Granite clogs

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Divulgation

The historic buildings in Madrid bear witness to a particular use of granite, one that has so become an invariant of the capital city’s construction over the centuries. We are referring to the socle or plinth, that is, in a building, the inferior section of a wall, which is for that matter in contact with the ground. The Madrid socles are made of generously sized ashlars of grey granite to support a variety of masonries – usually brickwork, either coated with plaster or naked; only occasionally stonework, and, very common in older buildings, a typical type of mixed masonry in which brickwork piers alternate with rubblework fillings.

The socle acts as an interface between the terrain, of an irregular, amorphous nature, and the building system to be erected on it, governed by geometry. As a building element, it performs two essential tasks, that may be characterized as structural and tectonic. In the first place, it is an essential part of the foundations, being that the socle enacts the transmission of the building loads to the ground – as a matter of fact, when these loads are light or the terrain is very firm, the socle alone may become the foundation properly. Secondly, the plinth raises and levels the base of the walls, so that the masonries may be laid over an horizontal surface away from the ground’s dampness.

In a sense, socles are a sort of clogs behind the building, which thanks to them maintains its verticality without sinking into the ground, while preventing the water from coming into contact with the feet of its masonry – the runoff as well as the water contained in the terrain. Prosopopoeia would be similarly in the origin of the term itself, as ‘socle’ is derived from the latin soccuūlus, diminutive of soccus ‘clog’, so that it could be translated as ‘little clog’. As opposed to the illustrious crepidoma or stylobate in the Classical orders, the etymological origin of ‘socle’ alludes to an utilitarian, modest nature.

It is precisely this condition which explains the relative frugality that the Madrid buildings display in the use of granite, typically confined to their lower parts. It is well known that building is a matter of economy: in the case of the Madrid socles, granite was undoubtedly the best material to solve the equation framed by the usual building factors: use, availability, workability, durability and, of course, cost. The considerable distance to the nearer quarries, in the Sierra Norte (a mountain range located some 60-70 km to the north) hindered a more extensive use of granite in the capital city, which however did occur when the budget was not a critical issue, as in a number of official buildings – it suffices to recall impressive granite façades and walls of the Royal Palace. In this regard, from the late 19th century on, the granite is used more lavishly in all sorts of buildings, thus coinciding with the development of the railway network around Madrid – that would have entailed a considerable reduction of transport costs.

Besides economy, it may be thought that it is the hardness of granite, in comparison to that of other building stones, what makes it so apt a material for being used in socles or plinths. In the case of Madrid this isn’t entirely accurate, though: the granite’s great compressive strength alone cannot be deemed determinant, considering that the masonries above (be they brickwork, rubblework, or mixed) as well as the sandy-clay soil below are relatively much softer. It is rather the granite’s excellent performance in the presence of water what explains its being employed as socle. In a similar fashion to the wood in clogs, which, unlike leather or fabrics in other types of shoe, the water cannot soak or permeate, the granite’s special constitution prevents the water from getting through its matter, be it inwards or upwards.

We have seen in a previous post that the granite is formed under extremely high pressures, resulting in a material with a very compact internal structure, and henceforth much less porous than other rocks. ‘Porosity’ may be conceptualized in different ways: the physics of materials refer, for instance, to theoretical porosity, effective porosity, average porosity, hydraulic conductivity, etc. This is related to the fact that there are so many types and sizes of pores as ways into which these may be interconnected. It would not be possible to describe in detail here the granite’s porosity – or, more properly, porosities; it suffices now to say that its pores are very small and distant between each other, resulting in an impervious material by comparison to other stones. The same fact grants the granite a good performance against mechanical weathering by ice-edging, because the water barely penetrates into the material.

The pass of time shows that the election of granite as the building material for socles in Madrid has been more than a lucky one, as proved by the countless examples that we may now enjoy while wandering around.

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