Experiences at the Academy of Fine Arts of Brera in Milan, Italy: the application of laser-technology on three case studies of the historical heritage

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229
Lasers in the Conservation of Artworks XI,
Proceedings of LACONA XI, P. Targowski et al. (Eds.),
NCU Press, Toruń 2017, DOI: 10.12775/3875-4.16
Experiences at the Academy of Fine Arts of Brera in Milan, Italy:
the application of laser-technology on three case studies of the
historical heritage
Elisa Isella1*, Donatella Bonelli1, Silvia Cerea1, Francesca Mancini1, Veronica
Ruppen1, Alessandra Botteon2, Antonio Sansonetti2
1 Brera Fine Arts Academy, Milan, Italy – Via Brera 28, 20121, Milan
2 Institute for Conservation and Valorization of Cultural Heritage ICVBC – CNR, Milan, Italy – Via
Roberto Cozzi, 53, 20125 Milan
* Corresponding author: elisa.isella@libero.it
Abstract
Scientific research regarding the conservation of gypsum plaster heritage is quite scarce; cleaning is
oen a problematic issue on this kind of material due to its water sensitivity and mechanical features.
erefore, laser cleaning is oen a good option as some recent paper has reported.
e research presented here focused on laser cleaning on three gypsum plaster casts belonging to the
collection of Brera Fine Arts Academy and dated back to the 19 century; recalling the names of the
original marble statues, the items are the Flora Far nese, e Velletri Pallas and the Barber ini Faun. e
three casts had been exposed for many decades in the entrance hall of the Academy in a semi-confined
environment. Unfortunately, the continuous passage of students soiled the plaster surface with a grey
greasy layer localized especially on the lower parts and on the surfaces with a strong horizontal
component. It was possible to identify a superficial grime and traces of the original patina. Marks of
different nature (felt, pen, pencil, scratches) were also present. Cleaning tests were carried out using
under Art Nd:YAG able to emit both at 1064 nm and at 532 nm. e effects of laser cleaning were
studied with a diagnostic campaign carried out both before and aer the cleaning operation. To define
the aspects and the nature of the surface microsamples were sampled and analysed with XRD, XRF,
then observed with a scanning electron microscope SEM, and with optical microscopy.
Several tests were carried out on the Flora Far nese, calibrating the fluence in order to match the ablation
threshold avoiding any damage (from FL=1 – 1,45 J/cm² at 1064 nm to FL = 0,5 – 0,7 J/cm² at 532 nm
with repetition rate from 6 Hz to 10 Hz) in particular on these surfaces the removal of cement splashes
constituted a hard task. A comparison in between tests at 1064 nm and at 532 nm were performed.
Both tests were evaluated with the aid of humidification with free water (applied with brush) and
with the use of AgarArt® rigid gel. On the basis of laser-plaster interaction observed on the Flora, th e
tests on the other casts excluded the IR radiation, because of a yellowing effect. For what regards the
cast of the Pallas, one of the low relieves in the wooden support base was involved. e laser cleaning
allowed the perfect removal of the outer layers of scialbo.
For the cast of the Faun, laser cleaning proved to be the appropriate system to eliminate greasy deposits
and dark grey stains visible on the irregular surface, maintaining the original morphology. Hence it
was possible to avoid invasive mechanical and chemical systems, minimizing the interaction with the
matter and saving working time. e aim of this work is to provide a contribution to the scientific

