Immobilization of salts

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Author: Hans-Jürgen Schwarz
back to Salt conversion

Barium method

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

The conversion of gypsum , which particularily refers to the removal of gypsum efflorescence and gypsum crusts has been in use for several years [Hammer:1996]Title: Salze und Salzbehandlung in der Konservierung von Wandmalerei und Architekturoberfläche.
Author: Hammer, Ivo
and has proven to be effective, when used in appropriate cases. Gypsum conversion is carried out in five steps [Matteini:1991]Title: In Review: An Assessmant of Florentine Methods of Wall Painting Conservation Based on the Use of Mineral Treatments
Author: Matteini, Mauro
:

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 insoluble 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 corns 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]Title: In Review: An Assessmant of Florentine Methods of Wall Painting Conservation Based on the Use of Mineral Treatments
Author: Matteini, Mauro
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

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]Title: Salze im Mauerwerk - Möglichkeiten zur Entsalzung und zur Salzumwandlung
Author: Friese, Peter; Protz, A.
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Treatment with lead hexafluorosilicate

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" is 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)

Literatur

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crystallization of a dissolved salt