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New insights to assess the consolidation of stone materials used in built heritage: the case study of ancient graffiti (Tituli Picti) in the archaeological site of Pompeii

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Abstract Tituli Picti are an ancient form of urban graffiti very common in the archaeological site of Pompeii (Naples, South—Italy). They are generally made of red pigments applied on walls of Campanian ignimbrite. This paper deals with a scientific investigation aimed to their conservation. This is a challenging task since it requires a multidisciplinary approach that includes restorers, archaeologists and conservation scientists. The study has provided suggestions on the proper way to conserve Tituli Picti over time. In the present work, several specimens of Campanian ignimbrite were painted with red earth pigment; lime and Arabic gum have been used as binders as well. Such painted stones were treated with three consolidants: a suspension of reactive nanoparticles of silica, ethyl silicate and an acrylic microemulsion. Treated and untreated specimens were subjected to thermal aging, artificial solar radiation and induced crystallization decay. It has been assessed the colorimetric variations induced by treatments. Moreover, the micromorphologic features of the consolidated surfaces have been highlighted by means of electron microscope observations. The scotch tape test allowed to compare the superficial cohesion induced by the three used products. According to the results, ethyl silicate seems to represent the most successful product.
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Ruoloetal. Herit Sci (2020) 8:49
https://doi.org/10.1186/s40494-020-00393-0
RESEARCH ARTICLE
New insights toassess theconsolidation
ofstone materials used inbuilt heritage:
thecase study ofancient grati (Tituli Picti)
inthearchaeological site ofPompeii
Silvestro Antonio Ruffolo1*, Natalia Rovella1*, Anna Arcudi1, Vincenza Crupi2, Domenico Majolino2,
Massimo Osanna3, Rossella Pace4, Alessia Pantuso1, Luciana Randazzo1, Michela Ricca1, Nicola Ruggieri5,
Valentina Venuti2 and Mauro Francesco La Russa1
Abstract
Tituli Picti are an ancient form of urban graffiti very common in the archaeological site of Pompeii (Naples, South—
Italy). They are generally made of red pigments applied on walls of Campanian ignimbrite. This paper deals with
a scientific investigation aimed to their conservation. This is a challenging task since it requires a multidisciplinary
approach that includes restorers, archaeologists and conservation scientists. The study has provided suggestions on
the proper way to conserve Tituli Picti over time. In the present work, several specimens of Campanian ignimbrite
were painted with red earth pigment; lime and Arabic gum have been used as binders as well. Such painted stones
were treated with three consolidants: a suspension of reactive nanoparticles of silica, ethyl silicate and an acrylic
microemulsion. Treated and untreated specimens were subjected to thermal aging, artificial solar radiation and
induced crystallization decay. It has been assessed the colorimetric variations induced by treatments. Moreover, the
micromorphologic features of the consolidated surfaces have been highlighted by means of electron microscope
observations. The scotch tape test allowed to compare the superficial cohesion induced by the three used products.
According to the results, ethyl silicate seems to represent the most successful product.
Keywords: Tituli Picti of Pompeii, Conservation, Consolidation
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Introduction
Pompeii is considered one of the most important archae-
ological site in the world, it represents a popular tourist
attraction in Southern Italy, counting about 3.6 million
visitors in 2018 [1]. e archaeological site of Pompeii
belongs to the UNESCO World Heritage and it extends
up to about 66 hectares (22 hectares still remain unexca-
vated). e ancient city of Pompeii is located on a plateau
formed by the Vesuvius volcano’s lava flow, near Naples
[2]. Information about the origin of the city is uncer-
tain. Researchers believe that the town was founded in
the 7th–6th century  by the Oscans. It came under
the domination of Rome in the 4th century  and was
conquered and became a Roman colony in 80  after
it joined an unsuccessful rebellion against the Roman
Republic [3]. e city was unexpectedly buried in the
thick layer of ash and lava from the devastating erup-
tion of Vesuvius of 79 . Re-emerged from the darkness
of centuries at the beginning of the 16th century, dur-
ing remediation works in the Sarno Valley [4], it is one
of the most significant proofs of Roman civilization. It is
divided into several Regio (neighborhoods) and Insulae
(blocks) and each house is identified with Regio, Insulae,
Open Access
*Correspondence: silvestro.ruffolo@unical.it; natalia.rovella@unical.it
1 Department of Biology, Ecology and Earth Sciences, University
of Calabria, via P. Bucci Cubo 12B, Arcavacata di Rende, CS 87036, Italy
Full list of author information is available at the end of the article
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Ruoloetal. Herit Sci (2020) 8:49
street and number that recalls modern addresses (i.e. Via
dell’Abbondanza Reg. IX Ins. I, n. 16). Walking around
the streets, some writings on walls are visible, recalling
the graffiti in our modern cities. ese wall inscriptions
(Tituli Picti) are uniqueness in the Roman archaeological
heritage (Fig.1).
