- Research article
- Open Access
A new bio-based organogel for the removal of wax coating from indoor bronze surfaces
© The Author(s) 2019
- Received: 14 February 2019
- Accepted: 24 May 2019
- Published: 30 May 2019
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.
- Green organogel
- Indoor bronzes
- Wax-based coatings
The 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 [1–3]. To obtain a specific shade, the bronze surface can be treated with different chemical solutions, such as chlorides, nitrates and sulphates. Wax, lacquer, or varnish were applied to saturate the surface color and protect the patina and the metal surface from corrosion [4–6]. 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 . 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 . Usually non-polar solvents, such as dodecane, can be used to solubilize the wax . Some alkaline compounds could also be used to remove the waxes from the surfaces, subsequently treated with weakly acidic solutions to neutralize their action. This application has been limited due to its aggressiveness for the artwork . Additionally, traditional cleaning methods were based on the application of neat solvents. This cleaning approach may induce drawbacks mainly owing to the unrestricted action of the solvent, which leads to its penetration into porous matrices, producing undesired phenomena [11–14].
In the last decade, chemical and physical gels used for the removal of aged varnishes or coatings from artwork surfaces have gained considerable popularity. The main reason is the gel’s ability to retain solvents and provide a controlled and efficient superficial cleaning action [9, 15–18].
For the cleaning of metal objects, different type of thickeners and confining systems (such as micellar solutions or microemulsions) were reported . Recently, a new poly(vinyl)alcohol-based film has been proposed for the removal of corrosion products from historical bronzes . 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 biocompatible cleaning systems for paintings based on the use of fully green components [20, 21]. In particular, in the previous researches we have demonstrated the efficiency of poly-3-hydroxybutyrate (PHB)-based gels with γ-valerolactone (GVL) as solvent, for the removal of terpenic and synthetic varnishes from oil and water sensitive 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 carbonate (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 polyesters and similar to polypropylene (PP) . PHB-based gels can be obtained by heating the polymer in an appropriate solvent and then cooled . Therefore, the solvent 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. However, PHB is a highly crystalline polymer, whose solubilization can be guaranteed by just a few green solvents, among which there are different polar molecules .
Biodiesel is a mixture of alkyl esters with long chain fatty acids, biodegradable and produced by renewable sources . 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 solvent 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 temperature. Furthermore, DMC also has the advantage of being the same solvent used for the extraction and purification 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.
The 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 Donatello to validate this innovative cleaning system.
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
Composition of the gel formulations
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 measurements were carried out at 5% strain. The 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 in the chamber. The capacity of the gel network to retain the solvent and thus reduce the evaporation rate, was evaluated 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. Rheological 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), following 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. Then, 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 oxidation of the surface with flame.
Finally, the Pulpito della Passione (1460 A.C.) exhibited in the church of Basilica di Lorenzo and attributed to Donatello was submitted to the green gel cleaning procedure for the removal of an aged wax-based coating.
The gel was sandwiched between two sheets of rice paper and left from 5 min to 15 min (according to the thickness of the layer to be removed) in contact with the sample 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. Then the gel was removed, and the surface cleaned with neat DMC and dry cotton swabs. The 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. Then, 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 evaluation of the gel cleaning performances in terms of: cleaning 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 Thermo Scientific Nicolet iN10MX was used in total reflection mode to record spectra in the range between 675 and 4000 cm−1, with a spectral resolution of 4 cm−1 and an optical aperture of 150×150 μm. To obtain representative data, spectroscopic analysis was performed on 3 different areas treated with the same cleaning procedure 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.
The presence of solvent on treated surfaces was characterised by FTIR microscope in total reflection mode for qualitative characterization and by gas chromatography 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. The cyclohexane is commonly used for analysing and synthesising fatty acid methyl esters (main biodiesel constituents) by GC-MS. Thus, cyclohexane has been selected as a suitable solvent and its ability to solubilize biodiesel has been tested before the analysis. The GC-MS analyses of cyclohexane were performed using an Agilent HP 6850 gas chromatograph connected to an Agilent HP 5975 quadrupole mass spectrometer. The injection port temperature was 280 °C. Analytes were separated on a HP-5 fused-silica capillary column (stationary 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 s−1 linear velocity at 200 °C). Mass spectra were recorded under electron ionization (70 eV) at a frequency of 1 scan s−1 within the 12–600 m/z range. The temperature of the column was increased from 50 to 180 °C at 50 °C min−1 and then from 180 to 300 °C at 5 °C min−1. Methyl nonadecanoate (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. Chromatographic 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 cm−1. The instrument has a measurement spot of 6 mm in diameter and working distance of approximately 15 mm. 256 scans were acquired for each spectrum at a resolution of 4 cm−1.
Gel formulation and characterization
Organogels are stable as long as the solvent, or a fraction of it, does not evaporate. The DMC/BD 3:1-based gel showed a good stability over the time. Thus, the shelf life of the gels was estimated at a time interval of 2 weeks, keeping the gel closed between two Petri dishes in a laboratory environment. At that time the gel has unchanged mechanical properties and good cleaning efficiency. Furthermore, previous studies have demonstrated the possibility of recycling PHB after its use in gel formulations .
Evaluation of the gel’s retention power based on TGA results
Solvent fraction [%wt]
DMC in solvent mixture XDMC [%wt]
TGA weight loss WLTGA
∆ DMC b
Evaluation the cleaning efficacy and biodiesel residuals
These samples allowed us to assess the applicability of the gel on substrates presenting morphologies and compositions comparable to a real case of study. Thus, 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 reflection mode to verify the absence of wax-residues after the treatment. All the areas were also documented with microphotographs to determinate changes in morphology (Fig. 6b, c).
On the other hand, bands that characterizes to the original metal patina were better identifiable. The broad band at 1586 cm−1, ascribable to copper carboxylate salts and possibly formed due to the interaction between fatty acids of wax with copper salts, become more intense. In addition, the O-H stretching at 3547 cm−1, the N–O stretching at 1047 cm−1 and O-NO2 symmetric stretching at 1342 and 1421 cm−1 may be referred to the copper hydroxyl nitrate (Cu2(OH)3NO3), formed during the procedure of manufacture, based on the use of a silver nitrate solution. The weak bands ascribed to the C-H stretching are still present although with a significantly lower intensity. These bands could be related to the presence of organic materials used as a coating penetrated into the porosity 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 manufacturing have become visible. In addition, no bands related to the presence of BD residues were detected.
Application of the DMC/BD gel on a real case of study
To date, scientific research has mainly focused on the proposal of new cleaning systems for painted surfaces, while little attention has been paid to the evaluation of new solutions 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.
Thus, 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 efficacy of the gels and evaluating BD residues after the application. The results showed the good performance of new green gel proposed for the removal of fresh and aged beeswax coatings, avoiding problems related to solvent residues and ensuring safety for the works, the operators and the environment.
Jia Yiming thanks the China Scholarship Council (CSC) for Ph.D. scholarship.
All authors contributed to the analysis of the data and the drafting of the document. All authors have read and approved the final manuscript.
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.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
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