Artificial Patination of Copper and Copper Alloys in Wet Atmosphere

coatings

Article Artiﬁcial Patination of Copper and Copper Alloys in Wet Atmosphere with Increased Content of SO2

Richard Bureš 1, Martin Klajmon 2, Jaroslav Fojt 1, Pavol Rak 1, Kristýna Jílková 3 and Jan Stoulil 1,*

1 Department of Metals and Corrosion Engineering, University of Chemistry and Technology, 166 28 Prague, Czech Republic; [email protected] (R.B.); [email protected] (J.F.); [email protected] (P.R.) 2 Department of Physical Chemistry, University of Chemistry and Technology, 166 28 Prague, Czech Republic; [email protected] 3 Department of Glass and Ceramics, University of Chemistry and Technology, 166 28 Prague, Czech Republic; [email protected] * Correspondence: [email protected]; Tel.: +420-220-443-750

Received: 3 October 2019; Accepted: 6 December 2019; Published: 8 December 2019

Abstract: Natural copper patina is usually formed over several decades. This work investigates the possibility of obtaining a stable artificial patina based on brochantite in a more reasonable time. The patination process was based on patina formation from a humid atmosphere containing sulphur dioxide. The studied parameters were humidity (condensation and condensation/drying), sulphur dioxide concentration (4.4–44.3 g m 3) and surface pre-treatments (grinding, pre-oxidation and · − pre-patination) prior to the patination process. Samples were evaluated by mass change, digital image analysis, spectrophotometry, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). A resistometric method was employed in order to observe the patina formation continuously during the exposure. Conditions inside the chamber were monitored during the exposure (pH of water and concentration of SO2 in gaseous phase). According to XRD, it was possible to deliberately grow a brochantite patina of reasonable thickness (approx. 30 µm), even within a couple of days of exposure. The drying phase of the condensation cycle increased the homogeneity of the deposited patina. Formation kinetics were the fastest under a condensation/drying cycle, starting with 17.7 g m 3 sulphur dioxide and decreasing dosing in the cycle, with an electrolyte · − pH close to 3. The higher sulphur dioxide content above 17.7 g m 3 forms too aggressive a surface · − electrolyte, which led to the dissolution of the brochantite. The pre-oxidation of copper surface resulted in a significant improvement of patina homogeneity on the surface.

Keywords: copper patina; gaseous phase; artiﬁcial patina

1. Introduction The typical colour of clean copper is usually salmon-pink. After the exposure of copper to the atmosphere, this colour starts to change. These changes of colour are due to the formation of a protective layer called patina, which lends an aesthetic and historical value to the object. A formation of patina usually takes several decades under common outdoor conditions. Patina forms due to an interaction of copper with gaseous, aerosolised or solid particles of corrosive species, such as SO2, NO2,O3 and Cl−. Changes start with the darkening of the copper surface to brown or even black locally and ﬁnish with the formation of an aesthetically optimal green layer. The resulting products (patina) are insoluble. Patina is a layer consisting of corrosion products formed and retained on the copper surface. The rate of formation, composition and colour of patina is inﬂuenced by composition of the atmosphere, the level of air pollution and an alternation of wet and dry cycles [1–7].

Coatings 2019, 9, 837; doi:10.3390/coatings9120837 www.mdpi.com/journal/coatings Coatings 2019, 9, 837 2 of 18

Currently, the rate of air pollution has decreased compared to previous decades, which correlates with an increase of time to patination [8,9]. Natural patina is spatially heterogeneous and has diﬀerent layers of the copper corrosion products. This was proven during the study of over 300-year-old roof tiles from Queen Anne’s Summer Palace in Prague and of the surface of a bronze monument to Francis Garnier, Paris [10,11]. In the beginning of an exposure, the layer of cuprous oxide (Cu2O), called cuprite, appears on a surface of the copper object [12]. A thin layer forms, even under dry conditions, by direct oxidation [13]. This layer changes the colour of copper to brown and dark brown. The thickness of this layer is 5–10 µm [1,8,12–14]. The mechanism of the formation of cuprite is according to Equations (1) and (2). + Cu Cu + e− (1) → 2Cu+ + H O Cu O + 2H+ (2) 2 → 2 In the next step the natural patination continues with oxidative dissolution leading to the formation of cupric ions according to Equation (3). The ions react with pollutants in atmosphere and form the green phase of patina. The composition of these phases depends on the pollutants in the atmosphere. In industrial atmosphere, the green copper patina is usually based on sulphates, whereas patina formed close to the ocean is based on chlorides [15].

+ 2+ Cu O + 2H 2Cu + H O + 2e− (3) 2 → 2

The sulphates of copper are brochantite (Cu4SO4(OH)6), antlerite (Cu3(SO4)(OH)4), posnjakite (Cu (SO )(OH) H O), stranbergite (Cu (SO ) (OH) 4H O), and langite (Cu (SO )(OH) 2H O). 4 4 6· 2 5 4 2 6· 2 4 4 6· 2 Posnjakite, strangbergite and langite are precursors and they can transform to the more stable phases brochantite and antlerite. Copper ions with chlorides can form nantokite (CuCl) instead of cuprite and subsequently form atacamite (Cu2Cl(OH)3)[12–14,16–18]. The main components of outdoor patinas are cuprite, brochantite, antlerite, posnjakite and atacamite [4,13,18]. Fonseca et al. have studied copper corrosion in urban and marine atmospheric conditions. They observed that patina formed in an urban atmosphere is less porous and more adherent compared to marine patina. Marine patina is heterogeneous resulting in more probable film spalling, probably due to strong winds and the high degree of wetness during almost all of the year [4]. The time to form patina from sulphates takes almost a year but chloride patinas can be found even after a month of exposure [19]. According to Greadel et al. [16] sulphates are more stable than chlorides, so even in a marine atmosphere, we can find more brochantite than atacamite. For example, the patina on the Statue of Liberty, which is surrounded by the sea, has as the most common compounds brochantite and antlerite, which are typically formed in acidic conditions and sheltered areas. Antlerite is even more stable than brochantite at lower pH levels. This composition is probably a consequence of acidic precipitations in New York Harbour [8,19–22]. This work is aimed at forming an artificial patina based on brochantite, as it is the most stable phase of copper patina. Brochantite is formed on the top of the cuprite layer, and the thickness is usually 5–40 µm. Brochantite is formed from cuprite according to Equation (4) [1].

