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A new bio-based organogel for the removal of wax coating from indoor bronze surfaces

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In this research, we propose an advanced system for the cleaning of wax-based coatings applied on indoor bronzes. To this aim we developed a new kind of eco-friendly gel based on PHB (poly-3-hydroxybutyrate) used as thickening agent, biodiesel (BD) and dimethyl carbonate (DMC). BD is a mixture of methyl esters obtained from palm oil, which acts as cleaning agent while DMC was added as additional solvent to partially solubilize PHB and forming a gelly phase. For the first time a PHB-based gel obtained by mixing two solvents with different proprieties was proposed, expanding the range of possible formulations, that can be used according to the specific restoration purpose. After the preliminary characterization of chemical and physical properties of the gel, an ad hoc analytical protocol was implemented to evaluate both the cleaning efficiency and the release of residues on the treated surfaces. Standard samples were prepared following ancient recipes and submitted to spectroscopic and chromatographic analysis before and after the cleaning procedures. Finally, the performances of PHB-DMC/BD gel were assessed on a real case of study presenting a wax-based coating: the Pulpito della passione attributed to Donatello and dated back to 1460. In situ analysis demonstrated the high cleaning efficiency of the proposed systems also for the removal of aged coatings.
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Yimingetal. Herit Sci (2019) 7:34
https://doi.org/10.1186/s40494-019-0276-8
RESEARCH ARTICLE
A new bio-based organogel fortheremoval
ofwax coating fromindoor bronze surfaces
Jia Yiming1,3, Giorgia Sciutto1*, Silvia Prati1, Emilio Catelli1, Monica Galeotti2, Simone Porcinai2,
Laura Mazzocchetti4, Chiara Samorì5, Paola Galletti5, Loris Giorgini4, Emilio Tagliavini5 and Rocco Mazzeo1*
Abstract
In this research, we propose an advanced system for the cleaning of wax-based coatings applied on indoor bronzes.
To this aim we developed a new kind of eco-friendly gel based on PHB (poly-3-hydroxybutyrate) used as thickening
agent, biodiesel (BD) and dimethyl carbonate (DMC). BD is a mixture of methyl esters obtained from palm oil, which
acts as cleaning agent while DMC was added as additional solvent to partially solubilize PHB and forming a gelly
phase. For the first time a PHB-based gel obtained by mixing two solvents with different proprieties was proposed,
expanding the range of possible formulations, that can be used according to the specific restoration purpose. After
the preliminary characterization of chemical and physical properties of the gel, an ad hoc analytical protocol was
implemented to evaluate both the cleaning efficiency and the release of residues on the treated surfaces. Standard
samples were prepared following ancient recipes and submitted to spectroscopic and chromatographic analysis
before and after the cleaning procedures. Finally, the performances of PHB-DMC/BD gel were assessed on a real case
of study presenting a wax-based coating: the Pulpito della passione attributed to Donatello and dated back to 1460.
In situ analysis demonstrated the high cleaning efficiency of the proposed systems also for the removal of aged
coatings.
Keywords: Green organogel, Polyhydroxyalkanoates, Indoor bronzes, Wax-based coatings
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Introduction
e surface of indoor metal objects is a complex system
characterized by the presence of inorganic products and
organic coatings. Traditionally, at the end of the casting
process, the bronzes were cleaned and the rough surface
usually treated to obtain the desired surface finish [13].
To obtain a specific shade, the bronze surface can be
treated with different chemical solutions, such as chlo-
rides, nitrates and sulphates. Wax, lacquer, or varnish
were applied to saturate the surface color and protect
the patina and the metal surface from corrosion [46].
Among these different materials, natural wax - such as
beeswax - is one of the mostly used coating for indoor
bronzes, thanks to its properties such as low water
vapor permeability and low gloss [7]. Waxes could also
be applied as a maintenance treatment. Consequently,
today one of the most common goals in the restoration
of bronze objects is the removal of degraded wax-based
coatings from objects.
