The Ten Books on Architecture
walls can no longer support. Fresh pitsand, however, in spite of all its
excellence in concrete structures, is not equally useful in stucco, the
richness of which, when the lime and straw are mixed with such sand,
will cause it to crack as it dries on account of the great strength of
the mixture. But river sand, though useless in "signinum" on account of
its thinness, becomes perfectly solid in stucco when thoroughly worked
by means of polishing instruments.
CHAPTER V
LIME
1. Sand and its sources having been thus treated, next with regard to
lime we must be careful that it is burned from a stone which, whether
soft or hard, is in any case white. Lime made of close-grained stone of
the harder sort will be good in structural parts; lime of porous stone,
in stucco. After slaking it, mix your mortar, if using pitsand, in the
proportions of three parts of sand to one of lime; if using river or
sea-sand, mix two parts of sand with one of lime. These will be the
right proportions for the composition of the mixture. Further, in using
river or sea-sand, the addition of a third part composed of burnt brick,
pounded up and sifted, will make your mortar of a better composition to
use.
2. The reason why lime makes a solid structure on being combined with
water and sand seems to be this: that rocks, like all other bodies, are
composed of the four elements. Those which contain a larger proportion
of air, are soft; of water, are tough from the moisture; of earth, hard;
and of fire, more brittle. Therefore, if limestone, without being
burned, is merely pounded up small and then mixed with sand and so put
into the work, the mass does not solidify nor can it hold together. But
if the stone is first thrown into the kiln, it loses its former property
of solidity by exposure to the great heat of the fire, and so with its
strength burnt out and exhausted it is left with its pores open and
empty. Hence, the moisture and air in the body of the stone being burned
out and set free, and only a residuum of heat being left lying in it, if
the stone is then immersed in water, the moisture, before the water can
feel the influence of the fire, makes its way into the open pores; then
the stone begins to get hot, and finally, after it cools off, the heat
is rejected from the body of the lime.
3. Consequently, limestone when taken out of the kiln cannot be as heavy
as when it was thrown in, but on being weighed, though its bulk remains
the same as before, it is found to have lost about a third of its weight
owing to the boiling out of the water. Therefore, its pores being thus
opened and its texture rendered loose, it readily mixes with sand, and
hence the two materials cohere as they dry, unite with the rubble, and
make a solid structure.
CHAPTER VI
POZZOLANA
1. There is also a kind of powder which from natural causes produces
astonishing results. It is found in the neighbourhood of Baiae and in
the country belonging to the towns round about Mt. Vesuvius. This
substance, when mixed with lime and rubble, not only lends strength to
buildings of other kinds, but even when piers of it are constructed in
the sea, they set hard under water. The reason for this seems to be that
the soil on the slopes of the mountains in these neighbourhoods is hot
and full of hot springs. This would not be so unless the mountains had
beneath them huge fires of burning sulphur or alum or asphalt. So the
fire and the heat of the flames, coming up hot from far within through
the fissures, make the soil there light, and the tufa found there is
spongy and free from moisture. Hence, when the three substances, all
formed on a similar principle by the force of fire, are mixed together,
the water suddenly taken in makes them cohere, and the moisture quickly
hardens them so that they set into a mass which neither the waves nor
the force of the water can dissolve.
2. That there is burning heat in these regions may be proved by the
further fact that in the mountains near Baiae, which belongs to the
Cumaeans, there are places excavated to serve as sweating-baths, where
the intense heat that comes from far below bores its way through the
earth, owing to the force of the fire, and passing up appears in these
regions, thus making remarkably good sweating-baths. Likewise also it is
related that in ancient times the tides of heat, swelling and
overflowing from under Mt. Vesuvius, vomited forth fire from the
mountain upon the neighbouring country. Hence, what is called
"sponge-stone" or "Pompeian pumice" appears to have been reduced by
burning from another kind of stone to the condition of the kind which we
see.
3. The kind of sponge-stone taken from this region is not produced
everywhere else, but only about Aetna and among the hills of Mysia which
the Greeks call the "Burnt District," and in other places of the same
peculiar nature. Seeing that in such places there are found hot springs
and warm vapour in excavations on the mountains, and that the ancients
tell us that there were once fires spreading over the fields in those
very regions, it seems to be certain that moisture has been extracted
from the tufa and earth, by the force of fire, just as it is from
limestone in kilns.