2 + 2Cu O + SO − + 4H O CuSO 3Cu(OH) + 2H + 4e− (4) 2 4 2 → 4 · 2

Brochantite is the main phase of copper patina in atmospheres polluted by SO2 and it seems that the amount of brochantite depends on the concentration of SO2 in the atmosphere. However, there is a limit concentration of SO2 above which brochantite does not form and too aggressive a layer of aqueous electrolyte dissolves the patina on the copper surface. In the work of Zittlau [23], there is a range where brochantite is thermodynamically stable. Thus, acid rains can be harmful for patina instead of supporting the formation of a new aesthetic layer on copper. For the artiﬁcial ageing of copper by artiﬁcial acid, the rain is commonly a solution with pH < 4.5 [18,21,24]. According to the Coatings 2019, 9, 837 3 of 18 work of Nassau, the limit of pH for the dissolution of copper patina and other copper compounds is 2.4 [18]. The important role of SO2 in the atmosphere is due to homogeneous and heterogeneous oxidation of sulphur dioxide to sulphuric acid. The homogeneous oxidation primarily starts through the reaction with a hydroxyl radical. The heterogeneous oxidation mainly occurs in cloud droplets or at air–water interfaces of aerosols [25]. The solubility of SO2 is determined by the following dissociation processes in droplets as shown in Equations (5)–(7) [26].

SO (g) SO (aq) (5) 2 → 2 + H O + SO (aq) HSO − + H (6) 2 2 ↔ 3 2 + HSO − SO − + H (7) 3 ↔ 3 Currently, many ways of the artificial patination exist. It is used in architecture, restoration or experimentally. Artificial patina is mostly based on chlorides and nitrates. Basic copper nitrate (gerhardite, Cu2NO3(OH)3) has low stability and it transforms readily to other corrosion products [27]. Chlorides are very stable, but their colour is somewhat bluish compared to brochantite. Common patination processes combine treatment by heating, immersion in solution, exposure in acidic water vapour or the application of acidic pastes [3,5,28]. There are also minor procedures for sulphates, but they usually show low reproducibility. This work is focused on patina formation in a wet atmosphere with increased SO2 content, due to lack of knowledge of stable artificial patina based on sulphates, especially brochantite, and different conditions were tested. Compared to the other methods, patination from gaseous phase represents a process of patina formation more similar to the natural one, which should theoretically render patina with better properties, such as color, adhesion and durability. These properties are given by the composition of patina and the artificial patina formed in condition of atmosphere with increased SO2 content would be based on sulphates like the natural patina formed usually in industrial areas. Sulphate patina is very stable in atmospheric conditions and the main phase is brochantite [1,8,16].

2. Materials and Methods This work investigates formation of copper patina based on brochantite from a gaseous phase. According to the studies of atmospheric corrosion of copper, speciﬁc conditions such as level of pollution, level of precipitation and time of exposure are required [4,29]. In order to reach right conditions for the experiment, a special chamber was designed at our department. The corrosion chamber has a volume of 30 dm3 and it is made of polypropylene. It allowes the control of temperature by rod heater with thermostat, blowing air by ventilator placed inside the chamber, humidity and the volume of gas (SO2) dosed in the beginning of exposure. SO2 is dosed manually and the volume of gas is set by pipette and manometer with liquid medium. Demineralised water was used as a feed water on the chamber bottom for condensation conditions (100% RH), and the feed water was changed for 3 every new inlet dose of SO2. The volume of the water/solution was 5 dm and the level was always above the rod heater, which was placed at the bottom of the chamber. Copper sheets of dimensions 5 5 cm2 were exposed under the 60 inclination and the eﬀects of the concentration of SO , pH, × ◦ 2 cyclic condition and pre-treatments of the copper surface on the kinetics and formation of patina were observed. At least four copper samples were tested under each condition. Four surface states of samples were studied, ground and three pre-exposed. Samples were ground by P 120 emery paper. Since natural patina has a layered structure, the pre-exposed surface was also employed. Pre-oxidation and pre-patination solutions were chosen due to the possible formation of copper oxides and copper sulphides. Pre-patination was based on immersion into solution 1 (100 mL NH (conc.), 33 g Na S 9H O and 250 mL demineralised water) or solution 2 (commercial solution 3 2 · 2 Coatings 2019, 9, 837 4 of 18

Sulka-K©–based on calcium and potassium polysulphides). Solutions were chosen according to restoration practice as solutions contain sulphur (not chlorides or carbonates) commonly used for patination of copper object. The third type of pre-exposed surface was pre-oxidation in the oven at 150, 200 or 300 ◦C for up to 6 h. The first experiment started with 24 h exposure at 40 ◦C with different inlet doses of SO2 (8.9, 17.7, 26.6, 35.4 and 44.3 g m 3). The pH (glass electrode Elteca 1220) of feed water inside the chamber, · − patinated area of the surface (QuickPhoto Industrial, 3.2) and mass gain of the samples were evaluated after the exposure. Samples with patina were documented by the Nikon COOLPIX AW130 camera (Tokyo, Japan). These pictures were cut on sample dimension. Software could recognize pixels with green color of patina and made calculations of the green area. Ion chromatography (DIONEX ICS 1000, eluent: 1.8 mmol dm 3 sodium carbonate +1.7 mmol dm 3 sodium bicarbonate) was used to verify · − · − the content of sulphites and sulphates in feed water. Samples of feed water were taken after dosing of SO (inlet dose 17.7 g m 3) in steps 5, 10, 15, 30, 45, 60, 90, 120, 180, 240, 300, 360 min. 2 · − The second experiment was carried out at 40 C for 1 week with 8.9 g m 3 SO and with different ◦ · − 2 surface pre-treatments. Pre-oxidised samples were also exposed for 24 h with the inlet dose of 17.7 g m 3 SO . The pre-patinated samples were analysed by X-ray diffraction (XRD, X’Pert Pro, · − 2 PANalytical, EA Almelo, Holland) and the samples pre-oxidised at 200 ◦C were evaluated by means of X-ray photoelectron spectroscopy (XPS, ESCAProbe, Omicron Nanotechnology, Taunusstein – Neuhof, Germany) in their original state. The observation of the samples after the exposure was also done by mass gain and patinated area evaluation. The next experiment was focused on patina formation kinetics and the influence of humidity conditions. Samples were exposed in one week cycle at 40 C with 8.9 g m 3 SO . The observed ◦ · − 2 parameters were tested at two levels: continuous condensation and 20 h condensation/4 h drying. Samples were studied by means of spectrophotometry (CM-700d, Minolta, Tokyo, Japan) and scanning electron microscopy (VEGA3 LMU, TESCAN, Brno, Czech Republic) (surface and cross section). The last experiment was carried out in order to optimise the process. One-week campaigns with cycles of condensation and drying were applied. A different content of sulphur dioxide dose was tested. There were constant contents during the exposure as well as optimal decreasing dosing within the exposure. The contents used in these experiments were in the range of 17.7 to 0 g m 3. Patinas · − were evaluated by means of XRD and patinated area evaluation. Resistometry was employed for the optimisation of the process, because this method allows a continuous record of the copper track mass loss during the exposure. The method is based on a change in electrical resistance of metal track due to mass loss with time. The resistometric sensor has two parts: reference and measuring. Reference part was covered by epoxy resin. Thickness of the measuring part decreases in time due to corrosion during the exposure and its electrical resistance increase. Adhesion of the patina was tested in a T-bend test, which is a method for evaluating the flexibility and adhesion of a coating on a metallic substrate. Samples with patina were bent around itself in prolonged direction. The experiments are summarised in Table1.

