CHAPTER 8
Maintaining the Copper Alloy Surface
The vessel, though her mast be firm, beneath her copper bears a worm.
Henry David Thoreau
INTRODUCTION
The alloys of copper are some of the most corrosion resistant metals in common use today. They achieve this by rapidly producing a layer of inert corrosion products on the surface. In the context of art and architecture, however, this tendency for rapid change in surface appearance and luster can be daunting. Unprotected copper alloys and those whose coatings have been breached or stained will change in appearance, often to a form that is less aesthetically pleasing.
The surfaces of copper and copper alloys undergo a change in their morphology, meaning that they will change in appearance as oxidation proceeds until a stability is achieved. For copper alloys with significant zinc content, this change in morphology can lead to physical degradation of the metal itself as zinc is selectively removed in the corrosion phenomenon known as dezincification. Bronze sculptures, particularly when exposed to chlorides, can develop what has been referred to as bronze disease, in which the surface is selectively attacked. The surface will continue to corrode if the chloride exposure is not contained.
Much of what occurs, if addressed early, can be removed, stopped, or restored. Keeping surfaces clean requires either protecting the surface with a substance such as a wax or lacquer or performing regular surface cleaning to remove foreign substances that have found their way to the surface.
PROTECTING THE NEW COPPER ALLOY SURFACE
When copper alloy surfaces are expected to be displayed with their natural metal luster, they will either require a coating of some form or another or regular cleaning and polishing. The coating will usually be either an organic coating (such as a clear lacquer or wax) or an inorganic coating (such as a chromate or silane coating). Inorganic coatings are not as commonly used in art and architecture, and are usually considered when thermal conditions could prematurely damage an organic coating. Organic coatings have a shorter life than inorganic coatings but they usually can be removed and restored, while inorganic coatings can't be restored but have a longer expected life.
One of the biggest challenges with organic clear coatings used to protect copper alloys is the potential development of tarnish under the film. This tarnish, which will develop later in the life of the coated copper alloy, is from peroxide anions that accumulate as the organic coating begins its first stages of degradation. The anions develop early in the life of the coating from trapped moisture below the film or residual solvents decaying under ultraviolet exposure.
As with other metals, one can evaluate the condition of the copper alloy surface from the point of view of these three categories:
- Physical cleanliness
- Chemical cleanliness
- Mechanical cleanliness
ACHIEVING PHYSICAL CLEANLINESS
To be physically clean, the copper alloy surface must be free of tarnish and light oxidation, fingerprints, soils, and other substances that have yet to engage with the base metal itself. If an artificial patina is to be generated on the surface of the copper alloy, the surface must first be physically clean. A physically clean surface is the condition nearest to a new metal surface: free of and unfettered by coatings or oxides.
Tarnish
All of the copper alloys will develop a light oxide on the surface when exposed to the air. This oxide will continue to grow. The oxide is a form of cuprous oxide (Cu2O).
Figure 8.1 shows the tarnished surface of a door made from copper plate. The surface is exposed to the outside environment and has developed a light oxide. The colors seen are induced from the light interference effects of the thin film.
This tarnish forms on the surface of clean, unprotected copper alloy surfaces with the exception of aluminum–bronze alloys. The aluminum‐bronze alloys form an aluminum oxide on their surfaces and are resistant to tarnishing. Tarnish is the natural first step toward mineralization of the copper surface as oxygen and hydroxides combine with the copper surface. They are weakly bonded at first so they can be removed more easily.
The tarnish can be removed using mild oxide removers designed for copper alloys. There are numerous tarnish removers for copper, copper alloys, and silver that work quickly and effectively. It is critical, however, to remove all remnants of the cleaning solution from the surface and from the recesses and seams of a fabricated assembly. If the cleaning solutions are allowed to remain in the seams, they can etch the surface and develop corrosion conditions favorable to corrosion. Commercial tarnish cleaners are composed of acids, bases, or neutral pH cleaners. Some have mild abrasives and assist in polishing the surface. It is critical to remove all remnants of the cleaners, otherwise they can continue to corrode the copper alloy. Alkaline‐based cleaners will remove tarnish well, but they also can corrode the copper if they remain on the surface. The corrosion may be superficial, but subsequent cleaning to remove the corrosion products will be more difficult. If the pieces are cleaned and placed in storage, it is even more critical to ensure that cleaning fluids and pastes have been thoroughly removed. These will continue to react with the copper surface and cause corrosion. Worse yet, they can create conditions of dezincification in the high‐zinc brass alloys.
FIGURE 8.1 Tarnish on copper surface of a door made from thick copper plates.
FIGURE 8.2 Cleaning solution and handprint left on a copper alloy form.
Figure 8.2 shows remnants of cleaning fluids that have seeped onto the reverse side of a polished brass form. A visible handprint has oxidized the surface of the copper alloy.
Fingerprints
Clean, uncoated copper alloys fingerprint readily. The oils left behind by hand‐ and fingerprints are composed of organic oils and fats called lipids, which are composed of amino acids and water. They will lightly etch the surface and create a contrasting appearance. Often they will not be readily apparent until the relative humidity rises. They will not disappear on their own (Figure 8.3).
It is possible to remove this light oxidation from the surface of copper and copper alloys where the design called for a pristine, natural appearance by using commercial cleaners designed for copper. Some buffing and polishing may be beneficial to improve the gloss and shine of the original appearance. Coat them with wax or clear lacquer to sustain the appearance is recommended; otherwise, plan on ongoing maintenance to remove tarnish that develops.
Under‐Film Corrosion
Tarnish and handprints can also appear under the clear coatings applied to protect the copper alloy surface. This is due to moisture or other substances being on the metal prior to the application of the clear lacquer. The solvents in the lacquer can also undergo a transformation and cause an oxidation reaction. It can be a significant and costly problem. Initially the corrosion may not be visible, but it will grow with time and exposure.
FIGURE 8.3 Fingerprints on copper alloy surfaces.
Figure 8.4 shows brass plaques that were highlighted with a light statuary finish to darken the etched portions. As time passed and the temperature changed, the panels darkened under the film. Deicing salts left on the lacquer could also have been a contributing factor. Any porosity in the film would have allowed the small chloride atom to enter and react with copper alloy surface.
This can occur even with lacquers containing antioxidation additions such as sodium benzotriazole. This is because the oil, moisture, or handprint was between the copper alloy surface and the antioxidation compounds in the lacquer. The darkening begins as spots under the lacquer as shown in the right‐hand image in Figure 8.4.The only means of correcting this is to strip the protective coating from the surface and remove the oxidation. Removing the coating on large panels and objects can be a daunting challenge when lacquers or varnishes are used. Removing these will require a solvent capable of dissolving the lacquer. It may require that the panels be removed and soaked in a bath of the solvent. Disposal, health, and flammability issues are primary concerns to address before attempting this.
In order to combat the return of tarnish or under‐film degradation, or to prevent its development in the first place, thoroughly clean the surface once the lacquer is removed prior to coating or recoating the surface. Heat it slightly to drive moisture out of the copper alloy surface pores. Do not overheat it or it will oxidize when it cools. Heating will not damage the statuary finish or patina. Wipe the surface down with an inhibitor solution containing sodium benzotriazole. A solution with 1–2% sodium benzotriazole and distilled water will work, or a 2% solution of benzotriazole and alcohol will keep moisture out and not affect the lacquer application. Use a lacquer coating that has ultraviolet absorbers and antioxidation enhancers in the lacquer. Incralac, an acrylic lacquer developed from research sponsored by INCRA, is an excellent lacquer that is used on copper alloys for this purpose.
