kSIST FprEN 17891:2023
(Main)Conservation of cultural heritage - Desalination of porous inorganic materials by poultices
Conservation of cultural heritage - Desalination of porous inorganic materials by poultices
This document specifies one method for the desalination by poultices of porous inorganic materials constituting cultural heritage. The desalination methodology can be applied to salt-loaded porous inorganic materials either 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 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.
Erhaltung des kulturellen Erbes - Entsalzung poröser anorganischer Materialien durch den Einsatz von Kompressen
Dieses Dokument legt ein Verfahren für die Entsalzung poröser anorganischer Materialien, welche kulturelles Erbe darstellen, durch den Einsatz von Kompressen fest. Die Methodik der Entsalzung kann bei salzbelasteten porösen anorganischen Materialien angewendet werden, die entweder von Salzverwitterung betroffen sind, und/oder um Konservierungsbehandlungen zu ermöglichen, die mit einer Verunreinigung durch lösliches Salz/lösliche Salze unverträglich sind, oder um bei bekannter Verunreinigung Salzschäden zu verhindern. In allen Fällen ist das Ziel der Entsalzung die Verringerung des Salzgehalts.
Darüber hinaus enthält dieses Dokument die grundlegenden Anforderungen an den Entsalzungsvorgang sowie Leitlinien für die Wahl der am besten geeigneten Kompressenbestandteile entsprechend den Eigen-schaften des Untergrunds und den Arten/Mengen des/der vorhandenen Salze(s), um den Entsalzungsprozess zu optimieren.
Conservation du patrimoine culturel - Dessalement des matériaux inorganiques poreux par application de compresses
Le présent document spécifie une méthode de dessalement par compresses des matériaux inorganiques poreux constituant un patrimoine culturel. La méthodologie de dessalement peut être appliquée aux matériaux inorganiques poreux chargés en sel, soit affectés par l’altération par les sels et/ou pour permettre des traitements de conservation incompatibles avec la contamination par le(s) sel(s) soluble(s), soit pour éviter les dommages causés par les sels lorsque la contamination est connue. Dans tous les cas, le dessalement a pour objectif de diminuer la teneur en sel.
De plus, le présent document fournit les exigences fondamentales pour l’opération de dessalement et les lignes directrices pour le choix des composants de compresse les plus appropriés selon les caractéristiques du substrat et les types/quantités de sel(s) présent(s) afin d’optimiser le processus de dessalement.
Ohranjanje kulturne dediščine - Razsoljevanje poroznih anorganskih materialov z oblogami
General Information
Standards Content (Sample)
SLOVENSKI STANDARD
oSIST prEN 17891:2022
01-oktober-2022
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: prEN 17891
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
oSIST prEN 17891:2022 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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oSIST prEN 17891:2022
DRAFT
EUROPEAN STANDARD
prEN 17891
NORME EUROPÉENNE
EUROPÄISCHE NORM
August 2022
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 draft European Standard is submitted to CEN members for enquiry. It has been drawn up by the Technical Committee
CEN/TC 346.
If this draft becomes a European Standard, 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.
This draft European Standard was established by CEN 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, Turkey and
United Kingdom.
Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are
aware and to provide supporting documentation.
Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change without
notice and shall not be referred to as a European Standard.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2022 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN 17891:2022 E
worldwide for CEN national Members.
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Contents Page
European foreword . 3
Introduction . 4
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 6
4 Symbols and abbreviations . 7
5 Principles of poultices desalination . 7
5.1 General. 7
5.2 Advection-based poulticing methods . 8
5.3 Diffusion-based poulticing methods . 9
6 Test equipment . 9
7 Methodology for the determination of the operational parameters . 10
7.1 General. 10
7.2 Selection of the poultice/s component/s . 10
7.3 Poultice formulation and required properties . 10
7.4 Poultice preparation by water addition . 10
7.5 Trial evaluation of effectiveness . 11
8 Desalination process . 12
8.1 Environmental conditions of application . 12
8.2 Preparation of the substrate . 12
8.3 Poultice application . 12
8.4 Number of applications . 13
9 Test report . 14
Annex A (informative) Desalination poultices . 15
Annex B (informative) Advection and diffusion process . 17
Annex C (informative) Workability and consistency: flow test (EN 459-2:2001) Cone
penetration (EN 413-2:1994) of poultices . 19
Annex D (informative) Identification of salt species or ions according to EN 16455. 20
Annex E (informative) Number of applications . 21
Bibliography . 24
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European foreword
This document (prEN 17891:2022) has been prepared by Technical Committee CEN/TC 346
“Conservation of Cultural Heritage”, the secretariat of which is held by UNI.