Table 1. Experiments.

3 Experiment Duration Temperature SO2 Inlet Dose [g m ] Cycle · − 1 24 h 40 ◦C 8.9–44.3 Condensation 1 week 40 C 8.9 2 ◦ Condensation 24 h 40 ◦C 17.7 Condensation 3 1 week 40 C 8.9 ◦ Condensation/drying phase 4 1 week 40 ◦C 17.7–0.0 Condensation/drying phase

Bronze alloys were subjected to patination processes after the optimisation. Two typical bronze alloys were chosen for this experiment and casted: “bell metal” binary alloy 78 wt.% copper and 22 wt.% tin (CuSn22) representing high tin bronze, and “statue bronze” quaternary alloy 90 wt.% Coatings 2019, 9, 837 5 of 18

Coatings 2019, 9, x FOR PEER REVIEW 5 of 18 copper, 4 wt.% tin, 4 wt.% zinc and 2 wt.% lead (CuSn4Zn4Pb2) representing cheaper low tin bronzes. wereBoth alloyscast in werea standard cast in electric a standard furnace electric into furnace a 50 mm into thick a 50 brass mm thickmould brass at UCT mould Prague. at UCT Chemical Prague. compositionChemical composition was checked was by checked X-ray fluorescence by X-ray ﬂuorescence (XRF, ARL (XRF,9400 XP, ARL Thermo 9400 XP, ARL, Thermo Chemin ARL, de CheminVerney Switzerlandde Verney Switzerland)) analysis from analysis the core from of the cast. core Samples of cast. (50 Samples mm diameter, (50 mm 5 diameter, mm thick) 5 mm were thick) exposed were withoutexposed any without pre-exposed any pre-exposed treatment treatment of the surface of the to surface compare to compareonly influence only inﬂuence of compos ofition. composition. Surface wasSurface only was brushed only brushedby brass bywire brass brush. wire brush.

3. Results Results and and Discussion Discussion

3.1. Influence of SO Inlet Dose and pH of Superficial Electrolyte 3.1. Influence of SO22 Inlet Dose and pH of Superficial Electrolyte The experiment experiment began began with with a astudy study of ofthe the influence influence of sulphur of sulphur dioxide dioxide inlet inlet doses doses between between 8.9– 3 3 8.9–44.3 g−3 m (the level of SO in Czech Republic atmosphere is usually in the range 10–100 µg m−3 [9]) 44.3 g·m · (the− level of SO2 in2 Czech Republic atmosphere is usually in the range 10–100 μg·m· − [9]) forfor 24 h at at 40 40 °C.◦C. Results Results are are summarised summarised in in Figure Figure 11.. The highest mass gain was recorded inin samplessamples 3 exposed to 17.7 and 26.6 g m−3 , which corresponded also to patina surface coverages of 68 and 54%, exposed to 17.7 and 26.6 g·m· −, which corresponded also to patina surface coverages of 68 and 54%, respectively. The The patina patina was was spread spread the most the homogeneously most homogeneously at these SO at 2 thesedoses. SOThe2 pHdoses. measurement The pH measurementof the feed water of the at thefeed chamber water at bottomthe chamber after exposurebottom after had exposure the optimal had pH the range optimal for pH patination range for of 3 3.0–2.7. The pH was 3.2 under the dose of 8.9 g m , resulting in a− less3 aggressive electrolyte, lower patination of 3.0–2.7. The pH was 3.2 under the· − dose of 8.9 g·m , resulting in a less aggressive electrolyte,mass gain and lower worse mass patina gain surface and worse coverage. patina This surface low aggressive coverage. electrolyte This low probably aggressi didve notelectrolyte provide sufficient dissolution of substrate copper for patina formation. The SO doses of 35.4 and 44.3 g m 3 probably did not provide sufficient dissolution of substrate copper for2 patina formation. The· SO−2 dosesprovided of 35.4 surface and electrolytes44.3 g·m−3 provided with pH surface lower than electrolytes 2.4 which with led pH to the lower fast than dissolution 2.4 which of theled copper to the fastsubstrate dissolution as well of as the of copper the patina substrate and mass as well gain as decreasedof the patina significantly, and mass gain even decreased to mass loss significantly, under the evenhighest to SOmass2 dose. loss Thisunder observation the highest is SO in agreement2 dose. This with observation the work is of in Nassau agreement [18], whowith proposedthe work theof NassaupH value [18] of, 2.4who as proposed the limit forthepatina pH value dissolution. of 2.4 as the limit for patina dissolution.

Figure 1. InfluenceInﬂuence of of SO SO22 inletinlet dose dose on on patina patina surface surface coverage coverage (S (S in in %), %), mass mass ch changeange and and pH of surfacesurface electrolyte under constant condensation at 40 ◦°CC for 24 h h..

This first first experiment experiment was complemented complemented by two exposures exposures in the chamber chamber without without samples samples with with 3 SO2 dosesdoses of of 8.9 8.9 and and 17.7 17.7 g·m g m−3 forfor 24 24 h. h. Figure Figure 2.2 presents presents content content of of sulphites sulphites and and suphates suphates within within 6 2 · − hours6 hours of ofexposure. exposure. The The pH pH measurements measurements were were recorded recorded continuously continuously during during these these exposures. exposures. The 3 contentThe content of SO of2 in SO the ingaseous the gaseous phase was phase controlled was controlled for a dose for of a8.9 dose g·m of−3 by 8.9 a gdatam loggerby adata (Industr loggerial 2 · − Scientifi(Industrialc, Gas Scientific, Badge Gas Pro). Badge The Pro). decreases The decreases of pH of values pH values finished finished within within 2–4 2–4 h (Figureh (Figure 33),), which probably corresponded to the maximum absorption of sulphur dioxide into water. This observation was confirmedconfirmed byby directdirect measurementmeasurement of of the the SO SO2 2content content in in the the gaseous gaseous phase phase of of the the chamber chamber (Figure (Figure4). 4). The values of SO2 content were within 0.6 and 0.9 ppm after 4 h of exposure. Accuracy of data logger according to the manual is 0.1 ppm. This means that the concentration is stable approximately around 2.0 mg·m−3 with respect to initial inlet dose 8.9 g·m−3.

