A Survey of Historic Finishes for Architectural Aluminum, 1920 –1960

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A Survey of Xsusha Flandro and Helen M. Thomas- Historic Finishes Haney for Architectural Aluminum, 1920 –1960

Fig. 1. Satin finish is in the upper left Technical papers published in 1883 presented corner and right side. Sandblasted finish is on the upper right and aluminum as “the metal of the future,” stating that middle. A replicated deplated “aluminum, we are satisfied, is the coming metal, destined one day to play as finish is on the bottom. Samples are prominent a part in the arts of civilization, perhaps, as iron.”1 This statement from a subway station in New York, was realized fifty years later, when aluminum for architectural use was hailed as New York. Images by the authors a modern marvel, a new wonder material capable of withstanding weathering unless otherwise noted. and structural stresses. Prized for its light weight and white color, aluminum has been used for a wide variety of elements, including architectural features such as windows, spandrel panels, and other decorative elements. Aluminum may be used without a finish or protective coat, as it forms a natural protective oxide layer on its surface (Fig. 1). The spandrel panels in New York City’s Rockefeller This article looks at Center are unfinished aluminum. However, most often a supplementary coating historic aluminum finishes or finish is applied to impart corrosion protection while establishing a desired surface appearance. Recognizing these finishes and how they are applied will aid and conservation issues in making appropriate restoration decisions. This survey is limited to popular affecting each finish. architectural aluminum finishes utilized in the United States from the 1930s to the 1960s.

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Fig. 2. Cross sections illustrating the different types of corrosion in an aluminum copper alloy. From The Surface Treatment and Finishing of Aluminum and Its Alloys, p. 18.

Fig. 3. Finishing techniques for aluminum. From The Surface Treatment and Finishing of Aluminum and Its Alloys, p. 43.

Aluminum Production Sainte-Claire Deville refined the process silicon and copper, is required to in- and was able to produce a full bar of crease strength.5 Aluminum alloys can Compared to other metals, the discovery aluminum for the Éxposition Univer- be either wrought or cast. Wrought and use of aluminum is very recent. selle in Paris. Due to the difficulties in products can be divided into two Common metals such as copper and extracting aluminum, it was treated groups: standardized and engineered. iron are ancient. Copper was discovered like a precious metal, more costly than Standardized products include sheet, c. 9000 BCE, and iron was discovered gold or platinum, and its use was lim- plate, foil, rod, bar, wire, pipe, and c. 5000 BCE. Aluminum, on the other ited to novelties and jewelry.2 In 1886 structural forms. Engineered wrought hand, was not discovered until 1825. Charles Martin Hall in America and products include extruded shapes, Aluminum is the third most abundant Paul Héroult in France independently forgings, and impacts. Cast-aluminum element on Earth, after oxygen and discovered a low-cost extraction method products are produced by pressure-die, silicon. Like most metals, it never oc- that is still used today. Called the Hall- permanent-mold, green- and dry-sand, 6 curs in nature as the pure element. Pure Héroult process, it involves dissolving and plaster casting. aluminum is extracted from its naturally aluminum oxide in crylolite and then occurring ore, bauxite. Aluminum was passing an electrical current through 3 Aluminum Corrosion first processed in 1825 by Danish sci- the molten solution. By the turn of the entist Hans Christian Oerstead using a twentieth century, more than 6,800 tons Pure aluminum, when exposed to the 4 chemical reduction method, which was of aluminum were produced annually. atmosphere, is immediately covered with a thin oxide film of hydrated alu- further developed by German chem- Pure aluminum is not used for archi- minum oxide.7 This natural oxide film ist Fredrich Wohler. Both methods tectural applications as its mechanical is between 0.005 and 0.015 microns were difficult and yielded only small strength is too low. Therefore, deliber- thick. It can appear on the surface as a amounts of aluminum. In 1855 Henri ate alloying with other metals, typically

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white powder. While the oxide film is Table 1. Common Types of Aluminum Corrosion protective under ideal conditions, the breakdown of the film is accelerated by Corrosion Type Mechanism for Corrosion salts (anions) present in mortar, rainwa- Galvanic Corrosion Galvanic corrosion is a form of localized electrochemical ter, seawater, guano, and deicing salts; corrosion that occurs between two metals. In the presence through contact with materials such as of an electrolyte, which creates a closed circuit, the least masonry and other metals; and by sur- noble of the two metals will become the anode and face agitation and preferential runoff.8 corrode preferentially. Aluminum is less noble than most commonly used architectural metals, including steel, There are many different corrosion copper, lead, stainless steel, and bronze. The use of these mechanisms for aluminum alloys. For metals in direct contact with aluminum will cause the each, the process is complex, incorporat- aluminum to corrode preferentially. Galvanic corrosion ing many factors, and varying according occurs most frequently where ferrous fasteners or framing to the metal and specific operating con- systems are utilized and where lead-containing primers ditions.9 The most common types of cor- are applied directly to the metal. The two dissimilar rosion encountered in architectural