EN
As a result of the effect of corrosive environment upon historic stone artifacts and as a result of incorrect use of materials and agents for their conservation there emerge secondary products. Their chemical composition depends on the kind and advancement of factors determining a degree of endangering for objects. Of particular importance are here water-soluble salts that represent the main cause of the destruction of stone sculptures, elements and details of architectonic decor as well as bricked historic objects. The object of this article was to show endangerings posed by the presence of soluble salts in porous material of an historic object as well as a survey and evaluation of the methods used to reduce their action. The most important salts identified in stone artifacts are given in Table 1. The mode of a corrosive effect of water-soluble salts upon the stone is based on physical and chemical processes in which water plays the most important role. It brings about dissolving of salts and makes possible their translocation through migration, while in some cases it results in their hydrolysis. These processes are not as yet fully recognized and a destructive effect of the salts often manifests itself together with the efect of other destructive factors. The methods employed to reduce a destructive effect of salts may be divided into: — methods of desalting the stones, — methods of immobilizing soluble salts present in stone by a chemical way, — methods of immobilizing the salts by means of anti-humidifying barriers. While evaluating the usefulness of a given method in practice the following conservation requirements should be taken into account: — the method employed should make possible a quick and easy removal of salt from the stone or its complete immobilization without impairing the appearance and colouring of the monument, — the use of the method should neither reduce mechanical properties of the stone nor impair its surface or deeper layers; it should also not affect adversely a further conditio of the stone, — the stone should preserve its relatively high porosity and fine patina, — substances used cannot be hygroscopic; they should not also produce water-soluble by-products that would have a destructive effect upon the stone. Well-known methods of desalting may be classified in the following way: A. Desalting by the migration of salts to expanded environment. B. Desalting by salt diffusion. C. Desalting by the action of an electric field. D. Desalting by means of an electromagnetic field. The techniques that finally make it possible to reduce water movement include: — elimination of sources of water — protecting the walls against dampening — concealment of the dampening of the interior walls. The facts presented in this study allow us to state that water-soluble salts are an essential factor destroying historic stone artifacts. It should be noted at this point that the mode of destructive processes has not as yet been fully explained. At the same time a major source of salts is the use of salt-containing bricks and cement mortars. Recent studies on the adsorption of bases by means of gas chromatography, the confirmation of studies on electrokinetic phenomena in porous bodies as well as the development of theories and laboratory studies on the so-called active centres on the surface of capillaries (e.g. loamy substances, „edges” and walls of cristals) as well as a way of their blocking lead undoubtedly to a fuller explanation of the role and mode of the action of soluble salts and will allow us to work out effective methods protecting historic stone artifacts. The most favourable treatments that reduce the effect of soluble salts in a given object are desalting and some antihumidifying barriers. At the same time it should be pointed out that the problem of desalting small objects which can be transported to workshops and which contain a high percentage of macropores has been practically solved. In view of its effectiveness, efficacy and safety for the object one should distinguish desalting by means of a membrane electrodialysis; still, at its present state the method may be used only for small stone objects (with pores smaller than macropores) which can be transferred to workshops. In many cases methods of electric drying protect effectively against the transportation of water and water solutions inside the walls. Still, they are of small use in case of drying walls with a higher degree of salting. Prior to applying electrodrying for salt-containing walls the given object should be first desalted by means of special plasters with high capacity, which is a useful procedure because of the safety of a desalted structure, despite a low effectiveness of the method, and also because this procedure can be repeated several times. On the basis of the data presented in the work it may be said that in order to solve the problem of a destructive effect of salt in stone porous objects further studies should concentrate on the following subjects: — the mode of a corrosive effect of soluble salts, — working out a special formulation of mortars with regular porosity and distribution of pores’ diameters basing on the adhesives which will not be harmful for the object, — new solutions of electro-drying with the use of modified injectable hydrofobie solutions or plasters with high inside capacity, — working out a technique of electro-desalting the objects „in situ" with о possibility to use it to dry buildings.