The Arch of Constantine, located along the Triumphal Way between the Circus Maximus and the Arch of Titus, is the largest honorary arch that has come down to us and represents a synthesis of the ideological propaganda of the Constantine era. The arch celebrates the triumph of Emperor Constantine over Maxentius, which occurred on 28 October 312 AD following the battle of the Milvian Bridge. The inscription on the central archway recounts that the monument was dedicated by the Senate to the emperor in memory of that triumph and on the occasion of the decennalia of the empire at the beginning of the tenth year of his reign, 25 July 315 AD.
The decoration in marble slabs in relief was conceived and realised according to a unified design, using mostly spoiled materials from other imperial monuments. On the main faces of the arch and on the sides, reliefs from the age of Trajan, Hadrian, Marcus Aurelius and finally, in the lower section, from the age of Constantine alternated in symmetrical patterns. Coloured marbles adorned the surfaces: columns in antique yellow, mirrors in red porphyry housed Hadrian’s roundels, cipolin marble bases supported the statues of the Dacians in pavonazzetto marble, while a band of coloured marble, possibly serpentine, ran horizontally across the cornice.

• Conservation status

  Between February and June 2021, extraordinary maintenance work was carried out on the northern and western fronts of the arch, and restoration work on the remaining two fronts will begin shortly, completing the maintenance work.
  The Arch of Constantine has been the subject of various restoration and maintenance works, and the quality of the interventions carried out since the 1980s has ensured the good conservation of the surfaces until today, leaving the monument in good condition despite its exposure to vehicular traffic and atmospheric agents, which are among the main factors in the deterioration of the arch’s surfaces: in fact, while the exposure of one side of the arch to the south inhibits the formation of plant microorganisms, it also favours the crystallisation of salts on the surfaces, leading to the progressive disintegration of the marble. Conversely, the lack of sunlight on the north face favours the development of microorganisms and reduces the phenomena of marble disintegration. The current intervention concerned the north and east faces, on which a thick layer of algal patina had formed, which was detrimental to the conservation of the material and did not allow the polychrome marbles of the arch to be appreciated.
  In addition to the biological patinas, the arch suffered from the infiltration of rainwater, the loss of functionality of grouting and the development of lichens that damaged the surfaces aesthetically and structurally, eroding the underlying marble. There were areas where the marble had disintegrated and showed signs of collapse through the falling of superficial grouting and crumbling of the marble; moreover, the most projecting parts had become a resting place for birds that had covered them with guano, a dangerous droppings because it chemically attacks the stone. The deposition of atmospheric dust in the most sheltered areas, not washed away by rain, dirtied the surfaces and triggered the formation of black crusts.
  Even though the surfaces were not seriously compromised, it was no longer possible to appreciate the polychromy of the marble and the preciousness of the modelling of the bas-reliefs.

• The Restoration Intervention

  At the beginning of the intervention, the photographic documentation of the surfaces and mapping were carried out in order to record the state of conservation and locate degradation situations. A biocide and herbicide treatment was carried out to eliminate weeds and algal patina. Cleaning tests were carried out in order to fine-tune an effective intervention protocol suited to the different forms of degradation encountered: for this reason, tests were carried out at several representative points. To fine-tune the most effective system, which turned out to be… water!
  The surfaces were then cleaned with brushes, sponges and water to remove algal patina and atmospheric particulate deposits. Where surfaces proved too fragile, cleaning was conducted with laser instrumentation. This highly selective system proved to be essential for cleaning fragile surfaces on which it is not possible to act with traditional instruments, or to remove black crusts below which the surface is very disjointed. Laser ablation made it possible to clean capitals, pilasters, the delicate Hadrian and Trajan reliefs, and the surfaces of the Dacians’ pavonazzetto robes that, due to their decay and delicacy, could not be treated otherwise.
  The disintegrated areas were consolidated and the detached parts were glued: although the interventions carried out in past decades are distinguished by their quality and durability, it was necessary to return to gluing that had lost its functionality over time.
  Among the ‘silent’ but most important interventions is the capillary microstuccoing that the restorers patiently carried out, sealing the marble’s millimetric cracks and micro-cracks one by one, preventing the infiltration of water and pollutants. These also act as a sacrificial layer by degrading before the marble itself and inhibiting acid aggression.
  The restoration work made the marbles once again appreciable in their variety, and also made it possible to see details that were previously not visible, such as the engraving on the bases of the statues of the Dacians bearing AD ARCU, indicating the destination chosen for the statues prior to their installation.

  

  Particolare dell’incisione sulle basi delle statue dei Daci dell’espressione AD ARCU

  The bronze brackets that secured the marble crustae to the arch frame and the original mortar behind, composed of lime and pozzolan, were also found. The Hadrian’s roundels showed traces of a red pigment that was investigated to find out if it was due to a possible application of colour, investigations traced these stains to lead oxides due to a canopy that was later removed.
  The arch never stops to amaze!

  

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