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Elisa Isella et al.
the emission of short impulses in Q-switch
to avoid the excessive heating of the surface
usually associated with long pulse duration.
Furthermore, based on the research of basic
laser interaction mechanisms, recent bibli-
ographic references [2, 3] have suggested to
operate in the visible spectrum at λ = 532
nm to avoid the undesirable yellowing of the
substrate.
2. e Brera Academy of Fine Arts
e survey here presented was carried out at
the Conservation School in the Academy of
Fine Arts, an Italian Institute that is based in
Milan and was founded in 1776 by Empress
Maria Teresa of Austria. e Brera building
hosts various institutional bodies such as the
Ministry of Culture, the Fine Art Gallery, the
Botanical Garden, the Astronomical Obser-
vatory and the Braidense Library; they are
all enclosed together to create a network. e
overall collection is worldwide renowned. It
goes back to the 19 century and consists
in different kinds of works like paintings,
ancient books, drawings, sculptures and
plaster casts. e Conservation School was
founded in 1997.e Historical Heritage of
Brera Academy is composed by 900 gypsum
plaster casts positioned in several of school
spaces, both public and reserved to the
students. All the conservation activities are
devoted at the aim to create the new gypsum
gallery in the original old basement of the
Academy. e Heritage Collection could
researches by the point of view of a Fine Arts Academy, which is starting to build a database for the
conservation products and methods about such a known and used material as plaster, which is not
sufficiently studied.
Keywords: Nd:YAG, 532 nm radiation, plaster casts, guano, Newberyite, ablation threshold, safety
and security
1. Introduction
Cleaning artwork mainly composed by
gypsum is a hard challenge even for skilled
conservators. Gypsum is partly soluble
and moreover, it is very sensitive to any
kind of wet cleaning; furthermore, it is not
mechanically resistant to abrasive systems
and its resistance is dramatically decreased
even when it is slightly wet. However, the
interaction between the cleaning system
and the gypsum surface is influenced by
secondary components, especially when
there are organic additives such as animal
glues or other protein materials. Recently
the Agar cleaning has proved to be effica-
cious on this kind of artworks, even though
some research is still needed in this field.
Laser cleaning allows to remove soiling,
but at the same time avoiding damages
to the artwork as proved by some recent
papers [1].
is research focuses on the application
of laser cleaning on three gypsum plaster
casts of the historic collection of the Fine
Arts Academy of Brera, dating back to the
19 century; they are the Flora Farnese, the
Velletri Pallas and the Barberini Faun. Laser
cleaning was applied to remove the compact
grayish deposits present on the surface and
partially penetrated into the gypsum. e
Nd:YAG laser source was chosen for the
advantages offered by its versatility and broad
operating range of laser pulse durations. is
instrument was employed to complete a pre-
dominately mechanical ablative process with

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Experiences at the Academy of Fine Arts of Brera in Milan, Italy
Fig . 1. Flora Farnese before the
restoration.
Fig . 2. Velletri Pallas before the
restoration.
Fig . 3. Barberini Faun before the
restoration.
be split into two parts: the 19 and the 20
century artworks, different for size, materials
and techniques.
e oldest artwork category group
includes items of huge dimensions, which
were modelled by using the original marbles;
on the contrary, the recent heritage is mainly
formed by prototypes useful to the artist for
the final marble sculpture.
3. e case studies
e Flora Farnese, Velletri Pallas and Bar-
berini Faun (Figs. 1 – 3) are three gypsum
plaster casts dating back to the end of the
18 century: their sizes constitute a challenge
for the conservators, being in the range from
2,04 m to 3,55 m approximately. ey were
made from original marble statues which
are currently conserved in three different
museums: the Archaeological Museum of
Naples (Flora Farnese), the Louvre Museum
(Pallas of Velletri) and the Munich Glypto-
thek (Barberini Faun).
ese three sculptures were located for
the last decades in the entrance hall of the
Academy, in a large hallway leading to the
Academy classrooms. is semi-confined
environment favours the ventilation and acts
on the average of the temperature and the
relative humidity, which replicates seasonal
standards. Urban colonies of pigeons and
the constant flow of visitors and students in
the corridors created different processes and
patterns of decay.
Plaster casts surfaces were covered by
a thick layer of different exogenic material,
composed of pigeon dropping (guano)
(Fig. 4), solid particles coming both from
soil and pollution. As a result of the contact
with human hands, a greasy, grey layer had

