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The dating of the masonry of Ostia

Dating the masonry of Ostia and Portus is obviously essential, but during the last decades it has driven many archaeologists to despair.[1] For the harbours, the work by the following archaeologists is fundamental:

- Italo Gismondi and Herbert Bloch wrote summaries for the first volume in the series Scavi di Ostia.
- Herbert Bloch. Publications about brick stamps.
- Marion Elizabeth Blake. Publications about masonry from the earliest time to the Antonine Emperors. The work was interrupted by her demise.
- Giuseppe Lugli. A book about masonry from the earliest time to late antiquity.
- Thea Heres. A book about masonry from 200 AD to 640 AD, supplemented by many articles and contributions to other books.
- Margareta Steinby and Tapio Helen. Studies about brick stamps.
- Janet DeLaine. Studies about the economics of building techniques.

The description of the masonry is fraught with difficulties, because of the obvious problem of saying with words what is seen and perceived. What one archaeologist calls "an irregular row of brown tufa stones", may be a "very irregular row of dark-brown tufa stones" for another. As long as computers do not generate descriptions, we will have to find a way around this issue. The best way is probably using photographic recording, so that linguistic problems are reduced to a minimum.

In the 20th century literature a number of chronological developments has been suggested. It is important to note that a single characteristic is not sufficient for dating, many characteristics have to be used. For example, a shift has been noted from red to yellow in painting, from the Hadrianic to the Antonine period, and a contemporaneous shift in the colour of the bricks. That single argument is not enough for dating, although it might explain why a different colour started to be used. It is equally hazardous to link the decreasing use of fresh bricks and the increasing use of tufa and reused bricks to economic crisis. For example, extensive renovation might well lead to reusing bricks of demolished buidlings.

Results from one city should not be automatically applied to another. For example, "opus vittatum" (see below) is found in Campania more than a century before it is seen in Ostia. Needless to say, a distinction should also be made between public and private buildings because of the difference in investment, and within private buildings between guild seats and shops.



Painting from the burial chamber of Trebius Iustus in Rome. Fourth century AD.

Terminology

For the employment of bricks Heres uses "opus latericium". A "later" is a kiln-baked brick. "Opus testaceum" is used by others.[2] For the employment of tufa stones, the ancient authors do not provide terminology. Modern authors use "opus reticulatum" and "opus vittatum" (also called "opus listatum"). Heres uses the following terminology:

- Opus reticulatum: small tufa stones placed diagonally, on one corner. Sometimes oblong tufa stones are used simultaneously, for the framing of windows and doors, and for the reinforcing of corners of buildings. The earliest phases, with an irregular or semi-regular pattern, are called opus incertum and opus quasi reticulatum.
- Opus reticulatum mixtum (for short opus mixtum): this is opus reticulatum that is reinforced and/or intersected by brick bands; the reticulate and the bricks are sometimes interlocking. Note: a few authors use opus reticulatum mixtum also for opus reticulatum in combination with oblong tufa stones.
- Opus vittatum simplex: exclusive use of oblong tufa stones.
- Opus vittatum mixtum, type A (for short: opus vittatum A): alternating oblong tufa courses and brick bands, 1:1.
- Opus vittatum mixtum, type B (for short: opus vittatum B): alternating oblong tufa courses and brick bands, in all other combinations than 1:1.

Opus reticulatum Opus reticulatum mixtum or Opus mixtum Opus latericium

Opus vittatum simplex Opus vittatum mixtum A Opus vittatum mixtum B

The concrete core

Bricks and tufa stones are the facing of a concrete core. The bricks and tufa stones protect the concrete from the climate, but are not relevant for the load-bearing capacities of the concrete. Usually the core is inaccessible for inspection. It rested on a foundation that could be made of concrete, mortar, stones, brick fragments, sherds and packed earth. Regularly no foundations were made for walls that were not load-bearing.

