The degradation of historical plasters: causes, consequences and conservation strategies
The degradation of plasters, especially the historical ones, is a complex phenomenon influenced by the interaction of multiple factors. Contrary to a superficial view, often trivializing, historical plasters are not mere "sacrifice surfaces" intended for a cyclical substitution, but constitute a heritage of historical information and material culture and therefore must be preserved, in particular by preventing, as far as possible, damage conditions with maintenance activities constant over time. In fact, they can reveal constructive modifications, changes in taste, decorative patterns and colour design, constituting evidence of the figurative and technological culture of the past. Their preservation – when it is still possible – is therefore crucial from a cultural, technological and economic point of view.
The degradation of plasters, especially the historical ones, is a complex phenomenon influenced by the interaction of multiple factors. Contrary to a superficial view, often trivializing, historical plasters are not mere "sacrifice surfaces" intended for a cyclical substitution, but constitute a heritage of historical information and material culture and therefore must be preserved, in particular by preventing, as far as possible, damage conditions with maintenance activities constant over time. In fact, they can reveal constructive modifications, changes in taste, decorative patterns and colour design, constituting evidence of the figurative and technological culture of the past. Their preservation – when it is still possible – is therefore crucial from a cultural, technological and economic point of view.
Interactions between support and plaster are one of the most relevant causes. Differential movements due to different behaviours of materials, inadequate preparation of the support and errors of execution may compromise the integrity of the plaster. If the support is not adequately prepared in terms of roughness, cleaning, consolidation and moisture remediation, the adhesion of the plaster is compromised.
The atmospheric and biological agents play a decisive role in degradation. Rain, wind, frost and thermal changes, along with the action of biodeteriogens, accelerate the deterioration of plastered surfaces. Rainwater can cause deep erosions while the enmbibition of the surfaces, which are always very porous, leads to the formation of salts, increased porosity and sometimes even damage, in winter, where the freezing water, with increased volume, produces further porosity for disintegration.
The mechanical stress factors, such as vibrations, structural adjustments and excessive loads, can cause cracks and detachments. In addition, improper interventions, inadequate maintenance or improper use of the building contribute to the acceleration of degradation. Air pollution is another critical element. Carbon dioxide damages calcareous materials, while sulphur dioxide, combined with humid phenomena (rain, fog), promotes the formation of chalk, producing decoesionements. The atmospheric particle may contribute to the formation of black crusts, while pollutants catalyze acid attacks in the presence of moisture.
Another frequent cause is Incompatibility between materials. The use of modern products that are unsuitable, such as cement plasters or premixed on historic walls, can generate tensions, cracks and detachments due to the different coefficients of thermal expansion. Polymer painting, when applied on historical plasters, compromises the breathability of the supports, infragilates due to UV rays, are difficult to remove and compromise the adhesion of traditional colours (lime, silicates).
Historical plasters are particularly vulnerable to inadequate restoration. In particular, careful assessments should be made regarding technological compatibility in the case of integration of gaps and in cases of restoration of the cortical layers of decosis historical plasters, which must be carried out with compatible materials where the rule is that hardness must be decreasing and the permeability of the layers must be increasing, from those more internal than those more external. The indiscriminate substitution and inability to intervene with correct and durable conservation procedures leads to the loss of their documentary value, deleting essential historical traces such as original patinas or remnants of ancient colorations.
The cleaning must be performed with extreme caution by preliminary tests to identify the most suitable methods. Uncontrollable techniques such as high pressure water jets, can cause irreversible damage. The consolidation, cortical and deep, is necessary to promote the re-adherence of plasters detached to the wall support, using injections of adhesive mortars and, for cortical interventions, using suitable products and once again compatible. In the presence of Saline efflorescences, can be used Adsorbent packs for the extraction and elimination of soluble salts. Theintegration of gaps must be made with mortars similar to the original ones for composition, colour and surface texture.
To assess the causes and effects of the degradation of historic plasters, aIn-depth analytical analysis (chemical-physical, mineral-petrographic). The humidity measurement It is essential to evaluate capillary rise phenomena, typical in historic buildings with foundations in direct contact with the ground. The porosity affects the resistance of materials, determining their sensitivity to atmospheric agents. The analysis of soluble salts allows to identify the causes of degradation and to take targeted measures.
The understanding of conservation status of historical plaster therefore requires a multidisciplinary approach which consider the specificities of materials, construction techniques, causes and conditions of degradation. The Actions maintenance and restoration must be Scheduled carefully to ensure the preservation of this precious heritage, using techniques and materials compatible with the nature of the building and the technical characteristics of the surfaces. Only onein-depth analysis and a conscious approach; from a cultural and technical point of view, they can ensure the preservation of historical plasters over time as valuable and irreplaceable testimony of material culture.
