Repatination for Outdoor Bronze Sculpture Conservation:

A Comparative StudyWafaa A. Mohamed(1), Mai M. Rifai(1), Nabil A. Abdel Ghany(2)

and Mohammed S. Elmetwaly(1)

(1)Department of Conservation, Faculty of Archaeology, Cairo University, Giza, Egypt.

(2)National Research Centre (NRC), Cairo, Egypt.

International Conference

SPark: Conservation of Sculpture Parks

September 14 - 16, 2015, Sisak, Croatia

INTRODUCTION

MATERIALS AND METHODS

CONCLUSION

ACKNOWLEDGEMENTS

REFERENCES

RESULTS AND DISCUSSION The authors are deeply grateful to Dr. Lobna Abdel Azez (NRC), Prof. DR. Sayed Abdou Sleem, Rasha Abo Elela (NIS) and Abdel Rahman

Sleem for their help.

Bronze is one of the most popular materials used in outdoor sculptures. Henry Moore (1898–1986), the best-known British sculptor of

outdoor bronzes, said, "Bronze is a wonderful material, it weathers and lasts in all climates" [1], but especially after the Industrial

Revolution outdoor bronze sculptures were exposed to numerous chemical pollutants which catalyzed nature’s threats of moisture, heat,

oxygen, ultraviolet and biological attack. These sculptures suffered irrevocable changes from damaging and scarring corrosion [2].

For these reasons, protection from bronze corrosion is very important when trying to conserve the bronze sculptures situated in a hostile

environment. Minimizing the bronze corrosion can be achieved by applying coatings on the surface. Coatings provide a barrier between

the corrodents and the metal substrate [3] and prevent chemical reactions between substrate and surrounding media [4], These coatings

must resist a wide range of temperature conditions, ultraviolet radiation, pollutants, snow, acid rain, wind and particulate matter [5].

Therefore, the main aim of this study is to test the efficiency of coating systems for two substrates (Patinated and Bare Bronze) by using

outdoor exposure tests (weight gain and color change) and laboratory tests using electrochemical impedance spectroscopy (EIS).

Weight gain, color change measurements and electrochemical impedance spectroscopy (EIS) tests demonstrated that Benzotriazole BTA

(pretreatment) 3% in ethanol + Incralac + Microcrystalline wax (Cosmoloid H80) 13.33g\20 ml toluene top coat gave the best results for

patinated bronze coupons while Benzotriazole BTA (pretreatment) 3% in ethanol + Incralac gave the best results for bare bronze

coupons. Artificial patinas offers surface protection against corrosion and it also has an aesthetic value.

Bronze substrate:

Ele. Cu Sn Zn Pb Sb Ag Bi Fe Cr V Ti In Se

% 85.28 6.93 4.76 2.1 0.04 0.16 0.1 0.02 0.01 0.02 0.04 0.01 0.53

(Table 1): Composition (%) of bronze coupons by portable XRF.

Substrates preparation:

Coating systems and application method:

Methods of evaluation and weather conditions:

Application MethodCoating SystemsNo.

-------Blank1

Applied by brush. Dried after 72 hours.ORMOCER2

Applied by brush. Dried after 15 minutes.Incralac3

13.33g\20 ml toluene. Applied by brush. Dried after 10 minutes. The surface was

rubbed using a cotton cloth to achieve homogeneity.Microcrystalline wax (Cosmoloid H80)4

3% BTA (Pretreatment) in ethanol applied by brush and after an hour another layer of

BTA was applied and after an hour the surface was washed with deionised water to

remove BTA residual and after 48 hours Incralac was applied.

Benzotriazole BTA (Pretreatment)

+Incralac5

A layer of Incralac was applied by brush then after drying a layer of Microcrystalline

wax (Cosmoloid H80) 13.33g\20 ml toluene top coat was applied.Incralac+ Microcrystalline wax (Cosmoloid H80) top coat6

3% BTA (Pretreatment) in ethanol was applied by brush and after an hour another layer

of BTA was applied and after an hour the surface was washed with deionised water to

remove BTA residual and after 48 hours Incralac was applied. After drying a layer of

Microcrystalline wax (Cosmoloid H80) 13.33g\20 ml toluene top coat was applied.

