Degradation of monumental bronzes

Atmospheric Environment Vol. 26B, No. 2, pp. 207 215, 1992.

0957 1272/92 $5.00+0.00 © 1992 Pergamon Press Ltd

Printed in Great Britain.

D E G R A D A T I O N O F M O N U M E N T A L BRONZES JOHN D. MEAKIN* a n d DAVID L. AMES~" *Department of Mechanical Engineering and tCenter for Historic Architecture and Engineering, University of Delaware, Newark, DE 19716, U.S.A.

and DONALD A. DOLSKE Illinois State Water Survey, Champaign, IL 61820, U.S.A.

(First received 20 April 1991 and in final form 5 November 1991) Abstract--Under the United States National Acid Precipitation Assessment Program a metallurgical and corrosion study has been made of sample of the 50 bronze replicas of the Hiker statue cast by the Gorham Foundry, Providence, Rhode Island, between 1906 and 1966.To study corrosion damage on a microscopic scale a technique was developed to take molds of selected features on the statue. The molds were examined by optical and scanning electron microscopy and surface profilometry. The overall approach has the potential for quantitatively monitoring the local corrosion at selected sites over a period of years and also characterizing the impact of conservation procedures. In a parallel research program, run-off samples were taken from selected Brigade Markers in a series of bronze tablets in the Gettysburg National Military Park; parallel rainwater samples were collected. Analysisfor pH, metal content and various ion species in the run off yields information on the total rate of corrosion as a function of dry and wet precipitation.

Key word index: Corrosion damage, run-off samples, rainwater samples, statue.

CORROSION OF THE HIKER BRONZE STATUES

Introduction The many replicas of the Hiker statue that were cast over a 60-year period, and which are located at numerous sites in the United States, provide a unique resource for assessing the impact of environmental conditions on the degradation of a monumental bronze. Theo Alice Kitson sculpted the original Hiker and from 1906 to 1965, 52 replicas of the statue were cast by the Gorham Company of Providence, RI (Panhorst, 1991). In 1965 the metal masters from which all these castings were made, were themselves assembled and erected in Arlington, VA, as the final copy of the Hiker series. Following an initial study and cataloging of 25 Hikers, l l statues from the Greater Boston area and two from Pennsylvania were selected for more intensive study. Analyses have been conducted on the composition of the alloy in actual statues, the overall features of the corrosion patterns, the composition of the corrosion layers and the morphologies of the statue surfaces in the as-corroded state, as well as one statue after conservation treatment.

Metallurgical and corrosion studies Composition. Sample drillings were taken from the underside of the gun butt on five Hikers in the Boston group and from a range of locations on the Shamokin and Allentown statues. Analysis by X-ray emission AE(B) 26"2-E

spectroscopy revealed that the alloy used was ounce metal, generally now known as leaded red brass, alloy C83600, which is nominally 85% Cu and 5% each of Sn, Zn and Pb. Table 1 gives the results of the analysis. Figure 1 shows that although there is significant variability in the Zn content, the most volatile component, there is no discernible trend in composition over the time period covered by the available analytical results. The Sn and Zn is expected to be in solid solution, although there may be some segregation of the Sn due to dendritic formation; the Pb will be rejected as isolated spheres (ASM, 1985). The overall size scale of the microstructure is likely to be less than 50/~m. The quenched outer skin of the castings should be essentially the same within and between statues and the overall high Cu content common to all statues will render them very similar in corrosion behavior. Corrosion studies. Prior to erection bronze statues are usually patinated to result in a relatively uniform surface color that can range from a light to a dark brown. Degradation of the statues as a result of corrosion has resulted in regions becoming essentially black and others ranging through various shades of green (Lins, 1985). Streaking has occurred widely and corrosion pitting, resulting in a splotched or speckled appearance, is also commonly observed. A collection of color slides has been made using 35 mm and 2 lit formats. A macroscopic taxonomy based on these slides has been developed. In this paper attention is