FIGURE 8.4 Panels at the 9/11 Memorial darkening under the film.
Incralac has ultraviolet absorbers and benzotriazole included with an acrylic ester resin dissolved in a solvent.
The ability of benzotriazole to prevent tarnish from developing on copper alloys was discussed in Chapter 4, and an explanation of what is occurring can be found in the same chapter. Figure 8.5 shows the results of a test in which a C11000 copper tube was first thoroughly cleaned, after which one half of the length was coated with a 2% sodium benzotriazole solution, allowed to dry, and then dipped in a strong solution of potassium sulfide. The unprotected portion of the tube immediately darkened while the half that was coated remained unaffected.
FIGURE 8.5 Image of test results of ability of benzotriazole to prevent tarnish.
When a copper alloy part is coated with the sodium benzotriazole, a thin layer is deposited on the clean surface. The sodium benzotriazole coating is imperceptible to the eye once the surface is dry. It has been used as a protective oxide inhibitor in museum work and it is a common oxide inhibitor used by the industry when shipping copper alloys to be further processed in the fabrication chain.
Eventually the benzotriazole begins to wear off and tarnish will occur. Thus, using it alone will require repeated treatments; otherwise, use it with a wax or Incralac coating.
Other Substances on the Surface
Bird Waste
On exterior copper surfaces, in particular sculpture, there are several common substances that inflict damage on the surface of the metal. It is difficult to envision a sculpture without bird waste on the upper regions. Bird waste is a watery paste composed of large amounts of nitrogen and phosphorus. Birds do not produce urine, so their waste is a pasty watery substance composed of uric acid. Uric acid is for the most part insoluble and will adhere to the surface. It can become a baked‐on crust as the sun heats the surface and decompose into components that will react with the copper alloy surface and create streaks of greening corrosion products. It can also adhere to old wax and create stains on the sculpture (Figure 8.6).
FIGURE 8.6 Bird waste on copper wall panels and a bronze sculpture.
To prevent this, bird waste needs to be cleaned from the surface regularly. To break the adhesion, pressure washing the surface or physically scrubbing the surface is advised. Detergents can assist in lifting the waste off the surface, but the putty‐like substances need physical displacement.
If allowed to remain they will discolor the surface and damage the underlying patina as they decompose into ammonia and phosphorus‐based substances that will combine with the copper. Figure 8.7 shows a bronze and stainless steel outdoor sculpture designed by the artist James Surls and titled Six and Seven Flowers. The horizontal “arms” are perfect perches for the birds in the area. You can see where the bird waste is beginning to interact with the copper alloy to create a slight green tint.
If the bronze petals were protected with a clear lacquer and wax, it would be easy to remove the bird waste with an occasionally pressure rinse. Allowing the bird waste to interact with the copper alloy will create corrosion products that will demand more difficult cleaning. The corrosion particles will need to be dissolved, unfortunately, along with the original patina. This will require the restoration of the patina after cleaning.
FIGURE 8.7 Bird waste beginning to interact with the copper alloy used for
Source: Six and Seven Flowers by the artist James Surls.
Other substancesthat will affect the physical cleanliness of a copper surface are adhesives and glues. General dirt and grime would fall into this category, as well as old wax coatings. Even printing by the Mill on the copper surface before patination will affect the end appearance, and thus the need for physical cleaning, before moving to the steps of patination. Adhesives, glues, inks, and old wax can be removed with commercial solvents such as mineral spirits or naphtha.
Removing potentially damaging substances from the surface, whether on bare metal, patinated metal, or coated metal, is advised for good long‐term performance. Figure 8.8 shows conditions that need to be addressed to bring the copper alloy back to a level of physical cleanliness. The condition of printing, as in the image at the lower right in Figure 8.8, will require removing the patina or starting over with physically clean copper.
Deicing Salts
The use of deicing salts in northern climates is very common. Unfortunately, these salts find their way onto the surface of metals in proximity of walkways and doorways. Deicing salts are composed of chloride salts such as magnesium, calcium, and sodium chloride. When the salts are applied to icy surfaces, they melt the ice by lowering the freezing point of water, a chemical behavior referred to as “freezing point depression.” The salts dissolve in the water and become ions of chloride and sodium or chloride and calcium. As more particles are added, it becomes more difficult for the water to freeze. It is the chloride ion in the water that causes stains on copper if it is splashed onto the surface and allowed to remain there. In the cold of winter, when the salts are applied, the copper and chloride are slow to react. The telltale sign of deicing salts on a surface is a white stain, as shown in Figure 8.9. The stain adheres to the metal surface and can be difficult to remove. To remove it, a deionized water rinse is recommended. A light pressure wash will assist in the removal as well.
FIGURE 8.8 (Left) Bird waste, (top right) old wax, and (bottom right) Mill printing under the patina.
As the higher temperatures of spring warm the surface and dew condensates on the cooler metal in the morning hours, the chloride and the copper will react and form copper chloride. Figure 8.10 shows three surfaces where the chloride and the copper have started to react and form copper chloride on the surface. These stains are mineral forms of copper now and cannot be removed by pressure washing, deionized water, or solvents.
FIGURE 8.9 Deicing salt residue on surfaces.
It is always the lower sections of copper alloy surfaces that show the corrosion product copper chloride (CuCl2), a turquoise bluish green hydrated mineral. The stain is superficial and does little harm to the copper other than making an esthetic alteration to the color. The difficulty is in removal, because removing the stain will remove all oxide from the copper surface. You cannot remove the stain without removing the oxide on the base metal. It is a patina, a mineralized surface that will require either physical removal down to the base metal, dissolving in acid, or basic solution treatments. Once clean, the surface will need to be reoxidize to restore the original surface color.
Waxing and lacquers can protect the surface of copper alloys for a while, or at least long enough for maintenance processes to remove all surface chlorides as indicated by the salt residue. Deionized water will effectively remove ions of chloride and other ionic forms that are on the surface. Thoroughly rinsing the surface in early spring before the chloride and copper have time to react is highly recommended. If the surfaces are coated with wax or clear lacquers, inspect the coatings and do any necessary repairs. You can usually find areas where the coating has been breached indicated by spots of darkening.
FIGURE 8.10 Deicing salt damage to copper surfaces.
Old Wax
On sculpture more so than architectural surfaces, old wax can make a surface look drab and washed out. Figure 8.11 shows a surface of old wax that has yellowed in the left‐hand image and the same surface brightened, protected, and enhanced by new wax on the right.
The old wax is easy to remove, and as it comes off many potentially damaging substances will be removed with it. Removing old wax offers a chance to closely inspect the surface as well. Remove old wax by wiping the surface with clean rags soaked in mineral spirits. Mineral spirits are mild aliphatic solvents, but working with them still requires protection of skin and eyes as well as surrounding areas. Avoid breathing in the fumes as much as possible, and use them only in a ventilated area away from open flames.
There are different levels of quality of mineral spirits, which are also called white spirits or naphtha. It is important to use the better grades. Removing the wax will be easier and less messy. Museum grade mineral spirits work exceptionally well.
Once the old wax is removed, it is good practice to clean the surface with mild detergent and water. Allow the surface to dry thoroughly before applying the new wax. The use of a torch helps to ensure that water is out of the surface pores of the metal, and gently heating the surface promotes the flowing of the new wax into tight regions. Use a high‐grade wax that has ultraviolet inhibitors and dry it to a hard, durable coating. It is best to apply the wax in layers to insure good coverage. Later in this chapter there is an in‐depth discussion of the waxes used on copper alloys.
