FOR A Monumental Project

John Robbin

From NPS-28 Standards for Hlstonc and Prehistoric Struc­tures .. " Deteflorated archItectural features will be repaired rather than replaced whenever possIble. If replacement IS necessary, the new mateflal should match the old m compOSition, design, color, texture, and other Visual qualities such as weathering characteflstlcs. Repair or replacement mlssmg architectural features Will be based on accurate dupllcatJons rather than on conjectural desIgns or avallablllly of different archItectural

elements from other stllr,u:c~-~ .... -:-... ~~; tures." _

INTRODUCTION

In 1886, a novel architectural concept was erected in New York harbor. This structure, a gift to the

United States from thousands of pri­vate citizens in France, was sculpted by Auguste Bartholdi, whose design was inspired by the colossal "granite beings" of Egypt. Unlike the tradition­al commemorative structures of its daY,the Statue of Liberty incorporated the avant-garde curtain wall construc­tion that subsequently enabled the building of modern skyscrapers. The edifice was made of 300 separate sheets of virgin copper riveted to a complicated wrought iron skeleton de­signed by the renown Alexandre Gustave Eiffel.

In 1980, the French-American Com­mittee for Restoration of the Statue of Liberty formed to evaluate the severi­ty of environmental deterioration to the various statue components.' Degener­ation of protective coatings and gal­vanic coupling between the copper and wrought iron endangered the

structural integrity. A restoration project commenced in 1983

under the direction of the U.S. Depart­

ment of the

Interior-National Park Service, with funding from The Statue of liberty-Ellis Island Foundation, Inc.2,3

Editor ', Not.: Spec,al \IIaMS 0 tile Copper 0eveI0p­menl Assooa on 10< p/Iolographoc c:oninbuioOn5 10 5 altoCle ESIHS Jones and Konrad Kundog of eOA also 5uppl'ed valuable leehn,eal bae<grQund male"al. In add,toOfl. Susan W"son Assoe>ates. ~ember RestoratoOfl Coord,nat'"9 Comm,nee provoded e$Senl'.' ",lormatoOfl and personal ralerances for the camp, laton Of "aItOCle

COPPER SKIN

The red copper sheets forming the statue's exterior are passivated by a barrier coating of 3Cu(OH2 . CUS04 (brochantite). In certain small areas, pitch based material, used. to water­proof the inside of the statue at the turn of the century, seeped through the lapped seams of the sheets and dripped down onto the exterior skin. From comparative measurements of patinated and pitch covered copper, a copper loss on the order of .005 in 100 years has been estimated.4

The statue's brochantite conversion coating was idellfified by x-ray analysis in 1963.5 It is the familiar greenish residue that protected copper roofing and other structures for centuries before the designer of "Liberty" chose the fire­refined copper material for the exterior sheeting. More recent tests on the patina in 1980 identified a second cop-. per sulfate compound 2Cu(OH)2 . CUS04 (antlerite), which is more water soluable and inherently less protec­tive than the brochantite.6 This chemi­cal reversion to a less stable com­pound is speculated by some to be related to the acid rain in New York City (typical 3.5 pH measurement in summer rain). The National Park Serv­ice plans to ultrasonically monitor fur­ther material loss, which might neces­sitate the future application of a corro­sion inhibitor.

Copper sulfate patina has a tightly adhering onion skin structure. How­ever, in certain highly ornate areas of the torch, investigators observed po­rosity in the patina. In addition, there were fine cracks in the copper which

JOURNAL OF METALS· February 1985

may either be due to stress corrosion due to excessive plastic deformation during working of the high oxide con­tent copper 100 years ago. Micro­scopic examination revealed severe pitting and granulation of the conver­sion coating on the torch.

The torch is 10 years older than the rest of the statue and began patinizing in a different environment, having been erected outdoors at the Centennial Exposition in Philadelphia in 1876-77 and subsequently in New York's Madison Square during 1877-84. The torch is not water tight. It has suffered severe weathering and its re­placement is deemed necessary by the National Park Service. The new torch is made of a commercially prev­alent copper grade (C11000, electro­lytic tough pitch H01 temper). It may be artificially patinized to preserve the aesthetic match with the rest of the monument. Other sheets that must be replaced are to be made from the same grade of copper, ordered to ASTM 8152-83.

SUPPORT SYSTEM

The most serious deterioration is not due to the salt spray effects on the skin, but rather to the galvanic action between the wrought iron supporting bands (armature) and the copper.

The folds in the robe of the statue are designed to impart rigidity to the .100 inch thick copper envelope sheeting. During manufacture, these sheets were molded onto wood molds or mandrels by cold working, dramati­cally increasing their tensile properties, through strain hardening. In addition,

. ••

Micrograph (250X) of the corroded copper skin with a heavy patina coating. Note also, the presence of numerous oxide particles in the fire refined copper matrix.

