Natural Patinas Research Articles

Innovative comparative EIS study of weathering steel: 2-year and 44-year-old, natural and artificial patinas

An artificial patina on weathering steel produced with a 10% sulphuric acid solution and a natural patina formed over a period of 2 years in an urban environment were compared with a 44-year-old natural patina on an outdoor weathering steel sculpture. The samples and sculpture are exhibited and studied in two of the most polluted Croatian cities, Zagreb and Sisak. Various techniques used to examine the patina included scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR) and in-situ electrochemical impedance spectroscopy (EIS) using a paste electrolyte cell. A widely accepted criterion for the protective effect of the patina on weathering steel, the ratio of goethite to lepidocrocite (α/γ), was found to be 0.10 < 0.38 < 0.71 for 2-year-old natural, 2-year-old artificial and 44-year-old natural patina on the investigated sculpture. Equivalently, the corrosion rates determined by EIS were approximately 37, 22 and 12.5 μm year-1. The in-situ EIS technique allowed confirming the efficiency of artificial patination by H2SO4 in immediately attaining about twice as protective patina than that formed naturally after 2 years. It also confirmed about three times as efficient protection offered by a 44-year-old patina except on the moss-covered surface.

Laser diagnostics and processing of historical and artificial copper patina

Natural and artificial patination is crucial for the protective qualities and appearance of artefacts and roofings. Patina on historical copper roofing sheets from the Belvedere Palace built by Prince Eugene of Savoy in Vienna 1723 were compared with artificial cuprite and brochantite coated copper sheets employing scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) imaging together with profilometry based on EDX and laser-induced breakdown spectroscopy (LIBS) with 4 ns laser pulses at 266 nm. Non-destructive cleaning with this UV wavelength was discussed. A sufficient fluence cleaning window, the difference between the destruction and the cleaning threshold fluence, of more than one order of magnitude could be realized. The quantification of contamination states on the natural patina samples by colorimetry testing represented a useful tool for future quantification tests of laser cleaning and colour changes.

Evolution and Evaluation of Aesthetic Properties in Weathering Steel Accelerated Patinas: The Role of Lepidocrocite

Weathering steels are widely used in civil engineering, architecture and contemporary art due to their mechanical properties, their enhanced resistance to atmospheric corrosion as well as their aesthetic properties. Artists and blacksmiths often apply chemical treatments to obtain the appealing colors of a patina in a shorter period of time. However, the development of an accelerated patina may have an effect on the final appearance and color of the surface. With the aim of evaluating differences in color and studying the evolution of the surface, eight accelerated patinas were made and exposed to the atmosphere for periods of time of up to 24 months and were compared to a natural patina. The characterization studies showed the presence of lepidocrocite on the surface. A close inspection of the X-ray diffraction patterns showed the displacement of the (020) lepidocrocite reflection and asymmetric broadening of selective lines of this phase that were associated to stacking and twins faults, respectively. These faults decrease with the exposure time and are related to a maximum at 630 nm in the reflectance spectrum and the stabilization of the b* coordinate (yellow color). The colors of the accelerated patinas differ from the natural patina at short exposure times. However, they tend to converge at longer exposure times.

Characterization of the corrosion products formed on Michael the Brave’s equestrian statue in urban atmosphere

The present work is focusing on the study of the corrosion products (natural patina) formed on the surface of the equestrian statue of Michael the Brave, Principe of Walachia, and on the patina formed on the bas-reliefs that decorated the marble of the statue pedestal. The statue is placed in the Mihai – Viteazul Square in the center of Cluj-Napoca, Romania, since 1976 and was restored in 2018 after 42 years of exposure in the urban atmosphere. The morphology and the chemical composition of the statue and of the natural patina formed on the statue’s surface, on the bas-reliefs surrounding the statue, as well as on one assembling element exposed in the urban atmosphere for almost 50 years, were determined by SEM - EDX supported by micro-Raman and XRD analyses. The X-ray maps pictured the distribution of various elements in the patina samples collected from the equestrian statue’s surface. The selective dissolution of the Cu, Zn, and Sn observed in the patina layer, was quantified by the dissolution factor based on EDX analysis. The corrosion products indicated the presence of malachite and brochantite, along with other various minerals. It was concluded that the analyzed patinas were affected by the pollution of the urban atmosphere, but their composition was mostly influenced by the composition of the different copper alloy substrates (high-leaded tin bronze or brass).