alu- metals should be isolated either by a coating, such as a minum include galvanic attack, pitting, bituminous paint, or rubber gaskets. intergranular corrosion, exfoliation, and filiform (Table 1 and Fig. 2). Intergranular Corrosion, Intergranular corrosion, pitting, and exfoliation are Pitting, and Exfoliation symptoms of galvanic corrosion occurring within the aluminum alloy itself. Alloys containing copper have poor Finishes corrosion resistance. Intergranular corrosion occurs along Between the 1920s and the 1960s ar- the crystalline boundaries between the alloyed metals, chitectural aluminum was finished in a manifesting as microcracking invisible to the naked eye. variety of surface treatments, including Intergranular corrosion is usually not evident until pitting mechanical, chemical-conversion coat- occurs. Pitting appears as small divots, usually filled ings, electroplating, electrolytic conver- with a fine white powder or gel, and is present in the sion coatings, and organic finishes — all exposed surfaces of the aluminum. Exfoliation is a type of which continue to be used today. of advanced intergranular corrosion that occurs parallel The finishing of aluminum was done to the surface of the aluminum and along the grain to impart corrosion resistance and/ boundaries, giving it a layered appearance. or a specific appearance. Mechanical Filiform Corrosion Filiform corrosion appears as worm-like channels finishes remove the oxide layer; conver- under an organic coating on aluminum. It usually sion treatments and anodizing change propagates from a site where the protective coating has the oxide layer; and organic finishes are been compromised. The organic coating separates from coatings applied over the oxide layer the substrate and is pushed up by the aluminum oxide (Fig. 3). corrosion products. It is often misidentified as a cosmetic problem only and is left untreated. Not addressed, it can Mechanical finishes. Mechanical fin- rapidly develop into pitting and intergranular corrosion. ishes were the first finishes used on aluminum. Mechanical finishing is usually the first step undertaken after fabrica- original finish. Abrasion and ultravio- and satin, which can be found on build- tion to eliminate scratches, pits, or other let degradation are common causes of ings from the 1930s. superficial blemishes resulting from the lacquer failure. Failure of the lacquer Sandblasted. A sandblasted finish is manufacturing process. These types of coating encourages corrosion of the achieved by bombarding the aluminum finishes are achieved by various grinding aluminum substrate and deterioration of surface with an abrasive medium under and polishing operations that remove the mechanical finish. the high spots of rough surfaces by us- pressure to create a matte, even, rough, ing the cutting action of the abrasive. A Mechanical finishes are easily replicated pocked surface.11 The texture of the mechanical finish leaves the aluminum in the field. Degraded lacquers must surface is determined by the blast media without a protective oxide layer; there- first be removed using solvent-based used, the air pressure employed, and the fore, it is employed as a preparation for strippers before the mechanical finish is distance and angle of the blast nozzle anodizing or the application of organic reinstated. The new mechanical finish from the work.12 Silica sand was tradi- coatings.10 must again be protected with a lacquer tionally used; however, other abrasive coating. media include alumina, emery, steel par- As the mechanical finish is often the ticles, corn cobs, rice hulls, and walnut Common mechanical finishes on archi- desired finish, a clear lacquer coating shells.13 must be applied. Proper maintenance of tectural aluminum include sandblasted the coatings is essential to preserve the 15 APT BULLETIN JOURNAL OF PRESERVATION TECHNOLOGY / 46:1 2015

conversion coatings is the speed and economy at which they can be ex- ecuted.20 The presence of the chemical-conversion layer can be identified using a scanning electron microscope or through metallurgical analysis of a cross section of the metal; however, in addition there has been limited success with determining the chemical process used to create the conversion layer using X-ray photoelec- tron spectroscopy.21 Conservation of conversion coatings Fig. 4. Cross section of electroplating lel lines into an aluminum surface by includes removing porous oxides and over an anodized surface. From The scratching it with successively finer sealing the surface. Harsh abrasives, like Surface Treatment and Finishing of abrasives or rotating wire brushes until glass beads and pressures higher than Aluminum and Its Alloys, p. 595. a soft, smooth sheen is achieved.15 100 psi, remove the oxide coatings. However, softer plastic abrasives and Fig. 5. An anodic oxide layer. After the use of number 180 emery, the low pressures (15 psi) can remove the surfaces are buffed with muslin or felt porous oxides while leaving the conver- wheels and a greaseless polishing com- sion coating intact.22 pound. Various grades of compounds give a coarser or finer finish, depending Severe deterioration of a chemical-con- on the grain size of the abrasive conversion coating may require removing tained in the compounds.16 Again, this all remnants of the original coating and finish must be protected from corro- re-treating the surface. Most chemical- sion by an anodized or applied coating. conversion treatments rely on submers- Satin finishes can be seen on the First ing the metal in a bath, which may not National Bank Building (now called be possible for large architectural pieces. the First National Center), located in Applied conversion coatings may be vi- Oklahoma City, Oklahoma, and the 4th able options. There are several types of Avenue station in New York City. chemical-conversion treatments that are performed.