This document is currently submitted to the CEN Enquiry.
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Introduction
Salts often present in stones and other porous inorganic building material 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 encountered in building materials are sulfates, chlorides, nitrates and carbonates of the
cations 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
4 2 2 4 2 2 4 4 2
other hydrates, the chlorides halite (NaCl) and sylvite (KCl), the nitrates niter (KNO ) and nitratine
3
(NaNO ). and the carbonates thermonatrite (Na CO ·H O), trona (Na H(CO ) ·2H O. Less frequently
3 2 3 2 3 3 2 2
double salts can be observed e.g. aphthitalite (K Na(SO ) ), carnallite (KMgCl ·6H O). Minor occurrence
3 4 2 3 2
of 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 substrate of the object.
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 measure. 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 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 moisture penetration as part of a holistic
conservation approach.
Desalination is a decision that should be taken only after having adopted investigative measures which
take into account all the aspects related to damage by salts, such as the type of salts present, their
origin, their amount, distribution and location, as well as the surrounding environmental conditions.
Information on previous treatments is also necessary.
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 interior 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.
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Based on the above consideration this document describes 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 restoration workshop for movable objects.
Today a wide variety of poultices are available 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.
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1 Scope
This document specifies one method for the desalination by poultices of porous inorganic materials
constituting cultural heritage. The desalination methodology can be applied to salt-loaded porous
inorganic materials either 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 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
EN 16682, Conservation of cultural heritage - Methods of measurement of moisture content, or water
content, in materials constituting immovable 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 terminological databases for use in standardization at the following addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
3.1
advection
transport of a substance (solute) or quantity by the bulk flux of the water
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.
3.3
desalination
reduction of salt ion content in a material or substrate in order to decrease their concentration (to make
them less harmful)
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3.4
desalination poultices
appropriate water bearing materials applied on a porous inorganic material in order to reduce its
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 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
2
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
[SOURCE: EN 16455, 3.3]
4 Symbols and abbreviations
Y 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 apply the desalination poultice it is first soaked with deionized water. After that, it is applied
on the object (see Annex A).
The desalination process by poultice takes place into two steps:
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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 takes place in two different ways:
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;
b) diffusion process transports salt ions from the substrate into the poultice due to an unbalanced salt
ion concentration gradient. This is generally a slower process with respect to advection. [2]
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.
In a porous material there will be always a balance between advection and diffusion.
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 may 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 coarse pores substrate to a medium with fine
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pores, so the poultice should contain finer pores than the substrate pore sizes in order to ensure flow
from the substrate to the poultice.
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
The salt is 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
1 substrate
2 poultice
3 diffusion
4 advection
5 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, depending on the
substrate pore size range relative to that (from Pel, Heritage and Voronina [5]).
6 Test equipment
−1
a) ultra-pure water (specific conductivity ≤ 1 µS ·cm )
b) magnetic stirrer
−1
c) conductivity meter capable of measuring to ≤ 1 μS·cm
d) instrument for the analysis of anions and cations
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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 compound/s;
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, three 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 (artificial or natural aggregates as pumice, pozzolan, expanded glass, sand of different grain
size) – see Annex A “desalination poultices”;
d) other components, either mineral (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 mixtures can be prepared with varying ratios of different components to achieve these
requirements.
Each one of the possible poultice formulations will have different physical properties such as:
a) water capacity absorption (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.
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.
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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 with a drill and a mixing rod) 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 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 trial tests.
It is suggested to compare at least three different formulations in order to evaluate their relative
effectiveness.
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
2
desalination process (a minimum area of 25 cm is usually required, and larger is better, although with
reference to the size of the object).
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; (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.
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7.5.2 Measuring the amount of ions transferred from the substrate to the poultices
Transfer a dry poultice sample size 5x5 cm with 200 ml of deionized water, stir it for 3 to 5 min, 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 value of the specific conductivity is representative of the salt ions transferred from the substrate to
the tested poultice.
7.5.3 Measuring the ion content transferred from 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 a, b) allows a direct evaluation of the
effectiveness of the different poultices tested. After comparison select the appropriate poultice. [3, 9, 10,
11]
8 Desalination process
8.1 Environmental conditions of application
It is recommended to apply the poultice on dry rather than wet substrates. For many salt contaminated
porous materials 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. It is
recommended that the area is protected from rain or other water supply, and wind, before and during
the poulticing to allow the drying of the poultice.
Dry indoor conditions are be preferred (heating, ventilation.) during poulticing (RH < 60 %).
8.2 Preparation of the substrate
Before poulticing, it is recommended that dust deposits and any solid soluble or partially soluble
products (such as residu
...
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