232
Elisa Isella et al.
recent act of vandalism was inflicted on the
Barberini Faun in March 2014: the le leg
was detached due to an inappropriate move
caused by a student (Fig. 5), whereas the
Pallas was intentionally splattered with blue
paint (Fig. 6).
Previous restoration and masonry build-
ing maintenance severely worsened the state
of the conservation of the three sculptures;
especially the surface of Flora which was
damaged by splashes of a grey mortar. An
in-depth study of the composition of the
gypsum plaster matrix highlighted interest-
ing details about the execution of the three
casts; specific technical diagnostic methods
were used leading to the current phase of
conservation.
rough X-ray diffraction (XRD) and
thermal analysis (TGA-DTA), it was possible
to identify gypsum as the principal compo-
nent, with a presence up to 90%; the rest is
composed of traces of bassanite, quartz and
calcium oxalates. ere is also a minimum
percentage of organic substances, specifically
traces of protein highlighted by infrared
Fig . 4. e Velletri Pallas helmet covered with a thick
and uneven layer of guano.
Fig . 5. Barberini Faun with fragments of the le leg
aer the vandalism act.
Fig . 6. e vandalism act on the Pallas surface:
a detail of the blue stain splattered, observed at the
stereomicroscope.
been formed on the lower part of the plaster
casts; moreover, the artworks were damaged
by different acts of vandalism, which caused
the fracture of exposed extremities. e most

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Experiences at the Academy of Fine Arts of Brera in Milan, Italy
Figs. 7 and 8. Optical and SEM images in backscattered electrons of the uncleaned area. ey show the
stratigraphic sequence of the plaster.
spectroscopy (FTIR). e optical and the
scanning electron microscope (SEM) images
(Figs. 7 and 8) provided the stratigraphic
sequence of the plaster surface.
On the outer gypsum surface it was pos-
sible to observe a grey layer that at the naked
eye appeared to be a scialbo. However, this
hypothesis could not be confirmed by further
analysis; in fact, no specific distinctive layer
was present, but phosphorus, magnesium and
silicon have been detected by EDS spectra on
the surface and the near/surface region of the
Fig. 10. FITR spectrum of Newberyite aer irradi-
ation.
Fig . 9. SEM – EDS maps of magnesium, phosphorus, silicon and calcium.
gypsum matrix. It is possible to see tracks of
these elements in the maps (Fig. 9).
FTIR spectrum (Fig. 10) shows the
absorption pattern of unknown material
sampled from the grey layer. It displays
a good overlapping referring to the mineral
Newberyte. is is a magnesium hydrogen
phosphate which crystallizes with three
molecules of water [Mg(HPO₄)·3H₂O]. e
hydrogen phosphate ion is reported to be
prevalent in weakly basic conditions, but it
is still present in a significant amount at pH
7 [4]. Different forms of magnesium phos-
phates have been detected as crystalline forms