Waxes are mainly composed of long chain aliphatic
molecules containing 20–50 carbon atoms [8]. Usually
non-polar solvents, such as dodecane, can be used to sol-
ubilize the wax [9]. Some alkaline compounds could also
be used to remove the waxes from the surfaces, subse-
quently treated with weakly acidic solutions to neutralize
their action. is application has been limited due to its
aggressiveness for the artwork [10]. Additionally, tradi-
tional cleaning methods were based on the application of
neat solvents. is cleaning approach may induce draw-
backs mainly owing to the unrestricted action of the sol-
vent, which leads to its penetration into porous matrices,
producing undesired phenomena [1114].
In the last decade, chemical and physical gels used for
the removal of aged varnishes or coatings from artwork
Open Access
*Correspondence: giorgia.sciutto@unibo.it; rocco.mazzeo@unibo.it
1 Department of Chemistry “G. Ciamician”, Microchemistry
and Microscopy Art Diagnostic Laboratory (M2ADL), University
of Bologna, Via Guaccimanni 42, 48121 Ravenna, Italy
Full list of author information is available at the end of the article
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Page 2 of 12
Yimingetal. Herit Sci (2019) 7:34
surfaces have gained considerable popularity. e main
reason is the gel’s ability to retain solvents and provide
a controlled and efficient superficial cleaning action [9,
1518].
For the cleaning of metal objects, different type of
thickeners and confining systems (such as micellar solu-
tions or microemulsions) were reported [19]. Recently,
a new poly(vinyl)alcohol-based film has been proposed
for the removal of corrosion products from histori-
cal bronzes [19]. Conversely, limited attention has been
devoted to the impact that such cleaning systems might
have on environmental and human safety.
To introduce powerful and sustainable alternatives for
cleaning artworks, we have recently proposed new bio-
compatible cleaning systems for paintings based on the
use of fully green components [20, 21]. In particular, in
the previous researches we have demonstrated the effi-
ciency of poly-3-hydroxybutyrate (PHB)-based gels with
γ-valerolactone (GVL) as solvent, for the removal of ter-
penic and synthetic varnishes from oil and water sensi-
tive egg tempera paintings [20, 21].
In the present research, bio-based components were
selected to produce new organogels able to solubilize
old wax coatings from indoor bronzes with a controlled
action. To this aim, PHB was used as thickening agent
and mixed with biodiesel (BD) and dimethyl carbon-
ate (DMC). PHB can be obtained from bacteria through
aerobic conversion of various carbon sources, and it is
characterized by thermal and mechanical properties
comparable to synthetically-produced degradable poly-
esters and similar to polypropylene (PP) [22]. PHB-based
gels can be obtained by heating the polymer in an appro-
priate solvent and then cooled [20]. erefore, the sol-
vent used must be able to solubilize the polymer and at
the same time must allow the formation of the crystalline
phase which holds the structure of the gel together. How-
ever, PHB is a highly crystalline polymer, whose solubi-
lization can be guaranteed by just a few green solvents,
among which there are different polar molecules [20].
Biodiesel is a mixture of alkyl esters with long chain
fatty acids, biodegradable and produced by renewable
sources [23]. BD proved its ability in the removal of wax
coating, making it a perfect candidate to produce a gel
system active against non-polar coatings. However, PHB
was completely insoluble in BD, and for this reason it was
not possible to obtain a gel using these two components.
To overcome this drawback, new solvents mixtures
were evaluated, with the aim of expanding the range of
possible formulations that can be used, and possibly
allowing the ability to tune the hydrophobic/hydrophilic
character of the obtained gels for tailored applications.
To this aim, DMC was selected as additional sol-
vent because it is soluble in BD and previous works have
demonstrated that it is able to solubilize PHB at 70–90°C
and to form a gelly phase when cooled down to room tem-
perature. Furthermore, DMC also has the advantage of
being the same solvent used for the extraction and purifica-
tion of the polymer from bacterial debris [24, 25]. DMC is
characterized by a low toxicity, it is fully biodegradable and
not classified as volatile organic compound (VOC) (EPA).
In addition, DMC has a high vapor pressure (7.57 kPa
at 25°C), that guarantees a lower residual amount on the
treated surface.
e ability of the triplet DMC, BD and PHB to provide
gel was thus assessed and its performances were evaluated
on standard samples and on the indoor bronze surfaces of
the Pulpito della passione (1460 A.C.) attributed to Dona-
tello to validate this innovative cleaning system.
