SIST EN 17891:2023

SLOVENSKI STANDARD
01-december-2023
Ohranjanje kulturne dediščine - Razsoljevanje poroznih anorganskih materialov z
oblogami
Conservation of cultural heritage - Desalination of porous inorganic materials by
poultices
Erhaltung des kulturellen Erbes - Entsalzung poröser anorganischer Materialien durch
den Einsatz von Kompressen
Conservation du patrimoine culturel - Dessalement des matériaux inorganiques poreux
par application de compresses
Ta slovenski standard je istoveten z: EN 17891:2023
ICS:
71.060.01 Anorganske kemikalije na Inorganic chemicals in
splošno general
97.195 Umetniški in obrtniški izdelki. Items of art and handicrafts.
Kulturne dobrine in kulturna Cultural property and
dediščina heritage
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN 17891
EUROPEAN STANDARD
NORME EUROPÉENNE
October 2023
EUROPÄISCHE NORM
ICS 71.060.01; 97.195
English Version
Conservation of cultural heritage - Desalination of porous
inorganic materials by poultices
Conservation du patrimoine culturel - Dessalement des Erhaltung des kulturellen Erbes - Entsalzung poröser
matériaux inorganiques poreux par application de anorganischer Materialien durch den Einsatz von
compresses Kompressen
This European Standard was approved by CEN on 23 July 2023.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2023 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 17891:2023 E
worldwide for CEN national Members.

Contents Page
European foreword . 4
Introduction . 5
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
4 Symbols and abbreviations . 9
5 Principles of poultices desalination . 9
5.1 General. 9
5.2 Advection-based poulticing methods . 10
5.3 Diffusion-based poulticing methods . 10
6 Test equipment . 11
7 Methodology for the determination of the operational parameters . 11
7.1 General. 11
7.2 Selection of the poultice/s component/s . 11
7.3 Poultice formulation and required properties . 12
7.4 Poultice preparation by water addition . 12
7.5 Trial evaluation of harmfulness and effectiveness . 13
7.5.1 General. 13
7.5.2 Measuring the amount of ions transferred from the substrate to the poultices . 13
7.5.3 Measuring the ion content in the substrate before and after desalination . 14
8 Desalination process . 14
8.1 Environmental conditions of application . 14
8.2 Preparation of the substrate . 14
8.3 Poultice application . 14
8.3.1 General. 14
8.3.2 Thickness of poultice . 15
8.3.3 Time of application . 15
8.3.4 Removal of the poultice . 15
8.4 Number of applications . 15
9 Test report . 16
Annex A (informative) Desalination poultices . 17
A.1 Clay materials . 17
A.2 Cellulosic and wooden materials . 17
A.3 Gelling materials aqueous, non-aqueous, and mixed gels . 17
Annex B (informative) Advection and diffusion process . 19
B.1 Advection . 19
B.2 Diffusion . 20
Annex C (informative) Workability and consistency: flow test (EN 459-2) Cone penetration
(EN 413-2) of poultices . 21
Annex D (informative) Identification of salt species or ions according to EN 16455 . 22
Annex E (informative) Number of applications . 23
E.1 General . 23
E.2 Trial test removing ions from a brick wall masonry . 23
E.3 Trial test: case study on a mural painting surface . 24
Bibliography . 26

European foreword
This document (EN 17891:2023) has been prepared by Technical Committee CEN/TC 346 “Conservation
of Cultural Heritage”, the secretariat of which is held by UNI.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by April 2024, and conflicting national standards shall be
withdrawn at the latest by April 2024.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national standards body.
A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organisations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Croatia,
Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland,
Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of North
Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and the United
Kingdom.
Introduction
Salts are often present in stones and other porous substrates as agents of decay of chemical, biological or
anthropogenic origin. They can originate from surface deposition of atmospheric pollutants, from
capillary transport or from external sources such as wind driven marine aerosol and from the material
itself, and may be present due to previous, unsuitable, restoration interventions.
The salts most often encountered in building materials are sulfates, chlorides, nitrates and carbonates of-
sodium, potassium, ammonium, calcium and magnesium. Frequently present are the sulfates: gypsum
(CaSO ·2H O), mirabilite (Na SO ·10H O) and thenardite (Na SO ), epsomite (MgSO ·7H O) and other
4 2 2 4 2 2 4 4 2
hydrates, the chlorides halite (NaCl) and sylvite (KCl), the nitrates niter (KNO ) and nitratine (NaNO ).
