The Origin and Evolution of Copper Patina Colour

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The Origin and Evolution of Copper Patina Colour Article in Corrosion Science · May 2019 DOI: 10.1016/j.corsci.2019.05.025

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Corrosion Science 157 (2019) 337–346

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The origin and evolution of copper patina colour ⁎

T

C. Leygraf , T. Chang, G. Herting, I. Odnevall Wallinder KTH Royal Institute of Technology, Division of Surface and Corrosion Science, Department of Chemistry, Drottning Kristinas väg 51, SE 100 44 Stockholm, Sweden

A R T I C LE I N FO

A B S T R A C T

Keywords: A Copper B SEM, XPS C Atmospheric corrosion

The copper patina colour has been systematically explored through a large set of short- and long-term exposed copper metal samples. The initial brown-black appearance is attributed to semiconducting properties of cuprite (Cu2O) and fully attained at thickness 0.8 ± 0.2 μm. The characteristic green-blue appearance is due to the colour forming Cu(II)-ion in the outer patina layer which needs to be 12 ± 2 μm to fully cover the inner cuprite layer. No significant influence of atmospheric environment on patina colour is discerned. The green-blue patina colour on historic copper was attained after shorter exposures than in modern copper due to more inhomogeneous microstructure.

1. Introduction The surface appearance of copper or copper-based alloys is one of their most striking characteristics. The appearance is determined by different properties, mainly colour, lightness, gloss, texture and shape.

Colour is a perceptual phenomenon that depends both on the observer and the prevailing conditions during observation. Intensity and wavelength of the interacting electromagnetic radiation influence the colour when the surface becomes visible upon light illumination. Gloss is another important property to influence surface appearance of a metal. It is synonymous with the brilliance or lustre of the metal surface, and determined by the ability of the surface to reflect light. Texture and shape are other properties which influence the surface appearance, but they are left outside herein, where the focus primarily will be on colour and secondly on lightness for determining surface appearance of bare and naturally patinated copper.

Mankind has taken advantage of the colour of copper or copper alloys for thousands of years. Yet, no scientific paper seems to exist that provides a comprehensive analysis of the evolution of colour of copper patina, and how it is influenced by different exposure conditions.

Scattered efforts to describe, rather than to explain, the appearance of copper patina colour have been reported earlier. Papers by Franey and Davies [1], Livingston [2], Strandberg and Johansson [3], Morcillo et al [4] and FitzGerald et al [5] have described colour evolution of copper patina in different types of field or laboratory exposures. It is clear from these and other studies [6] that copper during field or laboratory exposures forms a brown-black layer of cuprite (Cu2O) next to the copper substrate and a blue-green layer of one or several Cu(II)-compounds next to the atmosphere. Depending on the actual environment and

⁎

exposure time the Cu(II)-containing outer layer may consist of, e.g., posjnakite (Cu4SO4(OH)6·H2O), brochantite (Cu4SO4(OH)6), antlerite (Cu3(SO4)(OH)4), atacamite (Cu2Cl(OH)3) or combinations thereof [6].

A more precise characterization of patina appearance and colour has become possible with the use of spectrophotometry, whereby the surface colouration can be measured in a more quantified way [7–11].

The distribution of reflected light of the coloured sample is measured as a function of wavelength in the visible region, and the colour is presented in a two-dimensional graph and the lightness as a separate value.

Such quantified data enable us to correlate patina colour with other patina properties and with environmental characteristics.

The colouring and patination of metals and alloys has been the subject of systematic analysis [12,13]. The emphasis in those studies has been on techniques and recipes for contemporary metalwork and design, rather than on the physicochemical origin of copper patina colour. The present study on copper patina has been motivated by producers and end users, including architects, of copper sheet materials to better understand the conditions for forming different copper patina colours. It was preceded by a survey of published work which showed that no systematic analysis ever had been performed on the origin and evolution of copper patina colouration. For this reason the authors have undertaken a systematic exploration of the colour of copper patina formed during short- and long-term exposures- ranging from one day to hundreds of years- in different types of environments. A starting point has been the authors´ rich access of copper and copper-based materials exposed in marine [14] and urban or rural environments [6,15]. Selected historic copper materials have also been included [16].

To provide generic and general information on this issue, the main emphasis in this investigation is on colour characterization combined

Corresponding author.

E-mail address: chrisl@kth.se (C. Leygraf).

https://doi.org/10.1016/j.corsci.2019.05.025 Received 22 March 2019; Received in revised form 7 May 2019; Accepted 25 May 2019 Available online 27 May 2019 0010-938X/ © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

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Table 1 Exposure site, corresponding environment, exposure length and main composition of the investigated copper patina.

