Salts Resulting from Restoration Materials

Author:Hans-Jürgen Schwarz
English Translation by Sandra Leithäuser

back to Origin of Salts

Restoration materials

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

Water glass is used to consolidate surfaces. 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 water glass substance, the potassium carbonate up to 20%.

Potassium water glass: potassium carbonate is very hygroscopic and it is hardly ever found in its crystalline form. A subsequent reaction into potassium nitrate in contrast nearly always leads to efflorescence.

Acids

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

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

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

[Wihr:1986]

Wihr, Rolf (1986): Restaurierung von Steindenkmälern. Ein Handbuch für Restauratoren, Steinbildhauer, Architekten und Denkmalpfleger., München: Callwey

Water glass is the common name for a liquid solution of alkaline silicates, also known as Sodium silicate Na2SiO3 or liquid glass.

crystallization of a dissolved salt