Desalination: Difference between revisions

From Saltwiki
Jump to navigation Jump to search
No edit summary
No edit summary
 
(38 intermediate revisions by 4 users not shown)
Line 1: Line 1:
<bibimport />
Author: [[user:Hschwarz|Hans-Jürgen Schwarz]]
Author: [[user:Hschwarz|Hans-Jürgen Schwarz]]
<br>
<br>
back to [[Measures]]
English Translation by [[user:SLeithaeuser|Sandra Leithäuser]]<br>
back to''' [[Measures]]'''
<br>
<br>


== Abstract  ==
== Abstract  ==


If there is an increased salt load, the reduction of the salts is a method to deal with. Salt reduction measures include the possibilities of cpompress desalination, alone or in combination with other methods. Reducing the salt content of an object in a water bath as well as by electric current supported means are other options. When choosing the method the protection of the object has always the first priority. By appropriate investigations the success of a measure has to be ensured.
Increased salt contamination can be reduced using different methods. These include poultice desalination, by themselves or in conjunction with other methods; the use of a water baths, or methods aided by electric currents. When choosing a method, the first priority should be towards not damaging the object. And they should be accompanied by appropriate investigations to ensure their applicability.


== Introduction==
== Introduction==


Desalination is the removal of salts and salt-forming ions out of the pore structure of porous materials such as natural rocks (sandstones, limestones, tuffs, etc.), brick or terra cotta, plaster and wall paintings, in situ at the object or on moveable objects in the workshop.
Desalination refers to the removal of salts, i.e., their ions, from the pore system of porous materials such as natural stone (sandstones, limestones, tuffs, etc.), brick or terracotta, renders/plasters, or wall paintings. Treatments can be carried out in situ, or in a workshop for movable objects.
 
The most commonly encountered salts are sulphates (Gipsum CaSO<sub><font size="1">4</font></sub><font size="1">•</font>2H<sub><font size="1">2</font></sub>O, Mirabilite (Thenardite) Na<sub><font size="1">2</font></sub>SO<font size="1">4</font>•10H<sub><font size="1">2</font></sub>O (Na<sub><font size="1">2</font></sub>SO<sub><font size="1">4</font></sub>), magnesium sulphate (MgSO<font size="1">4</font>•7H<sub><font size="1">2</font></sub>O u.a), chlorides (NaCl u.a.) und nitrates (Nitrokalit KNO<sub><font size="1">3</font></sub> u.a.). In individual cases, differnet salts can exist side by side, and a variety of salt-forming ions in the pore solution.
 
Salts are damaging the structure of porous materials and lead to dusting with sometimes great material loss. The amout of destruction and its appearence depend on the kind of crystallizing salts, the concentration of the salt solutions and the environment conditions. Particularly damaging are climate fluctuations around the[[deliquescence|Deliquescence Humidity]]point of the salts. In addition, water-soluble salts have an ingfluence on conservation measures such as strengthening, waterproofing, painting or plastering or make even such action impossible. For these reasons looking to the success and the durability of a measure the reduction of the salt content is an indispensable prerequisite.


The desalination/ salt reduction can be done in several ways <bib id="Sawdy.etal:2006" />. The use of plaster/ slurries with salt-contaminated objects <bib id="Auras:2008" /> is described [[Plaster/Slurries|elsewhere]].
The most commonly encountered salts are sulfates (Gypsum CaSO<sub><font size="1">4</font></sub><font size="1">•</font>2H<sub><font size="1">2</font></sub>O, Mirabilite (Thenardite) Na<sub><font size="1">2</font></sub>SO<font size="1">4</font>•10H<sub><font size="1">2</font></sub>O (Na<sub><font size="1">2</font></sub>SO<sub><font size="1">4</font></sub>), magnesium sulfate (MgSO<font size="1">4</font>•7H<sub><font size="1">2</font></sub>O and other hydrates), chlorides (e.g., NaCl) and nitrates (Niter KNO<sub><font size="1">3</font></sub> u.a.). In general, different salts can co-exist and a variety of salt-forming ions can be found in pore solutions.  