Coatings 2019, 9, x FOR PEER REVIEW 6 of 18

In addition, equilibrium thermodynamic calculations were performed within this study to verify the observed experimental data and to assist in explaining them. The calculations were done by means of the PHREEQC (version 3) hydrochemical software [30] with the application of the THERMODDEM (version 1.10) thermodynamic database [31]. In the PHREEQC calculations, the non-ideal behaviour of the aqueous phase was treated using the Lawrence Livermore National Laboratory aqueous model for activity coefficients [30]. Meanwhile, ideal gas behaviour was assumed for the gas phase, which could be considered reasonable for the system and conditions under investigation. Prior to the equilibrium calculations for the air–water–sulphur dioxide systems of interest, the ability of PHREEQC with THERMODDEM to describe the solubility of SO2 in water at various temperatures and SO2 partial pressures was successfully tested against experimental SO2 solubility data from the literature [32]. There were two data sets calculated for the geometry of the experimental chamber (5 dm3 water and 25 dm3 air). The first data set was calculated for different additions of sulphur dioxide at a constant temperature of 40 °C, whereas the second data set was calculated for a constant SO2 addition of 8.9 g·m−3 with temperature increasing from 40 to 90 °C. In addition, both data sets were calculated both with and without allowing possible oxidation of sulphites to sulphates (SIV/SVI) in the aqueous phase. The results are summarised in Tables 2 and 3. The pH values measured during the experiments at 40 °C (Figures 1 and 3) corresponded to pH values of the data sets calculated without SIV/SVI oxidation. On the contrary, according to ion chromatography the sulphites content is much lower than sulphates and oxidation occurred (Figure 2). The content of SO2 in gaseous phase for the inlet dose of 8.9 g·m−3 decreased significantly, within 4 h, under the theoretically calculated 2.4 × 10−2 kPa with the SIV/SVI oxidation suppressed (Figure 4). An oxidation of sulphites to sulphates, therefore, appears

Coatingsto take 2019place, 9, 837in the aqueous phase, given by the presence of oxygen; however, one should take6 into of 18 account the kinetic effects, which may cause redox reactions to run slowly, achieving the calculated equilibrium state after a sufficient equilibration time. In other words, experimental observations Thebeing values somewhere of SO2 contentbetween were the calculations within 0.6 and with 0.9 and ppm without after 4 hthe of exposure.oxidation Accuracymay indicate of data that logger some accordingprocesses (e.g., to the the manual oxidation) is 0.1 ppm.were Thisstill running means that to achieve the concentration the equilibrium, is stable although approximately the associated around 2.0 mg m 3 with respect to initial inlet dose 8.9 g m 3. changes· were− of a small extent, and were thus difficult· − to be observed/detected experimentally.

0.0025

0.002

0.0015 1SO 0.001 2SO Coatings 2019, 9, x FOR PEER REVIEW 7 of 18 ntent [mol/kg] ntent 0.0005 gaseous phase. The co calculated values of SO2 content in gaseous phases with oxidation allowed did not exceed very low values0 (~10−42 kPa), even at 90 °C (see Table 3.). Thus, there must be possible reversible SO2 absorption to the0 liquid phase2 acidified 4by the presence6 of sulphurous and sulphuric acid. The equilibrium of the system will be differentt [h] for each chamber geometry and liquid/gaseous phase ratio, etc. However, the process can be controlled by the adjustment of the pH of feed water between 2.7 and 3. between 2.7 and 3. Figure 2. Evolution of sulphites and sulphates in feed water.

Nevertheless, interesting increases in SO2 content in gaseous phase were observed, which corresponds with heater start-ups controlled by the external thermometer above the chamber feed water. The temperature on the surface of the heating rod (TROD) can be calculated according to Equation (8), assuming a heating power equal to heat flux of 500 W, a constant bulk feed water temperature (TWATER) of 40 °C, and a heat transfer coefficient for water with natural convection of 500 W·m−2·K−1. The surface rod temperature of 75 °C is approximately equal to the theoretical overheating of water at the interphase. The overheating itself cannot explain the increase of SO2 in

Figure 3. Evolution of pH in solution.

Figure 4. Record of the SO 2 detectiondetection inside inside the the chamber. chamber.

TableIn addition, 2. Results equilibrium of thermodynamic thermodynamic equilibrium calculations calculations were for performed the air–water within–sulphur this study dioxide to verify the observedsystems at experimental 40 °C. data and to assist in explaining them. The calculations were done by means of the PHREEQC (version 3) hydrochemical software [30] with the application of the THERMODDEM Oxidation not Allowed Oxidation Allowed 3 Oxidation not Allowed Oxidation Allowed (version 1.10)SO thermodynamic2 (g/m3) database [31]. In the PHREEQC calculations, the non-ideal behaviour of pH PSO2 (kPa) pH PSO2 (kPa) 4.4 3.4 1.9 × 10−3 3.1 6.1 × 10−44 8.9 3.2 6.9 × 10−3 2.9 4.0 × 10−43 17.7 2.9 2.4 × 10−2 2.6 2.3 × 10−42 26.6 2.7 4.7 × 10−2 2.4 6.3 × 10−42 35.4 2.6 7.5 × 10−2 2.3 1.2 × 10−41 44.3 2.6 1.1 × 10−1 2.2 2.1 × 10−41

Table 3. Results of thermodynamic equilibrium calculations for the air–water–sulphur dioxide system -3 at various temperatures and a constant SO2 inlet dose of 8.9 g·m-3.

Oxidation not Allowed Oxidation Allowed T (°C) pH PSO2 (kPa) CSO2 (mol/kg) pH PSO2 (kPa) 40 3.2 6.9 × 10−3 7.77 × 10−4 2.9 4.0 × 10−43 50 3.2 1.1 × 10−2 7.76 × 10−4 2.9 7.2 × 10−43

Coatings 2019, 9, 837 7 of 18 the aqueous phase was treated using the Lawrence Livermore National Laboratory aqueous model for activity coefficients [30]. Meanwhile, ideal gas behaviour was assumed for the gas phase, which could be considered reasonable for the system and conditions under investigation. Prior to the equilibrium calculations for the air–water–sulphur dioxide systems of interest, the ability of PHREEQC with THERMODDEM to describe the solubility of SO2 in water at various temperatures and SO2 partial pressures was successfully tested against experimental SO2 solubility data from the literature [32]. There were two data sets calculated for the geometry of the experimental chamber (5 dm3 water and 25 dm3 air). The first data set was calculated for different additions of sulphur dioxide at a constant temperature of 40 ◦C, whereas the second data set was calculated for a constant SO2 addition of 8.9 g m 3 with temperature increasing from 40 to 90 C. In addition, both data sets were calculated both · − ◦ with and without allowing possible oxidation of sulphites to sulphates (SIV/SVI) in the aqueous phase. The results are summarised in Tables2 and3. The pH values measured during the experiments at IV VI 40 ◦C (Figures1 and3) corresponded to pH values of the data sets calculated without S /S oxidation. On the contrary, according to ion chromatography the sulphites content is much lower than sulphates and oxidation occurred (Figure2). The content of SO in gaseous phase for the inlet dose of 8.9 g m 3 2 · − decreased significantly, within 4 h, under the theoretically calculated 2.4 10 2 kPa with the SIV/SVI × − oxidation suppressed (Figure4). An oxidation of sulphites to sulphates, therefore, appears to take place in the aqueous phase, given by the presence of oxygen; however, one should take into account the kinetic effects, which may cause redox reactions to run slowly, achieving the calculated equilibrium state after a sufficient equilibration time. In other words, experimental observations being somewhere between the calculations with and without the oxidation may indicate that some processes (e.g., the oxidation) were still running to achieve the equilibrium, although the associated changes were of a small extent, and were thus difficult to be observed/detected experimentally.