%" thick reinforcing puddled iron bars were bent to contour to impart rigidity by attachment to the superstructure This armature was fastened to the skin by means of copper saddles, fastened to the skin with C11 000 cop­per rivets. Nineteenth century design­ers recognized electro-potential differ­ences between the iron and copper and attempted to separate the two materials with strips of leather and other materials impregnated with shellac. This design also left enough clearance to accommodate differential thermal expansion. However, conden­sation collecting in joint crevices and moisture in pockets of the deteriorat­ing waterproof coating inside the statue permitted conditions of accelerated galvanic attack to develop. The resulting corrosion layer on the iron armature forced detachment of the riv­ets that fastened saddles to the skin. In many places, the iron banding was severely rusted, threatening skin distortion. Consequently, the entire ar­mature structure must be replaced.

The National Park Service consid­ered several alloys for this purpose. Commercial aluminum and manganese bronzes, as well as a 70-30 copper nickel alloy C71500, were examined for their electrochemical compatability with the copper skin.? High strength Ferralium 255® was also tested for corrosion behavior relative to copper.s Finally, the National Park Service se­lected a commercially available 316L

The interior stainless steel armature con­forms to contours of the statue's copper skin, and stiffens it against wind forces. In the new design, the copper saddles connecting the armature to the copper skin will be fastened to the skin with flush cop­per rivets.

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permanent / / warping of the envelope

: condensation • waters

tearing off of the

--!:::> saddle on the more corroded side

To allow for the difference in coefficients of thermal expansion between copper and iron, the copper envelope was secured to the ribs through riveted saddles. The sad­dles were designed to permit the ribs to slide freely. Unfortunately, this ingenious system no longer .functions. Despite pre­cautions taken originally to isolate the iron from the copper by means of a layer of insulating material, the iron came into con­tact with copper with an anodic reaction. Under the thrust of the resulting exfoliating rust, roughly one third of the saddles (about 600) and their rivets have been totally or partially pulled off from the cop­per envelope, leaving holes in the copper skin. Another third of the saddles is in uncertain condition.

stainless grade for armature bars and Ferralium 225 for flat bars which span between the armature and the sec­ondary frame. To verify alloy selection, samples of all the alloys proposed, as well as wrought iron in "couple" ar­rangement to simulate the saddle-skin armature connections are exposed at the Kure Beach, N.C. outdoor expo­sure station. To date, two sample re­trievals' show no corrosion of copper or 316L or Ferralium.

In the original statue fabrication, puddled iron was chosen despite the availability of mild steels at the time. The National Park Service's choice of 316L stainless, the material used for many years to replace parts in the human body, was based on three criteria. First, this austenitic stainless grade has about the same elastic modulus to density ratio as the origi-

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nal puddled iron. This enables replace­ment with identical sized sections. The 316L has a low enough yield strength (42ksi) to facilitate duplication of the intricate armature configurations at a fabrication shop located on Liberty Island.9 Second, the thermal expan­sion characteristics of the stainless are somewhat better matched to copper than the original iron (8.9 x 10-6 in/in OF OFHC copper). Furthermore, a Tef­lon® coating can be installed within the saddle-armature connections to permit some movement during skin expansion. Finally, the stainless is electrolytically compatible in contact with copper of the similar relative area when exposed to a humid-air chloride environment. Tests showed that a pos­sibility of polarization occurred be­tween copper-stainless when co'upled in a liquid solution, with the 316L exhibiting an increasingly cathodic be­havior as the pH of the test solution decreased.1o However, these extreme­ly acidic solutions are also corrosive to the copper alone and do not repre­sent the normal environment inside the statue.

To facilitate armature replacement, the restorers removed 11,000 square feet of waterproofing materials from the interior skin. Liquid nitrogen was used to crack the tar and heavy paint coating.11 Blasting with sodium bicar­bonate abrasive will remove the remaining paint. A special corrosion inhibitor, benzotriazole, can be applied to prolong the cleaned copper appear­ance inside the statue and deter fu­ture crevice corrosion.

Other aspects of the current work will help preserve the interior surface of the copper skin by reducing the moisture available to support corro­sion. The lapped seams of the skin will be made more weather-resistant with silicone building sealant. New me­chanical ventilation will reduce the hu­midity of the interior environment. The exterior surface of the statue will re­main untreated, and sites will be es­tablished for monitoring corrosion by non-destructive techniques.

STRUCTURAL Fabricators specializing in restora­

tion work are shaping 1,200 individual armature bars (18mm x 50mm) of donated 316L stainless. 12 In some places the bars must be cold formed to incorporate 30° angles. The bars are then annealed using a direct cur­rent technique, thus eliminating any stress corrosion problems. Bend test data showed appreciable work harden­ing of this material (12 pt RB).13

In many places, the secondary frame bars are incorrectly positioned to bear the weight of the staircases and copper envelope. Some bars have

Earlier attempts to repair the leaking frame of the torch and flame have actually worsened the condition. For example, steel screws driven through the copper sheath­ing are now rusting and staining the exterior. Traces of paint, putty, and other materials also disfigure the surface.

been drilled or cut and the reduced cross section impairs their load bear­ing capacity. Structural replacement and reinforcement of the secondary supporting framework utilizes Ferralium 255.