== [[Water Bath Desalination]] ==
Salts can damage the fabric of porous materials and lead to powdering of the surface, sometimes causing substantial loss. The amount of deterioration and its appearance depends on the type of salt(s) crystallizing, the amount of salt(s) present, and environmental conditions. Particularly damaging are climate fluctuations around the [[deliquescence|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 hindering them. For these reasons, reduction of the salt content is an indispensable prerequisite for the success and durability of a conservation measure as well as for reducing the deterioration rate of the object in question.


Diese Methode ist nur bei Objekten durchführbar, welche in eine Werkstatt transportiert werden können, im Regelfall also bei Skulpturen und Objekten, die zu einer Restaurierung abgebaut werden können.<bib id=Franzen.etal:2008/>  
The desalination/ salt reduction can be carried out by several different methods <bib id="Sawdy.etal:2006" />. The use of renders/ mortars on salt-contaminated objects <bib id="Auras:2008" /> is described [[Plaster/Slurries|elsewhere]].


Das mit Salzen belastete Objekt wird in ein Bad mit kaltem oder leicht erwärmten Wasser gelegt. Das Wasser kann umgewälzt und dabei entsalzt werden. Im einfacheren Fall wird das Wasser von Zeit zu Zeit ausgetauscht. Die Wirksamkeit der Entsalzung wird durch Messung der Leitfähigkeit des Wasserbades kontrolliert.
== [[Water bath desalination]]  ==


Grad und Geschwindigkeit der Entsalzung sind abhängig von der Größe des Objektes, der Materialart (z.B. feinporig oder grobporiger Stein), der Art der Salze und salzbildenden Ionen, deren Menge und Verteilung im Porengefüge. Oberflächennah konzentrierte Salze werden schneller entfernt als die aus tieferliegenden Bereichen. Allgemein muss bei lebensgroßen Figuren mit mehrwöchigen bis mehrmonatigen Anwendungen gerechnet werden.  
This method is only applicable for objects that can be transported to a workshop, usually sculptures and items that can be removed from their permanent location.<bib id="Franzen.etal:2008"/>


Entsalzungen im Wasserbad haben bei Eignung des Objektes generell gute Erfolgsaussichten. Besondere Risikofaktoren sind:
The salt contaminated object is placed in a bath of cold or slightly warm water. In doing so, the water can be recirculated through a deionizing system to enhance the desalination process. An easier, but less effective method is to change the water from time to time. The efficiency of the desalination is monitored by measuring water conductivity.


*die totale Durchfeuchtung des gesamten Porengefüges, Gefährdung von Farbschichten;
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 within the pores. Salts concentrated near the surface are removed faster than those found in-depth.  The treatment of life-size figures can take between a few weeks to several months.
*bei fortgeschrittenem Zerstörungsgrad: durch das Wasser wird die mürbe Oberfläche abgelöst;
*liegen Salze vor, die in mehreren Hydratstufen vorliegen können, so können Hydratationssprengungen ausgelöst werden, die zu Substanzverlust führen.


Eine Vorfestigung bei mürben Oberflächen mit geeigneten Festigungsmitteln ist u.U. möglich (z.B. Kieselsäureester). Die Entsalzung kann dadurch z.T. erheblich verzögert werden.
On suitable objects, desalination in a water bath can be successful. Specific risk factors are:


== [[Desalination compresses]]  ==
*Saturation of the entire pore structure with water: Risk for paint layers;
*Increased deterioration: Flaking of brittle surfaces;
*Presence of salts with several hydrate phases: Phase changes may induce mechanical stresses, which can cause a loss of substance to the object.