3 3
and the carbonates thermonatrite (Na CO ·H O), trona (Na H(CO ) ·2H O. Less frequently double salts
2 3 2 3 3 2 2
can be observed e.g. aphthitalite (K Na (SO ) ), carnallite (KMgCl ·6H O). Minor occurrence of
3 4 2 3 2
phosphates and nitrites can be found.
In general, several types of soluble salts coexist and the ionic species present, depending on the
conditions, can interact with each other to form complex salts or lead to crystallization phenomena within
and/or on the surface of the object.
The solubility of any individual salt within a given system varies greatly and is influenced by the
concentration and type of other salts in the system and on the temperature. Generally, the concentration
of soluble salts in the substrate is highest near the surface, though increased salt contents can occur inside
to considerable depths.
Salts can damage the fabric of porous inorganic materials and lead to different decay morphologies
sometimes causing substantial loss of material from the object. In addition, water-soluble salts have an
influence on conservation measures such as cleaning, consolidation, treatment with hydrophobic
materials and painting or plastering, often hindering them. The extent of deterioration and its appearance
depend, for a given material, on the type of salt(s) crystallizing, the amount of salt(s) present, and the
environmental conditions, as well as the presence of moisture, leading to crystallization cycles.
Reduction of the salt content (desalination) is an essential prerequisite for reducing the deterioration
rate of the object and for the success and durability of a conservation measures. However, it is also
recognized that in some systems desalination may at best be only partially successful.
Desalination by poultices is one of the most common methods used to reduce salt content from objects.
The term desalination is used to indicate a reduction in the ion content of water-soluble salts,
preferentially in the near surface layers of the substrate, rather than a removal of all salts from the
substrate at depth.
Before any intervention/application to reduce salts and their ensuing damage, it is advisable to consider
investigation and relevant interventions to prevent excessive moisture penetration as part of a holistic
conservation approach.
Desalination is a decision that should be taken only after having carried out exhaustive investigations
which take into account all the aspects related to damage by salts, such as the type of salts present, their
origin, their amount and distribution, as well as the surrounding environmental conditions. Information
on previous treatments can be necessary.
Desalination of painted substrates will require additional considerations in order to evaluate the
feasibility of desalination by poultices. Where there is a lack of cohesion, paint could be lost during
treatment and some pigments or support compounds may react adversely during the poultice treatment.
In an indoor environment, to prevent the occurrence of salt dissolution-crystallization cycles due to
relative humidity changes, it is recommended where possible to stabilize the indoor climate [1].
When all possible interventions to prevent ongoing salt contamination have been considered and carried
out, actions may be taken to reduce the quantity of salt(s) by a process of salt ion extraction also termed
“desalination”.
NOTE To mitigate the presence of salts, apart from poultices, other actions such as the use of water baths, or
sacrificial porous renders/plasters, or plant halophylous vegetation, or the application of sulphate reducing
bacteria, or crystallization inhibitors, or electrochemical methods can also be proposed. These methods fall outside
the scope of this document.
Based on the above consideration this document specifies a procedure to reduce the amount of soluble
salts/ions present in a porous inorganic material by a process of poultice desalination, outlining the
requirements for the selection of poultice components and the procedure for application and monitoring
the desalination.
Desalination by poultices refers to a removal of soluble salts, i.e. their ions, from the pore system of
porous inorganic materials such as natural stones, bricks, terracotta, mortar, render/plaster and wall
paintings. Treatments can be carried out in situ, or in a conservation/restoration workshop for movable
objects.
Today a wide variety of poultices are used as single products and mixed with argillaceous materials (clay
poultices, diatomaceous earth, bentonite, attapulgite, sepiolite) using rapid methods of application.
Desalination using poultices relies on the principle that salts dissolved in water are transported from the
salt-contaminated porous materials into the poultice. The transport of salt solutions can take place both
by ion diffusion and by movement of the fluid.
Very early on, the risk of removing the most soluble salts and leaving behind the less soluble ones (and
risking greater ensuing damage) needs to be mentioned.