Among several modifications intend­ed to enhance visitor access and appreciation, the engineers also plan to undertake strengthening measures in two areas. 14 First, the torch arm of the statue, originally erected about two feet off its design location, will be rein­forced with special stiffening members. In the original construction the struc­ture of the arm was not directly con­nected to the central pylon. Therefore, hoop stresses were resolved in such a way as to overstress some mem­bers and leave others completely unstressed. The condition of the torch permitted rain penetration, which ac­celerated corrosion of the framework and increased the danger from fatigue failure. As part of the restoration, gages will be installed to monitor the cyclic stresses associated with wind conditions. 15

The neck of the statue will undergo

The convulated form of the statue's flame provides pockets where moisture can collect. This complex form-plus ill-chosen earlier repair-prevented copper from devel­oping its high copper's natural corrosion resistance.

JOURNAL OF METALS· February 1985

Using a wand especially designed by the Linde Division of Union Carbide Corporation, a worker sprays liquid nitrogen onto the paiJlted interior surface of the statue. The dark surface area is coal tar, from which the paint has been removed. The light area is the painted surface. The fog near the work area is primarily water vapor in the air which condenses when hit by the ex­treme cold of the liquid nitrogen.

a similar reinforcement. Again, OrIgI­nal members will be preserved in place and made structurally adequate with new supplementary members. Other structural work includes tighten­ing and heat-straightening of the origi­nal guy rods that stabilize the central pylon and repairing conflicts between the guy rods and the lattice girders that support the base of the statue.

CONCLUSION Although somewhat paradoxical for

so major a preservation project, the statue is in generally good condition considering its mix of materials and environment, but restoration of even a small proportion of so great a struc­ture results in a tremendous amount of work. And of portions requiring im­mediate attention, the bulk of the res­toration work is concerned with the replacement or reinforcement of fer­rous materials; the copper skin is in generally excellent condition.

The statue's success during the first 100 years is a tribute to French artist­ry and ingenuity. The work now under­way is intended to satisfy the statue's structural and metallurgical needs for at least the next 200 years. From the care and attention demonstrated by the restoration team in research, de­sign and execution, this estimate is probably most conservative.

ACKNOWLEDGEMENTS 1. Design work is being completed by

the French-American Consulting Team under the sponsorship of the French-American Committee for Restoration of the Statue of Liberty­Ellis Island Foundation, Inc. The following firms comprise the Con­sulting Team: France Architect: G. Ph. Grandjean Engineers-Advisors: J. Levron, J.

Moutard, P. Tissier United States Architect: Swanke Hayden Connell

Architects Associate Architect: The Office of Thierry W. Despont

Structural Analysis: CETIM Engineers: Ammann & Whitney

2. U.S. Department of the Interior, Na­tional Park Service E. Blaine Cliver-Chief, North

JOURNAL OF METALS • February 1985

Atlantic Historic Preservation Center John Robbins-Historical Architect

3. The Statue of Liberty - Ellis Is­land Foundation, Inc. F. Ross Holland, Jr.-Director, Restoration and Preservation

4. Norman A. Nielsen-Consulting Metallurgist and Research Fellow E.I. duPont de Nemours & Co. (retired)

5. D. H. Osburn, "The Truth About Miss Liberty", Materials in Design Engineering, June, 1963

6. Norman A. Nielsen-Research at duPont de Nemours & Co., Engi­neering Technology Laboratory

7. International Copper Research Association, Inc. Toni Tabak-Senior Applications Engineer

8. Cabot Corporation-Wrought Prod­ucts Division A.I. Asphahani-Director of Tech­nology, Kokomo, Ind. N. Sridhar-Engineer, Technolo­gy Division, Kokomo, Ind.

9. Texas Instruments, Inc. R. Baboian-Head of the Electro­chemical and Corrosion Laboratory

10. Nab/Fiebiger-Joint Venture Nab Construction Co. and R. A. Fie­biger, Inc.

11. Linde Division of Union Carbide Corporation

12. Arm & Hammer Division of Church & Dwight

13. Members of Specialty Steel In­dustry of US Altech Specialty Steel Corporation

Armco Specialty Steels Division Carpenter Technology Corporation Crucible Specialty Metals Division Cyclops Corporation Joslyn Stainless Steel Division Repubic Steel Corporation Slater Steel Inc.

14. Norman A. Nielsen-tests at Abbott Mfg. Co., NorwOOd, New Jersey

15. Ammann & Whitney Consulting Engineers, New York

If you want more information on this subject, please circle reader service card number 212.

The shape of the old armature structure (made of puddled iron) is being copied exactly by skilled craftsmen. The new ar­mature will be made from flat bars of stain­less steel.

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