207

208

J.D. MEAKINet al. Table 1. Composition (wt%) of five statues in Massachusetts and two statues in Pennsylvania Location

Cu

Chelsea Everett Lynn Malden Medford Shamokin Allentown

Zn

88.2 93.4 90.5 87.2 87.9 88.9+0.8 87.4+0.3

Sn

4.93 2.63 5.76 5.76 5.48 3.82+0.4 2.80__+0.3

4.39 2.22 2.43 3.93 3.94 4.06+0.4 3.93+0.1

Pb 2.17 1.47 1.25 3.04 2.64 3.14+0.3 5.68+0.2

Trace amounts of Ag (~0.02%) were also detected in the samples.

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O Lynn

Everett

1923

1927

Medford 1928

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Malden Shamokin

1934

1938

1937

1938

Statue

Fig. 1. Composition of five statues in Massachusetts and two statues in Pennsylvania: Allentown and Shamokin. Note the scale change of x 0.1 for copper.

focused on microscopic studies of the corrosion that were carried out with a view to determining, if possible, the corrosion mechanisms occurring and the relation between corrosion and atmospheric pollutants. The Hikers can be viewed as multiple exposure tests on essentially the same test structure with exposure times ranging from close to a century to essentially no exposure for the Montgomery statue which has been housed in a museum. To gain insight into the corrosion of the statues, a replication technique was developed which allows laboratory study of the surface morphology resulting from corrosion. Identification of the corrosion products remaining on the bronze surface was attempted using X-ray diffraction from surface scrapings. X - r a y diffraction. Samples were scraped from a number of statues and subjected to powder diffraction analysis using both film and standard diffractometer techniques. Samples from the Allentown Hiker were analysed at the Smithsonian Institution by Daphne R.

Ross using diffractometry, and samples from Malden, Chelsea, Everett and Medford were analysed with a film technique. A salient feature of many of the samples was that lines from the bronze tended to be dominant. Expressed in other terms, the amount of corrosion product that could be removed while respecting the integrity of the statues was very limited, and as a consequence definitive identification of minor corrosion products cannot be achieved. Further difficulties arise as the major lines for some of the copper compounds of interest tend to overlap. As would be anticipated the most generally detected corrosion product was brochantite, a hydrated copper sulfate, CuSO4'3Cu(OH)2 (Graedel et al., 1987). There was also evidence for cuprite, Cu20, malachite C u C O a . f u ( O n ) 2 and antlerite, CuSO4' 2Cu(OH)2. The latter material is of particular interest as it has been proposed that in particularly low pH conditions antlerite should be the favored form of copper sulfate (Matyi and Baboian, 1986). Table 2 is illustrative of

