10.6092/issn.1973-9494/1393
Consolidation and protection are two of the principal kinds of treatments through which the decay of old statues, stone facades, plasters and mural paintings caused by both natural atmospheric agents and, above all in the last five decades, by atmospheric pollution, is faced.
The most traditional approach has been and is mainly based on the use of organic polymeric materials.
They offer the advantage of easy application procedures and the possibility to obtain, at short times, very satisfying results. Different is their behaviour at long times. Some drawbacks come out over time both under the esthetical point of view as well as to the durability, compatibility and efficacy.
Particularly critical is the situation when porous materials and soluble salts - gypsum above all - are simultaneously present. In such a situation inorganic treatments demonstrate to be much more appropriate. They assure durable and compatible results.
In the present paper two of the most efficient and appropriate inorganic methods are reviewed in detail: the barium hydroxide method, both as desulfating and consolidating agent, and the ammonium oxalate method as passivating agent, consolidant and as a treatment capable of improving the natural colour contrast of the stone, when it is lost due to decay processes.
application/pdf
1973-9494
1393
en
Mauro Matteini
Conservation Science in Cultural Heritage
Inorganic treatments for the consolidation and protection of stone artefacts
Text.Serial.Journal
https://conservation-science.unibo.it/
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Articles
2009-04-02
2008-04-17
2015-11-02
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Mauro Matteini
2008
10.6092/issn.1973-9494/1393
13
1973-9494
en
27
Inorganic treatments for the consolidation and protection of stone artefacts
https://conservation-science.unibo.it/article/download/1393/766
8
I
    NORGANIC TREATMENTS FOR THE CONSOLIDATION AND
    PROTECTION OF STONE ARTEFACTS AND MURAL PAINTINGS


Mauro Matteini
Istituto per la Conservazione e la Valorizzazione dei Beni Culturali (ICVBC)
Consiglio Nazionale delle Ricerche, Firenze




1. Principal decay mechanisms affecting different kind of stones

    The expression stone materials is referred here both to natural stones as well as to
artificial stones such as mortars used for plasters, etc.
    Stone materials are characterized by the following main properties: a prevalent or
exclusive mineral-inorganic nature; hydrophilic properties. Both these aspects are very
important when planning treatments for their conservation and have never to be forgot-
ten.
    When possible mechanisms of damage are considered as respect to stone materials,
two main categories have to be mentioned: acid attack (caused by rains and humidity
condense in polluted urban atmospheres); soluble salts cyclic crystallization. The first



                                                                                                   Conservation Science in Cultural Heritage, 8/2008
mechanism (acids) induces corrosion to carbonatic materials such as calcium and/or
magnesium carbonates based stones while silicatic stones are only poorly affected.
    The second mechanism (salts) is mainly active towards porous stones, independent-
ly of their nature. Less important is the effect towards compact low porosity stones. Decay


Table 1. Decay risk of different typologies of stone materials used in art objects.




                                                                                              13
                                                                                      Table 2. Provenance of decay agents.

                                                                                                             From outside                            From inside

                                                                                       Acids      atmospheric gas pollutants                                 /

                                                                                                                                          – capillary rise
                                                                                        Salts     deposit of atmospheric particulate       – water infiltration from neighboring
                                                                                                                                             architectonic structures



                                                                                      processes are also strongly dependent on the kind of porosity. Usually small pores
                                                                                      increase degradation caused by soluble salts.
                                                                                         The scheme in tab.1 summarizes and simplifies what has been considered above.
M. Matteini - Inorganic treatments for the consolidation and protection of stone...




                                                                                         The next scheme (tab. 2) puts in evidence the provenance of the decay agents (acids
                                                                                      and salts), a further important point that has to be taken into consideration.

                                                                                      2. Consolidation and protection

                                                                                         It is well known that consolidation and protection are among the most important oper-
                                                                                      ations that are usually carried out in conservation work on stone objects.
                                                                                         Consolidation (structural consolidation is not considered here) is an operation
                                                                                      addressed to stone materials affected by loss of cohesion, with the aim of re-building it.
                                                                                      This is normally obtained through the permeation of special agents in liquid form (mainly
                                                                                      solutions) in the bulk of the stone materials, close to the surface. In most of cases, loss
                                                                                      of cohesion is limited to regions located near the surface at a major or minor depth. After
                                                                                      the impregnation step, consolidation of the system occurs due to different mechanisms,
                                                                                      among which solvent evaporation and special chemical reactions are the more frequent-
                                                                                      ly used. A texture is formed which, at some extent, is able to re-build lost cohesion. The
                                                                                      final effect has never to prevail the original mechanical properties (cohesion) of the sound
                                                                                      stone material.
                                                                                         Protection is an operation through which the surface of a stone artefact is preserved
                                                                                      by the action of decay agents. We shall not forget that decay agents can reach the sur-
                                                                                      face both from outside (acid attack and salts deposits) and inside (soluble salts), in case
                                                                                      of porous materials (see tab. 2).