FIGURE 8.11 (Left) Old wax and (right) renewed wax on a bronze sculpture by the artist
Source: Kwan Wu.
Old Lacquers
Removing old lacquer coatings on copper surfaces can be challenging. The first problem is determining what exactly you are removing. Lacquer is an eclectic term covering a wide range of possible coatings. Acrylic‐based lacquers (such as Incralac) have been in common use on sculpture and most copper alloys since the 1970s. The makeup of Incralac and its use is described in more detail later in this chapter.
Other lacquers have also been used with varying levels of success, linseed oil, urethanes, and in more recent times, sol‐gel applications, have been used. Sol‐gel applications are showing promise as coatings for metals. Sol‐gel is a process of applying thin films of discrete particles of metal oxides, either silicon or titanium oxide, over the surface.
Prior to the 1970s, sculpture was often coated with a cellulose‐based compound. One such compound was cellulose acetate butylate. It was a coating used on some bronzes, particularly those exposed to a lot of moisture, such as artistic fountains. An example of this coating is shown in Figure 8.12. This sculpture, called the Muse of Missouri, was constructed in the early 1960s. It was designed by the artist Wheeler Williams and cast at the Modern Art Foundry of New York. It is 4.5 m tall and stands on top of a pedestal fountain 3.9 m tall. It has been cleaned and treated several times over the last 50 years, but the original coating protecting the patina on the pedestal had never been removed.
FIGURE 8.12 Muse of Missouri by Wheeler Williams.
The image on the upper right shows the whitish residue that had developed over the years. It was determined by several tests to be decomposed cellulose nitrate compound that had formed a thick yellowish crust from the decaying of the cellulose acetate butylate. Figure 8.13 shows a microscopic image of the surface on the left before removal of the crust and after removal on the right. Hard‐water deposits had also formed on the surface. The compound was very durable and could not be dissolved by treatments with strong solvents such as acetone or xylene or with acids such as phosphoric, citric, or gluconic acids. As the compound decayed, it had formed a plastic‐like substance on the surface.
Ethyl acetate was eventually used to dissolve the cellulose substance. Ethyl acetate soaked rags held onto the surface were found to soften the surface and allow for the substance to be removed. Refer to the lower right image of Figure 8.12. Below the crust the patina was in good shape and could be protected with Incralac and wax. For fountains, three layers of Incralac followed by a wax coating have been found to provide very good protection to the surface.
Stains from substances in the fountain's water, in particular rust, had also deposited on the surface and created appearance issues. The stains came off when the underlaying coating was removed. Subsequent removal of the wax will remove rust stains with it, allowing the sculpture to look and perform as it did when first installed.
FIGURE 8.13 Microscopic image of the decaying coating.
Figure 8.14 shows rust deposits on the copper alloy surface around the feet of the sculpture and the outlet where the water is emitted. Most likely these deposits are from corroding internal pipe fittings on the steel pipe used to deliver water for the fountain.
The approach to cleaning surfaces should be to start simple and work up to more complex cleaning approaches. As a first step, isopropyl alcohol can be used to wipe the surface to remove light soils, light grease, and fingerprints that have not yet interacted with the copper surface to develop light oxidation. This can be followed by cleaning with a mild, neutral detergent and a clean‐water sponge or wipe (Figure 8.15). Add pressure, if necessary, to dislodge adhered waste such as bird droppings or road grime. Note that high pressure can potentially damage a wax coating, so use this cautiously on sculpture work.
The point is to remove as much of the general dirt and grime, bird waste, and other potential reactive substances from the surface as possible. As discussed earlier, if deicing salts are left on the surface, they will eventually reach the copper metal and react. Bird droppings have an organic acidic nature that will also eventually react with the copper if allowed to remain.
A final rinse with deionized water is also good practice. The deionized water will remove salts from surfaces. Chlorides, sulfides, and nitrates will come off with a deionized rinse, and as the surface dries spotting will be reduced.
To remove old wax, use mineral spirits, which is a common name for naphtha, or better still odorless mineral spirits, which is safer to use due to the removal of aromatics. Mineral spirits will dissolve the wax without harming any underlying lacquer or patina. The wax can be removed by wiping the surface with a clean rag saturated with the mineral spirits. After this, new wax can then be reapplied to the surface.
FIGURE 8.14 Rust stains on the Muse sculpture from deposits in the water.
FIGURE 8.15 Cleaning copper alloy surfaces with mild soap and water.
TABLE 8.1 Levels in achieving physical cleanliness.
| Level | Procedure | Benefit |
| 1 | Wipe down with isopropyl alcohol | Removes light soils and fingerprints |
| 2 | Clean surface with mild detergent | Removes soils and dirt |
| 3 | Wipe with commercial tarnish remover (if needed) | Removes light oxide and fingerprint oxides from the surface |
| 4 | Pressure wash (if needed) | Removes bird droppings, dirt deposits |
| 5 | Mineral spirits (if needed) | Removes old wax, adhesives, glues, inks |
| 6 | Degreasers (if needed) | Removes heavy oil and grease deposits |
| 7 | Steam (if needed) | Loosen tough soil deposits, bird droppings |
| 8 | Deionized water rinse | Removes salts and chemical deposits (use after any lower‐level operations as a final step) |
Mineral spirits will also remove most gum, adhesives, and other sticky residues that might find their way to the surface. When removing wax with mineral spirits, it will take with it most substances on the wax. If dirt and grease deposits are heavy, the addition of a steam cleaner may be needed. The added heat will loosen the particles and allow the mineral‐spirit wipe to pull these from the surface.
Commercial degreasers can be added if the grease or oil deposits are heavy or are deep into the pores of the metal. These degreasers are often alkaline and must be thoroughly rinsed from the surface, otherwise they will create regions where corrosion reactions can occur. These would show as telltale green deposits on the surface after several weeks of exposure.
Table 8.1 shows cleaning levels for removing substances or addressing various surface issues and their order in the cleaning process.
ACHIEVING CHEMICAL CLEANLINESS
Achieving chemical cleanliness involves removing heavy oxides and stains that have chemically bonded to the copper alloy or copper alloy surface. This includes Mill scale, heat tinting at welds, oxides from cleaning solutions, and solder fluxes that have been left on the surface and have chemically bonded with the metal over time. It also includes free‐iron deposits that have been transferred from other surfaces and integrated into the copper oxide in the form of rust stains, as well as deicing salt stains and fertilizer salt stains, which will develop if these substances are allowed to remain on the surface (Figure 8.16).
FIGURE 8.16 (Right) Cleaning fluid left on a surface, (top left) iron rust stains, (bottom left) heavy oxidation.
These substances become a part of the oxide layer on the copper surface. They are an aesthetic issue for the most part because of their contrasting appearance but can become a corrosion concern if conditions of dezincification arise, as is the case in the right‐hand image shown in Figure 8.16, where cleaning fluid has been left on a surface. Dezincification can occur on brass alloys with 15% or more zinc. If allowed to remain on the metal surface, all of these substances can lead to light etching of the surface, dezincification corrosion, or pitting.
FIGURE 8.17 Dark streaks of cupric oxide visible on old copper surfaces.
Removal of the contrasting corrosion products will also remove the oxide back to the bare copper alloy. Their removal will require mechanical abrading of the surface or chemical dissolution of the oxide that has formed.
