810 ROME [TOPOGRAPHY AND The use of mortar with onus quadratum is a sign of an early rather than a late date. 1 It occurs in the Servian wall on the Aventine, in the Tabularium.and, most striking of all, in the Tul- **" lianum under the "Mamertine prison" cer- tainly one of the oldest buildings in Rome (see fig. 11). Under the empire massive blocks, whether of tufa, traver- tine, or marble, are set without any 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 to- gether, but simply to give the joints a Fio. 2. Example of construction in which many materials smoothly fitting are used ; upper part of one of the inner radiating walls surface. The actual binding together was done by clamps and dowels, as well as by the mass and weight of the great blocks used. Ex- cept in the ear- liest masonry, each block was under the cunei of the Colosseum. A, A. Marble seats on brick and concrete core, supported on vault made of pumice-stone concrete (C). B. Travertine arch at end of raking vault (C). D. One of the travertine piers built in flush with the tufa wall to give it extra strength. E, E. Wall of tufa concrete faced with triangular bricks, carry- ing the vaults of pumice concrete which support the marble seats. F. Travertine pier at end of radiating wall. G. Brick-faced arch of concrete to carry floor of passage. H, H. Tufa wall, opus quadratum. J, J, J. Line of steps in next bay. K, K. Surface arches of brick, too shallow to be of any constructional use, and not meant for orna- ment, as the whole was stuccoed ; they only face the wall (which is about 4 feet thick) to the average depth of 4 inches. 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). 2 In more ornamental marble work bronze clamps were often used. When concrete was employed it was faced either with blocks of opus quadratum (e.g., the Servian wall along the Aventine) or with opus incertum small irregularly shaped blocks of tufa 3 to 6 inches across, with pointed ends driven into the concrete while it was soft, and worked smooth on the face only (see fig. 3). Thirdly, in the 1st century B.C. opus reticulatum, 3 also of tufa, was largely used alone ; after that it began to be mixed with brickwork. It is very neat in ap- pearance, 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. SECTION OF ANCLE JHM The palace of Caligula has it with Flo . s.-Conerete wall heed with (A) quoins of brick facing. Though in opus incertum and (B) opus reticu- Rome opus reticulatum was always jatum. C shows the section, similar made of tufa, in the neighbourhood in both- of the city it was sometimes of peperino or even lava, where these materials were found on the spot. 1 Choisy (L'Art de bdtir chez let Romains, Paris, 1873) is mistaken in his denial of the early use of mortar by the Romans. 2 The expansion of the iron through rust, which caused the stone to split, has frequently been a great source of injury to Roman walk?, as well as the practice, common in the Middle Ages, of breaking into the stones in order to extract the metal. 8 These two kinds of stone facings are mentioned thus by Vitrnvius (ii. 8), " reticulatum, quo nunc [reign of Augustus] omnes utuntur, et antiquum. quod tncerfwm dicitur." Of concrete walls faced with burnt bricks no dated example Brie earlier than the middle of the 1st century B.C. is known. The facing. Pantheon (27 B.C. ) is the most important early specimen of certain date. The bricks used are always triangular in. shape, so as to present a large surface on the face with little expenditure of brick, and also to improve the bond with the concrete oehind (see fig. 4). Even party walls of small rooms, only 7 inches thick, are not built solid, but have a concrete core faced with brick triangles about 3 inches long. Owing to this method of form- ing the walls it was necessary to support the facing until the concrete was set, which appears to have been done with a wooden framing covered with planks on the inside. In some cases the planks were nailed outside the wooden uprights, as was x a' .x- to Fio. 4. Section of concrete wall, showing the use of bricks merely as a facing. lone with the unfaced concrete walls (see below), and then a series of perpendicular 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 deter- mining the date of Roman buildings ; it has been stated that this can be done simply by measuring the number of brick courses that go to a foot, the more the bricks the earlier the work. This, how- ever, is not the case. In early work the bricks are thick and the joints thin, while in later times the reverse is the case. Thus brickwork of the time of Severus and later usually has more bricks to the foot than that of the Flavian period. The following list gives a few characteristic specimens of different dates. Building. Average thickness of Bricks. Average thickness of Joints. Rostra of Julius Caesar, 44 B.C Inches. H 1} 1J to 1} 1 tolj U 1J 4 IJtolf Inches. Ito J to J to 1 1 tolj fto J IJtoH Pantheon of Agrippa, 27 B.C Pretorian camp of Tiberius, 23 A.D Aqueduct of Nero, c. 62 A.D Baths of Titus, c. 80 A.D Palace of Domitian, c. 90 A,D Hadrian's temple of Venus and Rome, c. 125 A. D. Palace of Severus, c. 200 A.D Aurelian's walls, c. 271 A.D The length of the bricks as it appears on the face is no guide to the date, owing to the fact that one or more of the sharp points of the brick triangles were very 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 ; a new gang of workmen or a batch of bricks from a fresh figlina might easily occasion this. 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. 4 Special care and neatness were always 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 largo 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 con- crete itself forms an excellent ground for the stucco coating or back- ing 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. 5 The quality of the mortar varies according to its date : during the 1st and 2d centuries it is of most remarkable hardness, made of lime with a mixture of coarse pozzolana of a bright red colour ; in the 3d century it began to be inferior in quality ; and the pozzolana used under the later empire is brown instead of red. Concrete was at first always made of lumps of tufa ; then traver- Cone ? tine, lava, broken bricks, and even marble were used, in fact all wall; < the chips and fragments of the mason's yard. Under the empire vaul the concrete used was of travertine or lava mostly for foundations, of tufa or broken bricks for walls, and of tufa or pumice-stone (for the sake of lightness) for vaults. Massive walls were cast in a
- Some of the bricks are as much as 2J inches thick, while 1J inches is the
usual maximum for Roman bricks. B The Roman method of applying stucco to walls with a wooden "float," ex- actly as is done now, is shown in a painting from Pompeii (see Ann. lust., 1881).
