Immobilization of salts

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Author: Hans-Jürgen Schwarz
back to Salt conversion


Barium method[edit]

The barium method bases on the theory, that in the presence of readily soluble sulfate salts, the sulfate precipitates with a readily soluble barium compound to become barium sulfate, and hereby removes the damaging salt from the system.


Gypsum conversion[edit]

The conversion of gypsum [1], which particularily refers to the removal of gypsum efflorescence and gypsum crusts has been in use for several years [Hammer:1996]Address: München
Author: Hammer, Ivo
Booktitle: Salzschäden an Wandmalereien
Editor: Pursche, Jürgen
Note:
Pages: 81-106
Publisher: Bayerisches Landesamt für Denkmalpflege
Title: Salze und Salzbehandlung in der Konservierung von Wandmalerei und Architekturoberfläche.
Year: 1996
Link to Google Scholar
and has proven to be effective, when used in appropriate cases. Gypsum conversion [2] is carried out in five steps [Matteini:1991]Author: Matteini, Mauro
Booktitle: 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
Editor: Cather, Sharon
Note: The materials that constitute wall paintings have a high and open porosity, which makes them vulnerable to the effects of both liquids and gases either found in their environment or used for their conservation. Therefore, wall painting conservation strategies involving mineral treatments should be preferred for compatibility, thus ensuring good, lasting results. This text examines problems of sulfation, discussing appropriate tests to evaluate and monitor them. It describes a methodology based on the use of ammonium carbonate and barium hydroxide that has been employed in Florence for some 20 years. -- AATA*Assesses the merit of Florentine methods of wall paintings conservation based on the use of mineral treatments. Discusses serious problems of sulphation, particularly the effects of gypsum, and reviews appropriate tests to monitor them. The methodology of cleaning and conservation using ammonium carbonate and barium hydroxide is outlined. Evidence is given of the first frescoes by Fra Angelico and Sogliani - treated with 'barium'. Details photographed before conservation in 1973 and 1975 are compared with photographs taken in 1987, which demonstrate and confirm a generally satisfactory result.
Pages: 137-148
Publisher: The Getty Conservation Institute
Title: In Review: An Assessmant of Florentine Methods of Wall Painting Conservation Based on the Use of Mineral Treatments
Year: 1991
Link to Google Scholar
:

1. Dissolution of gypsum

CaSO4•2H2O + (NH4)2CO3 → (NH4)2SO4 + CaCO3 + 2H2O

In the first step the application of an ammonium carbonate poultice leads to the conversion of gypsum into soluble ammonium sulfate. It migrates to one part into the poultice and to another part it stays in the the surface layer and eventually migrates into lower layers. If calcite forms under the surface of the plaster, it achieves a positive, consolidating effect; if it forms on the surface, it has to be removed dilligently. Excess ammonium carbonate decomposes to become ammonia, carbon dioxide and water. (Ammonium carbonate alters proteinaceous coatings).


2. Precipitation of the insoluble salts

(NH4)2SO4 + Ba(OH)2 → BaSO4↓ + 2NH3+ 2H2O

The soluble ammonium sulfate of the first reaction becomes the insoluble barium sulfate.


3. First consolidating reaction

Ba(OH)2 + CO2→ BaCO3↓ + H2O

Excess barium hydroxide with ambient carbon dioxide converts to barium carbonate. This has a consolidating effect.


4. The second consolidating reaction

Ba(OH)2 + CaCO3 → BaCO3↓+ Ca(OH)2

A heterogeneous reaction converts the outer regions of the calcite grains into calcium hydroxide gel.


5. Ca(OH)2 + CO2 → CaCO3↓+ H2O

A consolidating effect is achieved due to carbonation. (The reactions 4 and 5 have not yet been investigated well enough and need better understanding.)

The method should not be used when nitrate is present in high concentration, when organic binders are present and when an adhesive effect is required.

