Salts Resulting from Restoration Materials
Author:Hans-Jürgen Schwarz
English Translation by Sandra Leithäuser
back to Origin of Salts
Restoration materials[edit]
Many restoration materials contribute to the salt contamination of objects. Sometimes they can even be the main source of damage.
The most relevant are the alkaline and acidic cleaning or restoration products such as:
Water glass[edit]
Water glass was used in the past to consolidate surfaces and was the precursor to the current silicate esters based consolidans. Water glass is a sodium (or potassium) silicate and the following reactions can be visualized schematically:
Na2SiO3 (Na-water glass) + CO2 + H2O → SiO2 Gel + Na2CO3
K2SiO3 (K-water glass) + CO2 + H2O → SiO2 Gel + K2CO3
The resulting sodium carbonate can constitute up to 30% of the deposited material, and the potassium carbonate up to 20%.
Potassium carbonate is very hygroscopic and it is hardly ever found in its crystalline form. A subsequent reaction into potassium nitrate will lead to efflorescence of this latter salt.
Acids[edit]
Some acids are used to clean surfaces, e.g. hydrochloric acid, acetic acid, formic acid and hydrofluoric acid. Accordingly, these lead to the formation of salts.
Alkaline cleaning agents[edit]
These include sodium and ammonium bicarbonate and Na or K lye.
Carbonates may form due the application of alkaline agents. These then point to the corresponding salts. Further reactions, in the presence of nitrate and sulfate, may also point to the related salts.
The presence of an alkali-containing building material may be concealed in the presence of "gypsum" within the pore solution, because, due to calcite precipitation, no natrite or thermonatrite efflorescence occurs, but thenardite/mirabilite efflorescence occurs instead. The simplified reaction has been approximately summarized in the following equation
Na2CO3 + CaSO4 → CaCO3 ↓ + Na2SO4
Neutralization reaction[edit]
Neutralization is often carried out in the process of cleaning objects with alkaline detergents (or respectively acids). Hydrochloric acid, for example, is used for neutralizing some alkaline detergents. Apart from the fact that it is difficult to determine the success of neutralization on every part of the object, this process causes the formation of easily soluble salts, for example following this reaction:
NaOH + HCl → NaCl + H2O
Literature[edit]
| [Wihr:1986] | Wihr, Rolf (1986): Restaurierung von Steindenkmälern. Ein Handbuch für Restauratoren, Steinbildhauer, Architekten und Denkmalpfleger., München: Callwey |  |