Degradation of monumental bronzes

209

Table 2. Analysis of scrapings from the Chelsea Hiker Unknown d

Brochantite

I/I o

d

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6.28 5.28

40 80

3.87 3.27 3.14 2.876 2.637 2.549 2.488 2.423

100 10 40 10 20 10 40 10

6.38 5.36 5.15 3.90

40 40 6 85

2.886 2.628

2 8

2.243 2.161 2.113

10 10 10

2.053

10

1.9729 1.7939 1.7175 1.6836

20 10 20 10

1.6182 1.5752 1.5418 1.5166

10 10 10 10

2.421

2.127 2.099 1.9690

4

2 2

Antlerite

Malachite

d

I/10

d

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5.40

25

2.857

100

2.477 2.425

30 20

2.252 2.160 2.129

8 8 20

2.054

10

1.0690 1.7900

18 12

1.6910 1.6780 1.6160 1.5710 1.5410

25 14 18 14 8

2.566 2.503 2.439 2.430 2.259

85 25 6 14 14

2.131

20

2.065 2.034

18 20

8 1.7110 1.6870

6 10

1.6170 1.5660 1.5510 1.5250 1.5110

6 14 16 4 12

the type of evidence for various potential corrosion products that is available from X-ray diffraction. The sample was a scraping from the right-hand chest region of the statue in Chelsea, MA, and the analysis was made using a film technique. Other samples from Chelsea and scrapings from Malden and Allentown showed peaks corresponding to the bronze as the n'fost prominent which indicates how little corrosion product is obtained by superficial scraping. A major effort would be required to be able to draw any quantitative conclusions about the dominant corrosion products and even then the data may be of limited value. In order to draw firm conclusions about the corrosion process, and its variation from statue to statue or at different sites on the same statue, knowledge of the structure of the surface layers from unaffected bronze to the outermost surface would be needed. Such information would be impossible to derive from scrapings. Surface replication. Microscopic and surfacecharacterizing techniques are much more readily carried out in the laboratory than at outdoor Hiker sites. Some photography can be carried out in the field but this is very limited in the enlargement that can be achieved. Accordingly a replica technique was developed and applied to selected features on a number of statues. The replicating material is a vinyl polysiloxane, widely used for dental purposes and available from

Cuprite d

I/I o

1.5100

27

dental supply houses. The two components are mixed by hand to yield a putty-like material which can be pressed firmly against the surface to be replicated. At normal temperatures the material sets up to a firm, but slightly elastic, solid which can be peeled from the surface. Areas of about 20 cm 2 have been routinely replicated. Measurements were made on a trial replica to establish the degree of dimensional stability. Figure 2 shows that over about a month the change in dimension was _ 0.002" in 0.58" or < ___0.5 %. A check after almost a year shows that the molding material has remained completely stable. Replicas were taken of the "V" on the knapsack of a number of statues. It was initially hoped to measure the prominence on the replica (depth on the statue) of a selected location on the letter. This proved to be impossible to do reliably as no repeatable base line could be established on the surrounding curved surface. Replicas were also taken on the rifle barrel immediately under the front sight. This position is free of complex run-off effects and over a relatively short distance, about 3 cm, includes surfaces that go from almost horizontally up to horizontally down. A macroscopic photograph of four replicas is presented in Fig. 3. A qualitative assessment of the surface roughness and pitting can be obtained by microscopic examination of the replica surface. Optical microscopy is useable but is limited by the very small depth of field.

210

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Degradation of monumental bronzes More flexible microscopy, using magnifications between x 10 and x 1000, can be carried out using scanning electron microscopy (SEM). The replicas are lightly gold coated before examination. Figure 4 shows the appearance of the replicas from the vertical surface of the gun barrel immediately under the front sight. The contrast between the essentially un-pitted Washington Hiker and the heavily pitted Lynn, Everett and Chelsea statues is immediately apparent. Although it would be feasible to use these micrographs to measure the fraction of the surface which is pitted area, such data gives no information on the depth of the pits. From a degradation viewpoint the latter is an important metric of damage. Using the same replicas, a surface profilometer has been used to develop a quantitative measure of the elevation along a sample line along the replica (statue) surface. Figure 5 shows the resulting traces for statues showing a range of surface degradation. The Montgomery statue has been housed in a museum since it was cast and the trace shows small asperities, probably dust and detritus on the surface. The Washington statue has not been exposed for a sufficiently long time to develop any significant pitting. The Lynn and Chelsea statues are very heavily pitted and the traces show pit depths up to 0.3 mm. The replica of the Fitchburg statue was taken after a conservation treatment using walnut

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211

shells. The pitting is still quite evident but there appears to have been some reduction (rounding) of the fine structuring of the surface. A topography of the surface could be built up by scans along a set of parallel lines. However a more promising approach, with the potential of generating some quantitative measurement of corrosion severity, is to perform a power spectrum analysis on the traces to detect characteristic wavelengths and depths. This investigation is ongoing. Probably the most valuable aspect of the replication procedure is the ability to monitor the change in surface condition over a period of time. Repeated replicas could be stored and the growth of selected pits, or the initiation of new pits, monitored. Replication would also yield some quantitative measure of the morphological changes caused by any conservation treatment. Direct measurement. An attempt was made to measure the dimension of a specific feature on the Hiker with a view to cross-comparing between different statues. The features selected were the rifle barrel immediately behind the front sight (i.e. close to the location selected for one of the replica studies), the barrel near the rear sight and a pannier strap. Table 3 shows the result of this exercise. It was immediately obvious that the pannier straps varied

~

WASHINGTON

CHELSEA

LYNN

EVERETr

Fig. 4. Scanning electron micrographs of replicas from various statues. Marker line is 0.1 mm.