Raman approach to the forensic study of bronze patinas

Illicit trade in archaeology is a widespread phenomenon connected to the market of archaeological objects from clandestine excavations or faking manufacture, which includes bronze objects. As a consequence, authentication and provenance studies are required on objects of unknown provenance when they are recovered thanks to the investigation activity of the police, before returning them to the museums. Various strategies and approaches can be used to identify fake bronzes, as for instance patinas studies, since the corrosion can spontaneously occur over time, due to the interaction of the surface with the environment, or induced by artificial treatments, performed to mimic the archaeological aging effects in short time. The artificial patina has outward appearance and colours very similar to the natural patinas, but with different mineralogical composition. In this work, Raman spectroscopy is used to discriminate between artificial and natural patinas by a non-destructive approach, supported by the microanalysis SEM-EDS and optical microscope observations in the case of two ‘Etruscan-Roman’ bronze statuettes of illicit and unknown provenance, and demonstrates their recent and fake manufacture. Indeed, the patinas of both statuettes contain covellite, antlerite, langite, chalcanthite and rouaite, minerals or metastable chemical species traced back to corrosion induced by chemical attack and thermal treatment, as distinct from their stable polymorphs usually formed in burial condition. Moreover, we have not found evidence of the presence of malachite and azurite, typical of natural aged patinas. Finally, the abundance of Zn in the alloy suggest a modern manufacture.

Novel Procedure for Brochantite Based Pigment Production and Its Immobilization for Restoration of Historical Copper Objects

The article deals with the preparation of artificial copper pigment based on brochantite. The pigment was prepared by slow additions of sodium hydroxide solution into the solution of copper sulphate. The studied parameters were addition rate, solutions concentrations, ferrous ion addition, and filtration time. The prepared pigments were evaluated by means of X-ray diffraction and spectrophotometry. Subsequent pigment immobilization in an acrylate varnish was also studied. Observed parameters were influenced by solvent type, mechanical or chemical dispersion, and natural aging in an aggressive outdoor atmosphere. Pigment containing varnishes were evaluated by means of spectrophotometry, optical profilometers, and pull-off test. Slow hydroxide addition allows the formation of brochantite pigment. Fast filtration limits backward pigment decomposition. Ferrous ion improves pigment color closer to natural copper patina. The ideal immobilization procedure includes ethylacetate as a solvent and mechanical dispersion. The varnish filled with pigment provides sufficient adhesion to the metallic copper substrate as well as long-term color stability in the outdoor atmosphere.

Testing of pilot 2 m3 exposure chamber for formation of brochantite based patina on copper and copper alloys – objects of practical dimensions

Abstract Formation of natural patina on copper and copper alloys objects takes tens of years. There are solutions for patination, which are used in restorers’ practice. However, these artificial patinas are usually based on nitrates, carbonates or chlorides. Patina based on brochantite is the most stable phase under atmospheric conditions. This type of patina was successfully formed in laboratory in a small exposure chamber with higher content of SO2. This work is next step of the experiment to make this method become more practicable. It started with construction 2 m3 exposure chamber and simulation of ideal conditions for patination process. The length of drying phase, homogeneity of conditions, pH of feeding water, colour of patina, placement of samples and final appearance were observed. The chamber construction allows to achieve ideal pH value of feeding water, samples surface became dry during the ventilation and temperature during condensation was stable at 40 °C. These conditions are ideal for patination process.

