbel because both have been in use in different parts of the world, at times during the same period in history at others separated by centuries.

One finds the use of the trabeate among civilisations that did not seem to have any contact with each other — like the builders of the Stonehenge 3100 BCE and the Greeks, whose first contact with the British Isles was in 1300 BCE or thereabouts. And both built trabeated structures, as did many other people. One finds the same pattern of use in the case of the corbelled arch, it is in use in almost all ancient civilisations at the same or at different times and just as was the case in the use of the trabeate we find that many of these civilisations did not have any contacts with each other.

How is it that they found the same solution to the problem of cracking beams all over the world is an issue that needs some more studies and we should not be surprised if we find a common link in our ancient past, just as anthropologists and genetic scientists have zeroed in on one ancestor who lived in Africa and was the mother of all Homo Sapiens.

Despite being a great improvement on the trabeate and the fact that it made it possible to build without fear of the roof collapsing, it was limiting and it was not possible to make really huge doors, also it relied mostly on large blocks of stone and the technique was used extensively as long as builders relied on huge blocks of stone fitted onto each other through what has been called the Pin and Hole method. Small sockets were carved in one layer of stones, small projections in the next layer filled the holes and the sheer weight of rocks kept them from moving even when earth quakes struck, that is how we built in India as well and rarely used any binding material.

Egyptians, the builders of the awe inspiring pyramids were not familiar with the slow drying, even slower maturing limestone mortar used by the Greeks, the Egyptians used a mixture of gypsum and quartz with small quantities of lime, this mortar could only work in the extreme dry conditions of the desert, the structures built using this mortar would have collapsed in the kind of rain that other parts of Africa, south of the desert lands or even on the banks of the Nile.

The next big step in the evolution of architecture was the discovery of the hardening quality of limestone once it was soaked in water and allowed to dry. The mixing of lime stone and brick dust that could have happened by accident or as a result of trial and error testing, led to the creation of a very stable mortar once it had dried.

Once again it is difficult to say who was the first civilisation to use it, but lime stone plaster was being used by ancient Greeks as also by Europeans. This brick and lime stone mortar was introduced to India by the Turks, they also brought with them the technology of building with rubble and it is to the Turks and Central Asians that we ought to be grateful for the next big leap in construction technology. We talk of that and more in the next piece of this mini-series.

The author is a historian, and organises the Delhi Heritage Walk for children and adults