Conservation of Historic Renders and Plasters: From Laboratory to Site

In interventions on historic renders and plasters, the first step is to decide upon the strategy: repair or substitution, based on an evaluation of the cultural value of the render or plaster, of the building itself and on a careful diagnosis of the typology of defects, their quantity and reparability. New renders or repaired renders should fulfil the main functions they are required to, especially protection and aesthetic functions. Compatible materials should always be used. Compatibility is needed for durability, not of the render, but of the wall as a whole, and also for preserving the documentary and symbolic value of the building as well as its image. Compatibility is defined in relation to the substrate and the existing mortars. Therefore tests need to be carried out on the old materials and on possible solutions, to compare characteristics and assist in the selection of the best. It is acceptable to begin using non-destructive or slightly destructive in-situ tests, because with them it is possible to collect useful information quickly and without destruction of the historic renders. Simple mechanical and physical tests can be carried out on the old mortars and a few chemical tests can also be performed, with portable equipment. If rigorous and complete tests are needed, some samples can be collected and tested in the laboratory, using methods adapted to non-regular, possibly friable specimens. The characteristics of the mortars to use can be established, based on the results obtained, in order to fulfil both functionality and compatibility. However, sometimes it is not possible to obtain enough data about old materials, especially concerning masonry as a whole, which is more difficult to test than mortars. For this situation, some general requirements have been established, based on previous work carried out on Portuguese historic masonry buildings, which can be used without risk of damaging existing materials. Decisions concerning the materials to use, especially binder materials, should also take into account the climatic and environmental conditions. Appropriate application techniques, workmanship and curing conditions are indispensable in achieving good aesthetic, physical and mechanical results. Therefore it is important to know what conditions are available for the application phase. An effective knowledge of the historic materials and of the possible compatible solutions, of their characteristics and problems, is essential; tests are an important tool but the interpretation of their results in order to take useful decisions is a complex task, requiring a multidisciplinary team efficiently coordinated.