                                                                                      3. The organic polymeric products

                                                                                         Traditionally, the problem of consolidation and protection of stone materials has been

                                                                    14
mainly faced with the use of organic hydrophobic products, both natural products, in the
past, and artificial synthetic products, more recently.
    The reasons of that are various:
–   organic materials (in solution form) are easy to be applied;
–   positive effects are immediately visible;
–   organic materials are usually free from immediate secondary negative effects;
–   they are said to be reversible;
–   the formation of a coating satisfies the general concept that protection has to be
    assured with respect to external decay agents.
    Together with pros, as in any restoration treatment, cons have to be considered.
Problems arise in particular situations and especially over time:
–   while short-term effects are generally good, long-term behaviour is often not satisfy-
    ing (because of alteration of aspect, loss of performance, etc.);
–   reversibility is often misunderstood or impossible. Reversibility is practically impossi-




                                                                                                     Conservation Science in Cultural Heritage




Figure 1. Behavior of not treated porous stones is compared with that of stones treated
with surface hydrophobic organic agents used for protecting or consolidating them.

                                                                                                15
                                                                                           ble in the case of consolidation because the operation itself (rather than the product)
                                                                                           is irreversible. In the case of protection it is often impossible because the product itself
                                                                                           (poly-syloxanes, for instance) is irreversible;
                                                                                      –    the most serious problems arise when objects made of porous materials exposed out-
                                                                                           doors are considered. In that case the effect of soluble salts sub-florescences is evident.
                                                                                           In fig. 1 the drawing puts in evidence the mechanism through which porous stones
                                                                                      affected by soluble salts may loose their integrity due to disaggregating processes
                                                                                      caused by internal crystallization of soluble salts (subflorescences) favoured by surface
                                                                                      hydrophobic treatments.

                                                                                      4. The mineral inorganic approach: the barium hydroxide method

                                                                                           At the end of the ’60 Enzo Ferroni, professor of chemistry at the University of
M. Matteini - Inorganic treatments for the consolidation and protection of stone...




                                                                                      Florence, together with Dino Dini, private restorer, found out a new treatment for de-sul-
                                                                                      fating frescoes. Since that time, that treatment is known as “Method of Barium Hydroxide”
                                                                                      or “Method of Ammonium/Barium”.
                                                                                           As it will be seen later the Barium method revealed not to be only useful for desulfat-
                                                                                      ing frescoes but also for efficiently consolidating them.
                                                                                           It was not the first time that mineral-inorganic treatments had been proposed for the
                                                                                      conservation of stone artefacts. In the second half of the XIXth century many patents
                                                                                      appeared with the proposal of various inorganic products for the consolidation of stone.
                                                                                      Among them sodium and potassium silicates, fluosilicates and barium hydroxide itself
                                                                                      may be mentioned.
                                                                                           Nevertheless, given that at the end of the century they were no longer mentioned we
                                                                                      have to think that most of them evidently had failed.
                                                                                           Coming back to the barium hydroxide method, as it has been anticipated, Ferroni and
                                                                                      Dini, initially, proposed that kind of treatment, not for consolidating but for de-sulphating
                                                                                      frescoes affected by gypsum cyclic crystallization and consequent disgregation.
                                                                                           Only after the first applications they realized of the other important function, the con-
                                                                                      solidating effect assured by the carbonation of barium hydroxide:

                                                                                      1)                            Ba(OH)2 + CO2 → BaCO3 + H2O

                                                                                      Anyhow, sulfation and loss of cohesion are closely related.
                                                                                           Sulfation mainly refers to destructive effects caused in stone materials (plasters
                                                                                      above all, but porous stones as well) by gypsum cyclic crystallization. Gypsum is a spe-
                                                                                      cial salt, according to its water solubility.

