CONSTRUCTIONS IN STONE
Stone is a natural material, appearing as rock or ground into small pieces, ultimately sand. As a material, stone is hard to dress, but it is very durable. It is a good conductor of warmth or chill, in a wall it is bad insulation.
Stone constructions can be used as collected, raw (without orthogonal shapes), as broken strata with a constant thickness, as hewn stones in regular shapes, and finally as hewn stones with connecting material.
Lintel is the simplest plain composition. Stone with small dimensions in profile and long length is hard to find. Such elements are very rare. Overhanging is the only possible composition of bridging two posts or walls with smaller elements. Corbelling is a symmetrical overlapping system: a system with vertical stresses only. The result is a false arch.
An arch is composed of hewn stones over a span, with the final element: a topstone. Each stone is hewn in two directions and the composition has vertical and horizontal stresses.
Spatial compositions include a false dome with corbelling (Rovero 2014), and a real dome or cupola with the use of an arch.
Drystone: collected and selected stone
The simplest composition can be built with raw stones, collected on stony terrain. Collection can be the result of clearing fields and pastures or of special collecting for building. Collected stones are without any exact shape: the selection of them is skilled work, as is the building itself. Each stone in the second layer has to 'sit' in the right place, overlapping two in the lower course.
Corbelling, as a spatial composition with horizontal courses of raw stones, is professionalism at the highest level. It can be found in prehistory: tombs between Saudi Arabia and Yemen are from the fifth millennium BC (Steimer 2001).
Drystone of broken strata
Stone elements of thin strata, broken in quarries, enable elements with the same thickness, which assure better constructions with a controlled shape. Raw stone cannot compose even walls or exact curves, stones with set dimensions can.
Rough stone courses have to be cut into smaller pieces. The stone is broken into the same thickness, but can be cut in optional dimensions and shapes. Hewing also allows additional elements, such as lintels, pinnacles or benches.
Constructions in hewn stone
Compositions made of stones with controlled thickness have regular building surfaces: vertical walls, precise roofs with pinnacles, also openings.
Buildings with controlled elements can be made more quickly and easily.
Stone constructions with binding material
Drystone can be fixed with other materials. Thin stone wedges can be used for levelling, and clay or mortar on the wall surfaces.
Small stone wedges are used only for levelling horizontal layers, not for construction. This system makes work easier.
Some more pretentious buildings in drystone use clay for the final covering, in whole or partially, as filler between stones on the front only. This is done for sanitary reasons: spaces between stones are ideal for nesting insects.
The arch and dome
Simple building construction starts, with a menhir: but this is not yet architecture. A single post cannot serve as architecture. A row of menhirs can show something more: direction or geometrical shapes as the first idea of an object with several purposes.
Two menhirs, posts or vertical plates, covered by a lintel, is a dolmen. Architecturally, this is the first shelter: covered and protected from three sides: left, right and above. Several dolmens can compose a room: a corridor, but it is narrow and long, mostly usable as a tomb in prehistoric monuments in Sardinia and in Brittany.
The next construction is corbelling, made of horizontal courses of stones, positioned one over another. The principle can be seen as a flat construction in cross section. The simplest usable object made in corbelling is a longitudinal corbelled construction. This can be seen in the grave cells of the Egyptian pyramids and in some wells in Spain.
The final composition of corbelling is a false dome with a square or circular ground plan. The most important thing is that it can be built without any scaffolding – just with hands and brains.
The Etruscans invented the arch in the second millennium BC. It opens possibilities for wider compositions. such as corbelling and, finally, a real dome.
An arch needs solid walls because of horizontal stresses. Important elements of this construction are two imposts, hewn stones and the topstone. All the stones are cut at least on two sides. The topstone is bigger, heavier and always ornamented: it shows its important role. The imposts are visible, but not so richly ornamented as the topstone.
All arch constructions need scaffolding and the most critical moment is with the removal of the scaffolding – when the quality of the composition can be proven.
The final composition using the principle of an arch, is a dome or cupola. It is an arch in three dimensions, rotating around the central vertical axis. The first dome was the Roman Pantheon, which was completed in the first century AD.
It is important to note the historical facts: the arch was invented by the Etruscans.The Romans conquered them and destroyed the nation in the first millennium BC, but took over their invention, which was developed in three dimensions.
All such constructions need solid walls. Walls can be built of massive stones or stone plates, and can be constructional or merely decorative. Stones can be built into walls as a dry construction or can be bound with other materials: some pieces of stones as wedges, sand or mortar, finally with concrete.