Materials andmethods
Dimethyl carbonate (DMC), poly-hydroxybutyrate (PHB)
and cyclohexane were purchased from Sigma-Aldrich.
Biodiesel (BD) from palm oil was purchased from Novaol,
Ravenna (IT). All the chemical reagents were commercially
available and directly used without treatment.
Gels synthesis andcharacterization
e green gel was synthesized by solubilizing poly-
hydroxybutyrate (PHB) into mixtures of dimethyl carbon-
ate (DMC) and biodiesel (BD), in a closed vial, by stirring
at 110°C for 5min, then cooled until room temperature in
a Petri dish. Two formulations of gel were prepared with
same procedure and further discussed in the present work
(Table1).
Oscillatory shear measurements were carried out on a
Paar Physica UDS200 rheometer working at 25°C (±0.1°C
Peltier temperature control system) using plate-plate
geometry (25 mm diameter). Frequency sweep measure-
ments were carried out at 5% strain. e storage and loss
moduli (G’ and G’’, respectively) and complex viscosity were
measured over the frequency range 0.1 to 100Hz. WAXS
were carried out at room temperature with a PANalytical
X’Pert PRO diffractometer equipped with an X’Celerator
detector (for ultrafast data collection). A Cu anode was
used as X-ray source (K radiation: λ = 0.15418nm, 40 kV,
40mA), and ¼° divergence slit was used to collect the data
in 2θ range from 2° to 60°. Micrographs of dried gels were
taken with a Scanning Electron Microscope (SEM) ZEISS
EVO 50 EP in Environmental mode with 100Pa pressure
Table 1 Composition ofthegel formulations
Gel PHB mg DMC mL Biodiesel mL
PHB-DMC/BD(3:1) 400 3 1
PHB-DMC/BD(1:1) 400 2 2
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Yimingetal. Herit Sci (2019) 7:34
in the chamber. e capacity of the gel network to retain
the solvent and thus reduce the evaporation rate, was eval-
uated by thermogravimetric analysis (TGA) using a TA
Instruments STD-600 apparatus. Analyses on gels (about
25mg) and neat solvents (sample weight about 25mg) were
performed under nitrogen flow. An isothermal run at 40°C
for 90min was selected as the one most like the exposition
condition of the restorer during cleaning practice. Rheolog-
ical and thermogravimetric measurements were performed
on a set of 3 replicates for each type of gel system studied
and the trend show no significant differences.
Standard samples andreal case studies
Standard bronze samples have been prepared by the
restorers of Opificio delle Pietre Dure (Florence), follow-
ing ancient recipes. In more detail, the metal surface of
a fresh cast bronze has been brushed with silver nitrate
solution and heated with blue flame, until the surface
turn to black. en, a thin layer of beeswax was applied
under soft flame.
Copper sheets were also used for the evaluation of
the cleaning approach. Copper sheets were prepared by
applying a thin layer of beeswax with soft brush after oxi-
dation of the surface with flame.
Finally, the Pulpito della Passione (1460 A.C.) exhib-
ited in the church of Basilica di Lorenzo and attributed to
Donatello was submitted to the green gel cleaning proce-
dure for the removal of an aged wax-based coating.
Cleaning procedures
e gel was sandwiched between two sheets of rice paper
and left from 5min to 15min (according to the thick-
ness of the layer to be removed) in contact with the sam-
ple surface. A light pressure guarantees a good adhesion
of the gel to the surface to be treated. Rice paper is used
to avoid the risk of PHB residues on the treated surfaces
and to further control the release (and evaporation) of
the solvents, without compromising the adhesion of the
gel even on curved or vertical surfaces. en the gel was
removed, and the surface cleaned with neat DMC and dry
cotton swabs. e neat solvent has been used for cleaning
with cotton swabs. In more detail, the cotton is soaked
in biodiesel and applied to the surface to be treated with
a slight mechanical action for a minute. en, a cotton
swab soaked with dimethyl carbonate has been used to
remove the residues of biodiesel for a few seconds.
Evaluation ofthecleaning performances
An ad-hoc analytical protocol was set up for the evalua-
tion of the gel cleaning performances in terms of: clean-
ing efficiency and presence of solvent residues after
the treatment. Non-invasive and micro-destructive
investigations were carried out on metal surfaces before
and after the treatments.