Poultices should be ideally chosen which do not leave behind any residues, particularly on fragile
surfaces. Any residues which do result need to be identified and removed, if possible, to prevent harm to
the substrate. Where there is any doubt that residue removal could not be carried out successfully, a
separating layer such as Japanese paper should be used.
1 Scope
This document specifies a methodology applying poultices for the desalination of porous substrate
constituting cultural heritage. The desalination methodology can be applied:
 to salt-loaded porous inorganic materials affected by salt weathering, and/or
 to allow conservation treatments incompatible with soluble salt(s) contamination, or
 to prevent salt damage where contamination is known to be present.
In all cases the desalination aims to decrease the salt content.
Furthermore, this document gives the fundamental requirements for the desalination operation and
guidelines for the choice of the most appropriate poultice components according to the characteristics of
the substrate and types/quantities of salt(s) present in order to optimize the desalination process.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
EN 15898, Conservation of cultural heritage — Main general terms and definitions
EN 16085, Conservation of Cultural property — Methodology for sampling from materials of cultural
property — General rules
EN 16455, Conservation of cultural heritage — Extraction and determination of soluble salts in natural
stone and related materials used in and from cultural heritage
3 Terms and definitions
For the purposes of this document, the terms and definitions given in EN 15898 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https://www.iso.org/obp/
— IEC Electropedia: available at https://www.electropedia.org/
3.1
advection
transport of a substance (solute) or quantity by the bulk flux of the water typically by capillary forces
Note 1 to entry: It does not include transport of substances by molecular diffusion.
3.2
conductivity
measure of the ability of water to conduct an electrical current
Note 1 to entry: It is dependent on the amount and types of dissolved salts (or other compounds) in the water.
Note 2 to entry: Second part of the sentence was deleted as redundant.
[SOURCE: EN 16455:2014, 3.2, modified - Note 2 to entry has been added]
3.3
desalination
reduction of salt ion content in a material or substrate in order to decrease their concentration (to make
them less harmful)
3.4
desalination poultice
appropriate water bearing materials applied to porous material in order to reduce their soluble salt
content
Note 1 to entry: Clay minerals, cellulose fibres, fine sand, gels etc. are usual compounds that are mixed in specific
formulation with deionized water to make effective desalination poultices fitted to the substrate properties.
[SOURCE: EN 17138:2018, Annex A for the definition of compounds]
3.5
diffusion
process resulting from random motion of molecules or ions by which there is a net flow of matter from a
region of high concentration to a region of low concentration
3.6
moisture content
amount of water in the material, as determined in accordance with a gravimetric method specified in
EN 16682
Note 1 to entry: The MC is expressed as a mass fraction in percent (%).
3.7
soluble salt
salt that readily dissolves in a solvent such as water in order to form a solution
Note 1 to entry: Within this document the term salt refers to soluble salts.
3.8
specific conductivity
conductivity of a solution measured between two electrodes 1 cm in area and 1 cm distant
−1
Note 1 to entry: The units are μS cm .
Note 2 to entry: Apart is substituted by term “distant” which is more precise.
[SOURCE: EN 16455:2014, 3.3, modified - Note 2 to entry has been added]
3.9
solubility
maximum amount of a solute which dissolves in a solvent
-1
Note 1 to entry: The units are g l .
Note 2 to entry: Within this document the term solvent refers to water.
4 Symbols and abbreviations
For the purposes of this document, the following symbols and abbreviations apply.
γ specific conductivity
−1
IC
ion conductivity in μS cm
5 Principles of poultices desalination
5.1 General
Desalination by poultices refers to the removal of soluble salts, i.e. their ions, from the pore system of
porous inorganic materials such as natural stones, bricks or terracotta, renders/plasters, or wall
paintings.
In order to carry out desalination, the poultice it is first soaked in water. After that, it is applied on the
object (see Annex A).
The desalination process by poultice takes place into two steps:
In the first step, the water penetrates into the porous material from the poultice and dissolves the soluble
salts (Figure 1 b)).
In the second step, the “salt extraction” moves the dissolved ions from the substrate into the poultice
(Figure 1 c)).