Table 2. Results of thermodynamic equilibrium calculations for the air–water–sulphur dioxide systems

at 40 ◦C.

Oxidation not Allowed Oxidation Allowed 3 SO2 (g/m ) pH PSO2 (kPa) pH PSO2 (kPa) 4.4 3.4 1.9 10 3 3.1 6.1 10 44 × − × − 8.9 3.2 6.9 10 3 2.9 4.0 10 43 × − × − 17.7 2.9 2.4 10 2 2.6 2.3 10 42 × − × − 26.6 2.7 4.7 10 2 2.4 6.3 10 42 × − × − 35.4 2.6 7.5 10 2 2.3 1.2 10 41 × − × − 44.3 2.6 1.1 10 1 2.2 2.1 10 41 × − × −

Table 3. Results of thermodynamic equilibrium calculations for the air–water–sulphur dioxide system at various temperatures and a constant SO inlet dose of 8.9 g m 3. 2 · − Oxidation not Allowed Oxidation Allowed T (◦C) pH PSO2 (kPa) CSO2 (mol/kg) pH PSO2 (kPa) 40 3.2 6.9 10 3 7.77 10 4 2.9 4.0 10 43 × − × − × − 50 3.2 1.1 10 2 7.76 10 4 2.9 7.2 10 43 × − × − × − 60 3.2 1.8 10 2 7.74 10 4 2.9 1.3 10 42 × − × − × − 70 3.2 2.7 10 2 7.71 10 4 2.9 2.1 10 42 × − × − × − 80 3.2 4.0 10 2 7.65 10 4 2.9 3.4 10 42 × − × − × − 90 3.3 5.5 10 2 7.58 10 4 2.9 5.2 10 42 × − × − × −

(T ) of 40 C, and a heat transfer coefficient for water with natural convection of 500 W m 2 K 1. WATER ◦ · − · − The surface rod temperature of 75 ◦C is approximately equal to the theoretical overheating of water at the interphase. The overheating itself cannot explain the increase of SO2 in gaseous phase. The calculated values of SO2 content in gaseous phases with oxidation allowed did not exceed very low 42 values (~10− kPa), even at 90 ◦C (see Table3.). Thus, there must be possible reversible SO 2 absorption to the liquid phase acidified by the presence of sulphurous and sulphuric acid. The equilibrium of the system will be different for each chamber geometry and liquid/gaseous phase ratio, etc. However, the process can be controlled by the adjustment of the pH of feed water between 2.7 and 3. Coatings 2019, 9, x FOR PEER REVIEW q 8 of 18 T = T + (8) ROD WATER α S 60 3.2 1.8 × 10−2 7.74 × 10−4 × 2.9 1.3 × 10−42 (q–heat flux; α–heat transfer70 3.2 coe ffi2.7cient; × 10S−–heating2 7.71 bar × 10 surface)−4 2.9 2.1 × 10−42 80 3.2 4.0 × 10−2 7.65 × 10−4 2.9 3.4 × 10−42 3.2. Effect of Copper Surface Pre-Treatment 90 3.3 5.5 × 10−2 7.58 × 10−4 2.9 5.2 × 10−42 Three surface states were studied in the first experiment. Samples were exposed in the chamber 3 for 1 week at 40 ◦C and constant condensation with a SO2 inlet dose of 8.9 g m− . One set of samples = + · (8) was pre-oxidised at 150 ◦C for 6 h, the second was pre-patinated × in Solution 1 and the third was pre-patinated in Solution 2 (see Experimental part). Pre-patinated samples had significantly thick (q–heat flux; α–heat transfer coefficient; S–heating bar surface) layers, thus they were analysed by XRD. The layers on samples from Solution 1 consisted mainly of3.2. cuprite Effect of (Cu Copper2O), whileSurface samples Pre-Treatment from Solution 2 had layers composing chalcocite (Cu2S) (Table4). The images of the samples before and after the exposure are in Figure5. The most uniform patina coverageThree was surface achieved states for were the studied pre-oxidised in the sample. first experiment. Samples were exposed in the chamber for 1 week at 40 °C and constant condensation with a SO2 inlet dose of 8.9 g·m−3. One set of samples was pre-oxidised at 150Table °C for 4. 6Characteristics h, the second ofwas layer pre obtained-patinated by pre-treatment.in Solution 1 and the third was pre- patinated in Solution 2 (see Experimental part). Pre-patinated samples had significantly thick layers, thus they were analysedFeatures by XRD. Pre-Oxidation The layers on samples Solution from Solution 1. 1 consisted Solution mainly 2. of cuprite (Cu2O), whileThickness samples [nm]from Solution 2 129 had layers composing 737 chalcocite (Cu2S) (Table 939 4.). The images Cuprite Cu O of the samplesXRD–composition before and after the Cuprite exposure Cu O are in Figure 5. The 2most uniformChalkocite patina Cu coverageS was 2 Djurleite Cu31S16 2 achieved for the pre-oxidised sample.

Figure 5. ExposureExposure of of samples samples with with pre pre-treated-treated surface. surface.

The next experimentsTable were 4. Characteristics focused on of thelayer pre-oxidation obtained by pre procedure.-treatment. The temperature was increased to 200 C and 300 C in order to reduce the time of pre-oxidation, and diﬀerent times ◦Features ◦ Pre-Oxidation Solution 1. Solution 2. of pre-oxidation up to 135 min were tested. Samples were exposed for 24 h at 40 ◦C and constant Thickness [nm] 129 3 737 939 condensation with a SO2 inlet dose of 17.7 g m− . The temperature of pre-oxidation of 200 ◦C allowed · Cuprite Cu2O XRD–composition Cuprite Cu2O Chalkocite Cu2S Djurleite Cu31S16

The next experiments were focused on the pre-oxidation procedure. The temperature was increased to 200 °C and 300 °C in order to reduce the time of pre-oxidation, and different times of pre-oxidation up to 135 min were tested. Samples were exposed for 24 h at 40 °C and constant condensation with a SO2 inlet dose of 17.7 g·m−3. The temperature of pre-oxidation of 200 °C allowed a very sensitive regulation of the pre-oxidised layer. Besides full results at 200 °C, there were results for samples pre-oxidised at 300 °C for 45 min for comparison, as it had the thickest oxidised layer. Results are summarised in Figure 6. The best patina coverage and mass gain was obtained for 200 °C and 25–40 min pre-oxidation. The patina coverage as well as mass gain decreased for longer times of

