Desalination: Difference between revisions
m (Protected "Desalination" ([edit=sysop] (indefinite) [move=sysop] (indefinite))) |
No edit summary |
||
| Line 1: | Line 1: | ||
<bibimport /> | <bibimport /> | ||
<br> | |||
Author: [[user:Hschwarz|Hans-Jürgen Schwarz]] | Author: [[user:Hschwarz|Hans-Jürgen Schwarz]] | ||
<br> | <br> | ||
| Line 8: | Line 9: | ||
== Abstract == | == 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== | == Introduction== | ||
Desalination | 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 ( | The most commonly encountered salts are sulphates (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 sulphate (MgSO<font size="1">4</font>•7H<sub><font size="1">2</font></sub>O u.a), chlorides (e.g. NaCl) und nitrates (Niter KNO<sub><font size="1">3</font></sub> 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 | 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|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 | The desalination/ salt reduction can be executed using several different methods <bib id="Sawdy.etal:2006" />. The use of plaster/ slurries on salt-contaminated objects <bib id="Auras:2008" /> is described [[Plaster/Slurries|elsewhere]]. | ||
== [[Water Bath Desalination]] == | == [[Water Bath Desalination]] == | ||
This method is practicable | 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.<bib id=Franzen.etal:2008/> | ||
The object | 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 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. | 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 | *the saturation of the entire pore structure with water: risk for paint layers; | ||
*advanced degree of destruction: flaking of | *advanced degree of destruction: flaking of brittle surfaces; | ||
*salts with several | *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 suitable strengthening | 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 | == [[Desalination Poultices]] == | ||
Desalination | 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). | ||
<!-- | <!-- | ||
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. | 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. | ||
| Line 76: | Line 77: | ||
''Zweckmäßigkeit der Entsalzung'' | ''Zweckmäßigkeit der Entsalzung'' | ||
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. | 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. | |||
''Schonung der Originalsubstanz'' | ''Schonung der Originalsubstanz'' | ||
| Line 86: | Line 88: | ||
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. | 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. | ||
--> | --> | ||
== | == 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: | 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 == | == Literature == | ||
Revision as of 07:22, 5 October 2011
<bibimport />
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
[Filter missing]