Chemical treatments and conversion Modified Bauer-Vogel (MBV) and Al- Chemical-conversion coat- coatings. rock processes. The first chemical oxide The sandblasted finishes are divided ings alter the aluminum surface through treatment was developed in 1915 by O. into three grades: coarse, medium, and either immersing or spraying the surface Bauer and O. Vogel in Berlin, Germany, fine. Heavy abrasive blasting can warp with a chemical solution. Chemical- where the aluminum piece was sub- thin sections of aluminum. The finer conversion procedures were patented merged in a bath of potassium carbon- finishes are often used decoratively or as in Germany as early as 1915 but are a ate, sodium bicarbonate, and potassium a preparation for satin-finished work, as topic whose history requires additional dichromate for two to four hours at the milder abrasive action reduces the research.17 In some cases, chemical 14 temperatures between 90° to 95°C, then subsequent grinding or polishing. The treatments etch the surface, but in oth- rinsed and dried. A light gray iridescent surface is then protected from atmo- ers, specialized films are formed.18 Some film is produced, which turns dark gray spheric corrosion by an oxide treatment of the most important types of chemical with longer exposure in the bath. This or coating. Sandblasted finishes can be treatments are intended to thicken the film can also be dyed. This treatment seen on the entrance doors to the Alcoa natural oxide layer present on the alu- was ultimately patented in 1923.23 Research Laboratories (now referred minum surfaces.19 to as A-K Research Park) located in In 1930 Dr. Gustav Eckert altered the In the United States, chemical-conver- Kensington, Pennsylvania, as well as the process using sodium carbonate and sion coatings were used to prepare the 4th Avenue subway station at 9th Street sodium chromate, making it easier aluminum surface for an additional and the Broadway Fashion Building at and faster to complete in three to five treatment, such as an organic finish or Broadway and West 84th Street in New minutes. It was renamed the Modified electroplating. The conversion coating York City. Bauer-Vogel Process, or MBV Process. is inert and unreactive and helps inhibit Depending on the alloy of aluminum, a Satin finish. A satin finish is produced corrosion if the secondary coating is light to dark gray coating is produced by imparting a series of minute paral- damaged. The main benefit of chemical-

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on the surface, which makes a suitable Fig. 6. Empire State Building, New base for an organic coating. A version of York City, designed by William F. this same process was patented by Alcoa Lamb and built between 1929 and and named Alrock.24 1931. Aluminum spandrel panels were designed with a deplated Caustic etch or frosted finishes. Frosted finish. finish or caustic etch is a decorative silvery-white finish chemically produced Fig. 7. Empire State Building, on aluminum. Caustic etching is derived deplated aluminum spandrels. from metal pickling, which utilizes a Some of the finish has deteriorated strong acid to remove surface impurities and the aluminum is corroded, from metal. However, unlike pickling, a but it appears that the dark strong base is used to perform a caustic gray deplated finish was in the etch. Sources as early as 1909 mention recessed portions of the design. caustic etching or caustic dipping as a finishing treatment for aluminum but caution that it does not result in an even appearance; if an even appearance were desired, a mechanical satin finish was to be utilized instead.25 Caustic etching removes the natural oxide film from the surface of the aluminum, preparing the surface for plating. The specimen is immersed in a 180°F solution of sodium hydroxide for approximately one minute. The piece is then rinsed with cold water and immersed in a bath of nitric acid to remove the dark smut deposited by the alkaline rosive environments such as door knobs solution and to neutralize the surface. and hardware.33 Finally, the metal is rinsed again with Chemical polishing. Chemical polish- cold water.26 ing is a process by which aluminum is There are four basic types of electroplat- given a bright, reflective finish without ing: plating over another base metal, Phosphatizing, Parkerizing, and Bonder- mechanical polishing. Developed by the direct plating, plating over chemically izing. Phosphatizing, or the phosphate Battelle Memorial Institute of Colum- etched surfaces, and plating over an- process, forms a thin, inert phosphate bus, Ohio, in 1948, the aluminum pieces odic and chemical oxide coatings. For coating on a base metal. The original are dipped in acid baths containing a all types the surface of the aluminum is process was first applied to steel in 