To evaluate the presence of wax residues after
the cleaning, an Infrared Microscope ermo Sci-
entific Nicolet iN10MX was used in total reflection
mode to record spectra in the range between 675 and
4000 cm1, with a spectral resolution of 4 cm1 and
an optical aperture of 150×150μm. To obtain repre-
sentative data, spectroscopic analysis was performed
on 3 different areas treated with the same cleaning pro-
cedure and 4 spectra were recorded before and after
treatment.
Dino-Lite Premier2 digital microscope type
AD4113T-I2 V with ×40 magnification was used
to record the morphological changes of the treated
surfaces.
e presence of solvent on treated surfaces was char-
acterised by FTIR microscope in total reflection mode
for qualitative characterization and by gas chroma-
tography mass spectrometry (5977 Agilent GC–MS)
for quantification. In this case, 1 cm2 of copper sheets
were treated with both gel and neat biodiesel and then
extracted with cyclohexane (10mL) under sonication for
20min. e cyclohexane is commonly used for analys-
ing and synthesising fatty acid methyl esters (main bio-
diesel constituents) by GC-MS. us, cyclohexane has
been selected as a suitable solvent and its ability to solu-
bilize biodiesel has been tested before the analysis. e
GC-MS analyses of cyclohexane were performed using
an Agilent HP 6850 gas chromatograph connected to
an Agilent HP 5975 quadrupole mass spectrometer. e
injection port temperature was 280 °C. Analytes were
separated on a HP-5 fused-silica capillary column (sta-
tionary phase poly(5% diphenyl/95% dimethyl)siloxane,
30m, 0.25-mm i.d., 0.25-μm film thickness), with helium
as the carrier gas (at constant pressure, 33cms1 linear
velocity at 200 °C). Mass spectra were recorded under
electron ionization (70eV) at a frequency of 1 scan s1
within the 12–600 m/z range. e temperature of the
column was increased from 50 to 180°C at 50°C min1
and then from 180 to 300°C at 5°C min1. Methyl non-
adecanoate (0.05mL of a solution 1000 ppm) was used
as internal standard for the quantitation, assuming a
unitary response factor for all the methyl esters. Chro-
matographic analysis has been carried out on 3 sample
replicas for each cleaning procedure.
Bruker Alpha portable FTIR spectrometer was applied
to monitoring the cleaning procedure on the real case of
study, with reflectance mode sampling and spectral range
400–7000cm1. e instrument has a measurement spot
of 6mm in diameter and working distance of approxi-
mately 15mm. 256 scans were acquired for each spec-
trum at a resolution of 4cm1.
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Results anddiscussion
Gel formulation andcharacterization
In order to assess the ability of a mixed solvent system
such as DMC/BD to produce a gel in the presence of
PHB, two gel formulations, obtained by varying the ratio
between solvents (DMC: BD, Table 1), were produced.
Preliminary tests showed that BD can dissolve wax, while
DMC is used mainly as a tool to promote gel formation
and does not seem to contribute to the cleaning. Both
the gels were thickened at the end of the procedure, lead-
ing to a gel with a whitish, opaque aspect (Fig.1). In both
cases the polymer was able to swell up roughly 10 times
its starting dry weight (400 mg PHB vs. 4 ml solvent
mixture) demonstrating an interesting attitude in acting
as solvent carrier to be used for conservation purposes.
Both gels were characterized to select the most suitable
as cleaning system. Indeed, materials suitable for the
cleaning procedures should be easy to be handle and to
be removed from the artwork surface, and they should
guarantee an adequate stability over time. Hence, micro-
structure, rheological behaviour and thermal properties
were characterized to evaluate the gel performance and
stability.
e mechanical properties of the two gels were inves-
tigated via rheological measurements. Figure 2a shows
the complex viscosity as a function of the applied shear
rate. For both samples, a decrease in complex viscosity
was observed with increasing frequency. However, the
gel containing a higher amount of DMC (DMC/BD 3:1)
display a viscosity which is higher by an order of magni-
tude if compared with the DMC/BD 1:1 gel. is means
that the gel is more rigid than the previous formulation
and that the simple modification of the solvent system
hardly impacts the system properties. Both samples dis-
played also a high storage modulus (Fig.2b), indicating
good mechanical proprieties in terms of gel stiffness,
allowing an easy handling and removal of the gel. e gels
presented a gel-like behaviour and in all the cases stor-
age modulus G’ was higher than that of the loss modulus
G’’. e storage modulus quantifies the elastic behaviour
of the gels, while the loss modulus shows the dissipation
ability of the polymer network. is feature proves the
ability of DMC to form the gel, building a 3D network.