                                                                    16
Table 3. Molar water solubility of salts commonly found in stone materials compared to
that of gypsum (logarithmic values).




   Table 3 illustrates the logarithm of molar solubility in water of various salts that can be
frequently found in stone materials, compared to that of gypsum. Gypsum is taken as a
reference soluble salt due to both its particular solubility as well as to its high diffusion in
all kinds of stone materials.
   Water solubility of gypsum is around 2% in water and does not practically change with
temperature. It can be considered a slight solubility (that means a high tendency to pro-
duce saturated solution and consequent crystallization) but enough great to cause dam-
age.
   Water solubility of the majority of the other salts is much higher than that of gypsum.
They can be considered highly soluble substances. Not frequently, their concentration                   Conservation Science in Cultural Heritage
within a porous stone can overcome the value of saturation. A minor group of salts has a
much lower solubility compared to that of gypsum. Actually, they can be considered insol-
uble salts, and for that not harmful.
   Now, what normally happens with gypsum?
   In normal climatic condition such as those usually found in churches, palaces and
other protected environments, crystal formation may be very slow. Crystal growth may
then prevail to nucleation, causing serious damages also when low amounts of gypsum
are inside the pores. Locally, the volume of gypsum crystals can exceed pore size, caus-
ing subflorescences and disgregation (fig. 2).
   The barium method, through the two following poultice treatments (with ammonium

                                                                                                   17
M. Matteini - Inorganic treatments for the consolidation and protection of stone...




                                                                                      Figure 2. Blistering caused by crystal growth of gypsum under the paint layer in a mural
                                                                                      painting (top left and right). A single pustule observed in cross section under optical
                                                                                      microscope (bottom left) and with SEM/EDS (bottom right).


                                                                                      carbonate, the first; with barium hydroxide, the second), is able to stop sulfation caused
                                                                                      by gypsum:

                                                                                      2)                    (NH4)2CO3 + CaSO4 → (NH4)2SO4 + CaCO3
                                                                                      3)                 Ba(OH)2 + (NH4)2SO4 → BaSO4 + 2 NH3 + 2 H2O

                                                                                      Successively, through reaction 1), cohesion lost due to sulfation is rebuilt.
                                                                                           The great success of this method for the conservation of mural paintings, in years and
                                                                                      decades after the first interventions, is justified by the double action guaranteed by the
                                                                                      treatment.
                                                                                           Now, as far as the consolidation mechanism is concerned, the following two further
                                                                                      reactions (lime neo-formation and carbonation), in addition to reaction 1), could also con-
                                                                                      tribute:

                                                                    18
4)                      Ba(OH)2 + CaCO3 → BaCO3 + Ca(OH)2
5)                          Ca(OH)2 + CO2 → CaCO3 + H2O

Laboratory testing is still in progress. Given the above reactions confirm to be important,
different operative condition should be adopted (i.e. much longer application times).
     A third mechanism is also worth of attention: direct reaction between ammonium car-
bonate and barium hydroxide.

6)                 Ba(OH)2 + (NH4)2CO3 → BaCO3 + 2 NH3 + 2H2O

Rapid ion reaction between barium and carbonate ions would not produce a consolidat-
ing effect but only an useless precipitate within the pores. Given this is experimentally
confirmed a further variation in the application condition should be adopted. A preliminary
treatment with distilled water poultice would be useful to cause migration of ammonium
carbonate toward the interior of the plaster, far from the surface, before barium hydrox-
ide is applied.
     Laboratory testing to ascertain the role of each of the above mentioned reactions is
in progress. It is not an easy task because of the necessity to create special conditions
able to isolate each single reaction.
     Now the question is: why the barium method, a kind of treatment that demonstrated
to be so appropriate and successful in the conservation of mural paintings, was only
episodically used for the consolidation of stone artefacts?
     Due to different reasons is not easy to give an answer. Actually, only episodic appli-
cations have been reported in the literature of barium on marble objects. No systematic
experimental testing and/or scientific investigation have been carried out, with the excep-
tion of an unique research work now in progress within the research European Project               Conservation Science in Cultural Heritage
EUARTECH; but about this, results are not yet available. According to my personal opin-
ion no objective obstacle should subsists to obtain with the barium method applied on
stone objects the same excellent results experimented on decorated mural surfaces. This
is probably true not only with regards to marble and limestones but also to sandstones
and much likely to clay based materials, such as adobe and similar. Only laboratory and
in situ testing is necessary and this is planned tobe done in the next months.