Effect of Sulfuric Acid Patination Treatment on Atmospheric Corrosion of Weathering Steel

The requirements for the formation of a protective patina on conventional weathering steels (WS) are well known in the scientific literature related to civil structures. However, these criteria are not always given due consideration when WS is used in cultural heritage, as in the case of sculptural work. An artificial patina was produced simulating artists’ working procedures using a direct patination technique, applying a solution of 10% H2SO4 on WS specimens. These were exposed for two years in the urban atmosphere of Madrid along with weathering steel specimens without artificial patina, called natural patina. The patinas generated have been analyzed using colorimetry, micro-Raman spectroscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD) and thickness measurements.The artificial patina color formed hardly differs from the color that the natural patina acquires from practically the beginning of its formation in the atmosphere of Madrid. After two years, the atmospheric corrosion rate of patinated WS is lower than 6 µm/year. The sulfuric acid treatment accelerates the protective ability of the patina with respect to the natural patina according to the ratio goethite to lepidocrocite (α/γ). Chromium-rich goethite is located in the inner part of the artificial patina as well as chromium-rich ferrihydrite. Ferrihydrite may act as a precursor of nanophasic goethite.

Copper and sulphur depth profile in patina layers using NRA and RBS

A combination of nuclear reaction analysis (NRA) and Rutherford backscattering spectroscopy (RBS) were utilized in order to obtain information on the depth distribution of sulphur and copper in artificially produced and natural patina layers. The copper profiling was performed by using the reaction 63Cu(p,p'y)6 3Cu and detecting the 1327 keV γ-ray deexciting the third excited state to the ground state of 6 3Cu produced. For the determination of sulfur the 2230 keV γ-ray was used, deexciting the first excited state to the ground state of 32S formed through the reaction 3 2S(p,p'y)3 2S, which exhibits three sharp resonances at projectile energies 3.094, 3.195 and 3.379 MeV. The relevant cross-sections were measured in the energy range between 3.0 and 3.7 MeV in steps of 20 keV at 125° to the incident proton beam direction. Supporting information on the depth distribution of oxygen and the other elements of the patina samples was obtained by p-RBS (Ep = 1.5 MeV; θ = 160°).

Virtues of Giambologna from Grimaldi Chapel Archaeometric Characterisation Part II: ‘Artistic’ and Natural Patinas

The scientific investigation on metal artwork is meant to expand the knowledge regarding the technical skills developed by artists in sculpture manufacturing. Moreover, all the gathered data support the speculation about the motivations behind the choices of certain material or a specific casting and/or finishing method (e.g., aesthetic, economic or technical reasons, or both) and give fundamental information for planning adequate restoration interventions. The subject of this study is the Virtues sculptural group made at the end of the XVI century by Giambologna to decorate the Grimaldi Chapel in the church of San Francesco di Castelletto (Genoa, Italy). Six life-size statues depicting Charity, Justice, Hope, Fortitude, Faith, and Temperance (i.e., the artwork discussed in this article); seven bas-reliefs; and six winged representations of putti are what remains of the original monumental project. Different micro-invasive analytical techniques (scanning electron microscopy and energy dispersive x-ray spectroscopy, SEM–EDS; micro-Raman Spectroscopy, μRS; gas chromatography–mass spectrometry, GC–MS) were applied to determine the chemical nature and the state of conservation of the “artistic” and natural patinas visible on the Virtues surfaces. Some patination tests were performed taking into account the collected analytical data and the indications provided by the scientific literature and the historical documents. The combination of the results obtained through the scientific investigations and the experimental work allowed for supposing the materials and the patination technique selected by the artist to adorn the sculptures and providing indications for the correct conservation of the six sculptures.

