Salt conversion: Difference between revisions
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== Abstract == | == Abstract == | ||
Damaging salts can be transformed into less damaging compounds using chemical salt conversion. | Damaging salts can be transformed into less damaging compounds using chemical salt conversion. Two methods can be used. The firs one 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]] == | == [[Removal of salts by salt conversion]] == | ||
Conservation practice uses various methods for the removal of soluble salts and salt crusts. | 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.<bib id="Hammer:1996" /> The use of hexafluorosilicic acid has also been documented. The Florentine method <bib id=Matteini:1991/> 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.<bib id="Pursche:2001" /> | ||
== [[Immobilization of salts]] == | == [[Immobilization of salts]] == | ||
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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. | 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, but formerly lead hexafluorosilicate was often used. | Today, barium solutions are employed most frequently, particularly for immobilizing sulphates, but formerly lead hexafluorosilicate was often used. | ||
<br> | <br> | ||
Revision as of 19:07, 26 December 2011
<bibimport />
Author: Hans-Jürgen Schwarz
back to Measures
Abstract
Damaging salts can be transformed into less damaging compounds using chemical salt conversion. Two methods can be used. The firs one 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
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