The qualities of Roman concrete have been much praised. Especially famous is the "pulvis puteolanus", or pozzolana. This is volcanic ash that will chemically react with chalk when water is added and then form something that we call concrete. It could settle under water. Research of the pressure resistance of Roman concrete from the first three centuries AD, from many locations in Germany, has shown that it was in the range of high-quality modern concrete.[3]

For dating purposes it is important to record whether a wall is structurally load-bearing or not. Less attention might be paid to masonry that was not load-bearing, so that there is only a terminus post quem.

Bricks

Bricks were produced as squares, but cut or sawn in triangles or trapezoids. The Roman foot that was used in Ostia has been calculated by Boersma to be 29.6 cm. For bricks the terms "bipedales", "sesquipedales", and "bessales" are used. The length visible in the wall is then two feet, 1.5 foot, or two-thirds of a foot, so 60, 45 and 20 centimetres, A length of c. 27 cm. is also found. The length of a normal brick is in the range of 18-28 cm. Between the bricks putlog holes for scaffolding are often seen.[4]



Drawing of part of a measure (Roman foot), made of bone.
Found in or near Via della Fontana in 1897.
Front (above) and back. NSc 1897, p. 524.


A bit more than half of the measure has been preserved. Originally it was 29.6 cm. long.
The original centre is at the right end and indicated by a lozenge.

Clay for bricks was present in abundance near Rome, in the Tiber valley. During some periods, for reasons that we do not fully understand, bricks were stamped. The stamps contain various information, for example about the production site ("praedia", "figlinae"), and sometimes even have consular dates. This makes the stamps very important for dating purposes, because bricks were not stored for a long time before being used, but of course bricks with stamps could be reused (many Hadrianic brick stamps were found in Aurelian's wall in Rome).[5]



Examples of brick stamps. Lugli 1957, fig. 128.


A brick stamp. Photo: Jan Theo Bakker.

Apart from the size the following characteristics may be recorded:

- Fresh or reused. Reused bricks were cleansed of the old mortar (it takes five minutes to clean a brick). As a result the edges are not sharp, and flakes are cut off the surface. Bricks may then be randomly broken, resulting in very short bricks.
- Surface. Smooth or rough? Bricks fired at a higher temperature are of a better quality.
- Colour. The colour of bricks depends on the composition of the clay: ferruginous clay leads to red bricks, calciferous clay and clay with much sand leads to yellow bricks. The colour was furthermore determined by the temperature at which the kiln was fired: the higher the temperature, the darker the brick.

Tufa stones

Tufa is found in abundance around Rome. It was quarried extensively for example in hills to the north of the Tiber.[6]

Apart from the size the following characteristics may be recorded:

- Fresh or reused.
- Surface. Smooth or rough?
- Shape. Sharp or rounded angles? Rectangular or oval?
- Colour.

Mortar

In Ostia simple, flat mortar joints are normally found. Mortar was made of lime and pozzolana. Like tufa, pozzolana was present in large quantities near Rome. The various kinds of pozzolana in Latium and the Bay of Naples created different colours. Red, black and brownish granules in the mortar are residues of pozzolana. Have a look at this explanation:

Apart from the height, the following characteristics may be recorded:

- Height. Is the height even? Obviously, the height cannot be even with bricks or tufa stones of varying sizes.
- Colour.
- Composition. Is the mortar tenacious or soft (which may of course have been influenced by centuries of moisture and sunlight). Is the mortar well-sifted? Does it contain granules or potsherds? Small pieces of marble are probably residues from kilns in which marble was burned to lime.

Mortar is especially important because it is the only part of the wall that cannot be reused in the wall (it can be used in a foundation). When Heres published her thesis, in 1982, chemical research of the mortar had not led to useful results.[7]

The modulus

A word that is encountered often in the study of masonry is "modulus", Latin for "a measured module". Heres applies it as follows:

Measurements are taken at heights 0.90, 1.40 and 1.90 from the ground.
- a-b. At these heights the length and height are measured of three bricks or tufa stones next to each other.
- c. At these heights, the height is measured of the mortar bed below and above each of the three bricks or stones.
- d. Working downward from each of the three horizontal rows of bricks or stones, the height of a stack of five layers of bricks or stones plus five layers of mortar is measured. This is the "modulus". It is determined five times, after moving down layer by layer. So at a single point, fifteen moduli will result: five moduli at heights 0.90, 1.40 and 1.90.