As discussed previously, the copper in copper alloys will oxidize in one of two states. One state is often referred to as copper (I) oxide and has the chemical formula Cu2O. This compound is known as cuprous oxide and is generally reddish to reddish brown in color. The other form of copper oxide is copper (II) oxide, also known as cupric oxide, and has the chemical formula CuO. This compound is black in color. You can see cupric oxide forming as black streaks on old copper exposed for years (Figure 8.17).
Dissolving the Oxide
Cuprous oxide can often develop on exterior surfaces and can lend a reddish color tone to brass items placed in an interior exposure as well. Citric acid–based cleaners work well on copper alloy to dissolve cupric oxide, but it will not dissolve cuprous oxide. The restoration of several large, intricately designed cast copper alloy doors provides an example of how these oxides may be removed (Figure 8.18). These magnificent doors were cast in the 1920s from alloy C83600. Sometimes referred to as European Bronze, 85‐5‐5‐5, or leaded red brass, the alloy contains nominally 85% copper, 5% zinc, 5% tin, and 5% lead. European Bronze is a cast alloy that was in extensive use in Europe and the United States in the late nineteenth century and early twentieth century.
These doors have been subject to an urban environment exposure and over the years have been exposed to deicing salts. They have also been sand blasted and possibly waxed a few times in their history. The oxides and surface corrosion created an esthetic issue with the owner. The corrosion on the cast surface created areas of golden yellow oxide and a ruddy reddish brown. On inspection there were no signs of dezincification, most likely due to the presence of lead and tin and the low zinc content.
To remove the oxides that had developed on the doors, they were first washed down with mild detergent and rinsed in clean water (Figure 8.19). Then they were dipped into a bath of concentrated lemon juice. Concentrated lemon juice is a source of citric acid. The bath had a pH of 2–2.5. The cast doors were allowed to sit in the bath for several hours.
FIGURE 8.18 Cast copper alloy doors after decades of exposure.
FIGURE 8.19 Initial removal of oxides.
The lemon juice worked more effectively than phosphoric acid or even a citric acid solution of a similar pH. There is another active component in the organic lemon juice that acted as a surfactant and helped to raise the oxide from the surface. Once the oxide was loosened, the doors were pressure washed with clean water to remove the residue. Figure 8.20 shows the cast copper alloy surface after it was removed from the bath.
Some of the surfaces retained the red cuprous oxide, since lemon juice does not dissolve this from the surface. The color of this copper alloy is golden yellow. To restore the entire surface to this color would have involved more potent treatments with phosphoric acid or more powerful acid treatments using sulfuric and nitric acid. Mechanical blasting would also have removed the oxide, but this would have had an undesirable effect on the copper alloy surface. For the application of the final statuary finish such treatments were not needed. The statuary finish would develop over the cuprous oxide and the natural base metal surface without any distinguishing mottling.
FIGURE 8.20 Much of the tarnish, old oxide, and patina was removed from the surface after a lemon‐juice bath.
Once the oxides were removed, the pieces were wiped down with phosphoric acid and alcohol. Figure 8.21 shows the initial development of a dark oxide treatment on the surface. The image on the right shows the oxide with a whitish film on part of the surface. The image on the left shows the initial application of the finish; some highlighting has been performed, as indicated by the bright edges on the cast features. Highlighting the surface brings out a contrasting appearance and gives an “old bronze” look and feel. The oxide used was a copper sulfide treatment that has a dilute phosphoric acid to act as an etchant and an electrolyte. The color goes dark and sometimes chalky at first. Scotch‐Brite pads were used to highlight the surface and remove some of the dark chalkiness.
Once the color was obtained and the highlights applied, the doors were coated with three layers of Incralac. The first coating of Incralac was a mixture of three parts xylene solvent and one part Incralac. This was allowed to dry before a second layer of Incralac, this time two parts xylene to one part Incralac, was applied. This coating was also allowed to fully dry before a third layer, also two parts to one, was applied.
Once the doors were hung (Figure 8.22), they were coated with two coats of wax. The wax used was Trewax® paste wax, which is a blend of carnauba and microcrystalline wax. When applying this wax, allow it to dry between coats and rub it into the surface to even it out. Light heat can be added to help the wax to flow into tight regions around the artwork.
This is one method to chemically clean a surface and then return it to the original patina appearance. There are other methods that involve dissolving oxides from the surface using selective electropolishing techniques. The left‐hand image in Figure 8.23 shows the selective electropolishing of a C26000 alloy surface. The oxidation of surface materials and dezincification deposits were removed by passing a selective electropolishing device over the copper alloy surface.
FIGURE 8.21 Developing the statuary finish.
FIGURE 8.22 The restored doors are hung.
FIGURE 8.23 (Left) Removing oxides from C26000 alloy and (right) repolishing the surface.
Once the oxidation was removed, the surface had a slight etch and needed to be repolished and buffed to bring back the original mirror finish. This is done using rouge coupled with a cotton buffing mop. There is an art to producing the correct polish. It is a messy operation, but buffing and polishing most of the copper alloys are easier than for other metals. The copper alloy surface will take well to buffing and polishing operations.
Electropolishing
Electropolishing and selective electropolishing of copper and copper alloys is an excellent method of removing light oxide deposits and heat tint from welding or brazing.
It is also a good method of removing tarnish and even stains caused by deicing salts.
Electropolishing processes are more commonly used in cleaning and preparing the surface of stainless steels. Copper alloys were some of the first materials to be electropolished in the early decades of the twentieth century. Electropolishing can be performed selectively, as indicated in Figure 8.23, or by immersing the surface in a tank containing an electrolyte, usually phosphoric acid. The copper is made the anode and immersed in a temperature‐controlled bath of electrolyte as a current is applied. A cathode is immersed into the solution to complete the circuit. As current is applied from a DC source, the surface protrusions on the copper dissolve by means of oxidation while a reduction process occurs at the cathode and hydrogen is released. The surface oxides are dissolved and high points on the copper alloy are smoothed slightly.
In selective electropolishing, or electrocleaning as it sometimes is called, a wand composed of graphite and wrapped in cotton is the cathode. The cotton is soaked in phosphoric acid and the copper object is connected to the positive terminal of a DC power source. A current is applied and as the wand is passed over the surface, the area under the wand is dissolved. The process of electropolishing copper is the opposite of electroplating : in electroplating the poles are reversed, making the copper electrically positive, immersed in a metal ion solution where the metal comes out of solution and forms onto the copper surface.
There are a number of other methods that can be used to bring the surface of copper alloys back to chemical cleanliness. Mechanical polishing and buffing (Figure 8.24) removes the surface and all oxides with it.
Abrasive Blasting
Other techniques for producing chemical cleanliness involve abrasive blasting of the surface with glass beads or fine sand, dry ice blasting, and blasting with walnut shells. The more aggressive of these cleaning methods, sand blasting, can shape thin sheet material and will create a coarse texture that may detract from the original appearance. This method creates airborne particles that may require special containment and collection.
For thin metals, such as copper roofing or wall sheathing, removing the patina and oxide using blasting techniques is the preferred method. It requires care to not damage the thin metal surfaces. Blasting with walnut shells and dry ice both work well, but dry ice blasting is not as messy. The difficulty of dry ice blasting lies in the fact that the dry ice hopper needs to be near the nozzle of the gun delivering the pellets to the surface. On roofs this may not be possible.
FIGURE 8.24 Mechanical polishing of a copper alloy surface.