Benzotriazole BTA (Pretreatment)

+Incralac+ Microcrystalline wax (Cosmoloid H80) top coat7

20% of Acryloid in toluene was applied by brushAcryloid B-448

(Table 2): Coating systems and application method.

(Figure 1): (A) Polished bronze (B) Patinated bronze.

(Figure 3): Weight gain process. (Figure 5): EIS cell and working bronze electrode.(Figure 4): Color change measurements.

Outdoor exposure

Weight gain

After 12 months of outdoor exposure, all coupons (bare and patinated

bronze) displayed different rates of weight gain, however the patinated

coupons showed a much less weight gain than bare bronze as given in

(Figure 5), because artificial patinas offers further surface protection

against corrosion [12]. The performance of the coating systems was

measured in (Table 4) and in (Figure 6). The results showed that

Benzotriazole BTA (pretreatment) 3% in ethanol + Incralac +

Microcrystalline wax (Cosmoloid H80) 13.33g\20 ml toluene top coat

gave the best results for patinated bronze coupons (98.45%), and

Benzotriazole BTA (pretreatment) 3% in ethanol + Incralac gave the

best results for bare bronze coupons (76.66%).

ORMOCER and Microcrystalline wax (Cosmoloid H80) displayed the

worst results for both bare and patinated bronze.

Coating systemsPerformance (%)

Bare bronze Patinated bronze

ORMOCER 25.78% 8.29%

Incralac 72.13% 67.36%

Microcrystalline wax (Cosmoloid H80) 15.33% 27.46%

Benzotriazole BTA (Pretreatment) +Incralac 76.66% 70.47%

Incralac+ Microcrystalline wax (Cosmoloid H80) top

coat74.56% 79.27%

Benzotriazole BTA (Pretreatment) +Incralac+

Microcrystalline wax (Cosmoloid H80) top coat75.61% 98.45%

Acryloid B-44 53.31% 44.56%

(Figure 5): Average of Weight gain for bare and patinated bronze

(blank and coating systems) after 12 months.

(Table 4): Performance of coating systems in bare and patinated bronze by weight gain.

(Figure 6): Coating systems performance after 12 months.

48 coupons were prepared, each measuring 4×5×0.3cm. The surface of

the coupons was polished with abrasive SiC paper strips and cleaned

using 10% sodium carbonate Na2CO3 by scrubbing with a clean brush.

After thorough rinsing in cold running water, the coupons were

additionally rinsed with ethanol followed by drying with filter paper.

Patination was carried out using 1% (w/v) ferric nitrate Fe(NO3)3 in

deionised water using the Torch Technique [6].

The coupons were divided into two groups and stamped with

punches: (A) 24 coupons (polished bronze) and (B) 24 coupons

(patinated bronze), (Figure 1).

Seven coating systems that meet both aesthetic appearance and protection requirements were applied onto the polished and patinated

coupons, (Table 2).

(Figure 2): Rack.

Laboratory tests using EIS: AUTOLAB302 NFRA32, Setup Software: Nova 1.10,

Netherlands). Electrochemical Cell consisted of three electrodes: working

bronze electrode (WE: 0.2cm2), Reference electrode “silver/silver chloride”

(RE) and platinum Counter electrode (CE), (Figure 5). Dilute Harrison electrolyte

(0.35 wt.% of (NH4)2SO4 + 0.05 wt.% of NaCl in H2O, [2] pH = 4.2) has been used

to test the behaviour of coatings exposed to acid rain [10,11].