5. A further mineral inorganic approach: the ammonium oxalate (AmOx) method
     AmOx as protecting passivating agent

     About 15 years ago the first experiments were carried out in the Laboratories of the


                                                                                              19
                                                                                      Opificio delle Pietre Dure in Florence in order to test the possibility of an innovative
                                                                                      approach to the problem of protecting marble and limestone monuments and artifacts
                                                                                      exposed outside.
                                                                                           In place of hydrophobing their surfaces by applying hydro-repellent barriers, it could
                                                                                      be convenient to make their surfaces acid-resistant through a passivating treatment.
                                                                                           The new approach could result particularly advantageous and important in case of
                                                                                      porous calcareous materials exposed outdoors, particularly in polluted urban centers.
                                                                                           This category of artifacts are highly exposed to acid rain and humidity condensation
                                                                                      and also affected by salts migration followed by cyclic crystallization in their porous struc-
                                                                                      ture with formation of very devastating sub-florescences, that are favoured by hydropho-
                                                                                      bic protective treatments.
                                                                                           An appropriate passivating acid resistant agent could be calcium oxalate as demon-
M. Matteini - Inorganic treatments for the consolidation and protection of stone...




                                                                                      strated by the long lasting naturally formed calcium oxalate patinas on the majority of
                                                                                      monuments.
                                                                                           Naturally formed patinas are usually coloured due to impurities that remain entrapped
                                                                                      in the oxalate matrix during the formation (fig. 3). Pure calcium oxalate (both the mono-
                                                                                      hydrate Whevellite CaC2O4.H2O, and the di-hydrate Weddellite CaC2O4.2H2O) is color-
                                                                                      less or whitish.
                                                                                           A suitable treatment to passivate calcium carbonate through a moderate and con-
                                                                                      trolled formation of calcium oxalate was found in the following reaction with ammonium
                                                                                      oxalate (AmOx).

                                                                                      7)              CaCO3 + (NH4)2C2O4 → Ca C2O4 + 2 NH3 + H2O + CO2




                                                                                                                                                      Figure 3. Cross section of a
                                                                                                                                                      typical yellowish natural
                                                                                                                                                      calcium oxalate patina on a
                                                                                                                                                      marble monument.

                                                          20
Figure 4. Logarithm of
molar water solubility of
calcium oxalate com-
pared to calcium carbon-
ate and gypsum.



Water solubility of calcium oxalate, is shown in the graph in fig. 4, compared to those of
calcium carbonate and gypsum. CaC2O4 has a very low solubility, which is maintained,
differently from CaCO3, also in a strong acidic environment. It is the ideal passivating
agent for limestones objects in urban acidic atmosphere.
   A significant test for demonstrating passivation was obtained by treating 5 small
Travertine cubes with the AmOx method. Couples of one treated and one not treated
Travertine samples were then immersed in 5 acidic solution at pH 5, 4, 3, 2, 1. In the
buffered acidic solutions Ponceau Red was also added. Ponceau Red is a highly sensi-
tive red staining agent for free Ca ions. The not treated samples strongly red stained at
pH 4, 3, 2, 1 while the treated samples did not stain up to pH 1.
   The passivation process can be interpreted as the formation of a calcium oxalate shell
on calcium oxalate pores surfaces till a certain depth from the surface. Precise analytical        Conservation Science in Cultural Heritage
mapping of the above distribution have not yet been obtained. EDS/SEM cannot distin-
guish among CaC2O4 due CaCO3. FTIR Microscope mapping is not enough sensitive.
MicroRaman Spectroscopy investigation seems to be promising. Work is in progress.
   Extensive laboratory investigation and testing have been carried out at the Scientific
Laboratories of the Opificio in Florence and, in the following years, at the Istituto di
Scienze e Tecnologie Molecolari (ISTM/CNR) in Perugia and at the ICVBC/CNR in
Florence and Milan.
   About 10 years ago the first applications started on monuments and artifacts in
Florence and many other parts of Italy. Among the many objects treated with this method
we want to mention the big marble statue ‘Eterno Padre’ by Baccio Bandinelli and the

                                                                                              21
                                                                                      graffiti façade of Palazzo Barbolani Montauto, both in Florence. A further more recent
                                                                                      example of AmOx passivating treatment is that of the Portale in Candoglia Stone of Santa
                                                                                      Maria delle Grazie in Milan.