Nitrates cause the formation of barium nitrate, which is slightly soluble and leads to visible crystallization on the surface. The organic binders in tempera or oil paintings do not tolerate the high alkalinity of barium hydroxide and lead to hydrolysis or saponification. Matteini [Matteini:1991]Author: Matteini, Mauro
Booktitle: 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
Editor: Cather, Sharon
Note: The materials that constitute wall paintings have a high and open porosity, which makes them vulnerable to the effects of both liquids and gases either found in their environment or used for their conservation. Therefore, wall painting conservation strategies involving mineral treatments should be preferred for compatibility, thus ensuring good, lasting results. This text examines problems of sulfation, discussing appropriate tests to evaluate and monitor them. It describes a methodology based on the use of ammonium carbonate and barium hydroxide that has been employed in Florence for some 20 years. -- AATA*Assesses the merit of Florentine methods of wall paintings conservation based on the use of mineral treatments. Discusses serious problems of sulphation, particularly the effects of gypsum, and reviews appropriate tests to monitor them. The methodology of cleaning and conservation using ammonium carbonate and barium hydroxide is outlined. Evidence is given of the first frescoes by Fra Angelico and Sogliani - treated with 'barium'. Details photographed before conservation in 1973 and 1975 are compared with photographs taken in 1987, which demonstrate and confirm a generally satisfactory result.
Pages: 137-148
Publisher: The Getty Conservation Institute
Title: In Review: An Assessmant of Florentine Methods of Wall Painting Conservation Based on the Use of Mineral Treatments
Year: 1991
Link to Google Scholar
is of the opinion, that in old paintings, these organic binders have largely transformed to become inorganic compounds (like calcium oxalate) and the above reactions will not necessarily take place, the use of this method can therefore be justified.

Magnesium sulfate conversion[edit]

Magnesium sulfate, like gypsum, can also be converted into the slightly soluble barium sulfate and ideally become magnesium carbonate, rendering harmless the damaging salt[Friese.etal:1999]Author: Friese, Peter; Protz, A.
Booktitle: Entfeuchtung/Entsalzung 10. Hanseatische Sanierungstage FAS - Schriftenreihe Heft 10
Editor: Venzmer, H.
Pages: 211-230
Title: Salze im Mauerwerk - Möglichkeiten zur Entsalzung und zur Salzumwandlung
Year: 1999
Link to Google Scholar
.


Treatment with lead hexafluorosilicate[edit]

A chemical salt conversion using lead hexafluorosilicate was sometimes recommended for the treatment with hydrophobic restoration plasters/mortars (link), because the restoration mortars are not hydrophobic, when they are freshly applied. Salts can therefore easily migrate into the pore structure of the plaster. To reduce the risk of a rapid salinization of the restoration mortar, the pretreatment of the substrate with a "salt treatment agent" was recommended. The different manufacturers suggest a variety of combinations. The most important ones are produced on the basis of lead hexafluorosilicate. Barium containing agents are also on offer. Even though a certain effectiveness of the treatment has been established, it must be stressed that soluble lead salts are a risk to the workforce and to the environment. For this reason, attempts should be made to achieve the same effect (the prevention of a rapid salt migration into the new plaster) through the use of impregnating agents, that have a hydrophobic and a compacting effect on the capillaries. Once the agents are activated and capillary compacting and hydrophobizing takes place, a reduction of salt migration to the surface is effected. Potential problems when applying the plaster are to be considered. As active ingredients sodium silicate compounds and potassium methyl siliconate/ fluate, i.e. salts of hexafluorosilicic acid are usually present.

Lead hexafluorosilicate reacts with sulfate and chloride compounds in complex reactions, forming a variety of products, which are nearly all hardly soluble or insoluble.

The reaction may be as follows:

Na2SO4 (s) + PbSiF6(s) → PbSO4 (sls)+ Na2SiF6 (sls)

Na2CO3 (s) + PbSiF6 (s) → PbCO3(is) + Na2SiF6 (sls)

MgSO4 (s) + PbSiF6 (s) → PbSO4 (sls) + MgSiF6 (sls)

2NaCl (s) + PbSiF6 (s) → PbCl2 (sls) + Na2SiF6 (sls)

(sls - slightly soluble; s - soluble; is - insoluble)

Weblinks[edit]

Literatur[edit]

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