Approximately 1100 Tituli Picti have been found, writ-
ten in different languages: Oscan, Greek and Latin [5, 6].
e support varies in the vast repertory offered by the
Pompeii’s inscriptions, from plaster to travertine, but
mainly they are written on Campanian ignimbrite, which
is a pyroclastic rock, having a high porosity and an high
silica content; red ochre or, more rarely, black pigments
are used. e widest spread dealt theme is of electoral
type in which the election of a citizen to a magistrates is
proposed, others are related to prohibition signals, invec-
tives, various acclamations, inscriptions concerning in
general the binomial relation love-death, property ads,
ads for selling building materials, requests related to lost
objects [7, 8]. e inscriptions are generally positioned
along the more intense traffic streets, where houses of
many influent personalities of the Samnite and Roman
town were located as well as close to the Forum, the par
excellence place for politic and electoral meetings [9].
e Tituli Picti are generally located at a height of about
3–4 meters, in order to ensure a full visibility of the
inscription to the potential readers and to make it diffi-
cult the overwriting as well [10].
e conservation of such inscriptions still represents
a challenge in the framework of the preservation of the
archaeological site. Ignimbrite is a fragile substrate
suffering several degradation processes [1113]. Many
Tituli Picti were studied by means of both non-invasive
and micro-invasive techniques [14]. e characterization
analysis highlighted the degradation phenomena affect-
ing the ignimbrite wall panels such as biological coloni-
zation, chromatic alteration and especially efflorescence
of sodium, calcium, magnesium sulphates, nitrates and
chlorides able to damage seriously the microstructure of
the rock until its fracturing. ese processes are encour-
aged by the high porosity of the material promoting the
uprising of the ground waters from the soil. Moreover,
the partial transformation of ignimbrite in clay miner-
als was revealed, in which the macroscopically observed
reddish colour is related to a diffuse presence of hydrated
Fe-oxides. In some cases, this phenomenon can lead to
a partial overlap with Tituli Picti, since they have similar
colours. e study on the pigments identified a red ochre,
revealing the hematite based composition, but not the
presence of any binder. is suggested that the pigment
was applied just by a water dispersion, but it is also possi-
ble that an organic binder, degraded over years, was used
for this reason there is not any evidence.
For conservation purposes, it is essential to establish
which are the best procedures to adopt in order to pre-
serve the Tituli Picti for as long time as possible, identify-
ing the most appropriate consolidating product.
e decay Tituli Picti consists in two processes, the
first one is related to the degradation of the superficial
portion of the stone which contains the pigment, while
the second one is represented by the degradation of the
stone itself. erefore, both aspects have to be taken into
account when the issue of Tituli Picti is faced.
For this reason, a laboratory experimentation was set
up in which specimens of ignimbrite were prepared,
and then a red ochre based paint with different binders
(Lime and Arabic gum), and without any binder as well,
was applied on such specimens. Once set-up, the painted
specimens were subjected to a washout test in order to
simulate the weathering due to the rainwater so leading
the paints to an insitu environment that was as close to
the real one. Several consolidating products were applied,
and then colorimetric tests, microscopic observations,
surface cohesion tests and capillary water absorption
measurements were carried out to check the suitabil-
ity of such treatments. In addition, salt weathering test
were conducted to evaluate the bulk cohesion provided
by consolidants versus salt crystallization, an important
degrading process observed in the archaeological site.