1859 prepared by first removing or thickening 27 mixture of typically phosphoric, nitric, and was patented in 1906. It is used and acetic acid at temperatures between the natural oxide film through chemical mainly to prepare aluminum surfaces for room temperature and 200°F for 10 sec- etching, anodizing, chemical conver- painting, anodizing, and electroplating. onds to 10 minutes. Following dipping, sion, or zinc immersion/disposition and This treatment is accomplished by im- the pieces are rinsed and dried. The then buffing the surface to remove any mersing or spraying the aluminum with resulting surface can either be plated or scratches or unevenness and thoroughly a 5 to 10% solution of phosphoric acid left as is.30 cleaning the surface of contaminants. at room temperature. The aluminum is The piece is then dipped in the electro- left in the bath/spray for approximately plating bath and a current is applied, 28 Electroplating. Electroplating uses five minutes, then rinsed and dried. electrical currents to form a coherent thereby depositing the desired metallic finish on the surface. The current am- The process was further refined by the metal coating on another metal. Modern perage and the bath contents are depen- Parker Company in 1918, when manga- electroplating started with the opening dent on the metal to be deposited on the nous phosphate was added to the bath, of the first electroplating plant, Nordde- aluminum.34 a process known as Parkerizing. In the utsche Affinerie, in Hamburg, Germany, 31 1940s Bonderizing or Bonderite 170 was in 1876. By 1894 experimentation was Successful electroplating of aluminum developed; in this process a complex taking place in the United States to plate is difficult because of aluminum’s im- 32 fluoride is used to accelerate the film steel with aluminum. However, elec- mediate oxide film formation, which formation of a crystalline iron-metallic troplating of aluminum did not become can reoccur during the surface prepara- phosphate film on the aluminum.29 common until the mid-1930s, when it tion stage; aluminum’s position in the was used mainly for small household electrochemical series, which can easily items and interior elements in noncor- 17 APT BULLETIN JOURNAL OF PRESERVATION TECHNOLOGY / 46:1 2015

Failure is manifested as blistering fol- The aluminum is first vapor-degreased lowed by exfoliation of the plating.36 and cleaned in a phosphate bath. The metal is then placed in the plating bath Before treatment, proper identification and a current is applied.38 of the base aluminum alloy, the base metal, and plating metal is required. Plating over anodized or chemically Analytical techniques, such as X-ray oxidized surfaces. Plating can be per- fluorescence (XRF), metallurgical analy- formed over an anodic or chemically sis, and cross-section microscopy, can oxidized surface. Similar to plating over be performed. Subsequent treatments a chemically etched surface, the anodic should be based on what is best for the or chemically oxidized layer provides a element and its surrounding compo- key onto which the deposit metal can nents. If the item cannot be maintained hold. The success of the plating depends in a controlled environment, and the on the quality of the oxidized layer (Fig. plating has failed (blistering or exfolia- 4).39 tion of the plating metal), it may be necessary to replate the item. Electrolytic oxide coatings. An important and once-revolutionary finish for aluminum is the electrolytic oxide coating, or anodizing.40 Anodizing aluminum on an industrial scale was first performed in 1923 to protect of Duralu- min sea planes, but it was not used for architectural elements until the 1950s.41 Today, anodizing is used to treat a wide array of aluminum products from architectural elements to jewelry. The oxide finishes are created by treating the metal anodically in various electrolyte baths to obtain a thin, inert, and durable aluminum oxide film on the metal surface. The oxide film is very Fig. 8. U.S. Plywood Building, 777 hard and offers excellent abrasion and Third Avenue, New York City, Plating over a base metal. Electroplat- corrosion resistance for the underlying designed by William Lescaze and ing over another deposited base metal aluminum. Some of the films are clear built between 1961 and 1963. The (usually zinc) is the most common and transparent; others are colored facade is clad in glass and dark method of electroplating aluminum in in varying degrees of brown, gray, or anodized aluminum. the United States, most often to deposit silver. The color and hardness of the chrome onto aluminum items. The alu- film will depend somewhat on the type Fig. 9. U.S. Plywood Building, dark minum surface is cleaned and then im- of aluminum alloy anodized. The films anodized aluminum. mersed in a zincate solution to produce