Exposure site (country)

Type of environment

Exposure length

Main compounds of the patina

References

Brest (France) Bohus Malmön (Sweden) Aspvreten (Sweden) Stockholm (Sweden) Old Town Church in Stockholm (Sweden) Otto Wagner Church in Vienna (Austria) Kronborgs Castle in Elsinore (Denmark) Helsinki Cathedral (Finland) Basilica in Maria Dreieichen (Austria) Drottningholm Castle in Stockholm (Sweden) Mausoleum in Graz (Austria)

Marine Marine Rural Urban Urban Urban Urban Urban Urban Urban Urban

3 months - 5 years 1 day - 1 year 1 day - 1 year 1 day - 1 year 100 years 110 years 120 years 150 years 160 years 300 years 390 years

Cu2(OH)3Cl, Cu2O Cu2(OH)3Cl, Cu2O Cu4SO4(OH)6·H2O, Cu2O Cu4SO4(OH)6·H2O, Cu4SO4(OH)6, Cu2O Cu4SO4(OH)6, Cu3SO4(OH)4, Cu2O Cu4SO4(OH)6, Cu3SO4(OH)4,Cu2O Cu4SO4(OH)6, Cu3SO4(OH)4, Cu2O Cu4SO4(OH)6, Cu3SO4(OH)4, Cu2O Cu4SO4(OH)6, Cu3SO4(OH)4, Cu2O Cu4SO4(OH)6, Cu3SO4(OH)4, Cu2O Cu4SO4(OH)6, Cu3SO4(OH)4, Cu2O

[14,17,18,19] this study [20,21] [20,21] [16] [16] [16] [16] [16] [16] [16]

the patina colour, with blue and yellow in the positive and negative direction, respectively.

The second way of representing patina colour is by plotting the reflected light percentage (%) measured as a function of wavelength (nm) in the range corresponding to the visual spectrum, i.e. from around 350 nm (violet) to around 750 nm (red). To obtain the reflectance for cuprite the relative reflectance value (%) at 670 nm was obtained versus the background value at 400 nm. Similarly, the reflectance for the Cu(II)-containing patina was obtained via its relative reflectance value (%) at 540 nm versus the corresponding background value at 400 nm.

The first representation will be used herein for more comparative purposes of different colours, while the second turns out to be useful for more mechanistic studies, e.g. when seeking to identify the properties of patina layer responsible for a certain colour.

with thickness determination of the different patina constituents.

Questions that we have aimed to explore include the following: - What thickness is required to fully obtain the brown-black cuprite layer and the characteristic green-blue patina colour, respectively?

- Which mechanisms determine the brown-black and the blue-green patina colour?

- Is patina colour dependent on the type of environment to which the copper metal is exposed?

2. Materials and methods 2.1. Exposures Before colour characterization the copper metal samples had been exposed mainly in various field sites with exposure times ranging from 1 day up to almost 400 years. A summary of test site characteristics has been compiled in Table 1, which includes the site name, type of environment, exposure length and main compounds found in the patina formed.

Freshly polished Cu metal samples (down to 1 μm) were also exposed in the laboratory in a climatic chamber (WEISS WK1000, Germany) following two different wet/dry cyclic procedures on a daily basis. The cycle time, relative humidity and temperature for each day of wet/dry cycle exposures are displayed in Table 2.

2.3. Analytical techniques for thickness characterization The thickness of the cuprite layer was determined by a combination of infrared reflection absorption (IRAS), galvanostatic cathodic reduction and scanning electron microscopy (SEM). IRAS has previously been able to estimate the thickness of the cuprite layer by monitoring the peak at 647 cm−1 and comparing the peak absorbance with another complementary acting technique [23]. In this work the complementary technique was galvanostatic reduction for cuprite thicknesses in the range 10–700 nm and SEM for thicknesses in the range 0.7–2.1 μm.

IRAS (FTIR spectrometer, Bruker Tensor 37) was employed to the estimate the cuprite layer thickness of the exposed copper samples. A grazing angle specular reflectance detector (80Spec, PIKE Technologies) was used with 528 recording scans for each spectrum and a resolution of 4 cm−1. The spectra were recorded in absorbance units (−log(R/R0)), where R is the reflectance of the exposed copper surface and R0 the reflectance of a gold surface, used as reference.

Galvanostatic cathodic reduction measurements were conducted to assess the cuprite thickness using a PARSTAT multichannel PMC Chassis instrument equipped with six PMC 1000 (AC/DC) channels and with an applied current density of −0.05 mA/cm2. 0.1 M KCl, a saturated Ag/AgCl reference electrode and a platinum mesh were used as the electrolyte, reference and counter electrode, respectively. N2 gas was purged into the solution for 30 min prior to the experiments to achieve oxygen-free conditions.

Cross sectional SEM investigations of the copper patinas were carried out using a LEO 1530 instrument with a Germini column, upgraded to a Zeiss Supra 55 (equivalent). The corresponding images were acquired by using a backscattered electron (BSE) detector at an accelerating voltage of 20 kV. The thickness of naturally formed copper patina usually exhibits considerable variations in thickness. For this reason 6 or 7 different parts of the patina were selected, and in each part 6 or 7 thickness estimates were made. The error bars for each thickness presented in different graphs is therefore the result of between 36 and 49 thickness estimates.

