‹‹‹ Vitr. 2.6.5 | Table of Contents | Vitr. 2.7.1 ›››
6Hence, in places where the mountains are not earthy, but of stone, the force of the fire escaping through the chinks burns that which is soft and tender, whilst that which is hard is left. Thus the earth of Campania, when burnt, becomes a powder; that of Tuscany a coal. Both of these are of great use in building, one species being very serviceable in land works, the other in works under water. In Tuscany, however, the quality of the material is softer than sandstone, but harder than earth; and from its entire subjection to the action of the sub-existing fire, it becomes that sort of sand which is called carbunculus.
6Therefore, where the mountains are not earthy but consist of soft stone, the force of the fire, passing through the fissures in the stone, sets it afire. The soft and delicate part is burned out, while the hard part is left. Consequently, while in Campania the burning of the earth makes ashes, in Tuscany the combustion of the stone makes carbuncular sand. Both are excellent in walls, but one is better to use for buildings on land, the other for piers under salt water. The Tuscan stone is softer in quality than tufa but harder than earth, and being thoroughly kindled by the violent heat from below, the result is the production in some places of the kind of sand called carbuncular.