Due to its abundance and the ease with which it can be extracted and cut, tuff has been one of the most used materials in the construction of public and private buildings in many regions of Italy, at least since the Etruscans and the Romans.

In Rome, seven qualities of volcanic tuff have been identified:[6]​[7][8]​.

to which we must add travertine (calcareous tuff).

In Pompeii and around Naples, there are three types of volcanic tuff:[6]​.

Volcanic tuff, formerly called "lithoid tuff", is found on the Capitoline Hill and several other hills in the interior of Rome. On the outskirts of the city it is mined at Monteverde, Pont Nomentano, Torre Pignatara outside the Porta Maggiore, Ardée and along the Via Ardeatina.

Vitruvius advised extracting these tuffs in summer and leaving them exposed for two years,[9]​ thus reducing the internal humidity of the stones and eliminating those that were not resistant.[6]​

Its color is reddish or yellowish brown with orange specks. The texture of this stone, quite resistant, is similar to that of pumice stone. It has been widely used in construction throughout Rome, in blocks of various sizes. In the time of the kings, it was used to build the Cloaca Maxima arcade and the walls of Servius. Sometimes it was cut into small, narrowly squared oblong cubes. With materials of this type, the walks of the Marcellus theater and the walls of the basilicas built in the time of Constantine were built[10].

In the Middle Ages, this stone, cut in the same way, was found in so-called Saracen constructions, such as the tower that forms the corner of the senatorial palace of the Capitol, the walls of the Leonese city, those of the "castrum Caetani" near the tomb of Cæcilia Metella and, finally, some of the repairs carried out on the wall of Aurelian. It was even used during the Renaissance, when the façade of the palace in Venice and the Rocca Pia in Tivoli, near Rome, were built with this stone.[10]​.

Volcanic tuff has been mined inside Rome, even in modern times. The Aventine Hill quarry, near Santa Prisca, provided some of the stones used to build the Braschi palace.[10]​.

The ancients called this tuff saxum quadratum, lapis quadratus, and sometimes tophus ruber, saxum rubrum, and saxum rubrum quadratum: these various names referred only to the volcanic tuff, which they usually used in square pieces. Therefore, it would be a mistake to assume that Vitruvius and Livy were referring with these expressions to any type of square stone other than that resembling the volcanic tuff of the Capitol and Monteverde.

In 1850, the tuff most commonly used came from quarries located outside the gates of San Paolo, San Lorenzo and San Sebastiano.[10]​.

Base material: volcanic ash

The material expelled in a volcanic eruption can be classified into three types:

Tephra is formed when the magma inside the volcano explodes due to the rapid expansion of hot volcanic gases. Magma usually explodes when the gas dissolved in it dissolves as the pressure decreases as it flows toward the surface. These violent explosions produce particles of material that can fly out of the volcano. Solid particles less than 2 mm in diameter (the size of sand or smaller) are called volcanic ash.[11]​[12]​.

Volcanic ash is further divided into fine ash, with particles of size less than 0.0625 mm in diameter, and coarse ash, with particles of size between 0.0625 mm and 2 mm in diameter. The tuff is further divided into coarse tuff (coarse ash tuff) and fine tuff (fine ash tuff or dust tuff). Consolidated tephra composed primarily of coarser particles is called lapillistone (particles 2 mm to 64 mm in diameter) or agglomerate or pyroclastic breccia (particles greater than 64 mm in diameter) rather than tuff.[12]​.

The composition of volcanic ash can vary greatly, so tuffs are in turn classified by the composition of the ash from which they formed. Ashes from volcanism with high silica content, especially in ash flows, are mainly formed by volcanic glass fragments,[13][14]​ and tuff formed predominantly by glass fragments is described as vitreous tuff.[15]​ Glass fragments are usually irregular in shape or approximately triangular with convex sides. They are the broken walls of countless small bubbles that formed in magma when dissolved gases quickly dissolved.[16]​.

Tuffs formed from ash consisting predominantly of individual crystals are described as crystalline tuffs, while those formed from ash consisting predominantly of pulverized rock fragments are described as lithic tuffs.[15]​.

The chemical composition of volcanic ash reflects the full range of volcanic rock chemistry, from high-silica rhyolitic ash to low-silica basaltic ash, and tuffs are equally described as rhyolitic, andesitic, basaltic, etc.[17]​.

