Capillary-rising salt pollution and granitic stone erosive decay in the parish church of Torre de Moncorvo (NE Portugal)-implications for conservation strategy

The study of a full-scale granitic building (the parish church of Torre de Moncorvo, NE Portugal, 16th-17th centuries) allowed the investigation of geochemical characteristics of capillary-rising salt pollution and their relation to the development of erosive-decay features on granitic building stones. There is visible evidence of erosive decay affecting both stone and mortars in the parish church, with height-related distribution patterns that indicate the influence of capillary-rising solutions in the decay processes. Salt efflorescences detected in the parish church and in several places of the town evidence a widespread salt pollution affecting granitic stones and other building materials. Erosive decay in the parish church is more pronounced and extensive in the walls, principally in the portals (east, north and south). The sampling program included decayed granitic stones (to study the characteristic of salt pollution affecting the stones), soils between pavement slabs and lime mortar joints (considering that these media constitute passive and cumulative indicators of salt-pollution sources and conditions). Results of water-soluble extracts indicate geochemical patterns with height that support the importance of capillary-rising pollution and which indicate that erosive-decay features affecting granitic stones occur associated with a range of total salt load and with diverse ionic compositions in a single monument. Distribution of stone erosive-decay features and geochemical characteristics of water-soluble extracts of decayed building stones, lime mortar joints and soils also indicate that cycles of capillary-rising contamination and drying of stones are predominant factors in the development of erosive decay features that affect the granitic stones, favouring salt fractionation with height and surface concentration of soluble salts. These processes are especially active in the walls (particularly in the portals), which are more exposed to cycles of capillary rise of solutions (namely, by rainwater accumulation near the walls). The existence of level differences between outside pavement and inside floor (as observed in the south wall) constitutes an additional solutions reservoir that favour stone decay. Lime mortar joints in the columns of the church's nave show accentuated salt pollution, but the granitic stones in these columns do not evidence the accentuated erosive decay features found in the walls, highlighting the influence of cycles of capillary rise of solutions and drying of stones. These results are relevant to the conservation strategy of this monument. Conservation measures must include isolation from soils (moisture- and salt-contamination reservoirs) and removal of mortar joints (salt-contamination reservoirs). Since intense and extensive erosion of ornamental stone elements is observed, almost obliterating their artistic value, replacement of some stones should be pondered. Remediation of the present salt-contamination (desalination) on the remaining stones needs to be considered and applied soon after isolation from solutions sources (or even begun simultaneously), since the removal of capillary-rising moisture would favour crystallisation of soluble salts from salt solutions inside the porous media of the stones and further promote stone decay. Consolidation procedures are proposed after the contamination-sources removal and desalination procedures. Given the size of the affected volume, a phased intervention is proposed, beginning with the portals.