Crushed walnut shells will work and not damage thin metal, but it can place a lot of dust and debris into the air. Collection and disposal of the walnut shells along with the dust particles created is an additional challenge. Both blasting metals are very load. Ear protection is mandatory.
Pickling
Pickling baths composed of sulfuric acid and nitric acid can remove thick oxides from the surface of copper alloys in controlled work areas. Working with these solutions is dangerous and requires a clear safety protocol, an understanding of working with acids, and a proper means of disposal.
You could say that dissolving the oxide on a surface with either lemon juice concentrate, citric acid, or phosphoric acid constitutes a milder form of pickling. These substances are all safer to use than strong acid pickling baths and work well on large surfaces. They do not work as rapidly as the sulfuric and nitric acid mixes, but they are much safer to use and easier to dispose of correctly. On large surfaces, acid baths are impractical.
Laser Ablation
Laser ablation techniques are another way to remove oxides from the surface of copper alloys. These techniques use a vacuum system that draws the fumes and released surface oxides into a filter trap. Laser ablation is a clean method of removing the oxidation and contaminants on copper alloy surfaces. Figure 8.25 shows the removal of patina from a perforated copper surface by means of a 1064‐watt laser ablation system. The laser ablation process can be performed in situ and uses no solvents or abrasives. The surface of the copper alloy has a light peening when viewed under a microscope.
FIGURE 8.25 Laser ablation of copper surface.
TABLE 8.2 Chemically clean surface levels for copper alloys.
| Levels | Procedure | Benefit |
| 1 | Immersion in acid bath | Remove oxides from surface |
| 2 | Selective electropolish | Weld tint, oxides, corrosion products |
| 3 | Immersion in stronger acids, blasting with dry ice or walnut shells, laser ablation | Remove heavy scale, heavy oxidation, iron stains, mineral stains |
| 4 | Mechanical finishing | Can selectively remove areas. Can remove scale and patinas. |
| 5 | Laser ablation | Waste is limited and contained by the vacuum. |
In laser ablation, the energy is absorbed into the oxide and it sublimates from the copper alloy surface. The energy effectively breaks the bond holding the oxide to the copper. The result is a chemically cleaned copper alloy (refer to Table 8.2).
ACHIEVING MECHANICAL CLEANLINESS
Achieving mechanical cleanliness involves restoring a surface that has been scratched or dented.
Because of the inherent softness of copper and some of the copper alloys, dents and scratches are a significant concern. When they occur, repair can be difficult. Copper is relatively soft and can be abraded by other materials. Table 8.3 lists a few materials and compares their relative hardness rating.
TABLE 8.3 Relative hardness of substances on the Moh scale.
| Material | Moh hardness rating |
| Diamond | 10 |
| Glass | 5.5 |
| Steel | 4 |
| Copper oxide | 3.5 |
| Copper alloys | 3 |
| Aluminum | 2.5 |
| Talc | 1 |
Scratches and Graffiti
Scratches and graffiti are an increasing issue with copper alloys used in proximity to human activity and contact. Unfortunately, many sculptures set in public spaces can attract people set on ignorant destruction. The soft metal and contrasting patina seem to attract the dissolute. Figure 8.26 shows examples of such wanton destruction.
FIGURE 8.26 Scratches through the protective layer and patina of three surfaces.
To restore scratched and marred surfaces to the artist's original intention can be a difficult procedure. First the coating and patina around the area must be removed down to the base metal. The process then involves grinding and polishing until the scratch is diminished sufficiently. The surface will be clean and smooth, with the base metal around the scratch exposed. The texture will be smooth and different from the surrounding metal. Initially you will need to match the texture of this surrounding metal, and this may require roughening with a small blasting tool; different media may need to be tested to arrive at a matching surface. A patina will also need to be developed to match the surrounding patina. This may not be possible, and the entire piece or segment of the sculpture may require stripping and repatination. Stripping may involve pressure blasting the surface with walnut shells or dry ice.
Thus, to achieve mechanical cleanliness some of the surface must be removed. For patinated or oxide surfaces, these need to be taken all the way down to bare metal. The scratch will first be sanded, then buffed to match the surrounding texture. Once this is achieved the patina or oxide can be applied.
Handling and Packaging
Copper alloy fabrications should be handled in such a manner as to not damage the surface or introduce substances that will initiate oxidation or lead to tarnish developing under the film if the surface is coated. Wear cotton gloves while handling the fabricated surfaces, store fabrications in dry, low‐humidity places, and protect the copper alloy with protective wraps during handling and processing. Corrosion‐inhibiting paper wraps that have benzotriazole‐saturated layers can be used to individually wrap copper alloy fabrications or to line the containers or crates used to ship or store the parts.
Careful packaging of copper alloys is critical to prevent moisture from entering their crates (refer to Figure 7.22). Copper and its alloys need special care, and planning for and preventing damage while in transit is an important step. Fabricated copper alloy parts should be separated to keep them from rubbing on one another. The parts shown in Figure 8.27 are being shipped overseas in a container. Each plate has been fixed to a rigid wood frame and accurately marked for placement on the project. An anti‐moisture dry pack of silica gel is placed in each crate to absorb condensation moisture during shipping and subsequent storage. The crates are also lined with a water‐deterrent film to shed any moisture that might get on the crates and seep into the interior.
These precautions should be performed even if the copper alloys are coated with a protective layer of lacquer. Copper alloys have such an affinity to moisture that it is paramount to keep the fabricated work dry during storage and shipping. Granted, if the copper is downpipe or metal roofing the precautions can be reduced, but it is very important not to allow moisture to collect on a copper alloy surface during storage and shipping.
FIGURE 8.27 Packaging for Prada being prepared for shipping.
Maintenance Needs and Reasons
There are any number of maladies that can inflict themselves onto the surfaces of copper alloy statuary.. Coarse surfaces make cleaning, waxing, and polishing the surface difficult. On exterior pieces, water can infiltrate some porous regions and slowly drain out, concentrating into streaks or resisting wax and lacquer adherence.
Wrought surfaces—those copper alloy sculptures that are made from sheet, plate, or wire—have maintenance needs similar to cast surfaces. These surfaces can also come into contact with substances that can cause a chemical reaction with the copper or the alloying elements in the metal, with zinc being one of the more critical of these elements.
In copper alloys, the presence of moisture is necessary to start the deterioration of the surface. The presence of moisture is all but unavoidable. Condensation, the relative humidity of a space, and moisture in other materials make isolation impractical. One can slow its access to the surface, but it will eventually arrive there. Moisture gives other substances that may be on the surface the ability to react. Moisture has both hydrogen and oxygen, so the preliminary substances needed to create an acidic or alkali environment are present. Corrosion will now depend on the other salts that may be on the surface and the temperature and humidity.
Corrosion products transferred from nearby steel structures can stain a copper surface. These stains will show as a contrast to the patina on the copper alloy. They are difficult to remove without damaging the existing oxide or patina on the surface. Figure 8.28 shows several examples of steel corrosion particles on the surface of copper work.
Foundries work to eliminate moisture from the surface by heating the metal up prior to applying the patina or protective waxes. This will greatly reduce the presence of moisture on the surface of the casting. One issue, however, can be moisture entering the casting due to porosity. Some castings may contain pinholes and cracks that allow moisture to enter into the hollow interior. If there are steel structures or pins used to anchor the castings, these can corrode and the steel corrosion products leach out, staining the surface (Figure 8.29). These are difficult to remove without harming the patina on the bronze casting. If there is still a decent wax coating or lacquer coating, then the rust runoff will be on the coating and not the patina. Removing the wax or the lacquer will take off the rust with it.