J.D. MEAKINet al.

212

widely between the statues and it would appear that any convenient piece of metal was pressed into service when the statue was assembled. The horizontal measurement of the barrel near the front sight seemed quite reproducible but it is now considered that the direct measurement approach will not prove fruitful for the following reason. In an outdoor environment the general rate of loss of bronze over time is reported to be of the order of 1-3/~m yr- ~ (Mattsson and Holm, 1982). Thus over say, 20 years the total overall loss will be about 50/~m or 0.002". As there are no precisely parallel features on the statues it is unlikely that repeated measurements on one statue, or cross-comparisons between statues, will yield data of the required precision. In contrast the localized pitting is occurring at rates up to 5/~m yr-1 and furthermore relocation of precisely the same pit is perfectly feasible.

Montgomery

RUN-OFF MEASUREMENTSAT GETTYSBURGNATIONAL MILITARYPARK

Introduction The Gettysburg National Military Park is in a rural area of Pennsylvania although there is substantial vehicular traffic created by the visitors. In about 1906 a series of Bronze Brigade markers were erected at numerous locations throughout the park. These markers are cast bronze plaques with raised letters and are mounted at 30° to the vertical on granite supports. They appear to have been cast by Bureau Bros. of Philadelphia and, according to a letter to the War Office dated 16 October 1906, were to be made of "Government Standard Bronze Metal of the best quality" (National Archives and Records Administration, 1906). Qualitative EDS analysis indicates a copper alloy containing small amounts of Sn, Zn and Pb suggesting that these markers are also ounce metal, nominally 5% Zn, Sn and Pb in Cu. A collection system was designed to sample the runoff solution following a sequence of rain event. A parallel rain sampling system was also operated and measurements made during the period 1986-1988. Analysis of the run-off samples produced measurements of pH, Cu and Zn content, and various ion species typical of acid rain.

Run-off and rain studies

Ilmm' A

Fig. 5. Profilometer traces for replicas from various statues.

Sampling. Three markers were selected for measurements and were fitted with a channel device that fed into an acid-leached polyethylene sample bottle. The markers selected were the II Corps, 3rd Division, 1st Brigade, designated ECBM; the III Corps, Artillery Brigade, WBMN; and the III Corps, 2nd Division, 2nd Brigade, WBMS. The rain sampling was in the region of ECBM on East Cemetery Hill, designated ECRS, and in the Wheatfield, designated WFRS, near WBMN and WBMS. Park staff and volunteers mounted the collection systems when a rain event was anticipated. In the event that rain did not occur in 12 h the collection bottle was recleaned to avoid contamination. Laboratory analysis was conducted using atomic absorption and ion chromatography; the techniques yield compositions to an accuracy of about 5% relative.

Table 3. Direct measurement of selected dimensions on Hiker statues Lynn Rifle sights Front Vertical ( i n . ) Horizontal Rear Straps

Chelsea

1.173-1.179 1.142, 1.152" 1.206 1.207, 1.208 2.369-2.374 0.118-0.122 0.272

Everett

1.131 1.206 2.371 0.193-0.197

Washington

1.167-1.172 1.220-1.221

*The Chelsea statue is mounted at ground level;hand polished by the neighborhood children.

213

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The size of the sample bottle, 500 ml, is such that a relatively small rainfall, namely about 0.1 mm or 0.04", will fill the bottle. As a result most of the run-off samples provide an averaged measure of the pH and ion content of the solution leaving the tablet. It would

be expected that the first run off, particularly following a prolonged dry period, will be particularly acidic and rich in leached species. Analysis. Computer-based files are now available that contain the results corresponding to 35 rain

J . D . MEAK1N et al.

