The old exclamation about Augustus finding Rome of brick and leaving
it of marble, deceives many. Ancient Rome was by no means a marble
city, although the quarries of Massa and Carrara are not far distant.
The staple-building materials of the Imperial City, even in its
palmiest days, were brick and travertine. The brick, however, was very
different from the porous cakes of crudely burnt clay of which the
modern metropolis of the world is built. I have examined on the spot a
great many specimens, and found them all to be of remarkably compact
structure, somewhere between the material of modern terra-cotta and
that of common flower-pots, and similarly intermediate in color. The
Roman builders appear to have had no standard size; the bricks vary
even in the same building—the Coliseum for example; all that I have
seen are much thinner than our bricks—we should call them tiles.
But the most characteristic material is the travertine. The walls
of the Coliseum are made up of a mixture of this and the tiles
above-mentioned. The same is the case with most of the other very
massive ruins, as the baths, etc. Many of the temples with columns and
facing of marble have inner walls built of this mixture, while others
are entirely of travertine.
I was greatly surprised at the wondrous imperishability of this
remarkable material. In buildings of which the smooth crystalline
marble had lost all its sharpness and original surface, this dirty,
yellow, spongy-looking limestone remained without the slightest
indication of weathering. A most remarkable instance of this is
afforded by the temple of Neptune at Paestum, in Calabria. This is
the most perfect ruin of a pure classic temple that now remains in
existence, and in my opinion is the finest. I prefer it even to the
Parthenon.
We have a little sample of it in London. The Doric columns at the
entrance of the Euston station are copies of those of its peristyle.
The originals are of travertine, the blocks forming them are laid
upon each other without mortar or cement, and so truly flattened
that in walking round the building and carefully prying, I could find
no crevice into which a slip of ordinary writing paper, or the blade
of a pen-knife could be inserted. Yet this temple was an antiquarian
monument in the days of the Roman emperors.
The rough natural surface of the stone is exposed, and at first sight
appears as though weathered, but this appearance is simply due to its
natural sponge-like structure. It appears to have been coated with
some sort of stucco or smoothing film, which, either by forming a thin
layer, or possibly by only filling up the pores of the travertine, gave
a smooth surface upon which the coloring was applied. This is now only
indistinctly visible here and there, and if I remember rightly, some
have disputed its existence.
But this travertine, though so familiar to the Italian, is such
a rarity here that some further description of its structure and
composition may be demanded. It is a limestone formed by _chemical_
precipitation. Most limestones are more or less of organic origin, are
agglomerations of shells, corals, etc., but this is formed by the same
kind of action as that which produces the stalactites in limestone
caverns. It has some resemblance to the incrustation formed on boilers
by calcareous water. Although the material of so many ancient edifices,
it is, geologically speaking, the youngest of all the hard rocks. Its
formation is now in progress at some of the very quarries that supplied
Imperial Rome.
On the Campagna, between Rome and Tivoli, is a small circular lake,
from which a stream of tepid water, that wells up from below, is
continually flowing. Its local name is the “The Lake of Tartarus.”
The water, like that of Zoedone, or soda-water or champagne, is
supersaturated with carbonic acid that was forced into it while under
pressure down below. This carbonic acid has dissolved some of the
limestones through which the subterranean water passes, and when it
comes to the surface, the carbonic acid flies away like that which
escapes when we uncork a bottle of soda-water, though less suddenly,
and the lime losing its solvent is precipitated, and forms a crust on
whatever is covered by the water.
When I visited this lake in the month of February it was surrounded by
a _chevaux de frise_ of an extraordinary character; thousands of tubes
of about half an inch to one inch in diameter outside, with calcareous
walls about one eighth of an inch in thickness. These were standing up
from two to three feet high, and so close together that we had to break
our way through the dense palisade they formed in order to reach the
margin of the lake. After some consideration and inquiry, their origin
was discovered. They are the encrusted remains of bullrushes that had
flourished in the summer and died down since. During the time of their
growth the water had risen, and thus they became coated with a crust of
compact travertine. This deposition takes place so rapidly that a piece
of lace left in the lake for a few hours comes out quite stiff, every
thread being coated with limestone. Such specimens, and twigs similarly
covered, are sold to tourists or prepared by them if they have time to
stop. Sir Humphry Davy drove a stick into the bottom of the lake and
left it standing upright in the water from May to the following April,
and then had some difficulty in breaking with a sharp pointed hammer
the crust formed round the stick. This crust was several inches in
thickness. That which I saw round the ex-bullrushes may have all been
formed in a few days or weeks. The rivulet that flows from the lake
deposits travertine throughout its course, and when it overflows leaves
every blade of grass that it covers encrusted with this limestone.
Near to the Lake of Tartarus is the _Solfatara_ lake which contains
similar calcareous water, but strongly impregnated with sulphureted
hydrogen; it consequently deposits a mixture of carbonate and sulphide
of calcium, a sort of porous tufa, some of it so porous that it floats
like a stony scum, forming what the cicerone call “floating islands.”
Lyell, in his “Principles of Geology,” confounds these lakes, and
describes Tartarus under the name of Solfatara.
The travertine used as a building stone is chiefly derived from the
quarries of Ponte Lucano, and is the deposit that was formed on the bed
of a lake like that of Tartarus. The celebrated cascade of the Anio
at Tivoli forms calcareous stalactites, and all the country round has
rivulets, caverns, and deposits, where this formation may be seen in
progress or completed.
It varies considerably in structure, some specimens are compact and
smooth, others have the appearance of a petrified moss, and great
varieties may be found among the materials of a single building. It
is, however, usually rough and more or less spongy-looking, as above
stated, but this structure does not seem to affect its stability, at
least, not in the climate of Italy. Whether it would stand long frosts
is an open question. The night frosts at and about Rome are rather
severe, but usually followed by a warm sunny day; thus there is no
great penetration of ice.
Every specimen I have examined shows a remarkable compactness of
_molecular_ structure in spite of visible porosity. All give out
a clear metallic ring when struck, and the intimate surface, if I
may so describe the surface of the warm-like structure it sometimes
displays, is always clear and smooth as though varnished. To this I
attribute its durability. Lest the above description should appear
self-contradictory, I will explain a little further. If melted glass
were run into threads, and those threads while soft were allowed to
agglomerate loosely into a convoluted mass, it would, as regarded in
mass, have a porous or spongy-looking structure, but nevertheless its
_molecular_ structure would be compact and vitreous; there would be
mechanical but not molecular, porosity. Travertine is similar.
Have we any travertine in England? This is a practical question of some
importance, and one to which I have no hesitation in replying, Yes.
There is plenty formed and forming in the neighborhood of Matlock,
but that which I have seen on the face of caverns, etc., is not so
compact and metal-like as the Italian. This, however, does not prove
the entire absence of the useful travertine. Not having any commercial
interest in the search, I have only looked at what has come in my way,
but have little doubt that there are other kinds besides those I saw.
I have also seen travertine in course of formation in Ireland, where I
think there is a fine field for exploration in the mountain limestone
regions, which have been disturbed by volcanic action of the Miocene
period. The travertines of Italy are found in the neighborhood of
extinct volcanoes.
The classic associations of this material, its remarkable stability,
and the faculty with which it may be worked, render it worthy of more
attention than it has yet received from British builders.