The use of mortar with opus quadratum is a sign of a comparatively early date. It occurs, e.g. in the “Servian” wall on the Aventine and in the Tabularium. Under the Empire massive blocks, whether of tufa, travertine or marble, are set without any Mortar. mortar. It must, however, be observed that in these early instances the “mortar” is but a thin stratum of lime, little thicker than stout paper, used not as a cement to bind the blocks together, but simply to give the joints a smoothly fitting surface. The actual binding together was done by clamps and dowels, as well as by the mass and weight of the great blocks used. Except in the Clamps. earliest masonry, each block was very carefully fastened, not only to the next blocks on the same course, which was done with double dove-tailed dowels of wood, but also to those above and below with stout iron clamps, run with lead (Vitr. ii. 8).[1] In more ornamental marble work bronze clamps were often used. Concrete is rarely found in connexion with opus quadratum; part of the “Servian” wall on the Aventine received a backing of concrete at a relatively late period. Up to the 1st century B.C. it was faced with opus incertum—small irregularly shaped blocks of tufa, 3 to 6 in. across, with pointed ends riven into the concrete while it was soft, and worked smooth on the face only (see fig. 2). From the beginning Opus reticulatum. of the 1st century B.C. opus reticulatum,[2] formed of rectangular tufa prisms laid in a regular pattern like a net (whence the name), is found. It is very neat in appearance, and is often fitted with great care, though it was generally covered with stucco. The so-called “house of Livia” on the Palatine is a good example of the earlier sort, when the quoins were made of small rectangular blocks of tufa. Under the Empire brick quoins came into use (as may be seen, e.g. in the so-called palace of Caligula). Though in Rome opus reticulatum was almost always made of tufa, in the neighbourhood of the city it was sometimes of peperino or even lava, where these materials were found on the spot.
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Fig. 2.—Concrete Wall faced with (A) Opus Incertum and (B) Opus Reticulatum. C shows the section, similar in both. |
Fig. 3.—Section of Concrete Wall, showing the use of bricks merely as a facing. |
Of concrete walls faced with burnt bricks no dated example earlier than the middle of the 1st century B.C. is The facing consisted at first of triangular fragments of tiles (tegulae), broken for the purpose and more or less irregular in shape and size, but from the latter part of the 1st century A.D. Brick facing. onwards triangular bricks were specially manufactured for wall-facings. This shape was adopted in order to present a large surface on the face with little expenditure of brick, and also to improve the bond with the concrete behind (see fig. 4). Even party walls of small rooms are not built solid, but have a concrete core faced with brick triangles about 3 in. long. In order to support the facing until the concrete was set, the Roman builders used a wooden framing covered with planks on the inside. Sometimes the planks were nailed outside the wooden uprights, as was done with unfaced concrete walls, and then a series of grooves appear in the face of the brickwork. Walls faced with opus reticulatum must have been supported temporarily in the same way.
The character of the brick facing is a great help towards determining the date of Roman buildings. In early work the bricks are thick and the joints thin, while in later times the reverse is the case, so that brickwork of the time of Severus and later has more bricks to the foot than that of the Flavian period.
The length of the bricks as it appears on the face is no guide to the date, since one or more of the sharp points of the brick triangles were frequently broken off before they were used. Moreover, varieties both in quality of workmanship and size of the bricks often occur in work of the same date. In the remains of Nero's Golden House great varieties appear, and some of the walls in the inferior rooms are faced with very irregular and careless brickwork.[3] Special care and neatness were employed in the rare cases when the wall was not to be covered with stucco, which in the absence of marble was usually spread over both inside and outside walls. All these circumstances make great caution necessary in judging of dates; fortunately after the 1st century A.D., and in some cases even earlier, stamps impressed on bricks, and especially on the large tiles used for arches, give clearer indications. The reason of the almost universal use of smooth facings either of opus reticulatum or of brick over concrete walls is a very puzzling question; for concrete itself forms an excellent ground for the stucco coating or backing to the marble slabs, while the stucco adheres with difficulty to a smooth facing, and is very liable to fall away. The modern practice of raking out the joints to form a key was not employed by the Romans, but before the mortar was hard they studded the face of the wall with marble plugs and iron or bronze nails driven into the joints, so as to give a hold for the stucco—a great waste both of labour and material.[4] The quality of the mortar varies according to its date: during the 1st and 2nd centuries it is of remarkable hardness—made of lime with a mixture of coarse pozzolana of a bright red colour; in the 3rd century it began to be inferior in quality; and the pozzolana used under the later Empire is brown instead of red.
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Fig. 4.—Example of Marble Lining, from the Cella of the Temple of Concord. A. Slabs of Phrygian marble. B. Plinth moulding of Numidian “giallo.” C. Slab of cipollino (Carystian marble). D. Paving of porta santa. E and F. “nucleus” and “rudus” of concrete bedding. G, G. Iron clamps run with lead to fix marble lining. H. Bronze clamp. J. Cement backing. |
Concrete was at first always made of lumps of tufa; then travertine,
lava, broken bricks and even marble were used, in fact all
the chips and fragments of the mason's yard. Under
the Empire the concrete used was made with travertine
or lava for foundations, with tufa or broken bricks for
Concrete walls
and vaults.
walls, and with tufa or pumice-stone (for the sake of
lightness) for vaults. Massive walls were cast in a mould; upright
timbers, about 6 by 7 in. thick and 10 to 14 ft. long, were set in
rows on each face of the
future wall; planks 9 to
10 in. wide were nailed
to them, so as to form a
case, into which the semi-fluid
mass of stones,
lime and pozzolana was
poured. When this was
set the timbers were removed
and refixed on the
top of the concrete wall;
then fresh concrete was
poured in; and this process
was repeated till the
wall was raised to the
required height. Usually
such cast-work was only
used for foundations and
cella walls, the upper
parts being faced with
brick; but in some cases
the whole wall to the top
was cast in this way and
the brick facing omitted.
In strength and
durability no masonry,
however hard the stone or
large the blocks, could
ever equal these walls of
concrete when made with
hard lava or travertine,
for each wall was one
perfectly coherent mass,
and could only be
destroyed by a laborious
process like that of
quarrying hard stone from its native bed. Owing to this method
of building the progress of the work from day to day can often be
traced by a change in the look of the concrete. About 3 ft. appears
to have been the average amount of wall raised in a day.
Marble linings were fixed very firmly to the walls with long clamps of metal, hooked at the end so as to hold in a hole made in the marble slab. Fig. 4 gives an example, of the time of Marble Augustus, fixed against a stone wall. The blocks were usually marked in the quarry with a number, and often Marble linings, &c. with the names of the reigning emperor and the overseer of the quarry. These quarry-marks are often of great value as indications of the date of a building or statue.[5] Metropolitan
- ↑ The expansion of the iron through rust, which caused the stone to split, has frequently been a great source of injury to Roman walls, as well as the practice, common in the middle ages, of breaking into the stones in order to extract the metal.
- ↑ These two kinds of stone facings are mentioned thus by Vitruvius (ii. 8), “reticulatum, quo nunc [reign of Augustus] omnes utuntur, et antiquum, quod incertum dicitur.
- ↑ Some of the bricks are as much as 212 in. thick, while 112 in. is the usual maximum for Roman bricks.
- ↑ The Roman method of applying stucco to walls with a wooden “float” exactly as is done now, is shown in a painting from Pompeii (see Ann. Inst., 1881, pl. H.).
- ↑ See Bruzza, in Ann. Inst. (1870), pp. 106-204; Hirschfeld, Die kaiserlichen Verwaltungsbeamten (1905), pp. 162 ff.