Materials andmethods
We used Campanian Ignimbrite for the experiments.
is rock is the product of the activity of Campi Flegrei,
along with the Ischia and Procida islands, a complex
Fig. 1 Examples of Tituli Picti
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Ruoloetal. Herit Sci (2020) 8:49
volcanic system constituted by an eastwest–oriented net-
work of small monogenic apparatus. It is very rich in sili-
con and aluminium and has a porosity ranging from 35
to 43% [14]. Raw material was cut (size 5 × 5 × 2cm) in
order to obtain the laboratory specimens. Paints mixture
were prepared by mixing red ochre with water, as well
as with water and several binders (lime putty, lime and
Arabic gum suspensions) (Table1). ese binders were
chosen, because they were very common in Roman age in
mural painting [15]. An average of 0.5g of each paint was
applied on stone surface (about 10cm2) by brush.
Samples were subjected to washout treatment for
168h. Tests were setup as follows: four specimens were
placed in a container filled with distilled water. A mag-
netic bar was dropped into the water and then the con-
tainer was placed on a magnetic stirrer at 1200 rpm
(Additional file1: Fig.S1). For consolidation assessments,
three commercial products were used: NanoEstel, Estel
1000, and Micro Acril (CTS, Italy). e first and the
second products were chosen because they have a good
chemical compatibility with stone, since both are based
on silica, while the choice of the third one was suggested
by restoration practice, which includes the use of organic
consolidants for the conservation of paintings. NanoEstel
is an aqueous suspension of reactive silica nanoparticles
having an average size of 30nm; Estel 1000 is composed
of TEOS (tetraethyl orthosilicate) diluted in white spirit
(a mixture of aliphatic hydrocarbons having a boiling
point of 145–250 °C). is molecule is hydrolysed by
water, then SiO2 is formed by condensation within the
porous structure, which can be bonded to the stone sur-
face, leading to a consolidating effect, this process is also
known as sol–gel. Micro Acril is a water based microe-
mulsion of acrylic polymer with a particle size of about
40 microns.
Consolidation products were applied at different con-
centrations of active compounds (3, 5, 10, 20%wt) (Addi-
tional file1: TableS1).
Colorimetric tests were carried out using a CM-2600d
Konica Minolta spectrophotometer to identify chromatic
variations induced by the treatments. Chromatic values
are expressed according to the CIE (Commission Inter-
nationale d’Eclairage) L*a*b* space, where L* is the light-
ness/darkness coordinate, a* the red/green coordinate
(+a* indicating red and a* green), and b* the yellow/
blue coordinate (+b* indicating yellow and b* blue)
[16]. Measurements were carried out using a 8.0 mm-
diameter viewing aperture, specular component excluded
(SCE), UV 0%, Illuminant D65 and 10° observer angle.
e colour changes were expressed in terms of E [15]:
All the given results are average values of ten measure-
ments taken on each specimen (five on the painted area,
five on the unpainted area). For each treatment, three
specimens were analysed.
Capillary water absorption tests were performed to
determine the amount of water absorbed by treated and
untreated specimen per surface unit (Qi) over time. Qi is
defined as: Qi = (mt m0)/S, where S is the area (in cm2)
of the sample exposed to water; mt and m0 represent the
weight of sample measured during the test, at the times t
and 0, respectively [17]. e test was aimed to check the
hydrophobic effect induced by treatments, as well as the
variation in porosity.
