Desalination

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Author: Hans-Jürgen Schwarz
back to Measures

Abstract

Increased salt contaminations can be reduced using different methods. These include poultice desalination, also in combination with other methods, the reduction of the salts using a water bath or methods aided by electric currents. When choosing the method, the protection of the object must always be the first priority. The measures should be accompanied by appropriate investigations to ensure their success.

Introduction

Desalination denotes the removal of salts and salt-forming ions out of the pore structure of porous materials such as natural stone (sandstones, limestones, tuffs, etc.), brick or terracotta and plaster or wall paintings. Treatments can be carried out in situ on the object, or on movable objects in the workshop.

The most commonly encountered salts are sulphates (Gypsum CaSO₄•2H₂O, Mirabilite (Thenardite) Na₂SO4•10H₂O (Na₂SO₄), magnesium sulphate (MgSO4•7H₂O u.a), chlorides (e.g. NaCl) und nitrates (Niter KNO₃ u.a.). In individual cases, different salts can exist beside one another, and a variety of salt-forming ions can be in the pore solution.

Salts can damage the fabric of porous materials and lead to powdering of the surface, sometimes causing substantial loss. The amount of decay and its appearance depend on the kind of crystallizing salts, the concentration of the salt solutions and the environmental conditions. Particularly damaging are climate fluctuations around the Deliquescence point of the salts. In addition, water-soluble salts have an influence on conservation measures such as consolidation, treatment with hydrophobic materials and painting or plastering, often making such action impossible. For these reasons, the reduction of the salt content is an indispensable prerequisite for the success and the durability of a conservation measure.

The desalination/ salt reduction can be executed using several different methods [Sawdy.etal:2006]Title: Desalination—rubbing salt into the wound?
Author: Sawdy, Alison; Heritage, Adrian
. The use of plaster/ slurries on salt-contaminated objects [Auras:2008]Title: Poultices and mortars for salt contaminated masonry and stone objects
Author: Auras, Michael
is described elsewhere.

Water Bath Desalination

This method is only practicable for objects that can be transported to a workshop, usually sculptures and objects that can be removed from their permanent location.[Franzen.etal:2008]Title: Water bath desalination of sandstone objects
Author: Franzen, Christoph; Hoferick, Frank; Laue, Steffen; Siedel, Heiner

The salt contaminated object is placed in a bath of cold or slightly warm water. In doing so the water can be desalinated and circulated to enhance the desalination process. An easier but less effective method is to exchange the water from time to time. The efficiency of the desalination is monitored by measuring the conductivity of the water bath.

Degree and speed of the desalination depends on the size of the object, the properties of the material (e. g. fine pores or coarsely porous stone), the type and amount of salts and salt-forming ions and their distribution in the pores. Salts concentrated near the surface are removed faster than those from deeper areas. The treatment of life- size figures can take between a few weeks to several months.

On suitable objects, desalination in a water bath has a good chance of success. Specific risk factors are:

• the saturation of the entire pore structure with water: risk for paint layers;
• advanced degree of destruction: flaking of brittle surfaces;
• salts with several hydrate phases: mineral hydration may be triggered, leading to an increase in the volume of the salt, which can cause a loss of substance to the object.

A pre-consolidation of brittle surfaces with a suitable strengthening agent, such as silicic acid esters may be possible. Due to this treatment the desalination can in some cases, be considerably delayed.

Desalination Poultices

Desalination using poultices relies on the principle that salts dissolved in water are transported from the salt-contaminated, porous, mineral building materials into the poultice. The transport of salt solutions can take place both by proper motion and by movement of the fluid. The motion of a fluid is usually triggered by a moisture gradient (capillary) or by temperature, density and pressure gradients (convection).

Control measures

For monitoring the success of the desalination, the salt content in stone, plaster or brick should be measured before and after the treatment.

There are only a few experiences on the desalination of entire buildings. Basically, a satisfactory desalination of any kind can only be expected, if the salts are concentrated near the surface at 1-2 cm depth. With both poultices and electrochemical methods the desalination will only reach a few centimeters into the materials.

Example: At "Nürnberger Tor" in Forchheim a NaCl contamination was successfully removed up to 90% with a bentonite / sand / cellulose poultice, which was applied twice to the splash zone of the structure. The success of the measure was due to the fact, that the salinization was confined to the uppermost centimeters.

Literature

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