Die Kompressenentsalzung <bib id="WTA_E 3-13-01/D:2003"/> beruht auf dem Prinzip, Salze in Wasser zu lösen und aus dem salzbelasteten, porösen, mineralischen Baustoff in die Kompresse zu transportieren <bib id="Sawdy.etal:2010"/>. Der Transport von gelösten Salzen kann sowohl durch deren Eigenbewegung als auch durch die sich bewegende Flüssigkeit erfolgen. Eine Flüssigkeitsbewegung wird durch einen Feuchtegradienten (Kapillartransport) oder durch Temperatur-, Dichte- und Druckgradienten (Konvektion) ausgelöst, wobei in der Flüssigkeit gelöste Salze mit dieser transportiert werden. Dagegen führen Konzentrationsgradienten zur Eigenbewegung der Salzionen (Diffusion). Der Kapillartransport (Advektion) wird durch das Porengefüge des Baustoffes bestimmt und kann in einfacher Weise durch den Wasseraufnahmekoeffizienten charakterisiert werden <bib id="Heritage.etal:2008"/>. Die Transportrichtung der Ionen verläuft entsprechend dem Feuchtegradienten vom feuchteren zum trockeneren Bereich. Die treibende Kraft für einen Ionentransport durch Diffusion ist ein Konzentrationsgefälle. Die Ionen diffundieren entsprechend dem Konzentrationsgradienten von der höheren zur niedrigeren Konzentration. Diffusion findet auch als Oberflächendiffusion an den Grenzflächen statt. Der konvektive Transport wird durch Druck-, Dichte- und Temperaturdifferenzen hervorgerufen und kann in einfacher Weise durch die Wasserdurchlässigkeit und andere Versuche überprüft werden. Dieser Transportprozess tritt bevorzugt in größeren Poren (<nowiki>></nowiki> 0,1 mm), Rissen und Hohlstellen auf.  
A pre-consolidation of brittle surfaces with a suitable strengthening agent, such as silicic acid esters may be possible. However, this treatment may considerably delay the desalination in some cases.


Die beschriebenen Prozesse treten gemeinsam auf. In welcher Größenordnung die einzelnen Transportprozesse zur Entsalzung beitragen, hängt von den Eigenschaften des Kompressenmaterials sowie den Umgebungs- und Verfahrensbedingungen ab. Die sich einstellenden Feuchte- und Salzströme werden darüber hinaus wesentlich vom komplexen Wechselspiel zwischen Feuchtezustand, Salzverteilung und Eigenschaften des Untergrundmaterials, insbesondere desser Porosität <bib id="Pel.etal:2010"/><bib id="Lubelli.etal:2010"/>, beeinflusst.
== [[Poultices for desalination]]  ==


Die Salzreduzierung durch Kompresssen ist die am häufigsten angewandte Methode <bib id="Bourges.etal:2008"/> <bib id="Verges-Belmin.etal:2005"/>. In den letzten Jahren wurden, insbesondere auch durch die Arbeiten im EU Projekt "Desalination", große methodische Fortschritte <bib id="Sawdy.etal:2008"/>erzielt.  
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 diffusion and by movement of the fluid. This is triggered by a moisture gradient, i.e., from the humid to the dryer area (capillary action, i.e., advection) or by temperature, density and pressure gradients (convection). In contrast, concentration gradients lead to the diffusion of the salt ions. Advection is determined by the pore structure of the building material and is characterized by the water absorption coefficient <bib id="Heritage.etal:2008"/>. Diffusion is triggered by concentration gradients, i.e., from higher to lower concentration. It can also takes place as surface diffusion at interfaces. The convective transport is triggered by pressure, density and temperature differences and can be checked via water permeability tests. This transport process occurs preferentially in larger pores (<nowiki>></nowiki> 0,1 mm), fissures and voids.
The three transport processes described above take place simultaneously. The scale on which the transport processes contribute to the desalination, depends on the properties of the poultice material, on the environmental and implementation conditions. Essentially, the moisture and salt currents are influenced by a complex interplay between moisture condition, salt distribution, the properties and particularly the porosity of the substrate and that of the poultice <bib id="Pel.etal:2010"/><bib id="Lubelli.etal:2010"/>. 
Salt reduction through the use of poultices is the most common method of desalination <bib id="Bourges.etal:2008"/> <bib id="Verges-Belmin.etal:2005"/>. In the last years, important methodical improvements have been achieved, especially due to the EU- project "Desalination" <bib id="Sawdy.etal:2008"/>.