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a very sensitive regulation of the pre-oxidised layer. Besides full results at 200 ◦C, there were results for samples pre-oxidised at 300 ◦C for 45 min for comparison, as it had the thickest oxidised layer. Results are summarised in Figure6. The best patina coverage and mass gain was obtained for 200 ◦C and 25–40 min pre-oxidation. The patina coverage as well as mass gain decreased for longer times of Coatings 2019, 9, x FOR PEER REVIEW 9 of 18 pre-oxidation. The compositions of the oxidised layers were studied by XPS. According to the observed colours,pre-oxidation. the oxidised The layers compositions had thicknesses of the oxidised of 10–350 layers nm [ 33 were–35 ], studied thus the by Ar XPS. sputtering According was excluded to the andobserved only the colours, received the surfaceoxidised was layers analysed. had thicknesses Measured of 10 spectra–350 nm were [33– evaluated35], thus the in Ar CasaXPS sputtering 2.3.15 softwarewas excluded (IMFP, RSf and etc only are the part received of the sofware surface library). was analysed. The data Measured for the chemical spectra state were evaluation evaluated werein obtainedCasaXPS from 2.3.15 the software NIST X-ray (IMFP, Photoelectron RSf etc are Spectroscopy part of the sofware Database. library). The XPS The spectra data for show the chemical three peaks ofstate copper evaluation 2p3/2 on were the surfaceobtained of from all pre-oxidised the NIST X-ray samples Photoelectron before exposure Spectroscopy in the Database. chamber The (Figure XPS 7). Thespectra peak atshow 932.6 three eV correspondspeaks of copper to binding 2p3/2 on energythe surface of metallic of all pre copper-oxidised and samples cuprite (Cu before2O), exposure the peak at 934.1in the eV chamber corresponds (Figure to tenorite7). The peak (CuO) at 932.6 and the eV peakcorresponds at 935.5 to eV binding corresponds energy to of cupric metallic chloride copper (CuCl and 2) orcuprite malachite (Cu (Cu2O),2 CO the 3(OH) peak 2 at)[ 36 934.1]. The eV last corresponds peak can beto attributed tenorite (CuO) to possible and the contamination peak at 935.5 during eV manipulation.corresponds The to cupric peak is chloride minor and (CuCl it did2) or not malachite change with (Cu2 theCO time3(OH) of2) pre-oxidation.[36]. The last The peak presence can be of attributed to possible contamination during manipulation. The peak is minor and it did not change metallic copper can be excluded according to Figure8 with the Auger spectra of copper L 3M4.5M4.5 peak.with The the kinetictime of energypre-oxidation. peak at The 916.9 presenc eV correspondse of metallic to copper cuprite can and be the excluded peak at according 917.6 eV correspondsto Figure to8 tenorite. with the TheAuger peak spectra of metallic of copper copper L3M4.5 wouldM4.5 peak. be situated The kinetic at 918.6 energy eV peak [36,37 at], 916.9 and eV it is corresponds missing, thus to cuprite and the peak at 917.6 eV corresponds to tenorite. The peak of metallic copper would be the layer is continuous. situated at 918.6 eV [36,37], and it is missing, thus the layer is continuous. There was obviously in the XPS spectra (Figure7) an increasing ratio between tenorite and There was obviously in the XPS spectra (Figure 7) an increasing ratio between tenorite and cuprite with time of oxidation (up to 75 min) and consequently the ratio is constant. Tenorite is in cuprite with time of oxidation (up to 75 min) and consequently the ratio is constant. Tenorite is in general less soluble than cuprite, thus it has an inhibitive eﬀect on oxidised layer dissolution and the general less soluble than cuprite, thus it has an inhibitive effect on oxidised layer dissolution and the patinapatina formation. formation. The The cupritecuprite layerlayer at 10 minmin pre pre-oxidation-oxidation was was too too thin, thin, thus thus the the optimal optimal time time for for pre-oxidationpre-oxidation was was 25–40 25–40 min. min.

Figure 6. Mass gain and surface coverage (S %) on samples pre-oxidised at 200 and 300 °C for different Figure 6. Mass gain and surface coverage (S %) on samples pre-oxidised at 200 and 300 ◦C for diﬀerent −3 timestimes [thickness [thickness ofof oxidicoxidic layer] layer] after after 24 24 h of h ofexposure exposure at 40 at °C 40 withCwith a SO2 a inlet SO doseinlet of dose 17.7 g·m of 17.7. g m 3. ◦ 2 · −

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Figure 7. X-ray photoelectron spectroscopy (XPS) peaks of copper 2p3/2 (binding energy) of samples FigureFigure 7. 7.X-ray X-ray photoelectron photoelectron spectroscopy spectroscopy (XPS)(XPS) peaks of copper 2 2pp3/23/ 2(binding(binding energy) energy) of ofsamples samples pre-oxidised at 200 °C for different times (10–135 min). pre-oxidisedpre-oxidised at at 200 200◦C °C for for di differentﬀerent times times (10–135(10–135 min).min).

Figure 8. Auger peaks of copper L3M4.5M4.5 (kinetic energy) of samples pre-oxidised at 200 °C for FigureFigure 8. 8Auger. Auger peaks peaks of of copper copper L L3M3M4.54.5M4.5 (kinetic(kinetic energy) energy) of of samples samples pre pre-oxidised-oxidised at at200 200 °C◦ forC for diﬀdifferenterent durations durations (10–135 (10–135 min). min).