e stiffness and rigidity are positively correlated with G’
and G’’. e DMC/BD 3:1 gel has higher modulus than
the DMC/BD 1:1, making the latter less stiff. Considering
that PHB is insoluble in BD, the higher viscosity recorded
for the gel with a higher DMC fraction might tentatively
be attributed to a stronger ability to interact with the
more akin solvent system, and such a stronger interaction
would in turn hinder the polymer chain mobility. On the
bases of this outcome, the DMC/BD 3:1- based gel seems
the most suitable candidate for cleaning purposes, since
stiffer formulations could be easier to peel off and leave a
lower (up to none) extent of residues behind.
Organogels are stable as long as the solvent, or a frac-
tion of it, does not evaporate. e DMC/BD 3:1-based
gel showed a good stability over the time. us, the shelf
Fig. 1 The aspect of a PHB-BD/DMC (3:1) gel; b PHB-BD/DMC (1:1) gel
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Yimingetal. Herit Sci (2019) 7:34
life of the gels was estimated at a time interval of 2weeks,
keeping the gel closed between two Petri dishes in a labo-
ratory environment. At that time the gel has unchanged
mechanical properties and good cleaning efficiency. Fur-
thermore, previous studies have demonstrated the pos-
sibility of recycling PHB after its use in gel formulations
[20].
While the capacity of both mixtures of solvents for
producing a gel-like structure was proved, the effect on
the polymeric component was assessed in terms of abil-
ity to crystallize. Wide-angle X-ray scattering technique
was used to evaluate the crystallinity of the gels under
investigation. As shown in Fig. 3, black diffractogram
is referred to the pristine PHB polymer, prior gelation,
while spectra in Fig.3a refer to the diffraction pattern of
freshly prepared DMC/BD gels. Wet gel samples DMC/
BD 3:1 and DMC/BD 1:1 diffractograms showed both
the presence of narrow reflections typical of a crystalline
phase besides the obvious amorphous halo expected for a
wet sample, where the solvent is still most of the analysed
volume. e comparison of such reflections with the
pristine PHB powder spectrum revealed a good agree-
ment of the 2θ positions, confirming once again, that the
presence of a gel-like phase is strongly connected to the
ability of the solvent system to allow polymer crystalliza-
tion, without contemporary forcing its complete precipi-
tation in a solid phase precipitate. Moreover, DMC/BD
3:1 wet gel (diffractogram b) seemed to be characterized
by a broader amorphous halo. When gels are dried of the
solvent system, the analysis of the PHB recovered still
displayed reflections positioned accordingly with those
of pure PHB. However, WAXS diffractograms recorded
after the complete drying of the gels (Fig.3b) showed a
broad amorphous halo, suggesting that the polymer is
not prone to re-crystallize again with a similar morphol-
ogy like the pristine polymer, as observed in the presence
of GVL, and previously recorded [20]. Such a hindering
effect towards crystallization ability might be due to the
sequential volatilization of the solvent that promotes a
preliminary evaporation of the DMC, thus leaving the
polymer in the presence of a non-solvent which not allow
mobility and reorganization of the polymeric chains in
a crystalline fashion. Additionally, the different solvent
systems can affect the ability to recrystallize after liquid
phase removal, and the presence of a smaller fraction of
BD worsen the quality of the PHB crystals, as observed
by the comparison of the shape and intensity of the dif-
fractogram peaks.
SEM analysis, performed to characterise the micro-
structure of the gels, was carried out on dried samples.
SEM images of PHB-DMC/BD gels with different DMC
and BD ratio of 3:1 and 1:1 are shown in Fig. 4, after
fracture. Both the systems display a compact irregular
surface, with some morphology typical of plastic defor-
mation. e gel DMC/BD 3:1 shows a finer structure
with micro-indentations typical of a more homogeneous
material. is behaviour well compares with the WAXS
study previously discussed, that shows a more amor-
phous system for the dried DMC/BD 3:1 gel.