2.2. Reflectometry for colour characterization In order to determine the surface coloration of copper patina colorimetric measurements were performed by means of a spectrophotometer, Minolta CM2500D, with a multi standard illuminant light source, D65 (represents the average midday light of Western/Northern Europe), at 10°. Patina colour is herein presented in two ways. In the first, the colour was registered in the CIELab colour reference space [22], originally defined by the International Commission on Illumination (CIE). The space allows for a three-dimensional digital representation of different colours with the axes L*, a*, and b*. Here, L* represents the lightness, where darkest black is at L* = 0, and brightest white at L* = 100. a* represents the green–red component of the patina colour, with green and red in the negative and positive direction, respectively. Analogously, b* represents the blue–yellow component of Table 2 Cycle time, relative humidity and temperature during each laboratory exposure.

Cyclic procedure 1

Cyclic procedure 2

t (h) RH% T (ºC) t (h) RH% T (ºC)

2 0 25 4 90 20

4 90 25 2 0 20

2 0 10 16 90 20

2 90 10

4 90 25

2 0 25

4 90 25

2 0 10

1 90 10 2 0 20

1 90 25

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In the Supplementary part Figs. 1–4 show how the raw data for cuprite thickness estimates were extracted. Fig. S1 exhibits IRAS spectra of copper exposed in three different outdoor conditions and Fig. S2 galvanostatic reduction curves of selected copper samples from Fig. S1 and from some laboratory exposed samples. Fig. S3 displays patina cross sections by means of SEM of selected copper samples presented in Fig. S1, and Fig. 4, finally, gives the overall correlation between the IRAS absorbance of cuprite and the corresponding thickness obtained with galvanostatic reduction (cuprite thickness range 10–700 nm) and with SEM (cuprite thickness 0.7–2.1 μm) respectively. This calibration line was then used to estimate the cuprite thickness of all field or laboratory exposed copper samples.

In order to reveal the colour characteristics as a function of patina thickness, one sample (Elsinore, see Table 1) was abraded in very small steps in order to gradually remove the patina in incremental steps. The mass of the sample was analysed after each abrasion step by a microbalance and the corresponding cross section analysed by SEM. Assuming that the outer removed layer consists of brochantite the estimated thickness after each removal step was consistent between the microbalance and SEM. This procedure allowed the gradual removal of the outer Cu(II)-containing layer in steps of around 1 μm.

Fig. 1. Relation between subtractive colour mixing and photo-induced excitation of electrons in a solid material. The left half shows the photon threshold energy in eV (and corresponding colour) above which all photons are absorbed by the material through some excitation process of electrons, and below which all photons are reflected (with corresponding reflected colour). The right-hand y-axis gives the corresponding photon wavelength in nm.

the visible region are absorbed and the material appears black. The colours corresponding to threshold energies between 1.5 and 3.5 eV are all given in the figure. For a photon threshold energy of, say, 2.5 eV (blue-green colour), the material appears yellow-orange because of the spectrum of reflected photons.

2.4. Complementary patina characterization In a few cases additional characterization of patinas after short exposures was performed by X-ray photoelectron spectroscopy (XPS).

This was accomplished with a Kratos AXIS UltraDLD X-ray photoelectron spectrometer (Kratos Analytical, Manchester, UK) equipped with a monochromatic Al X-ray source. Wide spectra and detailed high resolution spectra of C1s, O1s and Cu2p were monitored.

3.2. The yellow-red copper metal surface Fig. 2(a) displays the image of a polished bare copper metal surface (a), and the corresponding colorimetric characterization in the CIELab colour reference space (b) and, finally, the reflected light percentage of the visual spectrum (c). As stated above, the reason for the coloured appearance of copper is the difference in visual spectrum of the incoming light compared to the reflected light. Copper exhibits a high absorption at shorter wavelengths and a high reflectivity at longer wavelengths, resulting in its specific yellow-red colour. Silver, on the other hand, appears very close to the whitish metallic colour characteristic of a metal where the visual spectrum of the incoming light approximately equals that of the reflected light.

The reason for the specific colour of metals, such as copper, silver or gold, is their difference in how the electrons are excited by incoming visible light from lower, occupied, energy levels to higher, vacant, levels, and then de-excited to lower energy levels again with emission of photons. Upon excitement the metal electron response can be approximated by an oscillating plasma with a natural oscillating frequency [26]. For incoming light with frequency below the oscillating plasma frequency there is a strong absorption of light in the metal, whereby the propagating wave drops exponentially [26]. As a result of strong absorption, visible light only penetrates some 0.1 μm into the copper metal. At frequencies above the oscillating plasma frequency, the absorption of the wave in the metal is very small and the metal acts much more transparent. This is the reason why most metals are very transparent to X-rays.

Gold has an oscillating frequency corresponding to a photon energy of around 2.3 eV. This means that photons with frequency corresponding to 2.3 eV or lower energy are reflected from the gold surface, resulting in the characteristic yellow appearance (see Fig. 1). For copper the oscillating frequency corresponds to a lower energy than for gold, resulting in a more yellow-red appearance. For silver, finally, the oscillating frequency corresponds to a photon energy in the UV-region (4.0 eV), meaning that no photons in the visible region are absorbed.

From this follows that the incoming and reflected visual spectrum are very similar, resulting in its characteristic whitish metallic appearance.