== [[Electrochemical desalination]]  ==
== [[Electrochemical desalination]]  ==


Diese Methode kann in der Werkstatt oder am Objekt vor Ort durchgeführt werden. Durch Anlegen einer Spannung wandern die Salzionen zur Anode bzw. zur Kathode. Die Elektroden müssen ebenfalls in eine Kompresse eingebettet oder in einen Mauerwerkschlitz eingemörtelt sein.  
Electrochemical desalination can be conducted in the workshop or on site, on a building. When introducing electric tension to the object the salt ions migrate to the anode or cathode depending on their charges. The electrodes have to be laid into a poultice or mortared into a gap in the masonry.  
 
Basic principles of this method are the processes known as electrokinetics and electroosmosis, respectively. If an electric field is installed with the help of electrodes, the ions migrate to the oppositely charged poles. 
Grundlage dieser Methode sind die Prozesse der Elektrokinese bzw. der Elektroosmose. Wird mit Hilfe von Elektroden ein elektrisches Feld angelegt, so wandern die Ionen zu den entgegengesetzt geladenen Polen.  
The electrochemical desalination according to the principle of electroosmosis (including the AET- '''A'''ktive '''E'''ntsalzung und '''T'''rocknung- active desalination and drying method) has been a matter of controversial discussion in the literature. For an efficient desalination a number of rod-shaped electrodes are needed, that can be mortared into crevices or drill holes. The distance between electrodes should not be more than 30 cm. A better solution appears to be the use of net shaped electrodes, that are placed in a poultice onto the surface.
A major problem consists in producing a similarly good electrical transmission on every electrode. Otherwise the current only flows to one electrode, which cannot be verified without special circuits.
The applied tension has to be on the order of some 20-50 volts. This can lead to health and safety issues when used outdoors. The method of desalination only works if sufficient moisture is present and for this reason the objects have to be kept humid.


Die elektrochemische Entsalzung nach dem Prinzip der Elektroosmose z. B. nach dem AET-Verfahren ('''A'''ktive '''E'''ntsalzung und '''T'''rocknung) wird in der Literatur kontrovers diskutiert. Allgemein benötigt man für eine wirkungsvolle Entsalzung entweder zahlreiche stabförmige Elektroden, die in Schlitze oder Bohrlöcher eingemörtelt werden. Der Abstand sollte nicht über 30cm liegen. Besser geeignet scheinen netzförmige Elektroden, die in einer Kompresse auf die Oberfläche gelegt werden.  
Due to the very complicated and not well-established discharge reaction of the ions on the electrodes, a return migration of ion complexes can take place. This particularly applies to ions with amphoteric properties (e.g., magnesium ions).
 
Furthermore, the discharge reactions can cause strong pH fluctuations, leading to a very acidic or a very alkaline environments and to damages in the vicinity of the electrodes.  
Ein großes Problem besteht darin, an jeder Elektrode einen gleich guten elektrischen Übergang herzustellen. Sonst fließt der Strom nur über eine Elektrode, was ohne besondere Schaltungen nicht überprüft werden kann.  
 
In the event of a high salt contamination, a new method, developed by Friese can be applied (<bib id="Venzmer:1991" />). In such cases the influence of electric fields serves to stop the ion transport in the material. Hence, brick sized suction cups are placed on the surface of the wall. Under vacuum conditions a liquid is led past the sample surface. The liquid moistens the surface and sets the salt ion transport into motion. The salts are virtually washed off the surface and transported away.
Die angelegte Spannung muss in der Größenordnung von einigen Zehner Volt liegen, was bei Anwendung im Freien zu Sicherheitsproblemen führen kann. Die Entsalzung nach diesen Verfahren wirkt grundsätzlich nur, wenn ausreichend Feuchtigkeit vorhanden ist. Aus diesem Grund müssen die Objekte feucht gehalten werden.
 