Salt removal occurs in two different mechanisms that take place concurrently:
a) advection forces the transport of saline ions from the substrate to the poultice through capillary
water flow which is a relatively quick process. This process is dependent on the pore size distribution
of the poultice in relation to the substrate one; as well as on the rates of drying mechanism [2];
b) diffusion process transports salt ions from the substrate into the poultice due to an unbalanced salt
ion concentration gradient, that generates the outward ion movement. This is generally a slower
process with respect to advection [2].
In a porous material there will be always a balance between advection and diffusion.
The concentration difference between the salt solution within the object (high concentration) and the
water contained in the poultice (low concentration) generates an outward ion movement (diffusion).

Key
a substrate (in red) impregnated by salts
b poultice (in yellow) + substrate in the first phase
c poultice + substrate in the second phase
d substrate desalinated
Figure 1 — Desalination by poultices
The evaporation of water from the poultice to the surrounding air and the capillary transport (advection)
from the substrate is another factor of salt migration.
Finally, if salt contaminated water can be driven properly into the poultice, and if the drying front is
located within the poultice during the evaporation phase, salts can crystallize in the poultice [3].
5.2 Advection-based poulticing methods
The advection of water from a porous medium to another one can be described by the difference in pore-
size distribution (and hence capillary pressure) between the two porous media. Dissolved ions of the
salt(s) are transported by water flow during capillary suction.
During drying, the ions move towards the drying front by capillary flow (advection) and accumulate near
the surface. Salts are advected from a medium with coarser pores substrate to a medium with finer pores,
so the poultice should contain finer pores than the substrate pore sizes in order to ensure flow from the
substrate to the poultice, (second phase - Figure 1, c)).
Advection is generally quicker than diffusion, and so desalination treatments based on advection are
usually much faster.
The process of advection is described by the equation in Annex B [4].
5.3 Diffusion-based poulticing methods
Salts are transported through the water by diffusion due to the concentration gradient. For maximum
extraction efficiency the salt content of the poultice, at the beginning of the process, should be as close to
zero as possible. The process of diffusion can be described by a simple diffusion equation, which is also
referred to as Fick’s equation and successive modification (see Annex B).
Diffusion has the tendency to level off any accumulations, but in porous substrates it is a rather slow
process.
Key
S Substrate
P Poultice
D Diffusion
A Advection
X axis increasing pore size
Figure 2 — Schematic diagram illustrating the transport mechanisms (i.e. diffusion and
advection) by which aqueous ions can travel from a substrate into a poultice, relative to the
substrate pore size range (from Pel, Heritage and Voronina [5])
6 Test equipment
−1
6.1 ultra-pure water (specific conductivity ≤ 1 µS·cm )
6.2 magnetic stirrer
−1
6.3 conductivity meter capable of measuring to ≤ 1 μS·cm
6.4 instrument for the analysis of anions and cations
7 Methodology for the determination of the operational parameters
7.1 General
Several steps are required to optimize a poultice desalination system:
a) the selection of the poultice/s materials;
b) the actual poultice composition (mixture);
c) the amount of water to reach an appropriate workability and the adherence to the substrate.
Each of these factors has an influence on the efficiency of the desalination treatment.
7.2 Selection of the poultice/s component/s
Among the available materials, four main components can be used separately or in mixtures:
a) natural cellulose fibres (different lengths);
b) a mineral phase such as clay (e.g. bentonite, kaolin, attapulgite) and others, (e.g. amorphous silica);
c) fillers including artificial or natural aggregates as pumice, pozzolan, expanded glass, sand of different
grain size– see Annex A “desalination poultices”;
d) others, including inorganic (diatomite, fumed silica, rock wool) or organic materials (ion exchange
resins, gels, viscose sponges) [3].
Clays, cellulose and sand components are most frequently used [6]. Components should not be a primary
source of soluble salts; clay minerals or un-washed sand can themselves contain soluble salts. The salt
content of such materials shall be determined before selection and the result obtained should be close to
that of deionized water.
7.3 Poultice formulation and required properties
Components described previously should be mixed in appropriate proportions to achieve the required
properties for efficient salt extraction and proper adhesion to the substrate, as well as for ease of removal
after the desalination process.