Fig. 2(b) and (c) also display the optical characteristics of two copper-based alloys, brass (Cu4Sn), and bronze (Cu15Zn). It is evident that the alloying of different elements results in significantly different

3. Results and discussion 3.1. General Before showing more specific results on colour measurements of copper patina it is useful to provide some general information on the concept and conceiving of colour. For more detailed information we refer to text books, such as [24–26].

The visual spectrum to which the eye is sensitive is often defined as 750 nm (red) to 350 nm (violet). The most common light source for the perception of copper patina colour is ambient indoor or outdoor daylight, which by far extends beyond the visual spectrum. The colour perception, however, depends on different environmental parameters, including season, geographic location, time of day and weather conditions. If all light in the visible spectrum is reflected against an object, the object appears as white, and if all light is absorbed it appears as black. If only part of the visible spectrum (e.g. orange, 595–605 nm) is absorbed, then the object will exhibit its complementary colour (in this case green-blue, 480–490 nm) according to so-called subtractive colour mixing.

The metallic lustrous appearance of bare copper metal, the brownblack appearance of cuprite and the green-blue appearance of the fully developed copper patina are all results of subtractive colour mixing.

However, the physics behind the selective absorption processes are different for each case, as will be discussed in the next sections. Fig. 1 illustrates schematically the relation between subtractive colour mixing and excitation of electrons by photons in the visible region. The threshold energy for photon excitations is here defined as the photon energy above which all photons are absorbed by a material through excitations of electrons in the material from a ground state to an excited state, and below which no photons are absorbed, but rather transmitted or reflected by the material. The figure illustrates that if the threshold energy is higher than around 3.5 eV no absorption of photons in the visible region occurs and the material appears colourless. If, on the other hand, the threshold energy is below around 1.5 eV all photons in 339

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Fig. 2. Stereo micrograph images of bare polished copper metal, brass and bronze (a). The colorimetric characterization in the CIELab reference space of copper metal, brass and bronze (b), and the corresponding reflected light percentage of the visual spectrum (c).

b-data points continuously move towards origo, and in Fig. 3(c) where a reduction in reflectivity with increasing exposure time is displayed. The fact that the reduction in copper colour with cuprite growth approximately follows a straight line towards origo, suggests that the dark cuprite layer reduces the copper colour uniformly over the whole visual spectrum, i.e. without possessing any selective absorption properties itself.

We consider next the origin of the cuprite colour. Cuprite as a mineral often exhibits red colour, which is explained by its semiconducting properties. As a semiconductor, the electrons can be distributed in a valence and a conduction band [27]. Photons with higher energy than the band gap are able to excite electrons from the valence band to the conduction band resulting in a selective absorption of the high energy part of the incoming photons. Analogously with the discussion for Fig. 1, if the bandgap of the semiconductor corresponds to photon energies for visible light (around 1.5 to 3.5 eV), the result will be that the semiconductor appears coloured. An important example is cuprite with a bandgap of around 2.1 eV, corresponding to a wavelength of 590 nm (yellow). This means that photons with energy higher than yellow are absorbed and the non-absorbed visible light with energy less than 2.1 eV is transmitted or reflected. Some of the excited electrons can drop back to a level different from the level they came

visual appearances, as a result of different electron excitations and oscillation frequencies for each alloy.

3.3. The brown-black cuprite (Cu2O) patina Corrosion products are generally more transparent to photons than metals and appear coloured because of a variety of possible absorption processes in the corrosion products. Similar to metals, the photon absorption processes are the result of different kinds of electron excitations, and the specific colour that appears is a result of combinations of such electron excitations.

Fig. 3 displays the images of bare copper metal after different exposure times up to one year in the marine test site Bohus Malmön, Sweden, under sheltered exposure conditions (a). Separate analysis of these samples have shown that the layers formed largely consist of cuprite (IRAS) and to a very small extent also of some Cu(II)-containing compounds, as evidenced by XPS. The corresponding colorimetric characterizations in the CIELab colour reference space (b) and reflected light percentage (c) are also displayed. The figure shows the common observation that the copper metal surface undergoes a characteristic darkening as a result of the continuous growth of a cuprite (Cu2O) layer (3,5–6). This darkening is also illustrated in Fig. 3(b) where the a- and 340

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hydrogen.

Another reason may be that very small particles are finely dispersed into the cuprite structure, which then changes the optical properties of the substance and allow new electron excitations to occur. This phenomenon is based on theories proposed by Mie more than 100 years ago, who developed different theories for light scattering [34]. The authors have previously used this phenomenon to explain the black staining of Al-Zn alloys [35]. Following the Mie-based theory, which later was extended by others, very small metal particles embedded in the matrix of aluminium oxyhydroxides formed on Al-Zn can give rise to the black staining, a phenomenon that turns out to be very sensitive to actual exposure conditions [35]. Support for a Mie-based explanation of the black colouring of cuprite can be found in [36,37], where the observed black colour of cuprite was associated with finely dispersed gold particles in the cuprite matrix. It should be added, however, that no particles so far have been observed with SEM in the naturally formed cuprite layers investigated by the authors. However, this cannot exclude the existence of sub-micron sized particles embedded in the cuprite matrix and originating from different sources. For more details on atmospheric particles and their involvement in atmospheric corrosion, see [38].