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Coatings 2019, 9, x FOR PEER REVIEW 11 of 18 3.3. Kinetics of Patina Formation and InfluenceInfluence of Drying Phase 3.3. Kinetics of Patina Formation and Influence of Drying Phase −3 One-weekOne-week exposuresexposures atat 40 40 C°C with with a SOa SOinlet2 inlet dose dose of of 8.9 8.9 g m g·m3 were were performed performed in thisin this part part of the of ◦ 2 · − work.the work TheOne. The first-week first experiment exposures experiment wasat 40 was carried °C carriedwith out a SO without2 inlet with constant dose constant of condensation. 8.9 condensation. g·m−3 were The performed secondThe second consistedin this consisted part of of four of four cycles (over four days) 20 h condensation at 40 °C with SO2 doses and consequently 4 h of drying cyclesthe work (over. The four first days) experiment 20 h condensation was carried at out 40 ◦withC with constant SO2 doses condensation. and consequently The second 4 consisted h of drying of in ambientin fourambient cycles atmosphere. atmosphere. (over four These days) These four 20 four h cyclescondensation cycles were were followed at followed 40 °C bywith by 3 daysSO 3 day2 doses ofs constantof andconstant consequently condensation. condensation. 4 h of The drying The loss loss of metallicof inmetallic ambient copper copper atmosphere. during during these These these experiments four experiments cycles was were recordedwas followed recorded by by resistometry 3 dayby sresistometry of constant (Figure condensation. 9(Figure). The record9). The The showed recordloss continuousshowedof metallic continuous but copper decelerating butduring decelerating dissolutionthese experiments dissolution of substrate was of substraterecorded copper during copperby resistometry continuous during continuous (Figure condensation 9). condensationThe exposure.record Theexposure.showed process continuousThe with process drying but with decelerating stage drying showed stage dissolution as showed fast dissolution of as substrate fast dissolution as copper continuous during as continuous condensationcontinuous condensation condensation but stopped but duringstoppedexposure. the during drying The process the stage, drying thuswith allowingstage,drying thusstage the allowingshowed formation as the offast patina formation dissolution nuclei of as at patina continuous different nuclei sites condensation at promoting different but better sites stopped during the drying stage, thus allowing the formation of patina nuclei at different sites homogeneitypromoting better of lateral homogeneity coverage. of lateral coverage. promoting better homogeneity of lateral coverage. The results of all these exposures are summarised in Figure 10, as sample colours were evaluated The results of all these exposures are summarised in Figure 10, as sample colours were evaluated by spectrophotometry. Samples were compared to the patina of the roof sheets from Queen AnneAnne´s´s by spectrophotometry. Samples were compared to the patina of the roof sheets from Queen Anne´s Summer Palace in Prague, Prague, which which is is more more than than 300300 yearsyears old. old. PatinaPatina formedformed under constant constant Summer Palace in Prague, which is more than 300 years old. Patina formed under constant condensation conditions, providedprovided less uniform patina coverage comparedcompared to a cycle whenwhen a drying stagecondensation was involved conditions,. With cyclic provid conditioned less uniform the patina patina coverage coverage was compare better,d and to a the cycle colour when approached a drying stagestage was was involved. involved With. With cyclic cyclic condition condition thethe patina coverage coverage was was better, better, and and the the colour colour approached approached a natural patina. a naturala natural patina. patina.

FigureFigure 9. 9. Resis ResistometricResistometrictometric record during during exposures exposures.exposures. .

Figure 10. Spectrophotometry results displayed in CIELab coordinates and their comparison to Figurenatural 10. 10. patina, SpectrophotometrySpectrophotometry a—the red/green results results coordinate, displayed displayed b— inthe CIELabin yellow/blue CIELab coordinates coordinates coordinate and their. and comparison their comparison to natural to patina,natural a—thepatina, red a—/greenthe red/green coordinate, coordinate, b—the yellow b—the/blue yellow/blue coordinate. coordinate. Samples obtained from exposure with constant condensation and cycling with drying stages wereSamplesSamples evaluated obtainedobtained in from detailfrom exposure exposure by scanning with with constant electronconstant condensation condensation microscopy and ( SEMand cycling) /cyclingenergy with- dispersivewith drying drying stages X -stagesray were evaluatedwerespectroscopy evaluated in detail (EDS byin ). scanning detailSamples by electronof scanningthe exposure microscopy electron with (SEM) drying microscopy/energy-dispersive stages were (SEM collected)/energy X-ray aft spectroscopy-dispersiveer each drying (EDS). X-ray Samplesspectroscopystage. The of the e xperiment( exposureEDS). Samples with constant drying of the stages exposurecondensation were with collected was drying complement after stages each drying withwere two collected stage. separate The aft experimentexposureser each drying for with constantstage.2 and The 4condensation edaysxperiment (Experiment with was complementconstant 3). Appearance, condensation with patina two separate wasmorphology complement exposures and with cross for 2 two- andsection separate 4 days are (Experimentcompared exposures in for 3).

2 and 4 days (Experiment 3). Appearance, patina morphology and cross-section are compared in

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Appearance,Figures 11 and patina 12. A morphology thin layer of and cuprite cross-section (approx. are 1 μm) compared was formed in Figures under 11 constantand 12. A condensation thin layer of µ withincuprite 2 (approx. days of 1exposurem) was (Figure formed under12). Sulphate constant based condensation patina started within to 2 daysform ofafter exposure 4 days (Figure exposure 12). aboveSulphate the based former patina cuprite started layer. to form These after compounds 4 days exposure were identified above the by former XRD cuprite as the layer.brochantite These compounds were identified by XRD as the brochantite precursors strandbergite (Cu (SO ) (OH) 4H O), precursors strandbergite (Cu5(SO4)2(OH)6·4H2O), also called kobyashevite,5 and4 2 posnjakite6· 2 also called kobyashevite, and posnjakite (Cu (SO )(OH) H O). They have flower-like morphology, (Cu4(SO4)(OH)6·H2O). They have flower-like4 morphology,4 6· 2 which is similar to the natural patina whichformation is similar described to the by natural FitzGerald patina [8] formation. These flower described crystals by grow FitzGerald in the [latter8]. These exposure flower and crystals they µ formgrow in locally the latter thick exposure islands and of patina they form up tolocally 30 μm thick after islands 1 week of patina of exposure, up to 30 whenm after brochantite 1 week of exposure, when brochantite (Cu (SO )(OH) ) begins also to be present in the patina. The kinetics of (Cu4(SO4)(OH)6) begins also to be4 present4 in6 the patina. The kinetics of patina formation were much patinafaster formationwith drying were stages much inserted faster with to drying the patination stages inserted process. tothe It patination allowed the process. formation It allowed of new the formation of new nucleation sites at the surface after each new inlet dose of SO . The increasing number nucleation sites at the surface after each new inlet dose of SO2. The increasing2 number of nucleation sitesof nucleation allowed sites faster allowed and faster laterally and more laterally homogeneous more homogeneous growth. growth. There There were were obvious obvious islands islands of sulphatesof sulphates already already after after 2 days 2 days’’ exposure. exposure. The The number number of of islands islands was was much much higher higher when when compared compared µ to previous constant condensation after 4 days. The The islands islands had had thicknesses thicknesses of of approximately approximately 20 20 μm,m, and since the surface was covered covered significantly significantly by them, them, t thehe consequential consequential growth growth was was rather rather lateral. lateral. The thickness did not grow further and patina uniformuniform inin thicknessthickness waswas formed.formed.

Figure 11. SurfaceSurface appearance, appearance, patina patina morphology and and element concentration profileproﬁle of samples exposed for 2, 4 and 7 days in process with constant condensation (without drying stages).stages).

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Figure 12. SurfaceSurface appearance, appearance, patina patina morphology morphology and and element concentration profile proﬁle of samples exposed for 2, 4 and 7 days in process with drying stages.stages.

