Immobilization of salts

Author: Hans-Jürgen Schwarz
English Translation by Sandra Leithäuser

back to Salt conversion

Barium method

The so-called "barium" method is based on the fact that when a soluble barium compound is added to a solution containing soluble sulfate salts, the highly insoluble barium sulfate will be precipitated that will be immobilized given its low solubility. However, other soluble salts may remain from the counter ions of the original and barium compound added. These can in some cases be less damaging.

Gypsum conversion

The above approach is used for the conversion of gypsum , in particular for gypsum efflorescence and crusts, into barium sulfate. It 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 appropriately. The gypsum conversion is best 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

The first step is the application of an ammonium carbonate containimg poultice to the surface to be treated. This leads to the conversion of the relatively insoluble gypsum into soluble ammonium sulfate that partly migrates into the poultice. The remaining ammonium sulfate will eventually migrate and distribute itself in the material. The reaction will also form calcium carbonate )calcite) and this will have a consolidating effect. However, any calcite forming on the surface has to be removed diligently to avoid the whitening effect. Any excess ammonium carbonate decomposes into gaseous ammonia, carbon dioxide and water. It is to be taken into account that cmmonium carbonate may alter decorative mural paintings that contain proteinaceous additives.

2. Precipitation of the insoluble salts

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

The second step is the application of a barium hydroxide containing poultice. The soluble ammonium sulfate in the substrate will react to form the highly insoluble barium sulfate. The counter ions, hydroxyl and ammonium will evaporate as ammonia and water.

3. First consolidating reaction

Ba(OH)₂ + CO₂→ BaCO₃↓ + H2O

Any excess barium hydroxide will react with ambient carbon dioxide to form barium carbonate. This has a consolidating effect.

4. The second consolidating reaction

Ba(OH)₂ + CaCO₃ → BaCO₃↓+ Ca(OH)₂

There may be an exchange reaction between any existing calcite in the substrate and the barium hydroxide where the outer surface of the calcite grains turn into calcium hydroxide gel while barium carbonate may also accrue on other crystals.

5. Ca(OH)₂ + CO₂ → CaCO₃↓+ H₂O

Any calcium hydroxide formed will re-carbonate increasing the consolidating effect. The reactions 4 and 5 have not as yet been thoroughly investigated and require further studies.

IMPORTANT NOTE: The method should not be used in the presence of a high concentration of nitrates or organic binders. This approach will not provide an adhesive effect.

Nitrates will result in the formation of barium nitrate, a slightly soluble that 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 into inorganic compounds such as calcium oxalate and the above reactions will not necessarily take place, the use of this method can therefore be justified.

Magnesium sulfate conversion

Similarly, magnesium sulfate can also be converted into the highly insoluble 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.
.

Treatment with lead hexafluorosilicate

In the past, another conversion method based on the use of lead hexafluorosilicate was sometimes recommended for the treatment with hydrophobic restoration Plaster/Slurries, 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 offered.

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 and should be avoided. 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

[Friese.etal:1999]	Friese, Peter; Protz, A. (1999): Salze im Mauerwerk - Möglichkeiten zur Entsalzung und zur Salzumwandlung. In: Venzmer, H. (eds.): Entfeuchtung/Entsalzung 10. Hanseatische Sanierungstage FAS - Schriftenreihe Heft 10, 211-230.
[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.

crystallization of a dissolved salt