We next address the question how fast the copper surface darkens as a function of cuprite layer thickness. For this we have analysed three series of successively darkened copper samples exposed in rural (Aspvreten, Sweden), urban (Stockholm, Sweden) and marine (Bohus Malmön, Sweden) environments for times ranging from 1 day to 1 year.

For each sample the relative reflectance (i.e., the reflectance relative to the backgrouned) was measured and the relatively uniform thickness analyzed by means of IRAS, see further in Experimental. The overall result is shown in Fig. 4, with the decay in reflectance plotted against the thickness of the cuprite layer.

The fact that all data points follow the same curve suggests that the optical properties of cuprite largely are the same, irrespective of the environment (rural, urban or marine) in which the cuprite was formed.

In accordance with Beer Lambert´s law [39], the results furthermore show an attenuation of the underlying bare copper metal colour that increases with thickness of the cuprite layer, at least in the range from < 0.1 μm to around 0.8 ± 0.2 μm. According to the figure, a reflectivity loss of 1% is caused by a cuprite layer thickness of 10 nm.

Above 1 μm the curve levels off and the reflectivity reaches a plateau level. Fig. 5 is a corresponding graph displaying the loss in reflectivity for very thin cuprite layers, obtained by exposing freshly polished copper metal to laboratory exposures of varying time and repeated dry

Fig. 3. Stereo micrograph images of bare polished Cu after different lengths of exposures from one day to one year (a). The colorimetric characterization in the CIELab reference space of exposed copper metal (b), and the corresponding reflected light percentage of the visual spectrum (c).

from. In the end, the colour depends on the frequency distribution of both transmitted and reemitted light beams. This commonly gives cuprite a red colour [28]. This selective absorption process is illustrated in Fig. 1.

The question is then why a layer of cuprite formed next to the copper metal substrate during atmospheric corrosion appears brownblack rather than red, the colour of the bulk mineral? At least two processes may contribute to this. Cuprite is a metal-deficient, p-type, extrinsic semiconductor, with a rich variety of experimentally observed defect structures [28,29]. Hydrogen is one of the elements that may enter into cuprite through exposure in aqueous solution and influence the defect structure [30,31]. The scattering of light is not only governed by the location of different electron energy levels in the clean unperturbed cuprite structure. Impurities, such as hydrogen, can result in new energy levels which may narrow the band gap energy considerably by introducing new energy levels [32,33]. As Fig. 1 suggests, if the band gap energy is below 1.5 eV, the semiconductor appears black. Hence, one suggested reason for the dark appearance of thin films of cuprite may be a lowering of the band gap energy of bulk cuprite through new electron energy levels caused by additions of foreign atoms, such as

Fig. 4. The relative reflectance (ΔR, %) of gradually darkened copper metal plotted against the corresponding cuprite thickness (μm). The data are from short-term outdoor exposures up to one year in the rural site of Aspvreten, the urban site of Stockholm, and the marine site of Bohus Malmön, respectively. All sites are located in Sweden. The relative reflectance is defined as the reflectance (%) at 670 nm relative to the background reflectance (%) at 400 nm.

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oxides or ferric oxyhydroxides, which all exhibit characteristic brownorange colours [44].

An important implication of this is that the colour variation between different copper-containing compounds in the patina is relatively small, despite large variations in chemical composition between brochantite, antlerite, atacamite and other compounds. The reason is the presence of the optical F-centre associated with the colour-forming Cu(II)-ion.

Depending on the surrounding ligands the absorption characteristics of photons in the F-centre may vary slightly between different compounds but most likely not enough for us to see any relations between colour and chemical composition of patina constituents. For this to happen, the colours for brochantite, antlerite, atacamite and other minerals should vary more.

In most exposure programs of copper metal in different outdoor environments where the authors lately have been involved, a fully green-blue patina colour has hardly been attained, even after five years of outdoor exposure or more. The reason for this will be discussed in a later section. Since we wanted to analyse a full green-blue patina we started by investigating a copper metal sample which had been exposed for at least 120 years in the urban-marine environment of Elsinore, Denmark. This sample forms part of an investigation of several historic copper samples, and exhibits an average thickness of the inner cuprite layer of 4.1 ± 3.9 μm, and of the outer Cu(II)-containing layer of 15.4 ± 6.2 μm [16].

In order to reveal the colour characteristics as a function of patina thickness, the sample was abraded in very small steps following the procedure described in the Experimental section. Starting with the asreceived sample surface Fig. 6 exhibits the visual appearance of the Copenhagen sample after different consecutive patina removal steps.

The figure shows the measured reflectance of the remaining patina and the corresponding thickness of the Cu(II)-containing patina layer, based on SEM-analyses of the patina cross section. The figure clearly shows the gradual loss in reflectance of the green-blue peak with reduction of patina thickness. The reflectance loss starts at a thickness of the Cu(II)containing layer of around 10–12 μm and then continues towards lower thickness, until the reflectance approaches 0%.