234
Elisa Isella et al.
in animal’s guano. Depending on the guano
age, the presence of humidity, the content of
ammonium ion, a different mineral phase is
formed: Struvite [MgNH4PO46H2O], occurs
in fact in great abundance in the moist depth
of guano, while on the contrary, Newberyte
occurs in the older and drier parts of the
deposits. Its colour is described as greyish
or grey/brown [5]. As a general rule, the
hydrogen and di-hydrogen phosphates are
slightly more soluble than the corresponding
phosphates [6]. e low solubility product of
newberyte 1,5 10
– 6
at 25°C (Taylor, 1963),
compared to the Ksp of gypsum, 10 – 4.58 allows
to suppose that the mechanism explains
the current localization of phosphate and
magnesium in the near-surface region [7].
Subsequent cycles of condensation phenom-
ena, solubilize a greater amount of gypsum
in the outer surface, leaving the Newberyite
intact; the following re-precipitation of gyp-
sum in a drier season, moves it forward to
the outer profile of the surface. Newberyite
has been detected in cave minerals, in guano
and in kidney stones. In the field of cultural
heritage, the scientific literature reported
an occurrence of this substance on ivory
surfaces [8]. In this paper, the first detection
on gypsum statues was presented, as to the
authors knowledge.
4. Previous treatments and preliminary
laser test
e conservation works began in October
2014. e preliminary cleaning phases were
the following:
1. Dry cleaning (Fig. 24)
2. Wet cleaning (Fig. 11)
3. Agar Gel cleaning (Fig. 11 and Fig. 25)
Processing with a dry cleaning on
a decayed gypsum surface, using a synthetic
rubber or a sponge, kneads the soiling onto
the gypsum matrix. is is a drawback,
because it produces an uneven cleaning level,
which gives the impression of a stained sur-
face. Wetting the surface with a water based
system and adding water to the gypsum
material with a brush or a poultice did not
help at all; on the contrary, it enhanced the
possibility of permanent damages.
e Agar gel was not effective inside the
scratches and the poultice seemed to dam-
age the morphology of the plaster surface
(Fig. 11).
Fig. 11. Laser test (1) gel test
(2) solvent pad test (3).

235
Experiences at the Academy of Fine Arts of Brera in Milan, Italy
5. e laser cleaning
Eventually, laser cleaning tests were performed
(Figs. 12 – 13). As already mentioned, with
gels and poultice solvent (hydro- alcoholic
solution) the cleaning resulted in an unsat-
isfactory surface respect to the laser, which
on the contrary, proved a correct balance
between harmfulness and effectiveness.
Several laser tests (THUNDER ART – El.
En. – Florence, Italy) were carried out on the
Flora Farnese, calibrating the fluence (FL)
in order to match the ablation threshold
avoiding damages from FL = 1 ÷ 1,45 J/cm²
at 1064 nm to FL = 0,5 ÷ 0,7 J/cm² at 532
nm with repetition rates within the range 6
Hz to 10 Hz. A comparison in between tests
was performed at 1064 nm and at 532 nm.
Both tests were evaluated with some support
like wetting with clear water (applied with
a brush) and using the AgarArt® rigid gel as
an intermediary agent. A yellowing effect was
observed on the Flora surfaces, which led to
the exclusion of the radiation 1064 nm, on
the other two casts.
As it is possible to see in the maps of the
elements, the ablation at FL over 1.9 J/cm2
removed almost completely the magnesium
and phosphorus formations, clearly indi-
cating an overcleaning (Fig. 17). e visual
results displayed in Fig. 15 highlighted in fact
an unnatural white surface, where a portion
of the near-surface region has been removed
with the outcome that it damaged the surface
of the gypsum work (Figs. 14 – 17).
In conclusion, analyses were carried
out to evaluate the differences between the
untreated and cleaned laser zones; they
allowed to observe the partial lowering of
the Newberyte formation avoiding its com-
Fig. 12. Laser test at 1.5 J/cm². Fig. 13. Laser test at 0.7 J/cm².