                                                                                      AmOx to improve color saturation

                                                                                           A second positive effect induced by AmOx treatment is a moderate but very interest-
                                                                                      ing increase of the chromatic contrast (color saturation). This effect is probably due to
                                                                                      lowering of surface micro-roughness and consequent reduction of light scattering after
                                                                                      treatment.
                                                                                           In deep laboratory investigation has been recently carried out at the ICVBC in
                                                                                      Florence to investigate this point on more than 20 different types of limestones. Color
                                                                                      measurements, water absorption, crystalline structure of the surface under optical micro-
M. Matteini - Inorganic treatments for the consolidation and protection of stone...




                                                                                      scope, SEM, etc. were carried out. Results were presented at the Conference in memo-
                                                                                      ry of Cesare Brandi in Lisbon, May 3-5, 2006.
                                                                                           In the most of cases slight color change or no color change is observed. In some lim-
                                                                                      ited cases more evident change is caused. Apparently, it depends on the presence of free
                                                                                      Iron(III) ions in stones or mortars. When some discoloration is caused by the treatment,
                                                                                      it occurs immediately. It can be observed during preliminary tests on small areas and the
                                                                                      AmOx treatment is not applied. No long term discoloring effects have been observed.
                                                                                           As said above, usually, color saturation is often observed after the treatment and this
                                                                                      can be considered a positive effect. Actually, decay processes usually induce increase of
                                                                                      surface roughness in stone and decorated plaster surfaces with a consequent light scat-
                                                                                      tering effect (whitish appearance more easily visible in dark areas). AmOx treatment is
                                                                                      often able to reduce this negative optical appearance by rebuilding more homogeneous
                                                                                      surface texture.

                                                                                      AmOx as an efficient neutral desulfating agent

                                                                                           A third interesting action assured by AmOx treatment is desulfating action towards
                                                                                      gypsum. Gypsum is partially or totally converted into calcium oxalate, according to reac-
                                                                                      tion 8):

                                                                                      8)                   CaSO4 + (NH4)2C2O4 → Ca C2O4 + (NH4)2SO4

                                                                                           AmOx reacts with gypsum in a similar way it reacts with calcium carbonate. Slightly
                                                                                      soluble gypsum is transformed into very insoluble calcium oxalate, favored by a great sol-
                                                                                      ubility difference between the two species (fig. 5). Residual ammonium sulfate can be

                                                                22
Figure 5. Logarithm of
water solubility of calcium
oxalate compared to that
of gypsum and calcium
carbonate.


easily removed by washing the treated surface with de-ionized water (stones) or by des-
orbing it with cellulose powder or sepiolite poultices (plasters).
    Efficient de-sulfation is obtained with a final result comparable to that of barium
method, with the advantage of no alkaline environment. Actually, AmOx water solution is
practically neutral. Drawbacks can arise from a minor translucency of calcium oxalate
compared to that of calcium carbonate. When gypsum is located over the surface, some
whitish effect after the treatment may be observed. In that case, surface desulfation is
first needed, with anion exchange resins.
    The AmOx based desulfating method has been applied to the facades of the Pieve of
Santa Maria and the Church of San Domenico, both in Arezzo, with good results as
demonstrated by the analyses carried out by the ICVBC/CNR.

AmOx as a consolidating agent for marble and other limestones

    A further very promising action induced by AmOx treatment is a good consolidating             Conservation Science in Cultural Heritage
effect.
    The consolidating action of AmOx was first observed on highly disaggregated marble
(the so called ‘sugar-like’ marble). Preliminary experiments have been carried out both in
the laboratory and as pilot tests, in situ, on small areas. At the moment most of the
research in this direction has to be developed. Promising results have been recently
obtained on a big fragment of a heavy decayed marble, a “tortiglione’” column removed
from Florence Cathedral to be replaced with a new one. The column showed blackish
gypsum incrustation on most of its surfaces and a very serious de-cohesion of the mar-
ble underneath. The piece has been first cleaned with ammonium carbonate poultice in
order to remove gypsum incrustations and then treated with AmOx (5% water solution

                                                                                             23
                                                                                      dispersed in cellulose poultice, for 24 hours). Consolidating effect was measured by
                                                                                      means of the “drilling method ”. Evident increase of cohesion was demonstrated up to a
                                                                                      a depth of about 5 cm.