Transportation and lithification

The most direct way volcanic ash moves away from the chimney is in the form of ash clouds that are part of an eruptive column. These fall to the surface in the form of precipitation deposits that are usually well classified and tend to form a mantle of uniform thickness on the ground. Column collapse gives rise to a more dramatic and destructive form of transport, taking the form of pyroclastic flows and surges that are generally poorly sorted and accumulate in low-lying terrain. Sometimes, surge deposits present sedimentary structures typical of high-velocity flows, such as dunes and antidunes.[18] Volcanic ash already deposited on the surface can be transported as mud flows (lahars) when mixed with water from precipitation or eruption into a mass of water or ice.[19]​.

Volcanic ash particles that are hot enough will weld together after settling on the surface, producing a welded tuff. Welding requires temperatures above 600°C (1,100°F). If the rock contains scattered fragments the size of a pea or fiamme, it is called welded lapilli-tufa. Welded tuffs (and welded lapilli) can be of throwing origin, or deposited from ash flows, as in the case of ignimbrites.[20]​ During welding, glass and pumice fragments adhere to each other (at point contacts), deform and compact, giving rise to a eutaxitic fabric[21]​ Welded tuffs are commonly of rhyolitic composition, but examples of all are known. the compositions[22][23]​.

An ash flow sequence may consist of multiple cooling units. These can be distinguished by the degree of welding. The base of a cooling unit is usually unwelded due to cooling of the underlying cold surface, and the degree of welding and side reactions of fluids in the flow increases toward the center of the flow. Welding decreases toward the top of the refrigeration unit, where the unit cools more quickly. Welding intensity can also decrease towards areas where the deposit is thinner, and with distance from the source[20].

Colder pyroclastic flows do not weld and the ash sheets deposited by them are relatively unconsolidated.[21] However, cooled volcanic ash can lithify rapidly because it usually has a high volcanic glass content. It is a thermodynamically unstable material that reacts quickly with groundwater or seawater, leaching alkali metals and calcium from the glass. New minerals crystallize from the dissolved substances, such as zeolites, clays and calcite, which cement the tuff.[24]​.

Tuffs are further classified by their depositional environment, such as lacustrine tuff, subaerial tuff, or submarine tuff, or by the ash transport mechanism, such as precipitation tuff or ash flow tuff. Reworked tuffs, formed by erosion and redeposition of ash deposits, are usually described by the transport agent, as aeolian tuff or fluvial tuff.[25]​.

Due to its abundance and the ease with which it can be extracted and cut, tuff has been one of the most used materials in the construction of public and private buildings in many regions of Italy, at least since the Etruscans and the Romans.

In Rome, seven qualities of volcanic tuff have been identified:[6]​[7][8]​.

to which we must add travertine (calcareous tuff).

In Pompeii and around Naples, there are three types of volcanic tuff:[6]​.

Volcanic tuff, formerly called "lithoid tuff", is found on the Capitoline Hill and several other hills in the interior of Rome. On the outskirts of the city it is mined at Monteverde, Pont Nomentano, Torre Pignatara outside the Porta Maggiore, Ardée and along the Via Ardeatina.

Vitruvius advised extracting these tuffs in summer and leaving them exposed for two years,[9]​ thus reducing the internal humidity of the stones and eliminating those that were not resistant.[6]​

Its color is reddish or yellowish brown with orange specks. The texture of this stone, quite resistant, is similar to that of pumice stone. It has been widely used in construction throughout Rome, in blocks of various sizes. In the time of the kings, it was used to build the Cloaca Maxima arcade and the walls of Servius. Sometimes it was cut into small, narrowly squared oblong cubes. With materials of this type, the walks of the Marcellus theater and the walls of the basilicas built in the time of Constantine were built[10].

In the Middle Ages, this stone, cut in the same way, was found in so-called Saracen constructions, such as the tower that forms the corner of the senatorial palace of the Capitol, the walls of the Leonese city, those of the "castrum Caetani" near the tomb of Cæcilia Metella and, finally, some of the repairs carried out on the wall of Aurelian. It was even used during the Renaissance, when the façade of the palace in Venice and the Rocca Pia in Tivoli, near Rome, were built with this stone.[10]​.

Volcanic tuff has been mined inside Rome, even in modern times. The Aventine Hill quarry, near Santa Prisca, provided some of the stones used to build the Braschi palace.[10]​.

The ancients called this tuff saxum quadratum, lapis quadratus, and sometimes tophus ruber, saxum rubrum, and saxum rubrum quadratum: these various names referred only to the volcanic tuff, which they usually used in square pieces. Therefore, it would be a mistake to assume that Vitruvius and Livy were referring with these expressions to any type of square stone other than that resembling the volcanic tuff of the Capitol and Monteverde.

