Origin of Salts

<bibimport/> authors:Hans-Jürgen Schwarz

Back to Fundamentals

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 naturally occur in rainwater, seawater, groundwater or soil and sometimes they are 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, the use of road salt or cements. 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 found water can be seen as a more or less diluted salt solution. This can be proved by simple measurements of the electrical conductivity. Porous building stones, mortars and plasters contain salts in their pore space. Wherever water flows through porous materials and evaporates on a surface you can find more or less salt efflorescences.
Some salts or salt-forming ions are brought to the monument by humans. These can be alkaline building materials like water glass or Portland cement which were used in great quantities in the last century or road salt in wintertime. Some of the damages that were referred to as a result of acid rain are now known as the result of these building materials. [Klemm.etal:1999]Title: Schwefelisotopenanalyse von bauschädlichen Sulfatsalzen an historischen Bauwerken
Author: Klemm, Werner; Siedel, Heiner
Road salt is also one of the materials that can be detected on bridges and foundations, some buildings were even used as salt storages. Every natural material contains a certain amount of salt-forming ions that can be marginal, but also very high. These can be building materials like natural stone and bricks, adhesives like lime or cement, the building environment with its different kinds of soil and the ground- and surface water [Roesch.etal:1993]Title: Damage to Frescoes caused by sulphate-bearing salts: Where does the salt come from?
Author: Rösch, Heinrich; Schwarz, Hans-Jürgen
, but also the different coatings and paints on the surface. Beside natural emissions by volcanoes (H₂S, HCl, HF, SO₂ etc.), particulate material from sea (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 become more and more important. The extraction of resources, generation of energy, traffic and much more produce a great number of damaging salts. The most familiar substances are sulphur 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). It is still quite unknown, how ozone (O₃) and the large number of organic compounds participate in the destruction of cultural goods. Immissions mostly reach the objects by 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 by fortification on the building site.

Nitrates and organic salts like oxalate mostly have a microbiological provenance. While nitrates are transported into the object by humidity, oxalates are originated on the spot and stay there due to their low solubility.

Other sources for damaging salts can be fertilisers or detergents that are introduced by ground and surface water.

Literature

[Filter missing]

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