The procedure is repeated every few metres. The measurements result in a minimum, maximum and average. It is important to note that the modulus may "hide" characteristics: wide bricks and narrow mortar layers will give the same results as narrow bricks and wide mortar layers. Therefore, steps a, b and c are essential. Also, averages must not conceal that for example bricks of widely varying sizes were used.



Heres 1982, figs. 2-3.


Heres 1982, fig. 4.

Imitating masonry

In Ostia some rare examples have been preserved of paint imitating accurate masonry. The paint suggests that this masonry has red bricks and very thin, white joints. To achieve the effect, only part of the real mortar was painted white, the remainder plus the bricks red. Examples of real, extremely narrow joints are found in the burial places. Obviously a brick façade could be regarded as being beautiful. I have never been able to find a single trace of plaster or whitewash on the outer walls of the Barracks of the Fire Brigade. Nevertheless, the façades of many buildings in Ostia were at some point in time covered by plaster. In later antiquity Ostia was partly yellow and red (because of the bricks), partly brown (because of the tufa stones), and partly white (because of the plaster and whitewash).



Painted imitation of masonry. Thin white lines on the mortar, red paint on the bricks and mortar.
Caseggiato dell'Ercole (IV,II,2-4).

Chronological developments

Opus latericium
Walls made exclusively of brick occur throughout antiquity. Bricks appear during the reign of Augustus, but are then used only for specific constructions, such as corridors, wells, and nymphaea. The use continues until Theodoric, the early sixth century. The courses are usually well-levelled until c. 330 AD, with exceptions in the third quarter of the third century. In the late fourth century undulating courses appear.

Opus reticulatum and opus reticulatum mixtum
Opus reticulatum was used in the late Republican period and during the early Principate until Nero. The oblong tufa blocks used in combination with opus reticulatum were replaced at the end of the first century AD by bricks, in other words: opus (reticulatum) mixtum was introduced. The earliest opus mixtum is Flavian. It is very characteristic for the reigns of Trajan and Hadrian. Under Trajan the stones measure c. 8.0 x 8.0, under Hadrian 7.0 x 7.0 cm. The Trajanic work is not yet perfect, the Hadrianic work is immaculate. The use of opus mixtum declined with the Antonines, but it did not disappear altogether: the latest opus mixtum in Ostia belongs to the early fourth century (until c. 325 AD).[8] By then the tufa stones may have a very irregular pattern, but sometimes a wall could at first sight easily be assigned to the Hadrianic period.

Opus vittatum
Opus vittatum simplex was used in the Republican period until the reign of Nero. It re-appeared in the third century. In the first half of the third century the masons had started to place the small tufa stones of opus mixtum not on a corner, but in a horizontal-vertical pattern, and could do this throughout the third century. This was the birth of opus vittatum mixtum. Variant A (1:1) started to be applied in Ostia around the middle of the third century. It continued to be used throughout late antiquity. Variant B is rare in the second half of the third century, it belongs mainly to the fourth. In the third century the tufa stones are regular, with sharp corners. In the first half of the fourth century they are still well-cut, but in the course of the century they become irregular, with obtuse angles, eventually even becoming oval.[9] As a rule therefore, opus vittatum is later than c. 250 AD, but there are a few examples from the period 150-250 AD, especially in the Palazzo Imperiale and insula IV,II. Characteristic is then opus vittatum that is flanking panels of opus mixtum.

Height of the bricks
In the first century the bricks were rather thick, under Trajan about 3.8 cm. The height slightly decreased in the second century to 3.3 - 3.4 cm., and then fell below 3 cm. in the Severan period (2.7 - 3.2 cm. under Septimius Severus). Bricks remained thin in the third century. The height rose again under Constantine.

Colour
Domitianic bricks tend to be yellow. In the early second century (Trajan - Hadrian) the bricks are mainly red-orange to pink and yellow. Under the Antonines they are bright-yellow, under Commodus sometimes intersected by single red courses. Dark-red is characteristic for the reign of Septimius Severus. In the later third century the bricks are dark-red and yellow. Under Diocletian they tend to be yellow and orange, in the fourth century they may have any colour, from pale-yellow to dark-red.