In 1850, the tuff most commonly used came from quarries located outside the gates of San Paolo, San Lorenzo and San Sebastiano.[10]​.

Base material: volcanic ash

The material expelled in a volcanic eruption can be classified into three types:

Tephra is formed when the magma inside the volcano explodes due to the rapid expansion of hot volcanic gases. Magma usually explodes when the gas dissolved in it dissolves as the pressure decreases as it flows toward the surface. These violent explosions produce particles of material that can fly out of the volcano. Solid particles less than 2 mm in diameter (the size of sand or smaller) are called volcanic ash.[11]​[12]​.

Volcanic ash is further divided into fine ash, with particles of size less than 0.0625 mm in diameter, and coarse ash, with particles of size between 0.0625 mm and 2 mm in diameter. The tuff is further divided into coarse tuff (coarse ash tuff) and fine tuff (fine ash tuff or dust tuff). Consolidated tephra composed primarily of coarser particles is called lapillistone (particles 2 mm to 64 mm in diameter) or agglomerate or pyroclastic breccia (particles greater than 64 mm in diameter) rather than tuff.[12]​.

The composition of volcanic ash can vary greatly, so tuffs are in turn classified by the composition of the ash from which they formed. Ashes from volcanism with high silica content, especially in ash flows, are mainly formed by volcanic glass fragments,[13][14]​ and tuff formed predominantly by glass fragments is described as vitreous tuff.[15]​ Glass fragments are usually irregular in shape or approximately triangular with convex sides. They are the broken walls of countless small bubbles that formed in magma when dissolved gases quickly dissolved.[16]​.

Tuffs formed from ash consisting predominantly of individual crystals are described as crystalline tuffs, while those formed from ash consisting predominantly of pulverized rock fragments are described as lithic tuffs.[15]​.

The chemical composition of volcanic ash reflects the full range of volcanic rock chemistry, from high-silica rhyolitic ash to low-silica basaltic ash, and tuffs are equally described as rhyolitic, andesitic, basaltic, etc.[17]​.

Transportation and lithification

The most direct way volcanic ash moves away from the chimney is in the form of ash clouds that are part of an eruptive column. These fall to the surface in the form of precipitation deposits that are usually well classified and tend to form a mantle of uniform thickness on the ground. Column collapse gives rise to a more dramatic and destructive form of transport, taking the form of pyroclastic flows and surges that are generally poorly sorted and accumulate in low-lying terrain. Sometimes, surge deposits present sedimentary structures typical of high-velocity flows, such as dunes and antidunes.[18] Volcanic ash already deposited on the surface can be transported as mud flows (lahars) when mixed with water from precipitation or eruption into a mass of water or ice.[19]​.

Volcanic ash particles that are hot enough will weld together after settling on the surface, producing a welded tuff. Welding requires temperatures above 600°C (1,100°F). If the rock contains scattered fragments the size of a pea or fiamme, it is called welded lapilli-tufa. Welded tuffs (and welded lapilli) can be of throwing origin, or deposited from ash flows, as in the case of ignimbrites.[20]​ During welding, glass and pumice fragments adhere to each other (at point contacts), deform and compact, giving rise to a eutaxitic fabric[21]​ Welded tuffs are commonly of rhyolitic composition, but examples of all are known. the compositions[22][23]​.

An ash flow sequence may consist of multiple cooling units. These can be distinguished by the degree of welding. The base of a cooling unit is usually unwelded due to cooling of the underlying cold surface, and the degree of welding and side reactions of fluids in the flow increases toward the center of the flow. Welding decreases toward the top of the refrigeration unit, where the unit cools more quickly. Welding intensity can also decrease towards areas where the deposit is thinner, and with distance from the source[20].

Colder pyroclastic flows do not weld and the ash sheets deposited by them are relatively unconsolidated.[21] However, cooled volcanic ash can lithify rapidly because it usually has a high volcanic glass content. It is a thermodynamically unstable material that reacts quickly with groundwater or seawater, leaching alkali metals and calcium from the glass. New minerals crystallize from the dissolved substances, such as zeolites, clays and calcite, which cement the tuff.[24]​.

Tuffs are further classified by their depositional environment, such as lacustrine tuff, subaerial tuff, or submarine tuff, or by the ash transport mechanism, such as precipitation tuff or ash flow tuff. Reworked tuffs, formed by erosion and redeposition of ash deposits, are usually described by the transport agent, as aeolian tuff or fluvial tuff.[25]​.