3.4. Optimisation Optimisation of of Patination Patination Process Process DDecreasesecreases of pH values below 2.7 were recorded during cycling experiments when a new inlet 3 dose of SO2 17.7 g·m g m−3 waswas injected injected after after each each drying drying stage. stage. The The new new optimised optimised process process was was verified verified · 3 on samples pre pre-oxidised-oxidised for 40 min at 200 ◦°C.C. It beganbegan withwith aa standardstandard inletinlet dosedose ofof 17.717.7 gg·mm−−3 and 3 · continued with with decreasing decreasing inlet inlet doses doses of 8.9,of 8.9, 4.4 and4.4 and 0.0 g 0.0m− g·mafter−3 after each eachdrying drying stage. stage. However, However, before · 3 beforea final 3-daya final cycle3-day without cycle without a drying a drying stage, astage, higher a higher inlet dose inlet of dose SO2 of17.7 SO g2 m17.7− wasg·m− introduced3 was introduced again. · again.The process The process is described is described in Figure in Figure13, which 13, which also contains also contains the record the record of a resistometric of a resistometric copper copper probe. probe.Resistometric Resistometric probes probeshave been have described been described in detail in elsewhere detail elsewhere [38–50]. The[38– metallic50]. The trace metallic corroded trace corrodedapproximately approximately 1 µm in the 1 μm first in cycle, the first but cycle, all consequential but all consequential cycles dissolved cycles dissolved only 0.3 µonlym of 0.3 metallic μm of metallictrace. The trace pH. The was pH kept was in thekept range in the of range 2.7–3.0 of during 2.7–3.0 theduring entire the experiment. entire experiment. This optimised This optimised process processwas verified was verified for reproducibility for reproducibility in four independentin four independent replicates. replicates. The surface The appearancesurface appearance can be seen can bein Figureseen in 14Figure. The 14 patina. The patina was well was attached well attached to the to copper the copper surface. surface. The T-bendThe T-bend test wastest was used used to test to testthe adhesionthe adhesion of patina of patina and and no no cracks cracks were were observed observed after after the the third third bend bend (Figure (Figure 15 15).). TheThe patinatedpatinated area was approximately 80%. Th Thee surface surface coverage coverage was was uniform, uniform, and and it it contai containedned mostly mostly brochantite brochantite (see Figure 16).

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ExamplesExamples of bronzes patinated in anan optimisedoptimised processprocess are are given given in in Figure Figure 17 17. P. Patinaatina formation formation Coatingsisis significantlysignificantly2019, 9, 837 slowerslower on a bronze surface,surface, butbut patinationpatination isis possiblepossible and and the the prolongation prolongation of 14of the of the 18 processprocess toto severalseveral weeks could provide betterbetter coverage.coverage.

FigureFigure 13.13. ResistometryResistometry recordrecord onon coppercopper probe probe during during optimised optimisedoptimised process process.process. .

FigureFigure 14. 14.Examples Examples ofof samplesample (5(5 cmcm × 5 cm) appearances after after exposure exposure in in the the optimised optimised process process in in Figure 14. Examples of sample (5 cm× × 5 cm) appearances after exposure in the optimised process in Coatingsfourfour 2019 di different,ﬀ 9erent, x FOR replicates. replicates PEER REVIEW. 15 of 18 four different replicates.

Figure 15. T-bendT-bend test, 3rd bend.bend.

Figure 16. Example of diffraction patterns after optimised process patination.

Figure 17. Artificial patina on the surface of experimental bronze alloys samples (ø 50 mm), Experiment 4: “bell metal” CuSn22 (left) and cheap version of “statue bronze” CuSn4Zn4Pb2 (right).

4. Conclusions It was possible to obtain a well-adhering artificial patina based on brochantite prepared by the proposed process within one or two weeks. That was a much shorter time compared to several years in natural atmospheric conditions. The ideal process includes a condensation stage at 40 °C for 20 h and drying stages in ambient atmosphere for 4 h. The optimal pH of the feed water should be kept within 2.7 and 3, which is provided by the sulphur dioxide dosing decreasing from 17.7 g·m−3. Patina lateral homogeneity was supported significantly by surface pre-oxidation at 200 °C for 40 min.

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Coatings 2019, 9, 837 15 of 18 Figure 15. T-bend test, 3rd bend.

Figure 15. T-bend test, 3rd bend.

Figure 16. Example of di raction patterns after optimised process patination. Figure 16. Example of diffractionﬀ patterns after optimised process patination. Examples of bronzes patinated in an optimised process are given in Figure 17. Patina formation is signiﬁcantly slower on a bronze surface, but patination is possible and the prolongation of the process to several weeks could provide better coverage. Figure 16. Example of diffraction patterns after optimised process patination.

4. Conclusions Figure 17. Artificial patina on the surface of experimental bronze alloys samples (ø 50 mm), FigureIt was 17.possibleArtificial to patinaobtain on a thewell surface-adhering of experimental artificial patina bronze alloysbased samples on brochantite (ø 50 mm), prepared Experiment by the Experiment 4: “bell metal” CuSn22 (left) and cheap version of “statue bronze” CuSn4Zn4Pb2 (right). proposed4: “bell process metal” within CuSn22 one (left) or and two cheap weeks. version That of was “statue a much bronze” shorter CuSn4Zn4Pb2 time compared (right). to several years 4.in Conclusions natural atmospheric conditions. The ideal process includes a condensation stage at 40 °C for 20 h and drying stages in ambient atmosphere for 4 h. The optimal pH of the feed water should be kept It was possible to obtain a well-adhering artificial patina based on brochantite prepared by the withinIt was2.7 and possible 3, which to obtain is provided a well by-adhering the sulphur artificial dioxide patina dosing based decreasing on brochantite from 17.7 prepared g·m−3. Patinaby the proposedlateral homogeneity processprocess withinwithin was onesupportedone oror twotwo significantlyweeks.weeks. That wasby surface a much pre shorter-oxidation time comparedat 200 °C for to 40 several min. years inin naturalnatural atmosphericatmospheric conditions.conditions. The ideal process includes a condensation stage at 40 ◦°C for 20 h and drying stages in ambient atmosphere for 4 h. The optimal pH of the feed water should be kept 3 withinwithin 2.72.7 andand 3,3, whichwhich isis providedprovided byby thethe sulphursulphur dioxidedioxide dosingdosing decreasingdecreasing from from 17.7 17.7 g g·mm−−3.. Patina · laterallateral homogeneityhomogeneity waswas supportedsupported significantlysignificantly byby surfacesurface pre-oxidationpre-oxidation atat 200200 ◦°CC forfor 4040 min.min.

Author Contributions: R.B. and J.S. conceived of and designed the experiments; R.B. and J.S. are responsible for writing—review and editing, P.R. provided methodology and materials, M.K. provided thermodynamic equilibrium calculations, J.F. and K.J. provided formal analyses and analysed the data. Funding: This research was funded by Czech Ministry of Culture in programme NAKI II, grant number DG16P02H051. Acknowledgments: The authors are very grateful for the support from Czech Ministry of Culture in programme NAKI II under the project number DG16P02H051. Conﬂicts of Interest: The authors declare no conﬂict of interest. Coatings 2019, 9, 837 16 of 18

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