Since the time of rediscovery in 1900, the decay of the paintings in the presbyterySpace reserved for the clergy in Christian churches, situated in the central part of the church, behind the altar, especially on the south wall including the apse and in the adjacent Chapel of Theodotus has been a major concern.
A comparison of photographs from the excavations with the present situation emphasizes the severe alterations suffered by the painted surfaces in this area. Substantial losses of paint have occurred especially in the apseSemicylindrical niche in the back wall of the presbytery covered by a semidome and on the wall to its left. In some cases, e.g. the velum in the apse with circular ornaments from the time of Paul I (757-767), losses are almost total.
The walls and vaults of the presbytery and of the two side chapels were reconstructed in 1902, and in 1904 an air space was dug behind the apse " ten metres deep into the living rock, to eliminate the contiguity of the walls with the cliff of the Palatine." [Tea, 1937, p.10].
The fact that these structural works did not solve the conservation problems is indicated by the decision in 1945 to detach the panel of the Virgin and Child, Saints, Theodotus and Pope Zaccarias on the south wall of the Chapel of Theodotus, and in 1947, the Crucifixion in the niche of the same wall. Furthermore, the emergency stabilization of threatened fragments, carried out in 1952 by the Istituto Centrale per il Restauro (Central Institute for Restoration, Rome), probably refer to the paintings in the apse.
Thorough investigations conducted as part of this diagnostic project (2001-2002), in a pilot area that comprises the apse and the two adjacent walls, have shown that infiltration of dispersed rain water coming from the Palatine Hill is still an active phenomenon. Instrumental measurements of the water content in the masonry have defined an area with high levels of moisture (15.3 - 38.8%). This area is located below a roughly diagonal line extending from the lower right corner of the end wall through the apse and onto the entire wall to the left of the apse and part of the adjacent east wall of the presbytery.
In this area, surfaces are heavily disintegrated and affected by soluble salts. The decay is indicated by an almost continuous white patina of salt efflorescenceSurface formation of salt deposits, consisting of microscopic salt crystals which has strongly reduced the mechanical strength and provoked advanced surface disintegration.
Laboratory analyses have indicated that soluble salts consist of a mix of thenardite (sodium sulphate) as main component. Moreover, the microscopic examination of cross-sectionsThin layer produced of a micro-fragment of painting, which after smoothing and polishing can be examined under the microscope in reflected and/or transmitted light, reveal a mince layer of gypsum (calcium sulphate) on the surface of many samples which have engulfed atmospheric particulate leading to a strong chromatic obfuscation of the surfaces.
Thenardite originates from Portland cementThe most common type of grey cement, similar in colour to limestone found near Portland, U.K., which was used in large quantities during the 1900-02 intervention. It is an extremely dangerous salt because of its ability to form crystals, which according to fluctuating humidity assume varying volume, exerting continuous mechanical stress to the original material.
Equally damaging is gypsum, which apart from being generated by atmospheric pollution can also derive from Portland cement. The high moisture content in the lower part of the apse is also evident by the growth of green algae in some spots.
The surface above the most decayed area is more compact and less obscured by salt efflorescence. The major decay phenomenon in the better preserved area is an extensive lack of adhesion of the plaster layers, often combined with loss of internal mechanical strength.
Apart from the areas which suffer from surface disintegration, the paint layer is very stable. This can be accredited to the good original painting technique, which involved the application of colours on wet plaster and the use of lime milk as additional binder, resulting in pigments enclosed by a continuous calciteSynonimous of calcium carbonate matrix.
The evaluation of survey forms stress some general issues regarding the condition of wall paintings and historic plasters:
- The most diffused decay phenomenon is the lack of adhesion of the painted and unpainted plasters at various levels. Often, in areas with several superimposed layers, these are both detached from each other and from the brick masonry. In most cases the phenomenon exists in combination with severe reduction of the mechanical strength (loss of cohesion) of one or more layers. Lack of adhesion/cohesion account to about 60% of the total surface. In many areas, plasters are completely separated and require urgent stabilization.
- The paint layer is generally stable. Only about 6% of the painted surfaces are affected by flaking and/or powdering. These are normally found only locally, in areas suffering from salt decay. An exception are the most damaged areas of the apse wall, where the phenomenon of surface disintegration also involves the paint layer.
- Paintings that were not re-treated after the intervention of 1900-02 do not show any pictorial reintegration. The panel with paintings of the period of Pope Adrian I (772-795) detached from the west wall of the atrium in 1956-57 is heavily retouched. The wax fixative, applied probably on all painted surfaces in 1902, did not produce any perceivable chromatic alteration or other drawbacks in terms of conservation.
- About 40% of the examined surfaces are affected by crystallization of soluble salts. Out of these areas, about 64% show advanced salt decay. On the remaining surfaces only light whitish veils are visible.
- During the restoration intervention of 1900-02 more than one kilometre of cement filleting along the edges of fragments, about 690 cement fills of medium size (> 25 cm2), more than 2 sqm of larger cement fills and about 250 brass cramps were applied. The removal of these elements is an extremely delicate operation, which requires ample time and will thus have a relevant impact in budgetary terms.
- The most recent conservation-restoration interventions are those carried out in the 1980s, by ICCROMInternational Centre for the Study of the Preservation and Conservation of Cultural Property, Rome on the east wall of the atrium and by the conservators Giantomassi/Zari on the east wall of the left nave, the south side of the south-western pillar and the dividing wall in the presbytery. The paintings treated in that period correspond to about 25% of the total painted surfaces. The condition of the treated paintings is clearly better than of those which were not re-treated after 1900-02, and they only require control and maintenance. Some of the fragments in the atrium require a more thorough intervention, due to their exposure in an openair environment. An integral protection system comprising a roof covering the four sides of this area is undergoing design.
- The detached paintings do not have major problems in terms of conservation. The stability of the original material and the adhesion to the new support are, apart from a few isolated areas, satisfying. An exception is the painting representing the Virgin and Child, Saints, Pope Zaccharia and Theodotus from the Chapel of Theodotus. This panel, kept at the Forense Museum, is applied onto canvas. Although the condition of the painting is good, a transfer onto a rigid support is recommended.