236
Elisa Isella et al.
plete removal, except for the over cleaning
case.
In order to remove the grey layer, which
covered a wide range of the surface, it was
decided to complete the cleaning with the
laser THUNDER ART (El. En. – Florence,
Italy; wavelength = 532 nm; spot size 10 mm;
pulse duration ≈ 8 ns. (Fig. 26).
Working with the THUNDER ART Laser
allowed the completion of the complex phase
of cleaning by optimizing working time, but
also achieving better results compared to
the traditional cleaning methods previously
tested. anks to its high selectivity, the laser
system allowed full control and respect of
the surface characteristics. Moreover, the
laser beam offered the possibility to clean
the undercut areas such as the internal
folds of clothing, the flowers garland, and
other particularly delicate surfaces like the
carnation. e cleaning operations on the
three plaster casts necessitated approximately
fieen working days.
As a first step, the lower parts of the casts
were irradiated, specifically the bases made
of plaster because of their easy access. e
plaster bases of the Pallas and the Flora
have a smooth and simple morphology and
it was possible to reach an excellent level
of cleaning by using a fluence = 1.7 J/cm².
Regarding the Faun, it was necessary to
use a range of fluence from 1 to 1.7 J/cm²
(higher with respect to preliminary the tests)
because the plaster base had an irregular
rocky surface, which carries the working
marks, transferred on the plaster from the
original marble. During this first laser tests, it
was obligatory to take into consideration the
cleaning of the bas-relief located in the front
of the wooden base structure on which the
plaster cast of the Pallas stands (Figs. 18 – 19).
Fluence = 0.7 J/cm² was used to remove the
homogenos grey layer of the background,
while the figures were treated with fluence =
Fig. 17. Overcleaning. ese maps show that the
Newberyite is no longer present.
Fig. 16. Ablation threshold. ese maps show that the
Newberyite has decreased aer the laser cleaning.
Fig. 14. e sample before and aer a cleaning test
(the grey layer has become thinner).
Fig. 15. e laser removal shows an overcleaning
level.

237
Experiences at the Academy of Fine Arts of Brera in Milan, Italy
0.8 J/cm², also increasing the value on stains
up to a maximum of 1 J/cm². It was necessary
to modulate the values of fluence and the
repetition frequency, instead of using fixed
parameters, to be able to clean effectively the
draped dresses of the two female figures.
To clean the tunic of the Pallas which is
folded on the hips forming a large draping
triangle, it was needed to use a range of
fluence that varied from 1 J/cm², to 1.4 J/
cm², and a repetition frequency of 12 Hz.
Laser cleaning proved to be particularly
effective to eliminate the accumulated black
particles in the areas underneath the verti-
cal folds and under the laying zones of the
richly decorative elements of the breastplate
(depicting snakes and the head of Medusa)
(Figs. 20 – 21).
e technical difficulty in these two cases
was keeping the hand tool as perpendicu-
lar as possible to the modelled surface; on
the contrary, extra care had to be taken of
the Flora plaster cast in order to be able to
execute a uniform cleaning, respecting the
chiaroscuro effects, which give plasticity to
the figure. Her le forearm holds the elegant
draping of the himation (a type of clothing);
this area was irradiated with values of fluence
from 0.9 J/cm² to 1.4 J/cm² at a repetition
frequency of 18 Hz (Figs. 22 – 23). Excellent
results were achieved on the wreath of flow-
ers, where blackened areas were successfully
removed in the spaces among the flowers.
In correspondence with the anatomical
parts, it was necessary to be very careful and
delicate and to use the right manual skills. In
Fig. 18. Low relief before irradiation. Fig. 19. Low relief aer irradiation at 0.7 J/cm².
Fig. 20. e tunic of the Pallas before laser treatment.
Fig. 21. e tunic of the Pallas aer laser treatment.

238
Elisa Isella et al.
Fig. 22. Particular of the hand before laser cleaning.
Fig. 23. Particular of the hand aer laser cleaning.
particular, the nudity of the Faun regained
the full lapse definition of details, which pre-
viously were hidden by deposits irregularly
distributed on the modelled surface.
e expression on his face and the natural
rendering of the abdominal band re-emerged
using the laser with a range of fluence start-
ing with 0.7 J/cm² through 1.5 J/cm², with
a repetition frequency of 7 Hz (Fig. 26).
To remove the grey layer, which was easily
evident on the faces of the other two casts,
the laser parameters varied its distance from
the hand tool to the modelled surface. e
values in the interval between 0.5 J/cm² and
1.1 J/cm² were used to obtain a modulated
cleaning based on the state of the conserva-
tion of different surfaces.
e le leg of the Faun is considered
as a “special issue” because of the obvious
signs of deposits due to the interaction with
the environment, the plaster material and
the problems related to its previous recent
reconstruction. e signs of the deposits were
removed with fluence from 1 J/cm² to 1.7
J/cm², while the marks in correspondence
of the integration filling stucco needed to be
increased up to the high value of 3 J/cm².
In the previously mentioned target areas,
produced by the first laser tests, a blackening
was observed due to a focused laser irradia-
tion; continuing the irradiation in the same
points with increased fluence (1.7 J/cm² to
3 J/cm²) allowed removing the blackening.
is phenomenon was possibly provoked
by the presence of an acrylic resin, binding
a pigment and previously applied to uniform
the colour of the surface by a visual point
of view.