The question is if the relationship between reflectance and thickness obtained in Fig. 6 is representative for other environments as well. To elucidate this, a number of copper metal samples with partly or fully developed green-blue patinas were collected from different exposure programs. The samples had previously been exposed for extended times

Fig. 5. The reflectance (%) of slightly darkened copper metal plotted against the corresponding cuprite thickness (nm). The data are from laboratory exposures at cyclic wet/dry conditions for different number of days. The reflectance is here defined as the value at 670 nm.

and wet conditions, as described in Experimental. It is evident that also very thin cuprite layers of thickness range a few tenths of nm result in loss of reflectivity. After one day of wet/dry exposure the cuprite thickness has increased by 11 nm, with a loss in reflectance of 2%. After 14 days of wet/dry exposure the cuprite thickness has increased by another 8 nm, with a concomitant loss in reflectance of 1%.

In summary, the darkening effect of copper metal during initial stages of atmospheric exposure is caused by the formation of a cuprite layer, with optical properties largely independent of environment. In the cuprite thickness range from < 0.1 μm to 0.8 ± 0.2 μm, each incremental 10 nm of cuprite growth results in approximately 1% of reflectivity loss. The brown-black colour of cuprite is attributed to its semiconducting properties, where the bandgap of bulk cuprite (2.1 eV) has been considerably lowered due to introduction of other elements, such as hydrogen. Other causes of the dark cuprite colour cannot be excluded.

3.4. The green-blue Cu(II)-rich patina Much more than the brown-black inner cuprite layer, the characteristic visual appearance of copper patina is determined by its greenblue colour of the outer patina. Depending on exposure conditions this layer may consist of, e.g., brochantite, antlerite, atacamite and combinations thereof [6]. Images of these compounds as bulk minerals exhibit strikingly similar colours, as seen for instance in [40].

The reason for the characteristic green-blue colour of so many patina compounds are so-called colour-centers (or F-centers, from German Farbzentrum) associated with different metal ions [41,42]. Such centra can be said to be crystallographic defects in solid materials in which a vacancy of an anion is filled by one or several unpaired electrons.

Electrons in such vacancies often absorb light in the visible region (750350 nm), which creates characteristic colours. Often the electronic transitions are so called d-d transitions because the electrons involved have d-character [42]. Examples of colour centres in aqueous solutions with different transition metal complexes are [Fe(H2O)6]3+ which is red, pink [Co(H2O)6]2+, green [Ni(H2O)6]2+, blue [Cu(H2O)6]2+ and colourless [Zn(H2O)6]2+. Largely these colours correspond to the colours of different solid substances of the corresponding metal compounds. Hence, Cu(II)-containing compounds, such as most compounds in copper patina, exhibit their characteristic colour because of the presence of Cu(II), which acts as a colorant in the patina, often also referred to as a pigment (in solids) or a dye (in solutions) [43]. In a similar way many compounds formed on corroded iron contain Fe(III) that acts as a colorant or pigment in compounds of rust, such as ferric

Fig. 6. The loss in relative reflectance (ΔR, %) of a patina formed during more than 120 years of outdoor exposure in Copenhagen, Denmark, upon successive removals through careful abrasion. The relative reflectance after each removal step has been plotted against the remaining thickness (μm). It is defined as the reflectance (%) at 670 nm relative to the background reflectance (%) at 400 nm.

The red curve is a guide to the eye, and suggests that the loss in reflectivity starts at a thickness of the outer green-blue patina layer of 12 ± 2 μm.

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ratio between cuprite and Cu(II)-containing compounds (mainly brochantite and atacamite). This scattering in colour characteristics within each group hinders any conclusions to be drawn of possible colour differences between the groups, an observation which is in accordance with the fact that different Cu(II)-containing compounds exhibit similar colour.

To summarize, the consistent presence of the colour forming Cu(II)ion in the outer layer of any copper patina results in a characteristic green-blue colour which shows no significant variation with main compound (e.g., brochantite or atacamite). This implies that the copper patina colour is quite insensitive to type of outdoor or indoor environment. The Cu(II)-containing outer patina layer needs to be at least 12 ± 2 μm thick in order to completely cover the underlying cuprite layer and fully attain its full green-blue appearance.

3.5. Other colour-related aspects Fig. 7. The relative reflectance (ΔR, %) of different patinas, formed during shorter or longer outdoor exposures, plotted against the corresponding thickness (μm) of the outer Cu(II)-containing layer. It is defined as the reflectance (%) at 670 nm relative to the background reflectance (%) at 400 nm. The red curve is the same as seen in Fig. 6, but mirrored and drawn in the opposite direction. In view of the relative large error bars in the thickness estimates, the red curve can be said to agree with most observations. Hence, the data support the conclusion that an outer Cu(II)-containing layer with thickness of at least 12 ± 2 μm is needed to attain the full green-blue patina colour. Data points labelled Brest are from marine exposures of up to one year (12), and remaining data points originate from historic Cu samples exposed more than 100 years at the indicated sites (13).

Besides the physical effects described above that govern the colour of copper, cuprite and the outer Cu(II)-containing patina layer, there are a few other colour-related aspects to be briefly discussed. While previous effects all were based on excitation of electrons by photons in the visible region, a few other colour effects should be mentioned which are based on surface topography or regular surface periodicities in the nano- or micrometer range.