For outdoor exposure, the coupons were placed on an exposure rack on the

roof of the National Military Museum at Salah Eldin El Ayoubi Citadel in Cairo,

Egypt (Figure 2) for a period of one year. Orientation and position were

selected according to ISO 9223 standard: the coupons were facing south

exposed skyward in the position of 45 degrees from the horizontal [7]. Weight

gain (Figure 3) (0,000gm) was carried every month to evaluate the changes in

the weight. Color change measurements (Figure 4) (ΔE) by Color - Eye®

Spectrophotometer (OPTIMATCH 3100) were carried every two months on three

points for one sample for each coating systems. ΔE* documents the entire

change of surface appearance [8, 9].

Used (Data logger: LOG32) to measure temperature (C), relative humidity (%)

and Dew point (C)on the roof of the National Military Museum at Salah Eldin El

Ayoubi Citadel in Cairo, Egypt for the period of one year. (Table 3).

[1] Pullen, D. and Heuman, J. 2007. ‘Challenges and Advances. Modern and Contemporary Outdoor Sculpture Conservation’. Getty

Conservation Institute Newsletter, Vol. 22, No. (2): 4-10.

[2] Bierwagen, G. and Shedlosky, T. J. 2002. ‘Development and Testing of Organic Coating for the Preservation of Outdoor Bronze Sculpture

from Air Pollutant Enhanced Corrosion’, National Center for Preservation Technology and Training (NCPTT), Year (3): 1-26.

[3] Bierwagen, G. Shedlosky, T. and René de la pie, E. 2001. Final Report to the National Center for Preservation Technology and Training

(NCPTT). Grant Program. Research into Development and Testing of Organic Coating for the Protection of Outdoor Bronze Sculpture from

Air Pollutant Enhanced Corrosion, Year (1).

[4] Scott, D. A. 2002. ‘Copper and Bronze in Art "Corrosion, Colorants, Conservation”’, Getty Conservation Institute, Los Angeles.

[5] Shedlosky, T. Huovinen, A. Webster, D. Bierwagen, G. 4-8 October 2004. ‘Development and Evaluation of Removable Protective Coatings

on Bronze’, Proceeding of the International Conference on Metals Conservation, Canberra, National Museum of Australia, 400-413.

[6] Balta, I. Z., Pederzoli, S., Iacob, E. and Bersani, M., 2009. ‘Dynamic Secondary Ion Mass Spectrometry and X-ray Photoelectron

Spectroscopy on Artistic Bronze and Copper Artificial Patinas’, Applied surface Science (255): 6378–6385.

[7] Letardi, P. 4-8 October 2004. ‘Laboratory and Field Test on Patinas and Protective Coating System for Outdoor Bronze Monuments’,

Proceeding of the International Conference on Metals Conservation, Canberra, National museum of Australia, 379-387.

[8] Mohamed, W. A., Rifai, M. M., Ghany, N. A. A., and Elmetwaly, M. S., 4-5 December, 2014. ‘Conservation of an Outdoor Historical Bronze,

Open Air Metal, Outdoor Metallic Sculpture: from the XIXth to the beginning of the XXth Century, Paris, France, 176-185.

[9] Faltermeier, R. May 1998. ‘Colour Changes Induced when Treating Copper and Copper Alloy Archaeological Artifacts with the Corrosion

Inhibitors Benzotriazole and Aminomercapto – Thiadiazole’, SSC journal, Vol. 9, No. (2): 1-6.

[10] Ellingson, L. A. Shedlosky, T. J. Bierwagen, G. P. De la Rie, E. R. and Brostoff, L.B. 2004. ‘The Use of Electrochemical Impedance

Spectroscopy in the Evaluation of Coatings for Outdoor Bronze’, Studies in Conservation, Vol. 49, No.(1) : 53–62.

[11] Cano, E. Lafuente D. and Bastidas, D. M. 2010. ‘Use of EIS for the Evaluation of the Protective Properties of Coatings for Metallic

Cultural Heritage: A Review’, J. Solid State Electrochem,14 (3): 381-391.