239
Experiences at the Academy of Fine Arts of Brera in Milan, Italy
Fig. 24. Dry cleaning.
Fig. 25. Gel cleaning.
Fig. 26. Laser cleaning.
On the trunk of the Flora, using a fluence
of 1.1 and of 1.7 J/cm², was the only pos-
sibility to obtain the appropriate cleaning
level, not achievable with any other cleaning
method. In fact, the presence of mortar
patches on the side and the back of the trunk
damaged heavily the sculpture; the laser
cleaning system proved an extreme ability
to remove unwanted materials. rough
a careful calibration of the laser cleaning
system, it was possible to reveal the subtle
chromatic variations and the chiaroscuro
effects, enhancing the legibility of the plaster
cast surface.
e repetition rate changed importantly
as to the different plaster casts, ranging from
7 to 18 Hz; in fact, it was used as a valid tool
to face the uneven level of deposits on the
various surfaces and to obtain the correct
restitution of the surface brightness.
All the values of the fluence quoted in the
text are shown together with the correspond-
ence with plaster areas in the following table
(Tab. 1).
A very important aspect to stress is the
crucial importance of safety, not only for
the work of art but, in this special case for
the students, for the workers and the public.
All the students were trained specifically in
order to use the laser in safe conditions. ey
have employed Individual Protection Devices
during the restoration phases; they also
Table 1. Synoptic overview of used fluences [J/cm²]
Velletri Pallas Carnation Dress Basement Base Low Rilief
0.5 → 1.1 1 → 1.4 1.7 0.7 → 1.0
Flora Farnese Carnation Dress Basement Tree Trunk
0.5 → 1.0 0.9 → 1.4 1.7 1.1 → 1.7
Barberini Faun
Carnation Base
0.7 → 1.5
1.0 → 1.7Le Leg
1.0 → 1.7 AND 3

240
Elisa Isella et al.
underwent a medical examination before
and aer the project. e importance of the
detailed work plan is crucial when working
in a crowded hallway: in fact, students and
visitors frequently pass the hallway and we
were obliged to set-up the site with strict
laser safety barriers. e conservation site
is under the supervision of the Ministry of
Culture, which is located on the top floor of
the Brera building; inspectors of the Ministry
had a close supervision of the entire conser-
vation process.
6. Conclusion
e Conservation School of Brera Acad-
emy is in charge of the maintenance and
conservation of its own Cultural Heritage.
e three case studies here presented were
a pilot project for future works, taking into
account the overall difficulties in cleaning
gypsum based materials due to its solubility
and low mechanical properties. Newbe-
ryite was highlighted in the near surface
region as a new mineral formation, which
possibly comes from guano; it is supposed
to be related with a superficial grey layer,
but further investigation is still needed.
e laser cleaning technology was applied
with success aer preliminary treatments,
with the aim to lower the grey layer already
mentioned. e casts have been cleaned
with THUNDER ART @ 532 nm, in the QS
regime, ranging the fluence from 1.1 to 1.7
J/cm2 and the repetition rate from 12 to 20
Hz in the average. e performance of laser
cleaning treatments proved to be effective
in removing the grey layer from the sub-
strate.
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