As mentioned in the Introduction, gloss is an important property that affects the overall surface appearance. It is a measure of how well a surface reflects light in the specular direction, i.e., where the angle of incidence equals the angle of reflection. Three factors determine gloss: the surface topography of the material, its refractory index and the angle of incident light. A further optical property is apparent gloss, which is a measure of how large the specular reflection is compared to the diffuse reflection of light, i.e. the light scattered into other directions than in the specular direction [45].

Although presented in several figures in the CIELab colour reference space (Figs. 2, 3 and 8), a property not discussed so far is the lightness L*. This parameter represents the percentage of how much light is reflected from the copper surface, and is obviously a very important parameter for the overall appearance of bare or patinated copper. Information on lightness is hidden in the reflectance measurements, and is not directly seen there. During evolution of the patina film formation the surface starts with the bare copper metal surface which exhibits a high lightness. The lightness is strongly reduced upon the formation of the brown-black cuprite film, and is then increased again if the greenblue patina film appears. To demonstrate this, Fig. 9 displays two graphs. In the upper (a), the reflectance data from Fig. 5 (laboratory exposed polished copper metal forming very thin films of cuprite) have been plotted against the corresponding lightness data L*. The data exhibit a fairly linear relationship. In the second graph (b) data were taken from Fig. 4 (bare copper metal to cuprite formation, marine site) and from Fig. 6 (successive removal of green-blue copper patina, Elsinore) and plotted against the corresponding lightness data L*. The resulting loss in reflectance and concomitant loss in lightness for all three data sets deviate from each other, mainly due to differences in topography, colour and in index of refraction of the patinas formed in each environment [45]. Nevertheless they demonstrate that reflectance and lightness largely follow each other over a broad interval.

Surface topography, gloss and lightness, may be influenced by other processes as well. Soiling is one example. It is the result of deposits of atmospheric black (soot) particles onto the patina surface, mainly through local air pollution sources from anthropogenic activities, and may result in loss of reflectivity [46]. This is a phenomenon that may significantly influence the appearance of copper patina, in particular in urban environments. In the present study the effect of soiling cannot be excluded on some of the historic copper samples, and would probably result in slight loss in reflectivity, similar to the darkening effect caused by cuprite. However, due to lack of more complete data no efforts have

in marine [14] and urban [16] environments and were now analysed with respect to both reflectivity in the green-blue region (540 nm) and thickness of the Cu(II)-patina layer. The results are summarized in Fig. 7, this time with the Cu(II)-layer thickness in the forward direction.

The curve obtained in Fig. 6 has also has been included, but presented in the reverse direction. Similar to Fig. 6, the error bars in the thickness data are substantial because of the varying patina thickness layers within each sample exposed to outdoor environments.

Taking the accuracy in data into consideration the curve for the Elsinore sample obtained in Fig. 6 agrees well with the other samples and suggests that a fully saturated green-blue copper patina colour is obtained in the thickness range 12 ± 2 μm, irrespective of environment.

We next consider the variation in green-blue colour between copper metal samples exposed in different environments under free, unsheltered, conditions. For this we have compiled data from a very large colour investigation of copper patinas performed several years ago [7].

The patinas have previously also been analysed with respect to composition and environment in which the materials were exposed [6].

Based on environment and patina composition the 39 exposure sites were grouped into 5 statistically different groups. Colorimetric data was extracted from copper metal samples exposed for eight years from the following three groups, as previously described in [6]: Group 1 (5 exposure sites): high-polluted sites with respect to SO2 and NO2, mostly urban and industrial. Dominating compounds in patina based on quantitative XRD: cuprite and brochantite.

Group 3 (17 sites): medium-polluted sites, mostly urban and some urban-marine. Low in SO2, higher in NO2. Cuprite dominating compound in patina, followed by brochantite and atacamite at certain sites.

Group 4 (6 sites): low-polluted sites, mostly rural. Cuprite dominating compound, followed by brochantite.

In Fig. 8 all eight-year colour data have been compiled from the three groups of exposure sites. See also Supplement 1 in which the raw data and the chromatic variations of the coordinates can be seen. Based on measured a*-, b*- (upper part) and L*-(lower part) values of each sample, it is evident from the figure that the scattering in colour characteristics within each group is quite large, due to differences in 343

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Fig. 8. The CIELab plots (a*, b* in upper part, L* in lower part) of copper metal samples after eight years of unsheltered exposure in different sites that were categorized in statistically different groups according to differences in environmental characteristics, see further in (6). The data show that scattering of patina colour within each group is very large due to compositional patina variations. Hence, no influence of environment on colour patina can be discerned.

practically absent on the copper patinas studied herein.

Regular spacing on a surface may also result in colour effects through diffraction or absorption of light. Diffractive gratings split the white light into a spatially distributed colour sequence. This phenomenon has been used in photonic devices, light emitting diodes, and solar cells. Examples of periodic gratings and colour effects are seen in [53–55]. Such colour effects may be of relevance for the polished copper. If the surface structures are larger than the incident light wavelength they may change the optical properties of the copper surface through trapping of light or through diffraction effects, as has been demonstrated for coloured brass [56]. Hence, the shorter wavelengths of the visual spectrum are more likely to be influenced than the longer ones. A demonstration of this is the slightly bluish appearance of the copper metal surface upon diamond polishing. In nature such phenomena are also used and may result in bright colours in some butterfly wings or in flowers [57].