Mortar
Grey-black mortar is characteristic for the Antonine period. In the second quarter of the third century it changes from greyish to slightly violet, in the third quarter to whitish. Whitish and sometimes greyish mortar are characteristic for late antiquity. It is very tenacious in the early second century and still binding good in later periods, but gets softer at the end of the third century, and eventually soft and crumbly. The mortar beds are narrow until the beginning of the fourth century. Then the height increases, becoming very high at the end of the century.

Granules and particles in the mortar
Until the middle of the third century the mortar is well-sifted. In the first half of the third century red and black granules are seen. In the third quarter of the century the mortar is sifted irregularly, containing black and brownish granules and other particles. During the reign of Constantine the mortar is similar to that early in the third century, but then large black and red granules begin to appear, and small pieces of marble. At the end of the fourth century and in the first half of the fifth the granules are large and mostly black, and potsherds and pieces of marble appear.

Modulus
The modulus of the opus latericium decreased steadily from the Flavian to the Severan period: 30.9 (Flavian), 28.8 (Trajanic), 25.8 (Hadrianic), 25.0 (Antonine), 23.5 - 26.5 (early and late Severan). Then it started increasing: 28.2 (second and third quarter of the third century), 30.7 (Constantine), 30.9 (second and third quarter of the fourth century), 32.6 (last quarter of the fourth and first quarter of the fifth century), reaching 36.1 in the second half of the fifth century.

Reuse
In the second and third century large quantities of fresh bricks were used. Reused bricks are seen for the first time in the Severan period. Fresh bricks became rarer in the third century, making up about 50% of the bricks at the end of the century. Uniform reused bricks were then selected. In the early fourth century more fresh bricks were produced, but around 330-340 AD they became rare and were no longer uniform. Reused, square reticulate stones were used increasingly in the fourth century.[10]

Examples of masonry per period [11]
Vespasianus, Titus and Domitianus (69 - 96 AD)
Trajanus and Hadrianus (98 - 138 AD)
Antoninus Pius, Marcus Aurelius and Commodus (138 - 192 AD)
Septimius Severus, Caracalla, Elagabalus and Alexander Severus (193 - 235 AD)
The later third century (235 - 284 AD)
Diocletianus and Constantine (284 - 337 AD)
The later fourth century (337 - 400 AD)
The fifth century (400 - 500 AD)


Some more terminology

Opus signinum
Waterproof floor- and wall-revetment consisting of mortar,
mixed with terracotta sherds and crushed tiles or bricks.
Opus sectile
Decoration of walls or floors with marble slabs,
laid in a regular pattern.
Opus spicatum
A floor made of quite small, elongated tiles,
laid in a herringbone pattern.


(1) On the problem see Lavan 2018. The linguistic problem was already noted by Giuseppe Lugli, discussing the pioneering work of Esther Boise van Deman in the early 20th century, published in 1912.
(2) Extensive discussion of the terminology: H. Gerding, "Later, laterculus and testa: new perspectives on Latin brick terminology", Opuscula: Annual of the Swedish Institutes At Athens and Rome 9,1 (2016), 7-31.
(3) H.-O. Lamprecht, "Eigenschaften von römischen Beton", in: Opus caementitium. Bautechnik der Römer, Düsseldorf 1984.
(4) Booms 2007.
(5) Essential reading for the dating of Trajanic and Hadrianic brickstamps are Hetland 2007 and 2009.
(6) For geoarchaeological research on tufa see Lena 2011.
(7) For Ostia see also Morricone et al. 2013.
(8) Van Dalen 1991.
(9) David - De Togni 2019 is a short overview of vittatum in Ostia.
(10) On reuse in general see Underwood 2013.
(11) Most of the photos courtesy Daniel González Acuña and Klaus Heese.


[19-Jul-2020, Jan Theo Bakker; for this page I would like to thank Thea Heres]