Once the structure and morphology of the proposed
gels has been established, their ability to slow down sol-
vent evaporation was evaluated. e solvent evapora-
tion from gels was studied with TGA, in comparison to
the pure solvent behavior, i.e. in conditions that rule out
any possible thermal degradation contribution to the
recorded material weight loss. is investigation should
indeed provide proof of the ability of the gelled solvent
to be retained within the polymer 3D network. In the
applied conditions (90 isotherm at 40 °C) the weight
loss can only be attributed to some solvent evaporation,
which is mainly ascribed to DMC, since the biodiesel has
a low volatility and high boiling point. e relative weight
loss was calculated for each sample, referring to the sole
liquid components. is is due to the reason that the
Fig. 2 Complex viscosity (a) and Storage and loss modulus (b) as a
function of frequency for PHB-DMC/BD (3:1) gel (blue) and PHB-DMC/
BD (1:1) gel (green)
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Yimingetal. Herit Sci (2019) 7:34
polymer (PHB) will never volatilize in the applied con-
ditions, and results are resumed in Table2. Results are
resumed in Table2.
Results clearly show that the solvent is kept more effec-
tively in the gels under the same measuring conditions
(Fig.5). e bars in green show the difference between
the weight of DMC present in the pristine gel (discard-
ing the PHB fraction) and the actual weight loss recorded
during the thermogravimetric measurement, considering
only the solvent fraction. e blue bars represent the dif-
ference between the weight of DMC in the sole solvent
mixture and the total weight loss recorded during the
application of the isotherm in TGA. e solvent reten-
tion within the gels is higher for every ratio of dimethyl
carbonate and biodiesel. e higher is the biodiesel frac-
tion, the more efficient is the retention of DMC in the gel
system. is is a useful feature that can be used to modify
the evaporation properties of highly volatile materials
(such as DMC), which are generally more complicated to
use, while also ensuring greater gel stability.
Evaluation thecleaning ecacy andbiodiesel residuals
e evaluation of the cleaning efficacy of the gel was car-
ried out on standard bronze samples, which were melted
and covered with a beeswax-based by restorers of Opifi-
cio delle Pietre Dure in Florence (Fig.6a).
ese samples allowed us to assess the applicability
of the gel on substrates presenting morphologies and
compositions comparable to a real case of study. us,
representative evaluation on the performances of the
cleaning system was achieved. To this end, several areas
were treated with the DMC/BD 3:1 gel, selected on the
bases of rheological measurements as the most suitable
cleaning system for restoration purposes. Treated areas
were characterized by infrared microscopy in total
Fig. 3 WAXS diffractograms of pristine PHB powder (a) compared
with A wet gels PHB-DMC/BD(3:1) (b) PHB-DMC/BD(1:1) (c) and B
dried gels PHB-DMC/BD(3:1) (d) PHB-DMC/BD(1:1) (e). Vertical lines
are drawn to guide the eye about the position of pristine PHB (a)
crystal phase reflections
Fig. 4 SEM micrographs of dried gels PHB-DMC/BD(3:1) (a) PHB-DMC/BD(1:1) (b)
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Yimingetal. Herit Sci (2019) 7:34
reflection mode to verify the absence of wax-residues
after the treatment. All the areas were also documented
with microphotographs to determinate changes in mor-
phology (Fig.6b, c).
Results obtained by µFTIR analysis showed an effi-
cient removal of the coating in all areas in which the gel
was applied. us, the diagnostic bands of wax at 1474
and 1465cm1 (CH2 scissoring), the doublet at 731 and
721cm1 (CH2 rocking), as well as the C=O stretching
band at 1742cm1 were no longer visible after PHB-
DMC/BD gel application (Fig.7).