Microscopic observations were carried out by using a
FEI Quanta 200F electron microscope (Philips). e anal-
yses have been carried out at low vacuum regime (103
bar) and with an accelerating voltage of 20kV.
e salt crystallization test (or salt weathering test),
was performed by following the procedure described in
the existing standard [18]. Specifically, specimens under-
went 15 crystallization cycles consisting of: 2h of immer-
sion in a supersaturated solution of sodium sulfate (14%
w/w at 20°C), 16h of drying in an oven at 105°C, and
6h of cooling at room temperature. e weight of each
test sample was measured before the crystallization test
and after each cycle; the resulting weight loss was deter-
mined. Each measurement has been repeated on three
specimens.
e specimens were undergone to aging tests by
the means of two climatic chambers. e first artifi-
cial aging process regarded the solar radiation obtained
by a Suntest XLS + chamber (Atlas USA). e latter is
equipped with a Xenon arc lamp, tests occurred at 26°C
and at a radiation level of 53%. e total time of test was
4000h. After solar radiation aging, samples were hold for
1500 h in a temperature humidity controlled chamber,
with a temperature of 45°C and relative humidity of 70%.
Peeling test is a test aimed to quantify the adhesion of a
surface or a near to-surface layer to a substrate. Accord-
ing to the method proposed by Drdácký and co-workers
E
=
L2
+
a2
+
b
2
Table 1 Summary ofsample preparation
ID Red
pigment
(%wt)
Water
(%wt) Lime
putty
(%wt)
Lime (%wt) Arabic gum
suspension
(%wt)
P 38 62
PLPA 31 63 6
PLPB 33 65 2
PL 33 65 2
PAGA 24 38 38
PAG B 38 – – – 62
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Ruoloetal. Herit Sci (2020) 8:49
[19], a pressure-sensitive tape was applied to the inves-
tigated area and then pulled off. It was measured the
weight of material detached from the surface after peel-
ing off the tape. e test was repeated five times on
the same area. It is generally assumed that this amount
reflects the cohesion characteristics of the substrate. In
this specific case, the test was used to make a compara-
tive assessment of the cohesion provided by each treat-
ment. Measurements were performed on three samples
for each treatment, as well as before treatment.
Results anddiscussions
By a visual inspection, all paints seem to be poor adher-
ent to the stone; this is confirmed by washout tests, which
induced a variation of the colorimetric coordinates meas-
ured on the painted layers. is is macroscopically vis-
ible, since an aliquot of the pigmented layers is removed
thanks to the action of water. e pigment that pen-
etrates inside the stone seems to be less affected by the
washing action of the water. is situation is much more
similar to the real situation observed in Tituli Picti: there
is not any visible pigment layer on the surface, but there
are just pigment grains into the stone porous structure
[14]. Figure2 shows an overview of all paints and the vis-
ible effect of the washout as well. is result highlighted
that all paints are not well adherent on the stone and can
be removed by weathering, leaving some pigment in the
stone. For this reason, hereinafter we focussed our atten-
tion mostly on the paint made of the bare pigment.
Specimens painted with only the pigment and con-
solidated with the three products, and then aged, were
observed by means of electron microscope (Fig. 3) to
check the surface distribution of the products and assess
the penetration features as well. e distribution of the
consolidant was evaluated by morphological observa-
tion of the surface, since the application of the products
modifies the aspect of the surface; while the penetration
depth was determined by observation of cross sections of
the treated samples.
In samples consolidated with NanoEstel, the product
does not completely cover the surface; it appears con-
centrated especially into the voids. e coating appears
rather compact, although it is affected by some micro-
cracks. At the highest concentration, the product creates
a homogeneous surface coating having a thickness up to
40µm. e penetration depth does not exceed 20 µm.
e application of Estel has not produced a clear surface
coating, only in some points it has been possible to dis-
tinguish a significant film (up to 2µm of thickness). e
penetration depth is about 20µm. e Micro Acril-based
Fig. 2 Samples before and after washout test
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Ruoloetal. Herit Sci (2020) 8:49
treatment creates a “gloss effect” on the surface that, con-
sequently, appear covered by a compact and homogene-
ous coating, and without any microfractures. In this case,
a lower depth of penetration of the product is observed
(up to 7µm).