possess good absorption qualities for a very thin coating of pure zinc on the dyes and paints since most are minutely form immersion deposits in plating surface. porous.42 In anodizing, aluminum is the solutions; and the differences in coef- anode. It is placed within a bath with In the United Kingdom, most electro- ficients of expansion between aluminum an electrolyte (an acid) capable of yield- plating employed the Vogt process and its alloys.35 ing oxygen on electrolysis. A cathode (named after the developer, B. Vogt). is mounted inside the anodizing tank. Because aluminum is anodic to most The Vogt process relies on electrodis- When the electric current is passed, oxy- coating metals, the corrosion resistance persion of the zinc onto the surface of gen ions are released from the electro- imparted by the deposited metallic the metal instead of immersion. The lyte and an oxide coating is formed on coating can be drastically decreased if consistent zinc layer produced by this the surface of the aluminum. Progressive there are discontinuities in the coating. process is particularly important when oxidation takes place beneath the film If chemical etching solutions are not electroplated items are to be subjected just formed: that is, the film formed last properly rinsed from the surface prior to cold-working and heat treatment.37 is against the metal surface, while the to plating, a galvanic cell can form film formed first is the outermost layer. between the aluminum and the plating Direct plating. Direct plating involves The outer part of the film is less dense, metal. plating aluminum with another metal — usually copper, chromium, tin, or zinc. since it has been subjected to the most

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severe solvent action of the electrolyte. coating. The aluminum surfaces are The transparent films produced by the The aluminum oxide film is also slightly first sandblasted, followed by anodic sulfuric acid process make it the ideal porous. Thus, the current is able to pass oxidation by electrolytic treatment in a process for subsequent color dying.56 through the electrolyte and into the sulfuric acid bath of low concentration, The U.S. Plywood Building in New pores of the metal without much resis- to produce a very dark gray color, then York City contains dark, possibly origi- tance.43 Due to the porosity of the oxide coated with clear lacquer.49 The term nally brown, anodized aluminum piers film, dyescan be applied before the final disappears from use by the 1940s. It is and belt courses under the windows sealing stage (Fig. 5). After treatment, likely that the term and the concept of (Figs. 8 and 9). Films produced with the anodic coating is “sealed” by sub- deplating fell under the process of anod- sulfuric acid are thicker than those pro- merging the piece in boiling water. This izing. duced with chromic acid.57 The sulfuric treatment converts the aluminum oxide acid process is not recommended for use The Empire State Building, completed coating into the monohydrate as shown with assembled parts subject to stress or in 1931, contains aluminum window in the following formula: Al O + H O containing recesses or joints, as retained 2 3 2 spandrels finished with a dark gray “de- ➝ Al O •H O. The monohydrate has traces of sulfuric acid may cause cor- 2 3 2 plated” finish (Figs. 6 and 7).50 an increased volume, which essentially rosion.58 The escalators located in the closes the pores and destroys the ab- Chromic acid. The chromic acid process Marshall Field’s store in Chicago have 44 sorptive characteristics of the coating. was first patented by G. D. Bengough an Alumilite finish. and J. M. Stuart in 1923, and they are Cleaning anodized architectural alu- Electropolishing/Brytal process. Elec- minum is very important to retain the tropolishing, while referenced as early finish. If dirt is allowed to remain on Coatings provide protec- as 1912 for silver, did not become a the surface, corrosion cells can form. systematic treatment for aluminum until Anodized finishes on accessible surfaces tion either by creating a after World War II.59 Electropolish- that are maintained, e.g. shop fronts, physical barrier from the ing results in a reflective surface on the window frames, etc., have been shown aluminum. The degree of reflectivity to have an appreciably longer life than atmosphere or by active depends on the purity of the alloy. This surfaces that are cleaned infrequently. corrosion inhibition pro- technique essentially removes any rough Maintenance-cleaning products with edges of the metal, thereby lessening abrasives are unsuitable for anodized vided by pigments in the the differences between the valleys and aluminum, as they will damage the peaks on the surface. Electropolish- coating.45 However, if the anodized alu- coating. ing, also