Conservation of metals.
- Lucy Branch
The conservation of metals begins with the process of assessing the condition, stability, and losses or alteration that may have occurred to metal objects. Conservators are then in a position to manage risk and take preventative measures to protect the longevity of the material as well as consider a variety of treatments to repair any damage.
See also Metal
1. Challenges to the stability and longevity of metals.
Each metal has particular conservation issues related to the metal species or alloy used, how the object was made (cast, beaten, formed, rolled, machined, or extruded), whether or not a decorative finish was applied, and the nature of the environment to which the object was exposed. Condition issues can be identified as being either structural or surface related.
Structural instability can be the result of manufacturing flaws or physical damage. How the object was made can in turn dictate how an object should be treated. For example, a dent in a cast bronze sculpture should not be hammered out due to the crystalline structure of cast metal; the most likely outcome would be the formation of a crack (Selwyn 2004, 55).
Decorative finishes may include gilding, silvering, patination, painting, embossing, stamping, and polishing. A key characteristic of metals is their ability to change over time, and this is often seen as adding value to the object (Hughes 1993, 2–17). The unavoidable oxidation of metals which may enhance a sculpture’s intended patina provides their unique aesthetic qualities while also leading to their instability.
Apart from gold, all metals have a propensity to oxidize when in contact with air, water, and electrolytes such as salts or pollutants. This type of corrosion is electrochemical in nature, involving the exchange of electrons on the metal’s surface leading to mineralization (Selwyn 2004, 19). The corrosion layers formed can be specific to the environment to which the metal has been exposed. For example, if the electrolyte contained pollutants such as dissolved sulfur dioxide then the formation of copper sulfate on the surface is extremely likely. Corrosion proceeds inwards, permanently altering the surface of the metal.
Stability is not certain after a corrosion layer forms. Though some corrosion layers such as aluminum oxide are poor conductors and reduce the potential for further activity on a metal’s surface—a process known as passivation—others, such as silver or copper sulfides, are not and corrosion may proceed below the upper layer. Some electrolytes are also more destructive than others. The presence of chlorides and moisture can make a bronze surface very unstable and if left unchecked can result in significant metal loss (Logan and Selwyn 2007; Mattsson 1982, 9–19; Payer 1992, 103–121).
2. Assessing metals.
A good initial assessment of the condition of a metal object can be done by a trained eye since many of the most common corrosion products have distinct forms and coloration, but analytical instrumentation such as Raman spectroscopy, energy-dispersive X-ray spectroscopy (EDX), and Fourier transform infrared spectroscopy (FTIR) can be useful in conclusively identifying oxidation products. Polarized light microscopy can be helpful when dealing with objects having decorative finishes involving paint or lacquer layers. X-ray fluorescence analysis can be used to detect alloying elements including gold (Birnie 1993, 150). Endoscopes can be employed to investigate the condition of the internal walls of sculptures and any armatures if present (Selwyn 2004, 15). Finally, radiography is very helpful in assessing the overall condition of metal objects that cannot be perceived by the naked eye including internal cracks, previous repairs, and extant evidence of manufacturing techniques.
3. Environmental threats and preventative conservation.
The environment to which a metal object has been exposed dictates its present condition and prognosis for future stability once preventative steps have been taken.
In general, metal objects exposed to the elements will suffer the greatest damage due to a number of threats:
Rain and groundwater. Corrosion cannot occur without moisture and precipitation advances the degradation process in a particular way according to the impurities it has absorbed including natural gases such as carbon dioxide and those associated with industry and combustion: sulfur oxides, nitrogen oxides, hydrogen sulfide, soot, transition metals, ammonia, and sulfates (Sherwood 1992, 38).
Chlorides and other salts deposited during burial or by exposure road salting or marine air. In the presence of moisture, chlorides form hydrochloric acid and cause an aggressive, powdery corrosion product that causes pitting and can severely disrupt the surface.