Aufgrund der sehr komplizierten und noch nicht hinreichend bekannten Entladungsreaktionen der Ionen an den Elektroden kann es zur Rückwanderung von Ionenkomplexen in das Objekt kommen. Dies trifft insbesondere auf Ionen mit amphoteren Eigenschaften zu (z. B. Magnesiumionen).
 
Weiterhin können durch die Entladungsreaktionen an den Elektroden sehr starke pH-Wert-Verschiebungen zu stark saurem beziehungsweise stark alkalischem Milieu führen. Schäden im Bereich der Elektroden werden dadurch hervorgerufen.
 
Im Falle hoher Salzbelastung könnte ein neues Verfahren von Friese zur Anwendung kommen (<bib id="Venzmer:1991" />). In solchen Fällen kann allein durch elektrische Feldeinflüsse der Ionentransport nicht in Bewegung gebracht werden. Deshalb werden Saugnäpfe im Ziegelformat auf die Wandoberfläche gesetzt. Unter Vakuumbedingungen wird eine Flüssigkeit an der Probenfläche vorbeigeführt. Sie benetzt und ist in der Lage, den Salzionentransport in Gang zu setzen. Diese Salze werden damit quasi abgewaschen und wegtransportiert.


== Evaluation criteria==
== Evaluation criteria==


(nach <bib id="Snethlage:1994"/>)
(according to <bib id="Snethlage:1994"/>)


Salze sind fast immer ursächlich an der Zerstörung von historischer Substanz beteiligt. Bei der Entscheidung, ob eine Entsalzung durchgeführt werden muss oder kann, sind folgende Überlegungen anzustellen:  
The presence of soluble salts is nearly always part of the reason why historic building materials have deteriorated. To determine whether a desalination should be carried out, the following points need to be considered:


''Beurteilung der Substanzgefährdung des Objekts durch die Salzbelastung''  
''Evaluation (Risk assessment) of hazards to the substance of objects affected by salt contamination''


Es ist abzuwägen, ob die Maßnahme der Entsalzung u. U. einen größeren Substanzverlust nach sich zieht als das Belassen des gegenwärtigen Zustandes. Vertretbar ist die Entscheidung, keine Entsalzung vorzunehmen, dann, wenn beispielsweise das Klima so stabilisiert werden kann, dass die vorhandenen Salze keinen Auflösungs- und Kristallisationszyklen mehr unterliegen.  
It is to be balanced, whether the desalination measures may cause greater loss to the substance, than leaving the object in its present condition. If the climate can be stabilized to a degree, so that the salts are not subject to dissolution and recrystallization cycles, then not carring out a desalination is justified.


''Zweckmäßigkeit der Entsalzung''  
''Usefulness of the desalination''


Es ist zu prüfen, ob eine Entsalzung überhaupt erfolgreich durchführbar ist. Erfolg verspricht eine Entsalzung nur dann, wenn die Salze hauptsächlich in der Oberflächenzone auftreten. Gleichmäßige Versalzungen von annähernd 1 Gew.% über die gesamte Mauerwerksstärke, wie sie häufig bei Nitratversalzung auftreten, können nicht mit Aussicht auf Erfolg behandelt werden. Hier sind andere Lösungen, z. B. Nutzungsänderungen, anzustreben.  
It is necessary to assess whether it is at all possible to carry out a successful desalination. Only if the salts are situated near the surface, can a desalination process be relatively successful. A uniform distribution of salts at approximately 1% by weight throughout the wall thickness, which commonly occurs in the presence of nitrates, cannot be treated successfully. In such cases other solutions must be considered, e.g., giving the building a different use.  


''Schonung der Originalsubstanz''  
''Protection of the original substance''  


Bei einer Entsalzung können durch das angewendete Verfahren, wie oben beschrieben, Gefahren für das Objekt auftreten. Es ist vor allem zu berücksichtigen, dass falsch aufgebrachte Kompressen unter Umständen nicht mehr von der Oberfläche abgezogen werden können.  
During desalination treatments, risks to the object can arise. For example, incorrectly applied poultices may not be easily removed from the surface and their removal may cause more damage.  