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events and the associated rain and run-off analyses for the three bronze markers ECBM, W B M N and WBMS. Initial correlations have been tested for the following data pairs: Zn versus Cu for all marker run-off samples, the pH of the run off for all marker samples against the corresponding pH of the East Cemetery Hill rain sample, and the concentration of cor~r~er

versus sulfate in all marker run-off samples and the corresponding correlation between copper and nitrate. There are nine fewer points than the full data set in Fig. 6. The least square fit shown has a slope of 0.05 and a correlation coefficient, R 2, of 0.45. The points not included were extreme outliers, probably representing defective analyses. It should be borne in mind that sample collection is in a public area and is

Degradation of monumental bronzes not monitored for most of the time. Even including the outliers still gave reasonable correlation. It is concluded that actual dissolution of the plaque occurs without preferential removal of zinc from the copper base. The data is in agreement with a 5% zinc content that matches the alloy believed to have been used. Figure 7 shows an excellent degree of correlation of the copper and sulfate ion concentrations. The least square fit line shown has a slope of 0.62 and a correlation coefficient, R 2, of 0.90. If the copper is dissolved in the stoichiometric ratio for cupric sulfate, CuSO4, the anticipated slope would be 0.66, strongly suggesting that the concentrations of sulfate ions is determining the dissolution rate. The corresponding plot of copper against NO3, Fig. 8, shows much poorer copper correlation than Fig. 7. The least square fit line has a slope of 0.82 and a significant intercept of 11 ppm Cu; the correlation coefficient, R 2, is 0.5. This slope is very different to what would be anticipated if Cu(NO3)2 is the'controlling species, namely 0.51. The above conclusion concerning the controlling influence of the sulfate ion stands. Although a least square line has been put through the data of Fig. 9 there would appear to be essentially no correlation between the pH of rain and the pH of the run off occurring on the tablets at the same time. The correlation coefficient, R 2, was a meaningless 0.1. The run-off pH is almost certainly being dominated by the dry deposition that occurred since the previous rainfall.

CONCLUSIONS

The effects of acid precipitation on the corrosion of monumental bronzes has been studied using the Hiker series of status. Substantially different degrees of attack have been recorded. A mold technique has been

215

developed that has been used to compare the corrosion on different statues. The technique could also be used to monitor corrosion over a period of time and to determine the effect of conservation treatments. Run-off studies on bronze tablets at the Gettysburg National Military Park have indicated that the dissolution rate is controlled by the availability of sulfate ions. There appears to be little or no correlation between the acidity of the run off and the acidity of the rain falling on the tablet. Dry deposition between rain events are concluded to dominate the acidity of the run off.

Acknowledgements--This research has been supported by the National Park Service, cooperative agreement CA 0424-68003. The support and encouragement of Susan I. Sherwood of the National Park Service is gratefully acknowledged.

REFERENCES

ASM (1985) Metals Handbook, Vol. 9, 9th edn, pp. 637-645. American Society of Metals. Graedel T. E., Nassau K. and Franey J. P. (1987) Copper patinas formed in the atmosphere I. Introduction. Corros. Sci. 27, 639-657. Lins A. (1985) Outdoor bronzes: some basic metallurgical considerations. In Sculptural Monuments in an Outdoor Environment (edited by Naude V. N.), pp. 8-20. Pennsylvania Academy of the Fine Arts, Philadelphia. Mattsson E. and Holm R. (1982) Atmospheric Corrosion of Copper and its Alloys. Atmospheric Corrosion (edited by Ailor W. H.), pp. 365-382. Wiley Interscience, New York. Matyi R. J. and Baboian R. (1986) An X-ray diffraction analysis of the patina of the Statue of Liberty. Powder Diffr. 1, 299-304. National Archives and Records Administration (1906) Letter from Gettysburg National Park Commission, 16 October 1906, to the Secretary of War in 7393/46. Panhorst M. W. (1991) The popularity, patronage, production and placement of monumental bronze statue replicas of the Hiker (unpublished).