It has often been stated that earlier exposed copper patinas have developed their green-blue patina at a higher rate than today because of higher pollution levels of gaseous corrosion stimulators, such as SO2,

been made herein to analyse the effect of soiling on the overall visual appearance of copper patina, and the reader is referred to other studies [9,47].

The colour of metallic surfaces may also be influenced by other physical processes. Interference effects are one example. If a film of equal thickness is sufficiently transparent and formed on a relatively flat reflecting surface the conditions for interference are favourable.

This phenomenon is based on the superposition of two coherent waves, one incoming towards a metal surface on which the film has been formed, and one reflected towards the metal surface. Constructive interference between the incoming and outgoing waves occurs when the phase difference between the waves is an even multiple of Π (180°), while destructive interference occurs when phase difference is an odd multiple of Π. The colour generated depends then on the film thickness, the refractive index of the film, and the nature of the incident light.

Such optical effects are more commonly seen on films formed on passivating metals or alloys, such as aluminium [48,49], titanium [50,51], oxide coated metals [52] and stainless steels [53]. Due to lack of flatness and patina thickness uniformity, these effects are assumed to be 344

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4. Conclusions The origin and evolution of copper patina appearance in terms of colour have been explored by analyzing a large set of copper metal materials exposed from one day to almost 400 years in different unsheltered outdoor atmospheric environments. The emphasis has been on correlating patina colour properties with thickness of the inner brown-black cuprite patina layer and the outer green-blue Cu(II)-containing layer. The following conclusions could be drawn: - The darkening effect of copper during initial stages of exposure is caused by the formation of a cuprite layer, with optical properties largely independent of environment. Up to a cuprite thickness of 0.8 ± 0.2 μm, each incremental 10 nm of cuprite growth results in approximately 1% of loss in reflectivity.

- The brown-black colour of cuprite is attributed to its semiconducting properties. The bandgap of bulk cuprite (2.1 eV) is narrowed through introduction of impurities in the cuprite films formed during initial atmospheric corrosion, allowing photons in the whole visible spectrum to be absorbed.

- The characteristic green-blue patina colour is caused by the Cu(II)ion in compounds forming the outer patina layer, such as brochantite (Cu4SO4(OH)6), antlerite (Cu3(SO4)(OH)4) or atacamite (Cu2Cl(OH)3). The Cu(II) ion acts as a colour forming ion, and is part of all compounds in the outer patina layer.

- As a consequence of the colour forming Cu(II)-ion, no significant colour variation with main compound (e.g. brochantite or atacamite) can be seen, and no significant influence of type of atmospheric environment on patina colour.

- The Cu(II)-containing outer patina layer needs to be at least 12 ± 2 μm thick in order to attain its full green-blue colour.

- Due to the much more inhomogeneous microstructure in terms of inclusions of historic copper materials, the green-blue patina colour is postulated to have been attained at shorter exposure periods than on modern copper metal materials of more homogeneous microstructure and higher purity.

Fig. 9. Absolute or relative reflectance (%) data plotted against corresponding lightness (L*, %). Upper figure (a): Data from Fig. 5 (laboratory exposed copper metal with very thin films of cuprite). Lower figure: Data from Fig. 6 (field exposed copper metal, marine site with increasing cuprite thickness) and, again, from Fig. 6 (successive removal of green-blue copper patina through abrasion).

Data availability The raw/processed data required to reproduce these findings cannot be shared at this time due to legal or ethical reasons as it refers to an industrial project.

NO2 or O3 [38]. An earlier sentence commonly used among architects [58] “When a young architect covers his roof with copper it will turn green when his hair turns grey”, would hardly be true today. With the reduction in concentration of corrosion stimulating gases in many parts of the world, the atmospheric corrosion rates have been reduced, and so has the formation rate of green-blue coloured copper patinas. From the present study it is clear that the green-blue colour originates from the outer Cu(II)-containing patina layer. In a recent study [16] it was found that historic copper samples form much thicker outer layers of Cu(II)containing compounds compared to modern commercial copper materials. This difference was attributed to large inclusions in the bulk material integrated into the patina of the historic samples which trigger micro-galvanic corrosion effects and the formation of the green-blue Cu (II)-containing outer layer at the expense of the inner cuprite layer.

From this is evident that at least two reasons can be found for the higher formation rate of green-blue patinas on historic copper samples than on modern ones: larger inhomogeneities of historic copper samples and previously higher pollution levels, both triggering the formation of green-blue patina layers.

To summarize, besides the main colouring effects for copper metal, cuprite and the outer Cu(II)-containing patina layer discussed in previous sections, a few other effects can possibly have an influence on the copper surface or copper patina appearance and colour. They include diamond polishing of the bare copper surface which may result in a bluish metallic copper surface, and soiling of the patina surface, resulting in a partly darker surface.

Acknowledgements The authors are very grateful for the long-term support and collaboration with the European Copper Institute, Brussels, Belgium and the Scandinavian Copper Development Association, Espoo, Finland.

Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.corsci.2019.05.025.

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