known by the trade name the minum has been neglected, heavy-duty Brytal process (developed by the Brit- restoration cleaning will be required. ish Aluminum Company in the early recognized as being the inventors of Abrasive cleaning methods can be em- 1950s), consists of degreasing the alumi- anodizing.51 The chromic-acid process ployed to remove heavy pollutants, num and then immersing it in a carbon- produces a thin film particularly well- contaminants, and oxidation, followed ate/trisodium phosphate solution for 30 46 suited to the anodizing of assembled by the application of a coating. The seconds or less to remove the natural parts as the corrosion resistance of the coating can be clear or colored to match oxide film.60 An electrical current is then aluminum surfaces is not appreciably the original anodized layer. applied, and the voltage slowly raised affected by any chromic acid trapped in and lowered. This process produces a If the piece can be removed or disassem- the joints. The film also serves as an ex- thin film on the surface of the metal vis- bled, the worn anodized coating can be cellent paint base.52 Depending upon the ible as interference colors. The film is abrasively stripped and reanodized. Un- alloy used, chromic acid films vary from usually reinforced by anodizing.61 Elec- fortunately, this process removes metal light gray on pure aluminum to dark tropolished and Brytal process-finished and can affect profiles and details.47 purple-gray on alloys rich in silicon.53 items are usually found as smaller archi- Although aluminum may be anodized in Sulfuric acid. The sulfuric acid process tectural elements, such as hardware or phosphoric, boric, sulfamic, oxalic, and is the most common anodizing process sculpture. other acids, the chromic and sulfuric performed on aluminum. The first sulfu- acid processes are the most common. ric acid anodizing process was patented Organic coatings. Aluminum is often The chromic and sulfuric acid processes in England by C. H. R. Gower and Staf- painted both for protection against are similar, but employ different electro- ford O’Brien in 1927.54 Alcoa (Alumi- corrosion and for decorative purposes. lyte concentrations, bath temperatures, num Company of America) patented its Coatings provide protection either by voltage, and exposure times. own process called Alumilite in 1923.55 creating a physical barrier from the The oxide films vary from clear and atmosphere or by active corrosion in- Deplating. The term “deplated” ap- transparent to translucent and opaque, hibition provided by pigments in the pears in books and references from the 62 depending on the alloy composition, coating. Organic finishes are often 1930s.48 The deplated finish is a dark timing, and solution concentrations. popular, due to the low cost of modern gray, electrolytically formed oxide application techniques and their ability

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mechanically or chemically roughening resistant to corrosion, and an entire surfaces, or applying a suitable primer, chapter in Alcoa’s 1932 book entitled such as zinc chromate or titanium diox- Aluminum in Architecture is devoted to ide.63 Coatings are applied to aluminum ntenance.65 Deterioration of protective in the same manner as to other metals, finishes can cause the underlying alumi- including brushing, dipping, spraying, num to corrode. Conversely, corrosion electrostatic spraying, roller coating, of the underlying aluminum can cause and thermal spray. the finishes to fail. Alkyd, vinyl alkyd, vinyl, and thermo- Restoring deteriorated aluminum fin- setting acrylic paints have been applied ishes in situ can present several chal- to aluminum awnings and curtain walls. lenges and requires some creative Baked enamels have been applied to alu- thinking. Severe corrosion, layers of minum panels to provide vibrant color, over-painting, and inappropriate repairs such as on the McGraw-Hill Building can obscure the original finish. Several in New York City, constructed in 1931 finishes are created in a shop by dipping (Figs. 10 and 11). Bituminous paints, the aluminum piece in an oxidizing bath which are made from asphalt, driers and and sometimes by also introducing an drying oils in a solvent, have been ap- electric current. Often the aluminum plied to aluminum to protect it against structure or element cannot be disman- highly corrosive conditions, including tled and refinished off-site in a shop. embedment in soil or concrete, or im- Before any restoration or conservation mersion in salt water. Clear lacquers are efforts are undertaken, technical and most often applied over mechanically historic information must be combined polished or anodized surfaces to pre- with accurate material analysis in order ventoxidation from altering the appear- to develop appropriate conservation