Virtues of Giambologna from Grimaldi Chapel Archaeometrical Characterization

This scientific investigation on metal artwork is meant to expand knowledge regarding the technical skills developed by artists in sculpture manufacturing. Moreover all the gathered data support the speculation about the motivations behind the choices of certain material or a specific manufacturing method (e.g., economic or technical reasons, or both). The subject of this study is the Virtues sculptural group made at the end of the XVI century by Giambologna to decorate the Grimaldi Chapel in the church of San Francesco di Castelletto (Genoa, Italy). Six life-size statues depicting Charity, Justice, Hope, Fortitude, Faith, and Temperance (i.e., the artwork discussed in this article); seven bas-reliefs; and six winged representations of putti are what remains of the original monumental project. X-ray fluorescence, a nondestructive investigation method, was applied to determine the chemical nature of the metallic substrate and of the “artistic” and natural patinas. The achieved results allowed for distinguishing variations in the content of the major alloying elements that might correspond to a motivated will of the artist connected to technical and aesthetic aims: the production of defect-free and golden bronze artifacts.

Investigation of a naturally patinated bronze artifact originating from the outdoor statuary group of Mathias Rex

Abstract The present work aims to investigate the morphology and composition of natural patina formed after long-term atmospheric exposure (∼100 years) on the bronze screws used in assembling the statuary group of Mathias Rex, placed in the center of Cluj-Napoca, Romania. The chemical composition of the bronze and of the natural patina, formed on the screws’ surface were determined by X-ray fluorescence analysis, and the morphology of the latter was determined by SEM - EDX cartographies.

Cyclic wetting–drying ageing test and patina formation on tuffeau limestone

This paper presents the results of an ageing test based on cycles of wetting–drying of a French building limestone named tuffeau. During 3 years, 50 complete wetting–drying cycles with pure distilled water were carried out on the side of a cylindrical sample. The characterization of the stone shows that the ageing test applied induces changes in the surface appearance, with the formation of a patina with decrease in porosity and mineralogical changes due to dissolution, transport, and recrystallization of the dissolved mineralogical component of the rock as calcite. The decrease in the imbibition kinetics confirms the alteration of the tested stone sample. After 50 wetting–drying cycles, imbibition coefficients are almost halved. This alteration is not homogeneous as the patina covers only some sections of the exposed surface of the stone. On the well-formed parts of the patina with a brownish and varnished aspect, which represent about 20 % of the total surface, measurements show a decrease in porosity and roughness of about 25 %. The physico-chemical characterization shows that the patina formed in the laboratory is fully comparable to natural patina observed on monuments built of tuffeau.

Raman investigation of artificial patinas on recent bronze – Part I: climatic chamber exposure

In humid air, copper and its high copper alloys (bronze) tend to form an oxide layer (patina). Natural patinas protect copper and its alloys from further corrosion processes. On the other hand, artists have frequently deliberately patinated bronze for visual effects. Thus, it is of great importance to study the patina changing mechanism to follow its chemical changes and to predict in advance the likely corrosion processes. Green chloride and green nitrate patinas, applied over the brown artist's patina, were tested, and also brown patina and the patina that develops on bare bronze. The Raman spectra were studied after chemical patination, and after exposing the patina samples in a climatic chamber, which can produce an environment that resembles an industrial atmosphere, for 12 weeks. The structures of the patinas and of the corrosion products were characterized by scanning electron microscopy, Raman spectroscopy and X‐ray diffraction. Cuprite and cuprous sulfite were found on the brown patina, atacamite on the green chloride patina, and a mixture of gerhardite and rouaite on the blue to green nitrate type patina. After 12 weeks of exposure to humidity, a controlled concentration of SO2, and salt spray mist, the corrosion products changed. In general, clinoatacamite and paratacamite are the end corrosion products, after an intermediate brochantite stage on the green chloride and green nitrate type patinas. The end products of each patina type are given. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Micro‐Raman spectroscopy for the characterization of artistic patinas produced on copper‐based alloys