FIGURE 8.28 Steel corrosion products on several surfaces.
FIGURE 8.29 (Left, top right) Rust stains on cast sculpture; (top right) before and (bottom right) after removal of the protective coating.
Another issue that can develop given the porous nature of cast metal occurs when the foundry has left the core or portions of the core inside the casting. The core is the interior fill material that occupies the void in a bronze casting. The core material, called “luto,” is made of refractory material such as baked plaster and silica molds that have been crushed into a fine powder. Some of this will stick to the interior side of a casting when heated and needs to be broken off when the casting is completed.
When the cast sections are removed from the mold, the interior surface usually has the refractory material removed mechanically. Depending on how a casting is made, removing all the core material can be difficult. When core material remains, it can absorb moisture that enters though gaps and pores in the metal and then leaches out, creating areas where the copper, patina, and the core moisture react to form blisters. Figure 8.30 shows significant blistering of the surface of a sculpture that had core material left in it. The water leaching out from the interior has ruined this surface.
FIGURE 8.30 Blistering on the surface due to the core remaining inside the sculpture.
Such damage does not reveal itself initially. The sculpture in Figure 8.30 was set outdoors and it took a year for the damage to begin to appear. Once it did, however, it spread across the surface. This kind of issue requires moisture to manifest itself. If the piece had been placed inside, this would not have occurred—but in any case removing the core is standard practice for reputable foundries.
REPAIRING PATINAS
On the occasions where the patina has been damaged or needs to be restored, knowing what the metal constituents are or what the closest alloy is, is helpful. If any welding is needed you will want to know what alloy you are working with before you begin. If this information is not known, there are methods available to determine the metal constituents and the closest alloy type. One method commonly used by the recycling market to determine the various component materials being recycled is called X‐ray fluorescence (XRF).
Determining the Alloy
Figure 8.31 shows a light fixture before restoration on the left and after restoration on the right. To determine what the manufacturer used to produce the piece many decades before, XRF readings were taken of the piece. The analysis showed that the alloy used in the casting was C85800.
FIGURE 8.31 Antique light fixture before (left) and after (right) restoration.
In this case we used a portable XRF device. XRF analysis involves bombarding a surface with gamma rays and reading the “signature” photon of the surface atoms. Essentially, if the energy of the high‐energy gamma rays hitting the surface atoms of an object is greater than the ionization energy of the atoms, electrons in the inner shells can be dislodged. When this occurs, the atom will be unstable and electrons in the outer shells will fall into the inner shell to fill the gap left by the inner‐shell electron. This releases energy specific to the differences of the two electron orbitals. This specificity is a signature of metal atoms and the XRF compares this released energy to a database.
Figure 8.32 shows two readings taken of the top portion of the fixture. The XRF device reads the energy and compares it to a database of energy levels to identify the precise atoms and their percentages to determine which alloys closely match.
From the readings it was apparent that the small leaf forms were cast from a leaded brass that closely matched the C34000 alloy. More likely was that the cast alloy would have been close to C85800, also known as leaded yellow brass. This was a common alloy used to cast small ornamental parts in die cast molds. The size, shape, and detail of these leaves point to this. The top spun lid showed as C26000 (Cartridge Brass) in the XRF. This is most likely the case, since C26000 is a common spun alloy of copper. Readings are comparisons to what is in the data base. Further inquiry is needed to determine what adjustments are needed to interpret the readings correctly.
When restoring the patina, isolating the area as much as possible is suggested. That is, limit the surface to be restored to as small a region as practical, otherwise you will be stripping the entire surface and repatinating. Sometimes this might be the only choice.
FIGURE 8.32 XRF readings of the top section of the light fixture.
Once you know the metals, obtain samples of similar alloys and test the patina formulas that when applied will develop a close match to the color.
To begin, remove the existing patina using light abrasion and 60% phosphoric acid.
Laser ablation will also work well. Blasting the large surface area and taking it back to bare copper alloy will be required.
Correct any surface defects by sanding and polishing to match adjacent regions. Immediately after the base metal surface is exposed, begin applying the patina.
Develop the new patina, preferably with heat. Slowly build the color back to closely match the surrounding surfaces by stippling, brushing, or spraying the patina onto the warm surface. You need to keep the surface temperature around 90–95 °C (200 °F). Rinse the area and compare it to the rest of the surface in different light. Once you are satisfied that a close match has been obtained, coat the surface with clear lacquer or wax it. Again, compare the area to the untouched areas to see if the color gloss and sheen are acceptable. Often rewaxing or relacquering the entire surface helps to blend the appearance.
PROTECTING THE SURFACES OF COPPER AND COPPER ALLOYS
In the art world, cast bronze sculpture is often protected from the environment with a thin layer of clear wax. The general technique is to gently heat the surface of the sculpture and brush on a coating of wax. The heating is believed to open the pores on the surface, which the wax enters as it melts. The wax is applied in layers, and preceding applications of wax are allowed to dry before applying the next layer. Once applied the wax can be buffed with a clean cotton cloth to produce a smooth, continuous layer over the surface.
The heating of the surface also removes moisture that may reside in the pores or in recesses on the surface of the sculpture. Wax applications over moisture or oily residue will fail. Water or oil will prevent the wax from adhering to the surface of the copper alloy. Small remnants of moisture will create small pores through the wax, allowing the external environment to reach the copper alloy. Therefore, it is very important to have a clean, dry, oil‐free surface before applying wax to the surface.
Waxes
Waxes are hydrophobic, meaning they will not dissolve in water and water tends to bead and shed on a wax‐coated surface. Waxes are soluble in nonpolar solvents, such as mineral spirits. Wax coatings have good adhesion to metal surfaces and to oxides and patinas created on the surfaces. They do not adhere well to other wax coatings, though. One or two layers of wax are all that is needed. Beyond that, the surface texture gets gummy because of wax building up on the surface. A thin wax layer is more desirable than a thick wax layer. A buffed and polished wax surface will impart a clear, hard, elegant surface, adding depth and luster to the copper alloy.
When applying wax, the surface should be warmed. This aids in the flow of the wax deep into the metal surface. Allow the wax to dry for several hours before buffing the surface. Buffing the wax spreads the wax crystals out over the metal surface, forming a tight barrier of protection. Use clean, soft, cotton rags or buffing pads to work the wax into the surface and produce a smooth lustrous film. You can tell if the wax has not sufficiently dried if the surface has a gummy feel as it is rubbed.
Natural waxes are produced by plant and animals. These are long alkyl chains that contain alkanes, fatty acids, and alkyl esters. Beeswax is an example of an animal‐based wax, while carnauba wax is an example of a plant‐based wax. Beeswax is produced from the secretions of worker bees. A worker bee has glands on its sides that secrete the wax as the hive is created. Refined beeswax is white and noncrystalline. It is sensitive to temperature and humidity changes, and because of this its use as a protective wax on metals is limited. Additionally, beeswax can have a corrosive effect on copper alloys as it decays.
The carnauba wax is a general‐purpose hard wax used for protecting sculpture as well as floors and automobile surfaces. It is used in numerous other products, from chewing gum to lipstick. Carnauba wax comes from Copernicia prunifera, a Brazilian palm tree that is known as the wax palm or carnauba palm. This hard wax is extracted from the underside of the palm leaf. It has a high melting temperature and often is mixed with other waxes to increase their melting temperatures. Carnauba waxes can be applied to produce hard, hydrophobic surfaces. When they degrade, however, these plant‐based waxes can become acidic. For sculptures manufactured from copper alloy, this decrease in pH will usually not be a concern.