''Mögliche Beeinträchtigung anderer Konservierungsmaßnahmen''  
''Adverse effects on other conservation measures''  


Geringere Konzentrationen von Salzen gefährden nicht den Erfolg von Konservierungsmaßnahmen. Entsalzungen können dann entfallen. Allerdings läßt sich nur ein materialspezifischer Grenzwertbereich einer Salzbelastung angeben, da Porosität, Porenradienverteilung und Klima eine entscheidende Rolle spielen. Hier ist im Einzelfall nach Gutachten zu entscheiden. Es ist bekannt, dass Salze in höheren Konzentrationen eine Festigung mit Kieselsäueester oder eine Hydrophobierung beeinträchtigen. Auch die Dauerhaftigkeit der Maßnahme wird stark eingeschränkt.
Low salt concentrations do not compromise the success of other conservation measures. Desalination in such cases may therefore be omitted. However, only a material-specific threshold value for salt contamination can be specified, because the porosity, pore radius distribution and climate plays an important role. In individual cases decisions have to be made with reference to a survey or condition report. It is known that high concentrations of salts can have adverse effects on consolidation with silicic acid esters or on hydrophobic treatments. Also the durability of the measure will be affected and in the case of water repellent treatments may result in increased deterioration.


== Success control==
== Control measures==


Die Salzgehalte im Gestein, Putz oder Ziegel sind vor und nach der Anwendung zu messen, um den Erfolg der Entsalzung zu kontrollieren.  
To evaluate the effectiveness of the desalination, the salt content in stone, plaster or brick should be measured before and after the treatment.


Über Entsalzung an Gebäuden liegen nur wenige Erfahrungen vor. Grundsätzlich kann man eine befriedigende Entsalzung gleich welcher Art nur dann erwarten, wenn die Salze oberflächennah in 1-2 cm Tiefe angereichert sind. Sowohl mit Kompressen als auch mit elektrochemischer Entsalzung beträgt der Wirkungsradius nur wenige Zentimeter.  
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.  


Bisher existieren nur Musterflächen an wenigen Objekten. So konnte z.B. mit zweimaligen Kompressenansatz Bentonit/Sand/Zellulose eine NaCl-Versalzung im Spritzwasserbereich des Nürnberger Tores in Forchheim zu 90% entfernt werden, da die Versalzung nur auf den obersten Zentimeter beschränkt war.
Example: At "Nürnberger Tor" in Forchheim a NaCl contamination was successful in removing nearly 90% of the salt using a bentonite / sand / cellulose poultice, which was applied twice to the splash zone of the structure. The success of the measure was can be attributed to the fact that the salt was confined to the few outermost centimeters.


== Literature  ==
== Literature  ==
 
<!--
<bibprint />  
<biblist />  
 
-->
[[Category:Measures]][[Category:Desalination]] [[Category:Hschwarz]] [[Category:R-SLaue]] [[Category:inProgress]]
[[Category:Measures]][[Category:Desalination]] [[Category:Schwarz,Hans-Jürgen]] [[Category:R-SLaue]] [[Category:approved]]

Latest revision as of 16:53, 6 March 2024

Author: Hans-Jürgen Schwarz
English Translation by Sandra Leithäuser
back to Measures

Abstract[edit]

Increased salt contamination can be reduced using different methods. These include poultice desalination, by themselves or in conjunction with other methods; the use of a water baths, or methods aided by electric currents. When choosing a method, the first priority should be towards not damaging the object. And they should be accompanied by appropriate investigations to ensure their applicability.

Introduction[edit]

Desalination refers to the removal of salts, i.e., their ions, from the pore system of porous materials such as natural stone (sandstones, limestones, tuffs, etc.), brick or terracotta, renders/plasters, or wall paintings. Treatments can be carried out in situ, or in a workshop for movable objects.