In order to contribute to the improvement of restoration and conservation sciences of metallic artefacts belonging to Cultural Heritage, a wide research has been started on the study of artificial (artistic) and natural (corrosion or alteration) patinas of copper‐based alloys.Micro‐Raman spectroscopy (μRS) coupled with scanning electron microscopy (SEM) equipped with energy Dispersive X‐Ray Spectroscopy (EDXS) were applied for the characterization of a number of patinas made on metallic substrate reproducing the typical copper‐based alloys used for statuary. A non alloyed copper sheet has been also used to simulate the architectural cover typically used after a specific cold spray coating.To reach these goals, a number of patinas were experimentally produced in our laboratory using the torch technique and reactive solutions based on water as a solvent containing, respectively, copper nitrate, iron nitrate and potassium sulfide (‘liver of sulfur’). The only exception is a sample reproducing the architectural cover of a recent building nearly completely covered with copper sheets coated with copper salts.All the produced patinas were aged in a salt spray chamber and then studied with μRS, SEM and EDXS in order to understand their evolution between the two known terms and therefore the reactions involved in all patinas. μRS allowed us to detect as main constituents of the ‘artistic’ patinas, before and after ageing: copper oxides (Cuprite and Tenorite), copper nitrates (Rouaite and Gerhardtite), copper chlorides (Botallackite, Atacamite and Paratacamite), a copper sulfate (Posnjakite), iron oxides (Magnetite and Hematite) and an iron hydroxide (Limonite). Copyright © 2012 John Wiley &amp; Sons, Ltd.

An electrochemical impedance study of the corrosion protection of artificially formed patinas on recent bronze

The patina that covers bronze often has an aesthetic and/or historical value. It should therefore be protected from corrosion in aggressive urban environments. In the present study, two green chemically obtained bronze patinas were examined, as well as an electrochemical patina that resembles the natural patina. Different protective systems were applied to the samples: azole-based inhibitors in an ethanol solution, azole-based inhibitors in polymer coating or wax coating. The efficiency of these protective systems during immersion in simulated urban rainwater was monitored by electrochemical impedance spectroscopy. The results showed that the protection of patinated bronzes by the inhibitor/ethanol system was long-lasting and most effective for electrochemically patinated bronze. Protective properties of investigated inhibitor/Paraloid B44 systems were found to depend on inhibitor present in the coating as well as on properties of the patina layer on which it was applied. For all types of patina BTA/Paraloid B44 coating exhibited better protecting properties than MTI/Paraloid B44 coating. Carnauba wax was found to provide stabile and effective protection, especially for nitrate patinated bronze.

Noninvasive Solution for Electrochemical Impedance Spectroscopy on Metallic Works of Art

Metallic works of art of cultural relevance are continuously subjected to corrosion as the environment becomes increasingly polluted. A fast and simple method to in situ assess the conservation conditions is therefore required. This paper describes the development and performance of dry and gel-based electrodes which can be used to assess the surface conservation state without the need to move the artifacts and which do not cause any damage to them. The electrodes can be used with a portable electrochemical impedance spectroscopy system, without employing electrochemical cells. The proposed solution does not provide all the information that one can obtain using an electrochemical cell, but it can discriminate between protective coatings. It can be used to assess the protective capability of corrosion product layers and natural patinas, and it can therefore enable a noninvasive routine surface assessment to be conducted that could be extremely useful for people working in the field of conservation of cultural heritage.

Scientific Analysis of an Ancient Bronze Mirror Dating of Han Dynasty

A bronze mirror dating of Han-dynasty was studied in this paper. The in-depth structure and the composition of the natural patina and substrate alloy were determined by Electron probe micro-analyzer (EPMA) observation and EDS cartographies on a cross-section, the mineral composition of patinas were analyzed using XRD. The results show the layer structure of the patina has two obvious layers, and the inner layer ranges from 90µm to 260µm. This bronze mirror is a typical ancient Chinese black mirror Heiqigu, and the formation of high tin layer is due to special surface treatment by the ancients. Tthe composition of the patinas on the mirror and that of substrate alloy were found to be highly heterogeneous, the thin black patina on the mirror containing is cassiterite.