On the other hand, bands that characterizes to the
original metal patina were better identifiable. e
broad band at 1586 cm1, ascribable to copper car-
boxylate salts and possibly formed due to the inter-
action between fatty acids of wax with copper salts,
become more intense. In addition, the O-H stretching
at 3547cm1, the N–O stretching at 1047cm1 and
O-NO2 symmetric stretching at 1342 and 1421 cm1
may be referred to the copper hydroxyl nitrate
(Cu2(OH)3NO3), formed during the procedure of man-
ufacture, based on the use of a silver nitrate solution.
e weak bands ascribed to the C-H stretching are still
present although with a significantly lower intensity.
ese bands could be related to the presence of organic
materials used as a coating penetrated into the poros-
ity of the substrate as well as to the presence of copper
carboxylates. After the cleaning procedure, the surface
appeared to be less uniform and details on the manu-
facturing have become visible. In addition, no bands
related to the presence of BD residues were detected.
Comparative tests, performed with neat BD simple
applied with a cotton swab, seem to confirm the ability
of BD in the removal of wax coating (Fig.8). However,
while the CH2 rocking bands at 721 and 729cm1 dis-
appeared, wax and BD residues cannot be excluded due
to the presence of bands at 1746cm1 (C=O stretching
bands), at 1195 and 1175cm1 (O–CH3 stretching and
C–O–C symetric stretching) (Fig.8b). is result can
be explained by the less controlled action of BD, which
Table 2 Evaluation ofthegel’s retention power based onTGA results
a Evaluated discarding polymer fraction of the gel
b Evaluated as
DMC =100
(X
DMC
WL
TGA
)
X
DMC
Sample Solvent fraction [%wt] DMC insolvent mixture XDMC
[%wt] TGA weight loss WLTGA
[%wt] b
DMC
[%wt]
Gel PHB-DMC/BD(3:1) 90.9 78.9 67.3a15
Gel PHB-DMC/BD(1:1) 90.5 55.5 38.2a31
DMC/BD(3:1) 100 78.9 72.3 8
DMC/BD(1:1) 100 55.5 51.7 7
Fig. 5 Comparison of the solvent evaporation in the gel systems
(blue bars) and in pure solvents (green bars) based on TGA
experiments
Fig. 6 a Bronze mock-up sample made by Opificio delle Pietre
dure in Florence; Dino-lite microscope images b before and c after
cleaning by PHB-DMC/BD(3:1) gel for 5 min
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Yimingetal. Herit Sci (2019) 7:34
was not confined to the gel matrix, and by a less effec-
tive cleaning action of the neat solvent.
e evaluation of solvent retention by the treated sur-
face is a crucial point in the development of new clean-
ing systems [21]. Indeed, the persistence of solvent on
the treated surfaces may led to harmful interaction,
alterating the substrate and inducing further corrosion
phenomenas. To better investigate the amount of resid-
ual biodiesel after cleaning, quantitative GC-MS analy-
sis of biodiesel methyl esters (methyl esters of palmitic,
Fig. 7 Total reflection spectra of bronze mock-up sample made by Opificio delle Pietre dure in Florence collected from a treated area with
PHB-DMC/BD(3:1) gel a before cleaning; b after cleaning
Fig. 8 Total reflection spectra of bronze mock-up sample made by Opificio delle Pietre dure in Florence collected from a treated area with neat BD
a before cleaning; b after cleaning; c BD reference spectrum
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Yimingetal. Herit Sci (2019) 7:34
linoleic, oleic and stearic acid) were carried out on ad-hoc
samples obtained by covering an oxidated copper sheet
with a layer of beeswax and treated with PHB-DMC/BD
gel or neat BD. BD methyl esters were clearly visible and
detectable after the cleaning with neat BD applied by cot-
ton swab. e analysis performed on three sample repli-
cas treated with the same procedure allowed to quantify
the amount of BD in the substrate after the cleaning pro-
cedure (0.15 ± 0.03 mg/cm2). On the other hand, no BD
residues were detected after the application of DMC/BD
gel for 5 or 15min, confirming the gel capacity in solvent
retention (Fig.9), in accordance with FTIR investigations.
Application oftheDMC/BD gel onareal case ofstudy
After the preliminary evaluation of the gel performances
in terms of cleaning efficiency and solvent retention, the
PHB-DMC/BD gel was finally applied on the Pulpito
della Passione attributed to Donatello. e metal surface
presented a wax-based coating, probably applied dur-
ing a past restoration campaign. e coating appeared
extremely altered, possible due to the degradation phe-
nomena that occurred over time (Fig.10).