The crucial properties to achieve these requirements are:
a) water content (wetting capacity) and related drying shrinkage;
b) workability, (plastic range in the case of clay);
c) adhesion to the substrate;
d) pore size distribution and volume of pores;
e) thickness of the poultice.
7.4 Poultice preparation by water addition
There are desalination poultice products available on the market; some are ready to use, some others
need to be mixed with deionized water. The specifications of these products shall be carefully checked to
ensure suitability for each particular case.
When non-proprietary (self-made) poultices are preferred, once the components have been selected and
the relative compositional ratios have been established, in order to prepare the fresh mixture to be
applied (to the surfaces to be desalinated), it is necessary to gradually add deionized water, mixing
everything in a container of suitable size until the desired workability is obtained.
The absorbent compound/water ratio shall be noted in order to be able to repeat the mixing operation
consistently.
For self-made poultices it is useful to test workability and applicability on a vertical surface according to
the desired thickness. When drying, the poultice shrinkage should not be so excessive as to cause
detachment.
In order to obtain a balance between the workability, the surface adhesion on the substrate, the
appropriate thickness of poultice and the shrinkage of the applied poultice, some preliminary tests shall
be carried out weighing the dry poultice and the water amount necessary to satisfy the balance above
cited.
Frequently the removal of soluble salts requires many successive applications, so it is necessary to repeat
the application using the same ratio established by the preliminary tests in order to reproduce the same
operational conditions. This procedure allows an effective comparison to be made on the removal
efficiency of each application [7].
The water capacity absorption of the poultice and its workability and shrinkage, should be given in the
technical data sheet. Where non-proprietary poultices are used, it is possible to establish these
characteristics by laboratory testing (see Annex C).
7.5 Trial evaluation of harmfulness and effectiveness
7.5.1 General
Each method, depending on the constituent material of the substrates, their pore size distributions, the
pore volumes, the distribution of salts inside the substrate and the type of prevalent salts, can give
different results in terms of efficiency of the desalination process. For this reason, it is very useful, once
the formulation of more than one poultice has been selected, to start comparative trial tests.
It is suggested to compare at least three different formulations in order to evaluate their relative
effectiveness, and to repeat it in a different area, especially if the surfaces to be treated are widely
extended or differently located.
The surface tested should be large enough to be representative of the entire area to be desalinated, and
to allow several tests to evaluate the efficiency of the process, to be carried out before and after the
desalination process (a minimum area of 25 cm is usually required, and larger is better, although with
reference to the size of the object).
In order to achieve greater salts extraction, it is preferable to wait until the complete drying of the
poultice before removing it, although the level of drying should be evaluated according to the type and
condition of the treated area.
The evaluation of effectiveness of salt ion extraction from a substrate shall be done following at least one
of two different methodologies:
a) measuring the total amount of salt ions transferred from the substrate to the three poultices under
comparison, by conductivity measurements (as reported in Annex E);
b) measuring the ion content (quantitative evaluation by EN 16455) distribution at different
penetration depths of the substrate before and after the poultice under test have been applied.
Sampling shall be carried out in accordance with EN 16085.
The most accurate evaluation of effectiveness is by measuring the salt distribution within the substrate.
The evaluation of harmfulness shall be done by binocular examination and macrophotography of the limit
of treated and untreated surface before and after treatment, under natural light, raking light and near UV
light. The same kind of examination shall be achieved on the contact face of the dried poultice. The aim is
to evaluate the loss of original material and the remains of poultices left.
Oil or other organic components could be mobilised by poultice water resulting in discolouration. Test
areas should be carefully chosen to allow this risk to be identified prior to treatment.
7.5.2 Measuring the amount of ions transferred from the substrate to the poultices
The value of the specific conductivity is representative of the salt ions transferred from the substrate to
the tested poultice.
Transfer a dry poultice sample size 5 cm × 5 cm with 200 ml or 10 cm × 10 cm in 800 ml of deionized
water, stir it for 3 min to 5 min in most cases, let the suspension settle for 5 min, take a portion of the
supernatant solution and measure the value of its specific conductivity.
A reference blank shall be used by measuring the conductivity of poultices before any application
(according to EN 16455, see Annex D).
The final value of conductivity to be recorded is the value of the measured specific conductivity minus
the specific conductivity of the blank poultice.