Guano and detritus from the air, trees, insects, and birds. These substances often have a high concentration of compounds such as ammonia, urea, and phosphates that promote metal corrosion.
Both vandalism and adoration by the public. Graffiti, whether sprayed, written, or carved into the metal, can damage surfaces, while constant stroking and handling may alter or entirely remove patinas due to contact with sweat from hands. Over time, the metal itself may become thin due to abrasion.
Although they may be protected from the elements, metal objects kept indoors are also susceptible to damage due to the following factors:
Humidity fluctuations and condensation due to changes in temperature. This situation may provide the moisture required for metal corrosion.
Exposure to unstable storage and display conditions and materials. This may include other objects on display in proximity to the metal, display case materials, packing, and storage containers (Hackney 2016, 55–69). Concrete, wood, wood composites, adhesives, and textiles can release organic acids, formaldehyde, and hydrogen sulfide that promote corrosion (Pye 2001, 86–87).
Handling and transportation of objects. Handling, packing, shipping, and mounting always involve risks including knocks, scuffs, and scratches.
The best way to preserve metal objects housed internally is to maintain a stable environment with a relative humidity below 40%, isolate them from airborne pollutants and/or include absorbents in display cases to reduce their concentration (Ankersmit et al. 2000, 7–17), and ensure that display and storage case materials are free of damaging off-gassing materials. In some cases, a protective coating may be advisable.
Preventative care for outdoor metals should involve the periodic removal of deposited pollutants, guano, and other detritus from a surface followed by the application of a protective coating or barrier layer such as microcrystalline wax. Waxes are almost invisible and easily reversible. However, they are only partial barriers and need regular reapplication to be effective.
Harder films made of organic resins can be useful either as paints or clear varnish coatings. There are numerous proprietary formulas on the market that combine polymers, solvents, corrosion inhibitors such as benzotriazole, ultraviolet absorbers, and antioxidants (Copper Development Association Inc.). Lacquers are more robust than waxes, but these coatings tend to be visible, easily scratched, and harder to remove if they have been in place for an extended period of time. In addition, they are difficult to apply in an outdoor setting.
4. Remedial treatments.
The emergence of conservation, born out of a fusion of art restoration and material science, has permanently changed for the better the way that metal objects are treated. A greater respect for an object’s integrity and the value of its history have become recognized principles in conservation (Pye 2001, 53) and have directly affected the choices made by metal conservators. Techniques used to clean metals in the past were often aggressive, involving chemically stripping off corrosion layers down to bright metal surfaces, sandblasting, caustic cleaning with wire brushes, submerging in baths of alkaline Rochelle salts, or dilute sulfuric acid (Scott 2002, 354, 355). Today, corrosion layers are no longer automatically swept away as they may contain evidence of an object’s original surface and subsequent history. There are several processes and methods that are commonly used to treat metal objects today. Choosing a treatment will depend on many factors including scale of the object and the circumstances in which the treatment will take place.
Removing dirt and grime can be undertaken by hand with soap solution or solvents. Large metal objects might require steam cleaning or super-heated water. Corrosion removal can involve mechanical cleaning by hand using scalpels, abrasion with pumice pastes, copper wool, steel wool, or fine phosphor bronze brushes. Chemical cleaning using reagents applied in gel form, chelating agents, or buffered acids can work well if carefully controlled. Wet or dry air abrasion using pressurized air and an aggregate such as calcite, walnut shells, glass beads, and ice is particularly useful when cleaning large objects (Turner 2002, 31–33). Laser cleaning, which is probably the subtlest of all cleaning tools on metals, removes corrosion with an intense and pure form of light (Cooper 2002, 34–38; Naylor 2002, 40–42).
The methods touched on above have the potential to improve the condition of metals, but also in untrained hands can cause considerable damage. Many of the corrosion removal methods are irreversible treatments and so their employment must be fully considered. It is for this reason that remedial treatments of metals should not be attempted by those without specific metal conservation experience.
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