                                                                                      6. Conclusion and future research

                                                                                         Inorganic treatments received much more attention in the last decade compared to
                                                                                      the past. Barium Hydroxide and Ammonium Oxalate are at the moment the most promis-
                                                                                      ing agent for the consolidation and protection of stones and decorated plasters. They are
                                                                                      usually less simple then the organic polymeric products but they some important advan-
                                                                                      tages such as durability compatibility and more appropriateness in case of porous matrix-
                                                                                      es affected by salts crystallization.
                                                                                         The review illustrated above put in evidence the multiple action accomplished by
M. Matteini - Inorganic treatments for the consolidation and protection of stone...




                                                                                      these kind of treatments.
                                                                                         Nevertheless, a lot of further research is necessary in order to:
                                                                                      ü better understand their mechanisms of action;
                                                                                      ü study their behavior on matrixes different from the ones on which they were initially or
                                                                                         traditionally tested and applied;
                                                                                      ü investigate new procedures of application more appropriate to some of their multiple
                                                                                         functions (much longer treatment times; treatment by immersion, etc.).
                                                                                         Further research is in progress to improve the quality of results that can be obtained
                                                                                      with the use of these materials in the conservation of Cultural Heritage.

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                                                                                              25
                                                                                      Summary
                                                                                      Consolidation and protection are two of the principal kinds of treatments through which the decay of
                                                                                      old statues, stone facades, plasters and mural paintings caused by both natural atmospheric agents
                                                                                      and, above all in the last five decades, by atmospheric pollution, is faced.
                                                                                      The most traditional approach has been and is mainly based on the use of organic polymeric materi-
                                                                                      als. They offer the advantage of easy application procedures and the possibility to obtain, at short
                                                                                      times, very satisfying results. Different is their behaviour at long times. Some drawbacks come out
                                                                                      over time both under the esthetical point of view as well as to the durability, compatibility and efficacy.
                                                                                      Particularly critical is the situation when porous materials and soluble salts - gypsum above all - are
                                                                                      simultaneously present. In such a situation inorganic treatments demonstrate to be much more
                                                                                      appropriate. They assure durable and compatible results.
                                                                                      In the present paper two of the most efficient and appropriate inorganic methods are reviewed in
                                                                                      detail: the barium hydroxide method, both as desulfating and consolidating agent, and the ammoni-
                                                                                      um oxalate method as passivating agent, consolidant and as a treatment capable of improving the
                                                                                      natural colour contrast of the stone, when it is lost due to decay processes.


                                                                                      Riassunto
                                                                                      Consolidamento e protezione sono i due principali interventi con i quali si cerca di contrastare il natu-
M. Matteini - Inorganic treatments for the consolidation and protection of stone...




                                                                                      rale degrado dei manufatti lapidei sia naturali (statue, paramenti) che artificiali (intonaci, pitture murali,
                                                                                      etc.), fortemente peggiorato negli ultimi cinque decenni a causa dell’inquinamento atmosferico.
                                                                                      L’approccio più tradizionale e diffuso ha fatto e fa uso di trattamenti basati su polimeri organici di sinte-
                                                                                      si. Essi presentano i vantaggi della facilità d’impiego e della possibilità di ottenere, a breve termine,
                                                                                      risultati assai soddisfacenti. Diverso è invece il comportamento a tempi lunghi. Molti inconvenienti si
                                                                                      manifestano con gli anni, sia sotto l’aspetto estetico che sotto quello della compatibilità e dell’efficacia.
                                                                                      Particolarmente negati vi sono i risultati con i materiali lapidei porosi e in presenza di sali solubili, soprat-
                                                                                      tutto gesso. In queste situazioni i trattamenti inorganici risultano vincenti. Essi assicurano risultati dure-
                                                                                      voli e compatibili. Nello scritto vengono rivisti in dettaglio il metodo dell’idrossido di bario, nei due ruoli
                                                                                      di de-solfatante e consolidante, e quello dell’ossalato di ammonio, come agente passivante, de-solfa-
                                                                                      tante, consolidante e capace di migliorare la saturazione cromatica dei manufatti trattati.


