Salt conversion

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Author: Hans-Jürgen Schwarz
English version by Sandra Leithäuser
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Abstract

Damaging soluble salts can be transformed into less damaging ones by their conversion into insoluble salts. Two methods can be used. The first one converts soluble salts into poorly soluble or insoluble compounds, thus diminishing the number and degree of crystallization cycles. The second one transforms the less soluble salts into far 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, acids such as hydrochloric or acetic acid were often used to remove calcium carbonate crusts. However, 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
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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
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of "gypsum transformation" is based on the dissolution of the gypsum crust with ammonium carbonate. Ion exchangers have also been in use for many years to remove crusts.[Pursche:2001]Title: Konservierung von Wandmalerei, Reaktive Behandlungsmethoden zur Bestandserhaltung
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Immobilization of salts

Immobilization is the conversion of readily soluble salts into poorly soluble or insoluble salts that are therefore less deteriorating compounds. This appears to be simple but the problem is that not all ions form highly insoluble compounds, for example, there are no poorly soluble or insoluble nitrate compounds. Furthermore, masonry usually contains a mixture of ions, not only specific salts, such as calcium nitrate or magnesium sulfate. Thus, even when some salts crystallize out, a solution containing these and other ions will also be present. The most common anions are carbonates, nitrates, chlorides and sulfates, the most common cations are sodium, potassium, magnesium and calcium. This leaves chlorides, sulfates and some limited options for carbonates as the only possibilities for an immobilization treatment.

Currently, barium solutions, such as barium hydroxide, are the most frequently employed to immobilize sulfates. In the past, lead hexafluorosilicate was used to immobilize chlorides as well as carbonates, however the fluorosilicate will react with any calcium carbonate in the substrate by turning it into calcium fluoride, and can attack silicate minerals as during the reaction hydroflouric acid is released.


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.Link to Google Scholar
[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.Link to Google Scholar
[Pursche:2001]Pursche, Jürgen (eds.) (2001): Konservierung von Wandmalerei, Reaktive Behandlungsmethoden zur Bestandserhaltung, Bayerisches Landesamt für Denkmalpflege, München.Link to Google Scholar