Salts Resulting from Restoration Materials
back to Origin of Salts
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 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.
Some acids are used to clean surfaces [Wihr:1986]Title: Restaurierung von Steindenkmälern. Ein Handbuch für Restauratoren, Steinbildhauer, Architekten und Denkmalpfleger.
Author: Wihr, Rolf
, e.g., hydrochloric acid, acetic acid, formic acid and hydrofluoric acid. These can lead to the formation of salts as the acid reacts with the substrate material.
Alkaline cleaning agents
These include sodium and ammonium bicarbonate and Na or K lye, i.e., sodium or potassium hydroxide.
Carbonates may form due to the application of alkaline agents such as sodium or potassium hydroxide that will react with the CO2 from the atmosphere. These alkaline carbonates may react further in the presence or nitrate and sulfate, forming the corresponding 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 can represented by the following equation
Na2CO3 + CaSO4 → CaCO3 ↓ + Na2SO4
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, such as sodium chloride as shown by the following reaction:
NaOH + HCl → NaCl + H2O
|[Wihr:1986]||Wihr, Rolf (1986): Restaurierung von Steindenkmälern. Ein Handbuch für Restauratoren, Steinbildhauer, Architekten und Denkmalpfleger., München: Callwey|