The most commonly encountered salts are sulfates (Gypsum CaSO42H2O, Mirabilite (Thenardite) Na2SO4•10H2O (Na2SO4), magnesium sulfate (MgSO4•7H2O and other hydrates), chlorides (e.g., NaCl) and nitrates (Niter KNO3 u.a.). In general, different salts can co-exist and a variety of salt-forming ions can be found in pore solutions.

Salts can damage the fabric of porous materials and lead to powdering of the surface, sometimes causing substantial loss. The amount of deterioration and its appearance depends on the type of salt(s) crystallizing, the amount of salt(s) present, and 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 hindering them. For these reasons, reduction of the salt content is an indispensable prerequisite for the success and durability of a conservation measure as well as for reducing the deterioration rate of the object in question.

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

Water bath desalination[edit]

This method is only applicable for objects that can be transported to a workshop, usually sculptures and items 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
Link to Google Scholar

The salt contaminated object is placed in a bath of cold or slightly warm water. In doing so, the water can be recirculated through a deionizing system to enhance the desalination process. An easier, but less effective method is to change the water from time to time. The efficiency of the desalination is monitored by measuring water conductivity.

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 within the pores. Salts concentrated near the surface are removed faster than those found in-depth. The treatment of life-size figures can take between a few weeks to several months.

On suitable objects, desalination in a water bath can be successful. Specific risk factors are:

  • Saturation of the entire pore structure with water: Risk for paint layers;
  • Increased deterioration: Flaking of brittle surfaces;
  • Presence of salts with several hydrate phases: Phase changes may induce mechanical stresses, 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. However, this treatment may considerably delay the desalination in some cases.

Poultices for desalination[edit]

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 diffusion and by movement of the fluid. This is triggered by a moisture gradient, i.e., from the humid to the dryer area (capillary action, i.e., advection) or by temperature, density and pressure gradients (convection). In contrast, concentration gradients lead to the diffusion of the salt ions. Advection is determined by the pore structure of the building material and is characterized by the water absorption coefficient [Heritage.etal:2008]Title: How do conservators tackle desalination? An international survey of current poulticing methods
Author: Heritage, Adrian; Sawdy, Alison; Funke, Fredericke; Vergès-Belmin, Veronique; Bourges, Anne
Link to Google Scholar
. Diffusion is triggered by concentration gradients, i.e., from higher to lower concentration. It can also takes place as surface diffusion at interfaces. The convective transport is triggered by pressure, density and temperature differences and can be checked via water permeability tests. This transport process occurs preferentially in larger pores (> 0,1 mm), fissures and voids. The three transport processes described above take place simultaneously. The scale on which the transport processes contribute to the desalination, depends on the properties of the poultice material, on the environmental and implementation conditions. Essentially, the moisture and salt currents are influenced by a complex interplay between moisture condition, salt distribution, the properties and particularly the porosity of the substrate and that of the poultice [Pel.etal:2010]Title: Physical principles and efficiency of salt extraction by poulticing
Author: Pel, Leo; Sawdy, Alison; Voronina, V.
Link to Google Scholar
[Lubelli.etal:2010]Title: Desalination of masonry structures: fine tuning of pore-size distribution of poultices to substrate properties
Author: Lubelli, Barbara; van Hees, Rob P. J.
Link to Google Scholar
. Salt reduction through the use of poultices is the most common method of desalination [Bourges.etal:2008]Title: Comparison and optimization of five desalination systems on inner walls of Saint Philibert church in Dijon, France
Author: Bourgès, Anne; Vergès-Belmin, Veronique
Link to Google Scholar
[Verges-Belmin.etal:2005]Title: Desalination of masonries and monumental sculptures by poulticing: a review
Author: Vergès-Belmin, Veronique; Siedel, Heiner
Link to Google Scholar
. In the last years, important methodical improvements have been achieved, especially due to the EU- project "Desalination" [Sawdy.etal:2008]Title: A review of salt transport in porous media, assessment methods and salt reduction treatments
Author: Sawdy, Alison; Heritage, Adrian; Pel, Leo
Link to Google Scholar
.