4. Therefore, when different and unlike things have been subjected to
the action of fire and thus reduced to the same condition, if after
this, while in a warm, dry state, they are suddenly saturated with
water, there is an effervescence of the heat latent in the bodies of
them all, and this makes them firmly unite and quickly assume the
property of one solid mass.
There will still be the question why Tuscany, although it abounds in hot
springs, does not furnish a powder out of which, on the same principle,
a wall can be made which will set fast under water. I have therefore
thought best to explain how this seems to be, before the question should
be raised.
5. The same kinds of soil are not found in all places and countries
alike, nor is stone found everywhere. Some soils are earthy; others
gravelly, and again pebbly; in other places the material is sandy; in a
word, the properties of the soil are as different and unlike as are the
various countries. In particular, it may be observed that sandpits are
hardly ever lacking in any place within the districts of Italy and
Tuscany which are bounded by the Apennines; whereas across the Apennines
toward the Adriatic none are found, and in Achaea and Asia Minor or, in
short, across the sea, the very term is unknown. Hence it is not in all
the places where boiling springs of hot water abound, that there is the
same combination of favourable circumstances which has been described
above. For things are produced in accordance with the will of nature;
not to suit man's pleasure, but as it were by a chance distribution.
6. Therefore, 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.
CHAPTER VII
STONE
1. I have now spoken of lime and sand, with their varieties and points
of excellence. Next comes the consideration of stone-quarries from which
dimension stone and supplies of rubble to be used in building are taken
and brought together. The stone in quarries is found to be of different
and unlike qualities. In some it is soft: for example, in the environs
of the city at the quarries of Grotta Rossa, Palla, Fidenae, and of the
Alban hills; in others, it is medium, as at Tivoli, at Amiternum, or Mt.
Soracte, and in quarries of this sort; in still others it is hard, as in
lava quarries. There are also numerous other kinds: for instance, in
Campania, red and black tufas; in Umbria, Picenum, and Venetia, white
tufa which can be cut with a toothed saw, like wood.
2. All these soft kinds have the advantage that they can be easily
worked as soon as they have been taken from the quarries. Under cover
they play their part well; but in open and exposed situations the frost
and rime make them crumble, and they go to pieces. On the seacoast, too,
the salt eats away and dissolves them, nor can they stand great heat
either. But travertine and all stone of that class can stand injury
whether from a heavy load laid upon it or from the weather; exposure to
fire, however, it cannot bear, but splits and cracks to pieces at once.
This is because in its natural composition there is but little moisture
and not much of the earthy, but a great deal of air and of fire.
Therefore, it is not only without the earthy and watery elements, but
when fire, expelling the air from it by the operation and force of heat,
penetrates into its inmost parts and occupies the empty spaces of the
fissures, there comes a great glow and the stone is made to burn as
fiercely as do the particles of fire itself.
3. There are also several quarries called Anician in the territory of
Tarquinii, the stone being of the colour of peperino. The principal
workshops lie round the lake of Bolsena and in the prefecture of
Statonia. This stone has innumerable good qualities. Neither the season
of frost nor exposure to fire can harm it, but it remains solid and
lasts to a great age, because there is only a little air and fire in its
natural composition, a moderate amount of moisture, and a great deal of
the earthy. Hence its structure is of close texture and solid, and so it
cannot be injured by the weather or by the force of fire.
4. This may best be seen from monuments in the neighbourhood of the town
of Ferento which are made of stone from these quarries. Among them are
large statues exceedingly well made, images of smaller size, and flowers
and acanthus leaves gracefully carved. Old as these are, they look as
fresh as if they were only just finished. Bronze workers, also, make
moulds for the casting of bronze out of stone from these quarries, and
find it very useful in bronze-founding. If the quarries were only near
Rome, all our buildings might well be constructed from the products of
these workshops.
5. But since, on account of the proximity of the stone-quarries of
Grotta Rossa, Palla, and the others that are nearest to the city,
necessity drives us to make use of their products, we must proceed as
follows, if we wish our work to be finished without flaws. Let the stone
be taken from the quarry two years before building is to begin, and not
in winter but in summer. Then let it lie exposed in an open place. Such
stone as has been damaged by the two years of exposure should be used in
the foundations. The rest, which remains unhurt, has passed the test of
nature and will endure in those parts of the building which are above
ground. This precaution should be observed, not only with dimension
stone, but also with the rubble which is to be used in walls.
CHAPTER VIII
METHODS OF BUILDING WALLS
1. There are two styles of walls: "opus reticulatum," now used by