Suppose we want to discover what makes the clay of Impruneta so remarkable for the production of terra cotta. To do so, we have to look back on the rich and ancient history of the region.
Our voyage would begin about two hundred million years ago, in the middle of what is now the Tyrrhenian Sea. At the bottom of the Ocean of Tethys, thousands of meters deep, with extreme temperature and pressure conditions, where residues of the immersed surrounding lands' disintegration were accumulating. Coastal plains periodically invaded by tides, which bordered the east and south areas with landscapes similar to today's tropical regions, with calcareous and evaporitic deposits.
Movements of the earth's crust then caused deep lacerations through which magma could reach the surface of the sea, increasing the temperature of the water and contributing to the transformation of the rocks.
It is now recent history, a little more than twenty million years ago when two strips of the earth's crust began to roll towards the east to the point Corsica and Sardinia are located today.
For this reason, all the materials accumulated at the bottom of the sea began to compress themselves, forming blocks and scales that overlapped to create a new mountain chain: the birth of the Apennines.
Masses of twisted layers were displaced by enormous amounts of malleable sediment involved in orogenesis. It was soils of this type and development that formed a large part of our territory, conditioning, because of their very nature, the morphology of every single slope, and every single embankment studied for years to understand the complexity of the events that occurred. It is challenging for the geologists who intend to decipher this puzzle since they must try to penetrate what remains today, still visible with difficulty – unsettled and uneven masses of rocks still in movement, as recurrent landslides and earthquakes demonstrate. One understands why these geological units were charted using vague and generic definitions such as chaotic and undifferentiated as proof of how difficult it was to catalog them until a few years ago.
Even at a microscopic level, heterogeneity is the characteristic of this clay taken from the excavation and shattering of the rock. It can only be worked on during the summer months, treating it with continuous cycles of sunshine and sifting.
In this way, the larger stone blocks can be progressively removed, refining and dehydrating the raw material more and more before work in the furnaces. In this way, the shale becomes the clay that was deposited in the depths of the ocean more than two hundred million years ago but enriched, after the long journey, by the minerals of the many rocks that had formed in the meantime.
This remarkable genesis produces the best quality in the finished product through the various physical and chemical transformations of the mineral components under the effect of the temperature during the passage from raw material to a stable brick. It is only because of its particular mineralogical mix that cotto assumes its well-known mechanical characteristics, which render it unalterable to atmospheric agents and abrasion, qualities that have always been appreciated and valued for various uses as a building material.
The specific area of production of Ferrone, with its continuous outcrops of such particular clays and the presence of a rich course of water like the Greve river, was also able to develop because of other specific elements of geography history.
Already in Roman times, the old settlement was located along the route of the Hadrian Cassia, which ran into the Volterra road and consented rapid access towards Florence.
In particular, it was precisely the expansion of that city and the demographic and economic growth of its countryside with its splendid villas in the Renaissance period that later represented the reasons for constant development of the sector in terms of quantity and quality.
Therefore, it was almost inevitable that even Brunelleschi, called in for the construction of the Duomo of Florence, went as far as the ridge of San Giusto in Monterantoli to discover a marble of a particular reddish color which would embellish the façade of Santa Maria del Fiore. But probably not before having visited some of the furnaces situated along the Greve river, a few kilometers from there, to commission the bricks and tiles to cover his splendid Dome, which he then had constructed under his direct control. One can venture to say that the essential component that renders Florentine terracotta so special is an immaterial component such as time. One can get dizzy thinking that more than two hundred million years had to pass so that the deposits accumulated at the bottom of the ocean returned to the surface. It is only thanks to this very long interval that these materials were able to enrich themselves and modify themselves in terms of mineralogy and petrography.
It is precisely thanks to the time that one can exalt the quality of terracotta to the maximum – through its passage in the furnace, it finds the nature and essence of the rocks that it derives from.
To verify this, it is not essential to go to Brunelleschi's Dome. Fortunately, there are still some uninhabited ruins. It is here that one must look for our bricks and our tiles, the old pots at the entrance of the house, even more beautiful after decades of abandonment under the snow or under the sun, just waiting for time to pass.
Reference: Pavimenti e Prodotti in cotto by Manetti Gusmano & Figli