DECORATIVE LAYERS ON SILICON BRONZES

Decorative coatings artificially obtained on the surfaces of copper castings, commonly called artificial patinas in opposition to natural patinas, allow for adjusting the colour, the gloss, and the colour intensity of a casting to the customer demands. A decorative coating is a part of the artistic casting and the key element to the impression exerted by a sculpture. The colour, the type, and quality of a decorative coating depend on the author's intention, so that an artistic foundry should deal successfully with a variety of demands. Beside exerting the strictly artistic impression, patinas have one more goal to achieve - it is the corrosion protection. Natural patinas arising nowadays are characterised by Iow quality, irregular thickness, numerous cracks, and the dull taupe-brown colour. Therefore there exists need of generating decorative coatings in a controlled way. The present work is aimed to determine the influence of mechanical abrasive working applied to the surfaces of BK331 silicon bronze castings on the quality and durability of the obtained decorative coating for given recipes for patinating solutions.

Dynamic secondary ion mass spectrometry and X-ray photoelectron spectroscopy on artistic bronze and copper artificial patinas

To prevent the natural processes of decay and to develop and improve the treatments of conservation and restoration of artistic bronzes meaning statues and sculptures, it is important understanding the patination processes and the knowledge of artificially corroded surfaces. Chemical and physical characterization of artificial patinas obtained on artistic bronzes and coppers by using the 19th century Western traditional patination techniques and recipes by means of SEM–EDS, light microscopy and ATR/FT-IR has been done in previous studies [I.Z. Balta, L. Robbiola, Characterization of artificial black patinas on artistic cast bronze and pure copper by using SEM–EDS and light microscopy, in: Proceedings of the 13th European Microscopy Congress, 22–27 August 2004, Antwerp, Belgium, EMC 2004 CD-Rom Conference Preprints; I.Z. Balta, L. Robbiola, Traditional artificial artistic bronze and copper patinas—an investigation by SEM–EDS and ATR/FT-IR, in: Proceedings of the 8th International Conference on Non Destructive Investigations and Microanalysis for the Diagnostics and Conservation of the Cultural and Environmental Heritage, 15–19 May 2005, Lecce, Italy, ART’05 CD-Rom Conference Preprints]. Differences in morphology (structure, thickness, porosity, adherence, compactity, uniformity, homogeneity) and also in composition, on both artistic cast bronze and pure copper patinas, were clearly evidenced. Further in-depth investigation is required to be carried out in order to better understand the patinas mechanisms of formation and the layers kinetics of growth. The elemental and chemical analysis, either on a surface monolayer or in a depth profile, by using the Secondary Ion Mass Spectrometry (SIMS) and X-ray Photoelectron Spectroscopy (XPS) techniques, can provide this kind of information, unique at trace-level sensitivity. SIMS has proved to be a suitable analytical technique for analyzing small amounts of material with high atomic sensitivity (ppm or even ppb) and high depth/lateral resolution in the micron and sub-micron range [R.G. Wilson, F.A. Stevie, C.W. Magee, Secondary Ion Mass Spectrometry: A Practical Handbook for Depth Profiling and Bulk Impurity Analysis, Wiley & Sons, New York, 1989; M. Dowsett, A. Adriaens, The role of SIMS in cultural heritage studies, Nucl. Instr. Meth. Phys. Res. B 226 (2004) 38–52]. XPS has the ability to provide detailed chemical information on virtually each kind of solid sample, and elemental identification is therefore possible due to the core level photoemission. The most important advantage is the high surface sensitivity of the chemical information (a few monolayers) [E. Ciliberto, G. Spoto, Modern Analytical Methods in Art and Archaeology, John Wiley & Sons, Inc., New York, 2000]. In addition elements’ relative abundance can be made semi-quantitative or quantitative and information on chemical bonds can be derived. The aim of the present work is to highlight the advantages and the limits of XPS and Dynamic SIMS surface analytical techniques for the characterization of artistic bronze and copper artificial patinas. The results obtained on the analyzed samples allowed the distribution of the main elements in the corrosion patinas layers and the contribution of each elements present in bronze matrix to the color of the resulting patinas to be precisely revealed. This information could be used for comparative studies between artificial and natural patinas, and also for provenience and authentication studies for artistic and archaeological bronzes.