Inorganic treatments for the consolidation and protection of stone artefacts

Consolidation and protection are two of the principal kinds of treatments through which the decay of old statues, stone facades, plasters and mural paintings caused by both natural atmospheric agents and, above all in the last five decades, by atmospheric pollution, is faced. The most traditional approach has been and is mainly based on the use of organic polymeric materials. They offer the advantage of easy application procedures and the possibility to obtain, at short times, very satisfying results. Different is their behaviour at long times. Some drawbacks come out over time both under the esthetical point of view as well as to the durability, compatibility and efficacy. Particularly critical is the situation when porous materials and soluble salts - gypsum above all - are simultaneously present. In such a situation inorganic treatments demonstrate to be much more appropriate. They assure durable and compatible results. In the present paper two of the most efficient and appropriate inorganic methods are reviewed in detail: the barium hydroxide method, both as desulfating and consolidating agent, and the ammonium oxalate method as passivating agent, consolidant and as a treatment capable of improving the natural colour contrast of the stone, when it is lost due to decay processes