e colorimetric variations were assessed only on the
paint made with the bare pigment. In Fig.4 we showed
the results of such measurements, performed both on
the pigment and on the substrate. Micro Acril consoli-
dant induces stronger chromatic variations on both pig-
ment and substrate than Estel and NanoEstel. is could
be relative to the different nature of Micro Acril: it is an
organic consolidant in microemulsion, made, namely of
particles, which hardly penetrate into the stone, much
that cause the formation of a superficial slightly whit-
ish film. NanoEstel showed the best behaviour on the
Fig. 3 SEM images of stone surface treated with a NanoEstel b Estel and c Micro Acril
Fig. 4 Colorimetric variations induced by consolidation treatments on the painted (upper chart) and unpainted (lower chart) areas of the
specimens
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Ruoloetal. Herit Sci (2020) 8:49
pigment layer, while Estel seems to induce fewer varia-
tions in terms of colour on the substrate.
In Fig. 5, we reported the results of capillary water
absorption tests performed on treated samples, for com-
parison the values measured for untreated specimens
have been showed as well. For all treatments, the water
uptake decreases as the amount of consolidant increases.
is is due to two factors, the first concerns with the
decrease of porosity, while the second is related to the
hydrophobicity induced by the consolidant. e latter is
quite evident Micro Acril treatment, since it induced the
strongest decreasing of water sorption due to its organic
nature. NanoEstel and Estel are inorganic product, their
application behaved in similar way; it seems that they
induce a mild alteration of the porosity. However, it has
to be pointed out that Estel once cured has an inorganic
nature, but for several months after the application, there
is some solvent left into the consolidant, which provides
a transient hydrophobic effect [20].
Another crucial issue concerning the conservation of
the Tituli Picti is related to the possibility to provide a
better resistance of the superficial portion of the stone,
since it contains the pigment making visible the Tituli
Picti themselves.
anks to the peeling test, it was possible to assess the
surface cohesion of the treated stone. Measurements
were performed on three typologies of specimens: bare,
painted but not treated with consolidants, and con-
solidated. For the latter, measurements were performed
before and after artificial aging.
e aging should have an effect both on the micro-
structure of the consolidant (i.e. cracks and loose of cohe-
sion) and on the composition of the consolidant. is
latter one would occur only in the case of Micro Acril,
since it is an organic polymer. Results are summarized in
Fig.6. Unpainted/unconsolidated specimens showed an
average rm = 0.7 ± 0.2mg/cm2, while painted/unconsoli-
dated specimens reported rm values ranging from 2.0 up
to 4.3mg/cm2.
ese results suggest that the painted areas are those
more susceptible to lose material because of the peel-
ing action. Micro Acril seems to be more effective for all
paint mixtures, except for those paints containing Arabic
gum as binder. Probably, the organic origin of the gum
made difficult to the acrylic polymer to penetrate into the
stone and for this reason the final cohesion results to low.
NanoEstel achieved poor performances, in some cases
the treated specimens showed greater rm values with
Fig. 5 Capillarity water absorption test carried on treated and untreated ignimbrite specimens
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Fig. 6 Summary of scotch tape test measurements. The rm value observed for unpainted and unconsolidated specimens is 0.7 ± 0.2 mg/cm2. The
horizontal dashed line in each chart represents the rm value observed for painted but unconsolidated specimens
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Ruoloetal. Herit Sci (2020) 8:49
respect to unconsolidated ones. is is probably because
an aliquot of consolidant remains on the surface and is
easily removed by the peeling. Estel (ethyl silicate) seems
to be the most effective treatment, likely thanks to the
better penetration; it is worth to remind that it is a prod-
uct diluted in solvent, and not a suspension as for Micro
Fig. 7 Weight variations of samples measured during salt crystallization tests
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Acril and NanoEstel. e effect of the aging is not fully
clear, since in some cases the resistance of the surface
increase, while in some other it is quite lower, and some
other cases it seems to be the same as the unaged.
