One of the most spectacular sites of Villa Adriana is the Frigidarium of the Great Baths: it has a part of the cross vault suspended in the void, which defies the laws of gravity.
Comparison with an
Eighteenth century engraving by Giovan Battista Piranesi shows that in the eighteenth century it was already in these conditions. It has defied centuries and millennia, resisting bad weather, looting, collapses and earthquakes.
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The suspended vault is proof of the solidity of the opus caementicium, one of the most extraordinary and revolutionary inventions of the Roman construction technique.
Obviously it is "unreinforced" concrete; Vitruvius handed down the "magic formula", a mixture of pozzolana (a natural cement of volcanic origin found in the subsoil of the Villa itself), fragments of tuff or local stone, quicklime and river sand (because it has no salt).
By adding water to the quicklime a chemical reaction was triggered, and then the mixture was poured a little at a time onto a wooden cast. Within a few days
the mixture solidified, becoming almost indestructible, as proved by this extraordinary example.
The Frigidarium was the room for cold baths, which had two large water basins once covered in marble. The columns were in cipollino marble with Ionic capitals.
The vault was decorated with stucco, and in another room of the building part of it has survived. It seems that in the eighteenth and nineteenth centuries the English noblemen of the Grand Tour shot the stuccos to make some fragments fall down, to bring back at home as a precious souvenir.