Origin of Salts

<bibimport/> authors:Hans-Jürgen Schwarz

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Abstract

Salts that cause damage to wall paintings and cultural heritage made of stone, brick and other building materials, originate from natural sources or human activities. They are present in seawater, groundwater and even rainwater, as well as in soil or some stones, such as those in flood plains or near the sea. They may be part of the natural cycle of the chemical and physical deterioration of stone. Salts originating from human activities can be linked to emissions from burning fossil fuels, agricultural activities, salt or gun powder storage, some cements and the use of deicing salts. Salts are formed by the accumulation of ions in the wall or on the surface of specific parts of a building, e.g., plinths. The accumulation of ions can be due to soluble salts leaching out of building materials, soil, stone, wet or dry deposits from the atmosphere, but it can also be caused by the metabolism of micro-organisms or anthropogenic intervention.

Introduction

Every naturally occurring liquid water can be seen as a salt solution at different degrees of dilution. This can be established by simple measurement of its electrical conductivity. Porous building materials like stone, brick, mortar and plaster contain salts in their pore space. Wherever water flows through porous materials and evaporates from the surface, eventually some amount of salt efflorescence will be found.

Some salts or salt-forming ions are introduced into the monument by human activity. Apart from the historical storage of salt or gun powder, alkaline building materials like water glass or Portland cement, were used extensively in some countries over the last century. Some damages previously referred to as a result of acid rain are now known to be due to these building materials. [Klemm.etal:1999]Title: Schwefelisotopenanalyse von bauschädlichen Sulfatsalzen an historischen Bauwerken
Author: Klemm, Werner; Siedel, Heiner
Deicing salts should be added to the list of these materials, because they are still being used and they will affect mostly bridges, tunnels and foundations of buildings near roads. Other sources for damaging salts can be fertilizers and detergents that are introduced through ground and surface water. Finally, cleaning interventions, particularly used for brick masonry in some countries, is the use of alkaline materials followed by an acid rinse. As it is very difficult to estimate exactly the required amount of acid to neutralize the extra alkaline material, salts are generated.

All natural materials are contaminated with salt-forming ions, varying from low to very high levels. These include building materials, e.g., natural stone or brick, and binding agents, such as lime, cement and renders. The substratum, subsoil consistency, ground- and surface water also play a role [Roesch.etal:1993]Title: Damage to Frescoes caused by sulphate-bearing salts: Where does the salt come from?
Author: Rösch, Heinrich; Schwarz, Hans-Jürgen
. Besides natural emissions by volcanoes ((H₂S, HCl, HF, SO₂ etc.) and sea- water aerosols (NaCl etc.), [Steiger.etal:1994b]Title: Determination of wet and dry depostion of atmospheric pollutants on building stones by field exposure experiments
Author: Steiger, Michael; Dannecker, Walter
[Steiger.etal:1997]Title: Sea Salt in Historic Buildings: Deposition, Transport and Accumulation
Author: Steiger, Michael; Behlen, Andreas; Neumann, Hans-Herrmann; Willers, U.; Wittenburg, Christian
[Steiger.etal:2002]Title: Immissionsbelastung durch salzbildende Stoffe und Wirkung auf mineralische Baustoffe
Author: Steiger, Michael; Behlen, Andreas; Wiese, Utz
[Becker.etal:2005]Title: Luftschadstoffe und Natursteinschäden
Author: Becker, Karl-Heinz; Brüggerhoff, Stefan; Steiger, Michael; Warscheid, Thomas
, anthropogenic emissions are becoming more important.

The generation of natural resources and energy, emissions from traffic and so forth, produce a great number of damaging substances, of which sulfur dioxide (with the secondary product sulphate)[Wittenburg.etal:1993]Title: Ein Vergleich von Schwefeldioxid-Depositionsgeschwindigkeiten auf Naturwerksteine aus verschiedenen experimentellen Ansätzen (Comparison of sulfur dioxide deposition velocities on building stones from different experiments)
Author: Wittenburg, Christian; Mangels, Henning; Wolf, Falk; Steiger, Michael; Bothmann, Thomas; Dannecker, Walter
and nitrogen oxides (with the secondary product nitrate), are the most familiar. It is still not fully understood, how ozone (O3) and the large number of organic compounds participate in the destruction of cultural goods. Immissions mostly reach the objects through dry or wet deposition [Steiger.etal:1989]Title: Variability of aerosol size distributions above the North Sea and its implication to dry deposition estimates
Author: Steiger, Michael; Schulz, Michael; Schwikowski, Margit; Naumann, K.; Dannecker, Walter
by rainwater or fog, but also through accumulation in the subsoil.

Nitrates and organic salts such as oxalates mostly have a microbiological origin. While nitrates are transported into the object by humidity, oxalates are usually formed in situ, where they remain due to their low solubility.

Literature

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crystallization of a dissolved salt

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