                                                                                      Résumé
                                                                                      Consolidation et protection sont les deux principales interventions avec lesquelles on cherche de
                                                                                      contraster la naturelle détérioration des ouvrages pierreux tant naturels (statues, parements) qu’ar-
                                                                                      tificiels (enduits, peintures murales, etc.), fortement aggravés dans ces dernières cinq décennies à
                                                                                      cause de la pollution atmosphérique. L’approche plus traditionnelle et diffusée a employé et emploie
                                                                                      des traitements basés sur des polymères organiques de synthèse. Ils présentent les avantages de
                                                                                      la facilité d’emploi et de la possibilité d’obtenir, dans un bref délai, des résultats assez satisfaisants.
                                                                                      Dans des temps longs, au contraire, le comportement est différent. Beaucoup d’inconvénients se
                                                                                      manifestent avec les années, tant sous l’aspect esthétique que sous l’aspect de la compatibilité et
                                                                                      de l’efficacité. Particulièrement négatifs sont les résultats avec les matériaux pierreux poreux et en
                                                                                      présence de sels solubles, surtout plâtre. Dans ces situations, les traitements inorganiques s’avè-
                                                                                      rent être gagnants. Ils assurent des résultats durables et compatibles. Dans ce texte sont revues,
                                                                                      en détail, la méthode de l’hydroxyde de baryum, dans les deux rôles de dé-sulfatant et consolidant,
                                                                                      et celle de l’oxalate d’ammonium, comme agent passivant, dé-sulfatant, consolidant et capable d’a-
                                                                                      méliorer la saturation chromatique des ouvrages traités.


                                                                                      Zusammenfassung
                                                                                      Verstärkung und Schutz sind die wichtigsten Stichworte der Restaurationsarbeiten, die darauf zie-
                                                                                      len, dem natürlichen Verfall der Artefakten aus Stein entgegenzuwirken, sowohl der natürlichen
                                                                                      (Statuen, Paramente) als auch der künstlichen (Verputze, Wandgemälde), deren Verfall in den letz-

                                                             26
ten fünfzig Jahren wegen der Luftverschmutzung immer schlimmer geworden ist.
Die traditionellste und verbreitetste Methode basiert auf Behandlungen mit synthetisierten organi-
schen Polymeren, die einfach zu verwenden sind und die kurzfristig ziemlich gute Ergebnisse erzie-
len. Langfristig ist die Situation ganz anders, mit den Jahren kommt es nämlich zu Schwierigkeiten,
was die Ästhetik, die Verträglichkeit und die Wirksamkeit angeht. Besonders negativ sind die
Ergebnisse mit porösen Steinmaterialien und mit löslichen Salzen, vor allem Gips. In diesen Fällen
sind anorganische Behandlungen die beste Lösung, sie bringen nämlich zu langfristigen und ver-
träglichen Ergebnissen. In der Schrift werden zwei Methoden bis ins Detail betrachtet: die Methode
mit Bariumhydroxid, das entschwefelnd und verfestigend wirkt, und die Methode mit
Ammoniumoxalat, das passivierende, entschwefelnde und verfestigende Eigenschaften hat und das
in der Lage ist, die Farbsättigung der behandelten Artefakten zu verbessern.


Resumen
Consolidación y protección son las dos principales intervenciones con que se trata de combatir la
degradación natural de las piezas de piedra, tanto naturales (estatuas, paramentos) como artificiales
(enlucidos, pinturas murales, etc.), que en los últimos cinco decenios han empeorado a causa de la
contaminación atmosférica. El enfoque más tradicional y extendido siempre ha hecho uso de trata-
mientos basados en polímeros orgánicos de síntesis. Ofrecen las ventajas de la facilidad de uso y de
la posibilidad de obtener, en corto plazo, resultados bastante satisfactorios. También es distinto el com-
portamiento en plazos más largos. Muchos inconvenientes se manifiestan con los años, tanto bajo el
aspecto estético como bajo los aspectos de la compatibilidad y de la eficacia. Son particularmente
negativos los resultados con los materiales lapídeos porosos y en presencia de sales solubles, sobre
todo yeso. En estas situaciones, los tratamientos inorgánicos dan los mejores resultados. Aseguran
resultados duraderos y compatibles. En el escrito se revisan detalladamente el método del hidróxido
de bario, en sus dos funciones de desulfatante y consolidante, y el método del oxalato de amonio, como
agente pasivante, desulfatante y capaz de mejorar la saturación cromática de las piezas tratadas.




                                                                                                                  Conservation Science in Cultural Heritage




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