7.5.3 Measuring the ion content in the substrate before and after desalination
Soluble salt measurements (single ion species) after application(s) shall be done on dry substrate in
accordance with EN 16455 (see Annex D).
The comparison of the results recorded (according to 7.5.1 a, b) allows a direct evaluation of the
effectiveness of the different poultices tested. After comparison-the appropriated poultice should be
selected. See [3], [9], [10], [11].
8 Desalination process
8.1 Environmental conditions of application
Poultice should be applied on dry rather than wet substrates. For many salts contaminated porous
materials, especially in their dry state, the salts are concentrated beneath the surface and the water
supplied by the poultice is more easily absorbed in a dry substrate.
The procedure should be carried out at the optimal temperature (between 10 °C and 30 °C) throughout
the operation in order to facilitate the salt solubilization and to avoid too rapid evaporation. The area
should be protected from rain or other water supply, and wind, before and during the poulticing to allow
the drying of the poultice.
Low relative humidity indoor conditions should be preferred during poulticing (heating, ventilation etc.
with an RH < 60 %).
8.2 Preparation of the substrate
Before poulticing, dust deposits and any solid soluble or partially soluble products (such as residues of
mortar, salt efflorescence, black crusts etc.) should be removed with an appropriate cleaning technique.
Pre-wetting the surface is only recommended to improve the adhesion of the poultice if necessary. It shall
be limited in order to reduce the risk of driving salts that could be solubilized to greater depths in the
material during this pre-wetting phase.
8.3 Poultice application
8.3.1 General
The selected poultice will be applied to the surface to be desalinated normally by hand for flat surfaces
and/or artistically valuable surfaces, sometime applying a layer of Japanese tissue paper or soft tissue
paper between the substrate and the poultice, in order to obtain better removal and to avoid residues on
the surface.
When the substrate condition (i.e. fragile substrates) does not permit desalination a previous pre-
consolidation of the substrate can be considered.
During the application, it is very important to have good contact between the poultice and the whole-
surface of the substrate to obtain a homogeneous desalination all over the treated area.
The gun spraying application on large areas could be considered if:
a) the condition of the substrate allows it;
b) workability is measured and recorded;
c) the same effectiveness is achieved with gun spraying as by manual application.
The gun spraying should never be considered for painted or decorated surfaces.
8.3.2 Thickness of poultice
For a given formulation, the amount of water and its depth of penetration into the substrate are directly
linked to, but not only to, the thickness of the poultice.
In case of deep and/or serious contamination, it is important to apply a thick poultice to carry more water
into the substrate and have a higher absorbent porous media to store the salts. The thicker the poultice,
the higher the risk of its detachment from the substrate and its falling off due to its sheer weight, a lack
of adhesion, overloading and shrinkage. Application on overhanging surfaces requires special care.
Preliminary tests are necessary to optimize the thickness of the poultice which can be applied without
any risk of detachment or falling off.
The thickness of the applied poultice can vary from 0,5 cm to 2 cm. A thinner poultice may not be efficient
and a thicker one may be too heavy on the substrate and detach from it.
In any case, the thickness of the poultice shall be the same during the trials and the final application.
8.3.3 Time of application
The time required for application depends on the environmental conditions and is determined by the
drying rate of the poultice, as well as the area to be covered, and whether it is flat or carved.
The drying rate is affected not only by the environmental conditions (RH, T, air speed), but also by the
initial moisture content of the object.
8.3.4 Removal of the poultice
The poultice is removed with a soft tool (plastic or wooden spatula). According to the selection of the
poultice there are two kinds of desalination process, which require different times of poultice removal:
a) in case of desalination by advection (finer pores than the substrate) the poultice is removed when
dried or almost dried. As long as the contact with the substrate remains acceptable a desalination
process by advection is favoured by a longer drying process. Where there is a risk of detachment or
the poultice falling off before the end of the application, poultices should be removed when they are
still wet;
b) when desalination by advection is not possible, the poultice shall be removed frequently, to maintain
a high salt concentration gradient to get an efficient desalination by the diffusion process.
To enhance the effectiveness of both diffusion and advection mechanisms covering the poultice with a
plastic film should be considered. After a period of several days, removal of the film will allow drying
which is linked to advection of the salts.