Salt crystallization represents a threat for the integrity
of the stone [21, 22], in particular ignimbrite suffers this
kind of degradation [23]. e effect of the treatments as
a function of the resistance versus salt weathering was
explored in laboratory only on specimens painted with
pigment without any binder. Figure7 shows the weight
variations recorded after each cycle of salt crystalliza-
tion. An increasing of the weight is detected at earlier
cycles. is is due to the salt which crystallizes into the
porous structure without causing a detectable decay of
the stone. In untreated samples, a weight loss is evident
only after the fourth cycle, whereas, this evidence is post-
poned up the tenth cycle in treated ones. In some cases,
the break of the sample was observed rather than a sim-
ple loss of material; in these cases, the measurement was
stopped (i.e. NanoEstel 3% up the 9th cycle). All treat-
ments improve the resistance of the stone against salt
weathering, untreated samples lose up to 60% of their
weight. Treatments, in particular, those at higher concen-
trations lead to a lower weight loss and at later crystal-
lization cycles. Estel treatments seems to induce a better
resistance.
Conclusions
Tituli Picti represent an ancient form of urban graffiti in
the archaeological town of Pompeii. eir conservation is
still an open challenge. In this paper, we provided some
insights on the use of some consolidating products be
able to make such paints more resistant against degrad-
ing agents. e executive technique of the original Tituli
Picti is still unclear, so in our research, several hypoth-
esised paints mixture were reproduced on ignimbrite
specimens. In particular, red ochre was used as pigment;
lime putty, lime and Arabic gum were used as binders.
ese paints were washed out to reproduce the natural
aging of the Tituli Picti.
Our results support the evidence that they were made
just with pigment without any binder. Moreover, three
consolidating products were tested to identify the most
suitable to provide more resistance to the stone, with
particular reference to the superficial portion. Nanosilica,
ethyl silicate and an aqueous acrylic microemulsion were
applied in this experimentation. After microscopic obser-
vations, colorimetric tests, superficial and bulk cohesion
assessments, results suggest that ethyl silicate shows a
well-balanced behaviour and seems to be the most appro-
priate product for the consolidation and the conservation
of Tituli Picti.
Supplementary information
Supplementary information accompanies this paper at https ://doi.
org/10.1186/s4049 4-020-00393 -0.
Additional le1: TableS1. Summary of the consolidation treatments
carried out on the painted specimens. Figure S1. Arrangement of the
washout test, samples were immersed in deionized water and a vortex
were generated by means of a magnetic stirrer.
Acknowledgements
We would like to thank the reviewers for their comments helping us improv-
ing the article.
Authors’ contributions
MO, RP, NRu conceived the research; SAR, MFR, MR, VC, DM, VV designed the
research and wrote the original draft; LR, AA, NRo, AP performed the experi-
ments and statistical analyses. All the authors reviewed, edited, and approved
the final draft. All authors read and approved the final manuscript.
Funding
This research belongs to the framework of the project “Tituli Picti in the
archaeological site of Pompeii: diagnostic analysis and conservation strategies”
financially supported by the Planning Secretariat of the Archaeological Park of
Pompeii.
Availability of data and materials
All data generated or analysed during this study are included in this published
article [and its Additional file 1].
Competing interests
The authors declare that they have no competing interests.
Author details
1 Department of Biology, Ecology and Earth Sciences, University of Calabria, via
P. Bucci Cubo 12B, Arcavacata di Rende, CS 87036, Italy. 2 Department of Math-
ematical and Computer Sciences, Physical Sciences and Earth Sciences (MIFT),
University of Messina, Viale Ferdinando Stagno D’Alcontres 31, Messina 98166,
Italy. 3 General Director, Archaeological Park of Pompeii, via Plinio 4, Pompeii,
NA 80045, Italy. 4 Ecole Normale Supérieure—PSL, Laboratoire AOrOc Archéol-
ogie et Philologie d’Orient et d’Occident, UMR 8546, 45 rue d’Ulm, Paris 75005,
France. 5 Technical Planning Secretariat of the Archaeological Park of Pompeii,
via Villa dei Misteri 2, Pompei, NA 80045, Italy.
Received: 16 January 2020 Accepted: 16 May 2020
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