Another wax derived from plants is candelilla wax. This wax is made from the leaves of a small plant called the candelilla that grows in southern United States and northern Mexico. The wax is composed of hydrocarbons and is usually mixed with other waxes. It is yellow in color.
Synthetic waxes are produced during the petroleum refinement process. Two major types used in art and architecture as protective waxes for metal surfaces are paraffin waxes and microcrystalline waxes.
Paraffin waxes are large‐crystalline waxes produced by a dewaxing process in the production of paraffin distillates. These waxes have low toxicity and good water‐repellent characteristics, but the large crystalline structure makes them difficult to work into metal surfaces.
In common use as a protective coating for copper alloys are the microcrystalline waxes. Microcrystalline waxes are petroleum‐based waxes with good flexibility and hardness. They are made up of fine particulates (thus the name) and will coat a surface evenly. The small crystals are long branching carbon chains that give these waxes good flexibility. Sometimes mixed with carnauba wax to create blends, microcrystalline waxes are considered to be more stable than carnauba waxes or animal waxes and will not become acidic as they degrade. These waxes have good water‐repellant characteristics. A microcrystalline wax known as Renaissance Wax™ was developed by the British Museum in the mid‐1900s. This is a common wax for protecting metal surfaces and will remain at a neutral pH as it degrades.
The harder carnauba wax is better suited for outdoor sculptures than the basic microcrystalline wax. A blend of the waxes gives best results outdoors.
Butchers wax and Bowling Alley wax are blends of microcrystalline wax and carnauba wax. There are other blends on the market as well, but the combination of microcrystalline and carnauba wax has shown good durability and performance.
| Wax | Characteristic |
| Beeswax | Temperature sensitive |
| Carnauba wax | Hard and durable; hydrophobic; higher melting temperature |
| Candelilla wax | Hard; yellow in color; usually used in a blend with other waxes to facilitate hardening |
| Paraffin | Large‐crystalline structure; difficult to apply; synthetic |
| Microcrystalline | Synthetic wax with good durability; lower melting temperature; high molecular weight; should be blended if used on exterior sculpture |
| Blend | Improved characteristics by combining waxes |
The biggest drawback to wax protection of copper alloy sculpture and artifacts is the short life span of the compounds. Over time they will dry out and shrink. Degradation first appears as whitish or yellowing streaks on the surface. As further degradation occurs, the wax coatings crack and wear away from the surface, leaving the metal—or the metal oxide, in the case of a patina—exposed. You can expect a service life of two to five years when the wax is applied correctly. Temperature and humidity will determine the long‐term performance of waxes, as will abrasion from human interaction or other contact.
When salt‐laden moisture is able to pass under the wax coating and become trapped against the copper alloy sculpture, the potential for bronze disease can arise. Spots of pale green will appear as an indication that the wax coating has been breached and copper chloride compounds are forming.
Wax coatings can be more easily removed from the surface of the metal with the use of nonpolar solvents, such as naphtha‐based solvents like mineral spirits. This is a distinct advantage over lacquers, which entail more rigorous polar solvents. To remove the wax, heat the surface of the metal up to around 90 °C (200 °F). Moisten a clean rag with mineral spirits and wipe the surface down, the old wax will loosen and come off onto the rag. Exercise caution when using open flames near mineral spirits.
Organic Coatings: Lacquers
Lacquer coatings were created specifically for copper alloy surfaces. They contain oxide inhibitors and ultraviolet radiation absorbers. The oxide inhibitors are specially formulated to combine with copper atoms on the surface, effectively changing the energy level and the drive to combine with oxygen.
There have been numerous tests using azoles, amines, and amino acids.1
In art and architecture, the most commonly prescribed lacquer, Incralac, is composed of benzotriazole in an acrylic resin known as B‐44 and a solvent blend. Ultraviolet absorbers and a chelating agent to combat under‐film corrosion are usually included. Chapter 4 describes the application of this coating.
Benzotriazole is the combination of an organic benzene molecule with a triazole molecule forming the organic molecule C6H5N3. It is the nitrogen ion that appears to provide the molecule with the oxidation inhibitor for copper. The nitrogen ion has free electron pairs that bond with the copper ions on the surface. This effectively prevents the copper ion from bonding with oxygen. The mechanism is referred to as “chemiabsorption,” in which the surface of the copper bonds with the benzotriazole molecule at the nitrogen site. Refer back to Figure 4.11 for an image of the molecule and where it is thought to bond with the copper atom at the surface. Further, the triazole molecule fills voids and shields the copper with a very thin layer across the surface.
There are other azoles that have shown promise in inhibiting oxidation on the surface of copper alloys. These involve the tetrazole molecule. This synthetic molecule also has nitrogen in the compound (CH2N4). The compound phenyltetrazole C7H6N4 is also a good corrosion inhibitor and works in a fashion similar to benzotriazole.
There are two issues with acrylic lacquer coatings: under‐film oxidation in the form of small dark spots and under‐film oxidation along the edges in the form edge darkening. These issues only present themselves on copper alloys that are polished and intended to show natural copper alloy color tones. When darkened by statuary oxidation processes or patinated to develop rich color tones, this darkening may occur, but it will be concealed by the lack of contrast.
This type of oxidation is caused by improper preparation of the metal surface. Moisture left in the pores of the metal from rinse water or condensation that formed prior to the application of the lacquer will slowly darken the metal surface under the lacquer. The curing of solvents can also create this darkening. The solvents can react with ultraviolet radiation and cause oxidation reactions under the film. This problem can also arise if the chelating component of the lacquer has deteriorated or the coverage was insufficient.
It is important to use a good copper corrosion inhibiting lacquer with proper chelating and ultraviolet light inhibitors. It is extremely critical to start with a clean, dry surface.
When the lacquer fails, it must be removed from the surface. The oxide must be eliminated by treating the metal surface and this may entail restoration of the finish or patina on the copper alloy. This can be a costly exercise, in particular on large surfaces.
The best route is to prevent under‐film darkening from occurring in the first place. This can be done by heating the metal to drive the moisture out before coating it with the clear lacquer. Follow with a wipe or rinse of 2% benzotriazole dissolved in alcohol. This establishes an oxide inhibitor across the surface so that subsequent handling and temporary exposure will not allow moisture to take hold. Be cautious about using flammable substances such as alcohol and lacquers near heat sources. Apply the first coating of Incralac, diluted to a 3:1 ratio of Incralac to xylene or toluene. Allow this to dry thoroughly. Apply a second coating, usually less dilute, and then a final layer of the lacquer. Allow the Incralac to dry thoroughly between each coating.
Degradation of Organic Coatings
The organic coatings used on metals are made of polymers. A polymer is a long‐chain molecule made up of monomers and linked together by covalent bonds. As an example, polyethylene is a polymer of the monomer ethylene. These covalent bonds link the molecules together into chains.
In the acrylic lacquers used on metals the acrylic polymer has a photo stabilizer added to prevent photodegradation of the polymer chain. The benzotriazole helps to stabilize the acrylic against sunlight damage. So, it works both as an ultraviolet stabilizer for the acrylic and as an oxidation inhibitor for the copper.
When the acrylic coating fails, the failure begins first along the edges where the coating is usually thinnest. The coloration will appear darker than the rest of the surface. On patinated surfaces the darker color may be concealed, and eventually moisture and pollutants can change the color of the patina. But on natural polished or brushed surfaces and on light‐to‐medium statuary finishes there will be a darkening effect.
