Salt conversion

<bibimport /> Author: Hans-Jürgen Schwarz

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Abstract

Damaging salts can be transformed into less damaging compounds using chemical salt conversion. Two methods can be used. The first method converts soluble salts into poorly soluble or insoluble compounds, resulting in the salts being less damaging. The second one transforms insoluble salts into more soluble ones in order to remove them more easily.

Removal of salts by salt conversion

Conservation practice uses various methods for the removal of soluble salts and salt crusts. In the past, often acids such as hydrochloric acid or acetic acid were used to remove calcium carbonate crusts, but this can lead to subsequent damages if the correct procedure is not followed diligently.[Hammer:1996]Title: Salze und Salzbehandlung in der Konservierung von Wandmalerei und Architekturoberfläche.
Author: Hammer, Ivo
The use of hexafluorosilicic acid has also been documented. The Florentine method [Matteini:1991]Title: In Review: An Assessmant of Florentine Methods of Wall Painting Conservation Based on the Use of Mineral Treatments
Author: Matteini, Mauro
of "gypsum transformation" also includes the dissolution of the gypsum crust with ammonium carbonate. Ion exchangers have been in use for many years to remove crusts.[Pursche:2001]Title: Konservierung von Wandmalerei, Reaktive Behandlungsmethoden zur Bestandserhaltung

Immobilization of salts

Immobilization is the conversion of readily soluble salts into poorly soluble or insoluble salts and thus relatively harmless compounds. This appears to be simple and obvious but problems arise in the practical application. Masonry not only incorporates defined salts, such as calcium nitrate or magnesium sulfate, but also relatively complex mixtures, which may be partly crystalline, partly present as a salt solution. The most common compounds are carbonates, nitrates, chlorides and sulphates, the most common cations are sodium, potassium, magnesium and calcium. It should be noted, that nitrate compounds can not be converted into insoluble salts, as there are no poorly soluble or insoluble nitrate compounds. This leaves chlorides, sulfates and some limited options for carbonates as the only possibilities for immobilization treatment.

Today, barium solutions are employed most frequently, particularly for immobilizing sulphates, but formerly lead hexafluorosilicate was often used.

Literature

[Hammer:1996]	Hammer, Ivo (1996): Salze und Salzbehandlung in der Konservierung von Wandmalerei und Architekturoberfläche.. In: Pursche, Jürgen (eds.): Salzschäden an Wandmalereien, Bayerisches Landesamt für Denkmalpflege, 81-106.
[Matteini:1991]	Matteini, Mauro (1991): In Review: An Assessmant of Florentine Methods of Wall Painting Conservation Based on the Use of Mineral Treatments. In: Cather, Sharon (eds.): The Conservation of Wall Paintings: Proceedings of a symposium organized by the Coutrauld Institut of Art and the Getty Conservation Institute, London, July 13-16, 1987, The Getty Conservation Institute, 137-148.
[Pursche:2001]	Pursche, Jürgen (eds.) (2001): Konservierung von Wandmalerei, Reaktive Behandlungsmethoden zur Bestandserhaltung, Bayerisches Landesamt für Denkmalpflege, München