ance of the finish. strategies.66 Documentary research, on- Proper maintenance of the coating is site investigation, and laboratory exami- essential to preserve the original finish. nation of samples must be performed to Abrasion and ultraviolet degradation identify the type of aluminum alloy used Fig. 10. McGraw-Hill Building, New are common modes of failure. Failure of and the original finish installed. Armed York City, designed by Raymond the coating encourages corrosion of the with the knowledge of the original Hood and built in 1931. The first- aluminum substrate and deterioration of finishing technique and the limitations floor entrance is decorated with the finish. After addressing any underly- of field conditions, decisions regard- black and blue bands of enameled ing corrosion issues and completing a ing conservation and replication can aluminum panels. paint analysis to determine original col- be made. However, the best method of ors, organic finishes can be reinstated to preservation and conservation of these Fig. 11. McGraw-Hill Building, black match the historic appearance. finishes is appropriate periodic mainte- and blue bands of enameled nance, including gentle cleaning to pre- aluminum panels. Conclusion vent abrasion of the coatings or oxide films and limiting access to water, salts, to produce smooth, lustrous surfaces Finishing techniques provide both masonry, and strong acids or alkalis. without polishing. performance and aesthetic benefits for aluminum and its alloys. They serve the Xsusha Flandro is a senior conservator Various types of coatings can be easily dual purpose of increasing the corrosion at Jablonski Building Conservation. She applied to aluminum. Coating formu- resistance of the metal and achieving a holds a master’s in historic preservation lations change almost daily; however desired surface appearance. This paper from Columbia University and a BFA traditional coating materials include provides an overview of the common in ceramics and sculpture from the Uni- lacquers, enamels, varnishes, paints, and finishes applied to architectural alumi- versity of Utah. Flandro has been with porcelain. Normally, all that is required num from the 1920s to the 1960s and JBC for five years. Her interests include to prepare the aluminum for treatment associated conservation and mainte- architectural metals, early architectural is cleaning the surface with a solvent to nance issues. aluminum, glazed terra cotta, and his- remove any grease, oils, or other con- toric testing procedures. Flandro may be In the 1930s Alcoa and The Reynolds taminants that might affect the adhesive contacted at xcflandro@jbconservation. Company advertised aluminum as be- qualities of the finish coat. Improved com. adhesion can be obtained by first ap- ing resistant to corrosion by “defying plying chemical conversion coatings, the corrosive attacks of smoke and fumes.”64 However, aluminum is not

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Helen M. Thomas-Haney is a principal 12. Reynolds Metals Company, Finishes for 43. Ibid., 47. at Jablonski Building Conservation. She Aluminum (Louisville, Ky.: Reynolds Metals 44. Ibid., 48. Co., 1951), 30. earned a bachelor’s in historic preserva- 45. Wernick and Pinner, 394. 13. Ibid., 30. tion from the University of Mary Wash- 46. Alumitec Ltd., “The Maintenance and 14. Aluminum Company of America, Alcoa Preservation of Anodized Aluminum Facades,” ington in 1998 and a master’s in historic Aluminum in Architecture (Pittsburgh, Penn.: www.alumitecltd.com/what-we-do/anodized preservation from Columbia University Aluminum Company of America, 1932), 117- -aluminum, accessed Dec. 2014. in 2002. Thomas-Haney has more than 120. 47. Leslie W. Flott, “Metal Finishing: An 12 years of experience as an architec- 15. Reynolds Metals Company, 25. Overview,” Metal Finishing 2002 Guidebook tural conservator. She is also currently 16. Aluminum Company of America, Alcoa & Directory (Boston: Elsevier Science, 2002), Aluminum in Architecture, 123. 28. an adjunct professor in the historic-pres- 17. David Hallam, “Preventive Conservation 48. The authors were unable to determine the ervation program at Columbia Univer- and Maintenance of Aluminum Artefacts and inventor of the deplated finish. sity. Thomas-Haney may be contacted Collections,” presentation given at “Alumi 49. Aluminum Company of America, Alcoa at [email protected]. num: History, Technology, and Conservation” Aluminum in Architecture, 120. conference, Washington, D.C., April 7-9, 2014. 