[12] Chelaru, J. D. S. Mureşan, L. M. Soporan, V. F. Nemeş, O. and Kolozsi, T. April-June 2010. ‘A Study on the Corrosion Resistance of

Bronze Covered with Artificial Patina’, International Journal of Conservation Science )IJCU(, Vol. 2, Issue (2): 109-116.

(Figure 12): Coating systems performance (Patinated bronze) by using EIS.(Figure 10): Coating systems performance (Bare bronze) by using EIS.

(Figure 9): Nyquist plots for Blank and Coating systems (Bare bronze). (Figure 11): Nyquist plots for Blank and Coating systems (Patinated bronze).

Laboratory tests

Electrochemical Impedance Spectroscopy (EIS)

Color change measurements

(Figure 7): Color change measurements for Blank and Coating systems

(Bare bronze).

(Figure 8): Color change measurements for Blank and Coating systems

(Patinated bronze).

After 12 months of exposure, all coupons (bare and patinated bronze) displayed a color change with different rates but bare bronze

showed a much higher change than patinated bronze.

Incralac systems gave the best results, while Microcrystalline wax (Cosmoloid H80), ORMOCER and Acryloid B-44 gave the worst results

for both bare and patinated bronze as follows:

Bare bronze: Performance of coating systems ranked from best

to worse.

Benzotriazole BTA (pretreatment) + Incralac > Incralac +

Microcrystalline wax (Cosmoloid H80) top coat > Incralac >

Benzotriazole BTA (pretreatment) + Incralac + Microcrystalline

wax (Cosmoloid H80) top > Acryloid B-44 > ORMOCER >

Microcrystalline wax (Cosmoloid H80) > Blank. (Figure 7).

Patinated bronze: Performance of coating systems ranked from

best to worse.

Benzotriazole BTA (pretreatment) + Incralac + Microcrystalline

wax (Cosmoloid H80) top coat > Incralac > Incralac +

Microcrystalline wax (Cosmoloid H80) > Benzotriazole BTA

(pretreatment) + Incralac > ORMOCER > Acryloid B-44 > Blank >

Microcrystalline wax (Cosmoloid H80). (Figure 8).

Bare bronze: Performance of coating systems ranked from best to

worse.

Benzotriazole BTA (pretreatment) + Incralac > Acryloid B-44 > Incralac

> Benzotriazole BTA (pretreatment) + Incralac + Microcrystalline wax

(Cosmoloid H80) top coat > Incralac + Microcrystalline wax

(Cosmoloid H80) top coat > ORMOCER > Microcrystalline wax

(Cosmoloid H80). (Figures 9 and 10).

Patinated bronze: Performance of coating systems ranked

from best to worse.

Benzotriazole BTA (pretreatment) + Incralac + Microcrystalline

wax (Cosmoloid H80) top coat > Acryloid B-44 > Benzotriazole

BTA (pretreatment) + Incralac > Incralac + Microcrystalline wax

(Cosmoloid H80)> Incralac > ORMOCER > Microcrystalline wax

(Cosmoloid H80). (Figures 11 and 12).

Dew Point

(oC)

Relative

Humidity (%)

Temperature

(oC)Seasons

Mean

of Max

Mean

of Min

Mean

of Max

Mean

of Min

Mean

of Max

Mean

of Min

12.69.95831.830.322.1Spring

1917.165.438.535.126.3Summer

1513.87245.527.320.5Autumn

7.86.662.640.122.615.7Winter

(Table 3): Minimum and maximum rates for temperature (C), relative

humidity (%) and Dew point (C) during year.

The Nyquist Plots for coupons (Bare and Patinated bronze) show the performance of coating systems (Figures 9 and 11) in Dilute Harrison

electrolyte (0.35 wt.% of (NH4)2SO4 + 0.05 wt.% of NaCl in H2O, pH = 4.2).

Incralac systems gave the best results, while Microcrystalline wax (Cosmoloid H80) and ORMOCER gave the worse results for both bare

and patinated bronze as follows: