Clay Altars from Caerleon


Fragments of several Roman ceramic altars were discovered in 2007 during an archaeological evaluation by Monmouth Archaeology on land behind the Mission Chapel on the Bulmore Road. The site lies across the River Usk from Isca, close to a main Roman road and en route to the extra-mural settlement and cemeteries at Bulmore.

This unusual and intriguing find consists of the following:

  • Conjoining fragments of two mensae (sacrificial tables) of different designs;
  • Fragments of two stepped ‘bases’ matching the mensae in their proportions and broken from the altar shafts along their upper surfaces;
  • One separate ‘box’ altar base with incised decoration;
  • One fragment of an altar shaft, inscribed with text, of a thickness and form matching the proportions of the mensae and stepped bases rather than the ‘box’ altar.

It is probable that this assemblage represents the remains of no less than three separate altars; two with stepped bases, shafts and separate mensae, and one one-piece ‘box’ altar. The shaft of at least one of the stepped altars was inscribed and it is probable that all three originally bore lines of text. Both of the Mission Chapel mensae retain evidence of burning in the centre of their upper surfaces and each also has rows of holes for candles along two of their four sides (one has rows of candle holes along the sides adjacent to the bolsters, while the other has holes along the front and back where there are no bolsters).

The ‘box’ altar base survives to approximately half its original height and is of similar proportions to the mensae and other altar fragments. This altar is decorated on three sides suggesting that it had been set up against a wall. The front face is decorated with a spray of palm leaves, the right side with palm leaves and a pole axe, while the left face shows a knife beneath an arch formed of two palm leaves leaning towards one another. The palm leaves represent victory while the knife and axe recall the instruments of animal sacrifice; the beast was usually stunned using the pole-axe and then stabbed with the sacrificial knife.

The survival of opposing corners of the mensae and the nature of their fragmentation suggest that they were deliberately broken with blows delivered near the centre of each. The ceramic altar fragments had been reused to form the walls of a drainage channel near to the corner of a Roman building of unidentified proportions or function, with the ‘box’ altar and other fragments nearby. A partial sheep or goat skull, bones and horn core were also found within the ‘drain’, which was covered by a capstone. All of the bones are of sheep or goat and include a partly burned fragmented skull, three juvenile lower mandibles, two mature upper mandibles, and a fragment of goat horn core. It is believed that these bones were in place when the capstone was put in place and it is possible that these are the remains of sacrificial animals.


Clay altars are known from other examples from Britain and other parts of the Roman Empire, but these are not common finds and it was more usual to construct altars in stone. A very close match for the Mission Chapel sacrificial tables was discovered during excavations of a Mithraeum in Linz in Upper Austria (Egger, 1958, Karnitsch, 1956). The fragments of the Linz mensa formed half of a large square ‘plate’, or ‘platter’, of fired clay measuring approx 38cm square, with raised edges and holes of varying sizes to receive candles. It was inscribed with a Mithraic inscription to Jupiter, the planet and god associated with the Leo grade among followers of the oriental god Mithras. Like the decoration and inscription of the Caerleon examples, the inscription of the Linz mensa was scratched into the wet clay, before firing, using a stick or similar object.

In the third book of his Saturnalia, the fifth century grammarian and philosopher Macrobius defined the purpose of the sacrificial altar table:

Mensa, in qua epulae libationesque et stipes reponuntur

‘The mensa, by which means the libation feast and small offerings are staged again’

Macrobius, Saturnalia III, 11, 442

There are many depictions of the sacred altar fire from the ancient world, including examples on reverses of its coinage. Fine sculptural depictions include the beautiful, and well known ivory ‘Symmachi’ diptych panel, dating to the late fourth century AD. There is also a depiction of the sacred fire from Caerleon, on the Bonus Eventus and Fortuna sculpture now displayed at the National Roman Legion Museum (Brewer 1986).

The altar fragments from Mission Chapel are the subject of on-going research and a paper describing these finds in more detail will be published shortly (Lewis, M., Clarke, S. and Bray, J. 2008. ‘Roman Clay Altars from Caerleon’, The Monmouthshire Antiquary, Volume XXIV).


Brewer, R.J. 1986. Corpus Signorum Imperii Romani Corpus of Sculpture of the Roman World, Great Britain. Volume 1, Fascicule 5, Wales. Oxford: British Academy and Oxford University Press.

Egger, R. 1958. ‘Bescheidene Ex-votos’, Bonner Jahrbuch 158: 73-80.

Karnitsch, P. 1956. ‘Der heilie Bezirk von Lentia’, Historisches Jahrbuch der Stadt Linz, 189 ff., das Mithraum 205 ff. mit Plan Taf. II.

Written by Mark Lewis M.Sc., National Roman Legionary Museum

Mensa of clay altar from Caerleon (© Amgueddfa Genedlaethol Cymru National Museum Wales)

Mensa of clay altar from Caerleon (© Amgueddfa Genedlaethol Cymru National Museum Wales)

Base of clay box altar from Caerleon (© Amgueddfa Genedlaethol Cymru National Museum Wales)


















Mensa from a Mithraeum in Linz, Austria (Eggers 1958)