50. Margot Gayle and David W. Look, “Part 18. Etching tends to lower the corrosion I. A Historical Survey of Metals” in Metals Notes resistance of the metal by removing the natural in America’s Historic Buildings: Uses and 1. “The Metal of the Future,” The Manufac oxide film and therefore is usually followed by Preservation Treatments (Washington, D.C.: turer and Builder (March 1883): 51. This an additional treatment. U.S. Dept. of the Interior, 1980), 84. source states that “technical papers for the past 19. Christian Vargel, Aluminum Corrosion 51. The Aluminum Development Association, two months have been full of accounts of a (Boston: Elsevier Science, 2004), 186. Anodic Oxidation of Aluminum and Its Alloys, new process of manufacturing aluminum”; the 20. T. S. N. Sankara Narayanan, Surface Pre Information Bulletin no. 14 (London: The mentioned technical papers themselves are not treatment by Phosphate Conversion Coatings: Aluminum Development Association, 1948), 5. identified in the article and were not identified A Review (Chennai, India: Advanced Study 52. Ibid, 48-50. through further research by the authors. Center Co., Ltd., National Metallurgical 53. Ibid., 20 2. James Ashby, “The Aluminum Legacy: The Laboratory, Apr. 22, 2005), 1. 54. Gayle and Look, in Metals in America’s History of the Metal and Its Role in Archi 21. Chris Adams and David Hallam, “Finishes Historic Buildings, 84. tecture,” Construction History 15 (1999): 81. on Aluminum: A Conservation Perspective,” 55. Penny Sparke et. al., Aluminum by Design 3. Ibid. Saving the Twentieth Century: The Conserva (Boston: Harry N Abrams Inc., 2000), 256. 4. U.S. Geological Survey, “Aluminum Sta tion of Modern Materials, ed. David Grattan 56. The Aluminum Development Association, tistics,” http:// minerals.usgs.gov/minerals/ (Ottawa: Canadian Conservation Institute, 25. pubs/commodity/aluminum/, accessed Nov. 1993), 282. 2014. 22. Ibid., 283. 57. Reynolds Metals Company, 50. 5. The Aluminum Development Association, 23. Wernick and Pinner, 196-197. 58. Ibid, 50. The Properties of Aluminum and Its Alloys, 24. Ibid, 196-197. 59. Delstar Metal, Users Guide to Electropol Information Bulletin no. 2 (London: The Alu 25. Aluminum Company of America, Methods ishing (Houston, Tex: Delstar Metal, 2003), 2. minum Development Association, 1955), 11. of Working Aluminum (Pittsburgh, Penn.: 60. Willfred F. Coxon, Modern Developments 6. J. R. Davis, ed., “Chapter 1: Introduction,” Alcoa, 1909), 17. in Metal Finishing (Leighton Buzzard, Beds.: Corrosion of Aluminum and Aluminum Alloys 26. Reynolds Metals Company, 32-35. Arrow Press, 1950), 35. (Materials Park, Ohio: ASM International, 27. Wernick and Pinner, 236. 61. Wernick and Pinner, 84-85. 1999), 16. 28. Reynolds Metals Company, 37. 62. Musa, 56. 7. “Several distinct aluminum hydrates are 63. Reynolds Metals Company, 70-75. known, including the α- and β-monohydrates, 29. Wernick and Pinner, 236-239. Reynolds 64. Aluminum Company of America, Alcoa and the α- and β-Al2O3 respectively.” S. Metals Company, 35. Wernick and R. Pinner, The Surface Treatment 30. “Bright Finish for Metals,” The Science Aluminum in Architecture, 16. and Finishing of Aluminum and Its Alloys News-Letter 56, no. 15 (Oct. 8, 1949): 227. 65. Ibid., 133. (Teddington, England: Robert Draper Ltd., 31. M. Stelter and H. Bombach, “Process 66. Adams and Hallam, 282. 1964), 31. Optimization in Copper Electrorefining,” 8. A study completed in 1960 for the U.S. Advanced Engineering Materials 6, no. 7 Navy found that a water flow of 135 feet per (2004): 558. second over aluminum resulted in surface loss 32. Bruno Martin, The Practical Electroplater of 0.034 inches to 0.083 inches per year. J. L. (New York: E. Bruno, 1894), 72. Basil, “Corrosion of Materials in High Velocity 33. Aluminum Company of America, Finishes Sea Water,” Progress Report No. 1, Research for Aluminum (Pittsburgh, Penn.: Alcoa, and Development Report 910160A (Annapolis, 1938), 47. Md.: Naval Engineering Experimental Station, Dec. 30, 1960). 34. Ibid. 9. Ahmed Y. Musa, “Corrosion Protection of 35. Ibid. Al Alloys: Organic Coatings and Inhibitors,” 36. Wernick and Pinner, 581-605. The APT Bulletin is published by the Recent Researches in Corrosion Evaluation 37. Ibid. Association of Preservation Technology, an and Protection, ed. Reza Shoja Razavi (Rijeka, 38. Ibid., 588-590. interdisciplinary organization dedicated to Croatia: Intech, 2012): 52. 39. Ibid, 587-590. the practical application of the principles and 10. Wernick and Pinner, 42-43. 40. Ibid, 248. techniques necessary for the care and wise 11. The term “sandblasted” is a bit of a 41. J. D. Minford, Handbook of Aluminum use of the built environment. A subscription misnomer, since media other than sand can Bonding Technology and Data (New York: to the Bulletin and free online access to be used to abrade aluminum. However, all Marcel Dekker, 1993), 84 and 95. the trade documents refer to this finish as past articles are member benefits. For more “sandblasted.” 42. Reynolds Metals Company, 74. information please visit www.apti.org.

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