8.4 Number of applications
In order to reduce the salt content as much as possible it is necessary to repeat the poultice application
several times. After a few applications it is useful to check the distribution of soluble salts inside the
substrate.
After each application the amount of salt removed by the poultice is determined by conductivity or ion
content measurements and the data are reported in a graph. Desalination is stopped when 2-3 successive
measurements reached an asymptotic value [8], [9], [10] (see Annex E).
9 Test report
The test report shall contain:
a) the date of the application;
b) the environmental conditions;
c) the condition survey of the object and the substrate;
d) the characterization of the substrate;
e) the location and size of the treated surface area;
f) the shape and form of the treated area;
g) the poultice components and formulation, including amount of water used, and conductivity of water
used;
h) for each area, including the tested area, the number of desalination treatments and the duration of
each application;
i) the preliminary results of tested areas;
j) the ion content of the substrate before and after desalination process;
k) the ion content removal after each application and description of the kinetics on a graphic;
l) the pictures before and after desalination;
m) any deviations from the procedure;
n) any unusual features observed.

Annex A
(informative)
Desalination poultices
A.1 Clay materials
a) Sepiolite is a fine grey-white naturally occurring clay containing hydrated magnesium trisilicate.
b) Attapulgite is a natural clay mainly composed of a hydrated magnesium aluminium silicate. Its
structure comprises three-dimensional chains, which prevent it from swelling like common clay and
makes it capable of absorbing large quantities of liquids.
c) Kaolinite is a clay mineral with the chemical composition Al Si O (OH) . Kaolin is a term used for
2 2 5 4
both the natural clayey rock rich in kaolinite and the fine-grained industrial material produced from
the rock and supplied as a white dry powder of different grades. Kaolin is a soft, absorbent and micro-
porous product.
d) Synthetic silicates, generally they are closely related to the natural clay mineral hectorite Na (Mg,
0.3
Li) Si O (OH) a tri-octahedral sheet silicate. The free-flowing powder forms a thixotropic gel when
3 4 10 2
mixed with liquid, and stays in place when applied.
A.2 Cellulosic and wooden materials
a) paper fibres, (different length);
b) cotton wool;
c) wood or paper pulp.
These may be used as a poultice material. They are used with a liquid (often water) as an inert filler for
the preparation of cleaning mixtures. Pure cellulose fibres, of hydrophilic nature, are only partly swollen
by water without dissolving and are insoluble in most solvents. Poultices made up of cellulose fibres are
soft and easy to handle.
A.3 Gelling materials aqueous, non-aqueous, and mixed gels
In case of water-sensitive substrates, gelling agents can be used to limit the amount of water released.
Many different polymers have been used for this purpose.
a) Carboxymethyl Cellulose is used for the preparation of cleaning mixtures and poultices to which it
gives thixotropic and thickening properties allowing these to be applied to stone surfaces and mural
paintings. Used at lower concentrations they offer a good flexibility to develop products with a wide
range of flow and rheological properties. These polymers are available as powders and liquid.
b) AGAR is a mixture of polysaccharides extracted mainly from red algae of Gracilaria; it is actually the
resulting mixture of two components: the linear polysaccharide agarose, and a heterogeneous
mixture of agarose main chain with substitutes called agaropectin. At room temperature the gel is
rigid due to its macro- reticulate structure. It can be used in fluid form above 38 °C to 40 °C. The
structure also allows entrapping a high amount of water either bonded to the polysaccharide
molecules or free. Usually, agar is used at concentrations lower than 5 %.
c) Gellan gum is a water-soluble anionic polysaccharide produced by the bacterium Sphingomonas
elodea (formerly Pseudomonas elodea). At room temperature the gel is rigid due to its macro-
reticulate structure.
d) Xanthan Gum is a high molecular weight water soluble polysaccharide that is widely used as a
rheology control agent for aqueous systems. It increases viscosity, helps to stabilize emulsions, and
prevents the settling of solids in a wide variety of applications. A polysaccharide extracted from
bacteria, capable of giving pseudoplastic gels stable up to 60 °C.
® 1
e) Carbopol is composed of a neutralized polyacrylic acid fr
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