PHB-DMC/BD gel was sandwiched in between two
sheets of rice paper and applied for 15 min on a repre-
sentative area of a bas-relief. On-site spectroscopic inves-
tigations were carried out to monitor the cleaning effects
using a portable spectrometer. IR spectra in total reflection
mode were acquired before and after the cleaning pro-
cedure (Fig. 11). e untreated surface showed peculiar
bands ascribable to the fatty materials present on the sur-
face. In particular, bands at 1736cm1 (C=O stretching),
1480 cm1 (CH2 scissoring), 1188 cm1 (C–O stretch-
ing), and at 786cm1 (CH2 rocking) were visible. In addi-
tion, the presence of calcium carbonate at 716cm1 and
Fig. 9 Chromatograms of a cleaned sample with neat BD; b cleaned sample with PHB-DMC/BD(3:1) gel for 5 min; c cleaned sample with
PHB-DMC/BD(3:1) gel for 15 min
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Yimingetal. Herit Sci (2019) 7:34
879 cm1 probably ascribable to atmospheric deposi-
tion, and calcium oxalates (band at 1331cm1) were also
detected. After the cleaning procedure, only traces of
calcium oxalates were visible, demonstrating the high effi-
ciency of the proposed system in removing the wax-based
coating, without leaving solvent residues.
Fig. 10 Detail of the Pulpito della passione (Donatello, 1460, Florence), before (left) and after (right) the cleaning with PHB-DMC/BD(3:1) gel
Fig. 11 Total reflection spectra collected from the Pulpito di Donatello on the treated area of with PHB-DMC/BD(3:1) gel a before cleaning; b after
cleaning
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Yimingetal. Herit Sci (2019) 7:34
Conclusions
To date, scientific research has mainly focused on the pro-
posal of new cleaning systems for painted surfaces, while
little attention has been paid to the evaluation of new solu-
tions for the restoration of indoor and outdoor bronzes.
We developed a new fully sustainable cleaning method
based on the use of the PHB-DMC/BD organogel for the
removal of wax coating from indoor bronze surfaces,
increasing the number and types of PHB-based gels that
can be used for restoration purposes. In particular, we
demonstrated the possibility of tuning the hydrophobic/
hydrophilic character of the gels for tailored applications.
us, Biodiesel was selected as a perfect candidate for
the non-polar coatings, while DMC was selected thanks to
its ability to solubilize PHB at 70–90°C and to form a gelly
phase when cooled down to room temperature.
FTIR and GC-MS analyses allowed to describe the effi-
cacy of the gels and evaluating BD residues after the appli-
cation. e results showed the good performance of new
green gel proposed for the removal of fresh and aged bees-
wax coatings, avoiding problems related to solvent residues
and ensuring safety for the works, the operators and the
environment.
Acknowledgements
Jia Yiming thanks the China Scholarship Council (CSC) for Ph.D. scholarship.
Authors’ contributions
All authors contributed to the analysis of the data and the drafting of the
document. All authors have read and approved the final manuscript.
Funding
Not applicable.
Competing interests
The authors declare that they have no competing interests.
Availability of data and materials
The datasets used and analysis during the current study are available from the
corresponding author on reasonable request.
Author details
1 Department of Chemistry “G. Ciamician”, Microchemistry and Microscopy
Art Diagnostic Laboratory (M2ADL), University of Bologna, Via Guaccimanni
42, 48121 Ravenna, Italy. 2 Opificio delle Pietre Dure, Viale Filippo Strozzi
1, 50129 Florence, Italy. 3 Chongqing Cultural Heritage Research Institute,
Chongqing 400013, China. 4 Department of Industrial Chemistry “Toso
Montanari”, University of Bologna, Viale Risorgimento 4, 40136 Bologna, Italy.
5 Department of Chemistry “G. Ciamician”, University of Bologna, Via Selmi 2,
40126 Bologna, Italy.
Received: 14 February 2019 Accepted: 24 May 2019
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... Furthermore, chemicals employed during conservation interventions (e.g., solvents, acids, chelators) can be corrosive for metals if not accurately handled and retained [32,33]. However, encouraging research demonstrated the reliability and suitability of gels as cleaning systems for the adjustable and controlled removal of altered organic films or harmful corrosion [29,30,34,35]. ...
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