Electrochemical desalination[edit]

Electrochemical desalination can be conducted in the workshop or on site, on a building. When introducing electric tension to the object the salt ions migrate to the anode or cathode depending on their charges. The electrodes have to be laid into a poultice or mortared into a gap in the masonry. Basic principles of this method are the processes known as electrokinetics and electroosmosis, respectively. If an electric field is installed with the help of electrodes, the ions migrate to the oppositely charged poles. The electrochemical desalination according to the principle of electroosmosis (including the AET- Aktive Entsalzung und Trocknung- active desalination and drying method) has been a matter of controversial discussion in the literature. For an efficient desalination a number of rod-shaped electrodes are needed, that can be mortared into crevices or drill holes. The distance between electrodes should not be more than 30 cm. A better solution appears to be the use of net shaped electrodes, that are placed in a poultice onto the surface. A major problem consists in producing a similarly good electrical transmission on every electrode. Otherwise the current only flows to one electrode, which cannot be verified without special circuits. The applied tension has to be on the order of some 20-50 volts. This can lead to health and safety issues when used outdoors. The method of desalination only works if sufficient moisture is present and for this reason the objects have to be kept humid.

Due to the very complicated and not well-established discharge reaction of the ions on the electrodes, a return migration of ion complexes can take place. This particularly applies to ions with amphoteric properties (e.g., magnesium ions). Furthermore, the discharge reactions can cause strong pH fluctuations, leading to a very acidic or a very alkaline environments and to damages in the vicinity of the electrodes.

In the event of a high salt contamination, a new method, developed by Friese can be applied ([Venzmer:1991]Title: Sanierung feuchter und versalzener Wände
Author: Venzmer, Helmuth
Link to Google Scholar
). In such cases the influence of electric fields serves to stop the ion transport in the material. Hence, brick sized suction cups are placed on the surface of the wall. Under vacuum conditions a liquid is led past the sample surface. The liquid moistens the surface and sets the salt ion transport into motion. The salts are virtually washed off the surface and transported away.

Evaluation criteria[edit]

(according to [Snethlage:1994]Title: Entsalzung
Author: Snethlage, Rolf
Link to Google Scholar
)

The presence of soluble salts is nearly always part of the reason why historic building materials have deteriorated. To determine whether a desalination should be carried out, the following points need to be considered:

Evaluation (Risk assessment) of hazards to the substance of objects affected by salt contamination

It is to be balanced, whether the desalination measures may cause greater loss to the substance, than leaving the object in its present condition. If the climate can be stabilized to a degree, so that the salts are not subject to dissolution and recrystallization cycles, then not carring out a desalination is justified.

Usefulness of the desalination

It is necessary to assess whether it is at all possible to carry out a successful desalination. Only if the salts are situated near the surface, can a desalination process be relatively successful. A uniform distribution of salts at approximately 1% by weight throughout the wall thickness, which commonly occurs in the presence of nitrates, cannot be treated successfully. In such cases other solutions must be considered, e.g., giving the building a different use.

Protection of the original substance

During desalination treatments, risks to the object can arise. For example, incorrectly applied poultices may not be easily removed from the surface and their removal may cause more damage.

Adverse effects on other conservation measures

Low salt concentrations do not compromise the success of other conservation measures. Desalination in such cases may therefore be omitted. However, only a material-specific threshold value for salt contamination can be specified, because the porosity, pore radius distribution and climate plays an important role. In individual cases decisions have to be made with reference to a survey or condition report. It is known that high concentrations of salts can have adverse effects on consolidation with silicic acid esters or on hydrophobic treatments. Also the durability of the measure will be affected and in the case of water repellent treatments may result in increased deterioration.

Control measures[edit]

To evaluate the effectiveness 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 successful in removing nearly 90% of the salt using a bentonite / sand / cellulose poultice, which was applied twice to the splash zone of the structure. The success of the measure was can be attributed to the fact that the salt was confined to the few outermost centimeters.

Literature[edit]