Inorganic Coatings
Inorganic coatings used as oxide inhibitors have shown promising results. These are considered alternatives to the organic lacquer coatings because they do not degrade as the organic coatings do. Organic coatings have a workable useful life of from 7 to 20 years, depending on quality of application, exposure and maintenance. Some have performed outdoors for longer periods.
Silanes and siloxanes polymers are inorganic coatings used as corrosion inhibitors. These are organosilicon molecules that are very hydrophobic. These silicon‐based compounds are used as coatings to repel moisture. They are the basis for many anti‐fingerprint coatings. These coatings form a thin layer over the copper alloy surface that is only a few atoms thick. The major drawback is they cannot be removed easily. They will swell when addressed with solvents, but solvents do not dissolve them. Siloxanes cure into cross‐linked molecules that are highly chemical resistant. The only way to remove them is to break the silicon bonds. Acids and alkalis are ineffective. There are commercial silicon‐bond “digesters” available. They are applied to the surface and allowed to set for a period of time as they break the bond with the copper alloy surface. But, as with the organic coatings, this is impractical, if not impossible, for large scale surfaces.
Oils
During the past century, many bronze sculpture and even copper architectural surfaces were coated with drying oils. Several of the more common were linseed oil, lemon oil, paraffin oil, and castor oil.
Linseed oil was perhaps the most favored of these oils. It once was used as a sizing in gold‐leafing applications. Not in common use today, this oil has good hydrophobic qualities. Linseed oil is a natural hydrocarbon‐based oil derived from flax. When it dries it leaves a hard, clear coating. It has been used as a protective coating for copper roofing and sculpture in the past. Linseed oil consists of unsaturated fatty acids, stearic acids, and palmic acids. It dries and eventually weathers off of exposed surfaces.
The other oils mentioned—lemon oil, castor oil and paraffin oil—also act as hydrophobic coatings on the metal surface. These oils are infrequently used today on metal art and architecture.
CLEANING THE COPPER SURFACE
A clean copper alloy surface is necessary to ensure good contact and coverage of applied inorganic or organic coatings. The creation of statuary and patina surfaces requires a clean metal surface. All coatings, whether applied or chemically induced, must link to the metal surface. Oils, excess moisture, and other substances on the copper alloy surface will interfere with that contact. Copper alloys can be cleaned and degreased effectively by using alkaline solutions, such as degreasers with sodium hydroxide or potassium hydroxide. These will remove oils, grease, and other organic emulsions that can be left on copper surfaces during manufacturing and handling processes.
When copper alloys are heat‐treated with lubricants on the surface, they can become fixed into the metal and will require a robust treatment to remove them. Acid treatments consisting of a mixture of sulfuric and nitric acid work well in removing thick oxides. Mechanical means may also be needed to remove thicker oxides.
Light tarnish and oxide can be removed using milder acidic solutions, such as phosphoric acid and concentrated citric acids. These will remove cupric oxide but will not remove cuprous oxide.
The following is a list of various chemical solutions that have shown success in cleaning copper alloys:
- Ammonium citrate (5% solution)
- Citric acid (20% solution plus 4% thiourea)2
- Phosphoric acid (10–20% solution plus 1% thiourea)
- Ethylenediaminetetraacetic acid (EDTA) (4% solution)
- Potassium sodium tartrate (25% solution)
- Sodium hydroxide with glycerol (120 g/40 g per 1 l H2O)
- Polymethacrylic acid (10–15% solution)
Copper alloys can be electrocleaned effectively in 2–5% sodium hydroxide solution.
Selective electrocleaning with 60% phosphoric acid solution is another effective way of removing oxides, dezincification deposits, and cuprous oxide formations from the surface of copper alloys.
REMOVING COPPER STAINS FROM OTHER SUBSTANCES
As copper and some of the copper alloys develop their natural patinas, often some of the compounds involved will dissolve into moisture from rain and condensation. If there are porous, light‐colored substances below or in nearby proximity to copper surfaces, copper salts can redeposit into their pores by means of capillary action. Figure 8.33 shows images of a 100‐year‐old stoplight made from Monel. Monel is a nickel–copper alloy with excellent corrosion resistance once favored as a sheet‐metal material. The copper gave it good formability. As this alloy weathers it turns from a gray nickel tone to a ruddy brown color. Monel has about 30–33% copper and it was the copper that left the stain on the limestone base of the light shown in the left‐hand image.
These copper deposits can have a detrimental esthetic effect on appearance. Limestone, concrete, travertine, and other light‐colored, porous materials can show these streaks and stains. The deposits can be tenacious and removing them can be very difficult. Steam, pressure washing, and detergents have little effect in the removal of the stains. Current approaches involve introducing a solution or paste that combines with or displaces the copper and allows it to be lifted out of the pores.
There are several commercial treatments that have proven results. These involve ammonia: usually ammonium carbonate made into a thick paste or poultice. Other substances are added to act as a surfactant or chelating agent and allow the copper salts to be displaced.
EDTA and Laponite® are often added to aid in the displacement. The poultice is allowed to remain on the stone surface for several hours and then rinsed off. Due to the heavy ammonia base, other surfaces as well as the workers should be protected. Powerful ammonia applications will require appropriate ventilation and eye, skin, and lung protection at a minimum.
FIGURE 8.33 Cleaning the copper stain from a limestone base, before (left) and after (right).
There are additional treatments that are showing promise in removal of these stains by less hazardous means.3 One uses alanine, an amino acid. Amino acids are organic compounds; due to their makeup of both amino and carboxylic functions they can take on many forms. They are often used as release agents, detergents, and dyes, among other things. When accompanied with lesser concentration of ammonia, the alanine is found to be very effective in removing copper stains without presenting the risks associated with the handling and disposal of hazardous materials.
DETERIORATING PATINAS
There are occasions in which the patina on copper surfaces can deteriorate or fail. This can occur when other substances, such as iron or chemical sprays, have gotten on the patina surface. Muriatic acid, a dilute form of hydrochloric acid used to clean stone, brick, and concrete can cause an artificial patina to lift from the copper surface. Ammonia‐based cleaners used on glass or ceramic will affect the patina's color and can cause the patina to lift from the underlying metal. Even water, if it is allowed to sit on the surface, can damage a patina.
Furthermore, if the patina was improperly applied or the surface of the copper alloy was not correctly prepared and cleaned, it can lead to the patina or oxide sluffing off of the surface or becoming streaked. To address these conditions, it may be necessary to remove all or part of the patina from the surface.
Attempts to touch up areas can be futile. For sculptures, isolated areas can be addressed. The geometry of the sculptural form may allow for conditional limits that can conceal the transition from the older patina to the new patina. Many in conservation address small patina blemishes by utilizing infilling techniques that use paint or colored waxes. This will work for a while, but eventually the surface will need to be addressed with a more in‐depth restoration.
On architectural surfaces, it is ill advised to attempt applying fixes to a patina failing in place. The copper alloy surface may have a light oxide around where the patina has come away from the surface. Selective preparation of the surface will be difficult and matching an existing finish, in particular a shop‐produced finish, will not be possible due to the variables involved.
Removing the patina from the entire surface and either allowing it to grow back over time (which will take decades) or replacing it with shop‐prepared metal is recommended. Even if you decide to apply the patina on the surface in situ, then you must start with removing the oxide back to the bare metal surface.