Coin Preservation and Surface Treatment

Metallic artifacts, including currency, are susceptible to surface degradation due to environmental exposure and handling. Understanding the composition of coinage and the nature of surface accretions is crucial for informed preservation strategies.

Coinage Metals and Their Reactivity

• Gold (Au): Relatively inert, but may develop surface films.
• Silver (Ag): Prone to tarnishing due to sulfur compounds in the air.
• Copper (Cu): Readily oxidizes, forming patinas (copper oxides, carbonates, and sulfates).
• Nickel (Ni): Resistant to corrosion, but can develop surface discoloration.
• Zinc (Zn): Highly reactive, readily forms zinc oxide.
• Aluminum (Al): Forms a protective oxide layer, but susceptible to pitting in certain environments.
• Alloys: Combinations of metals with varying reactivities (e.g., brass, bronze, cupro-nickel). The behavior of alloys is complex and depends on the proportions of the constituent metals.

Types of Surface Contamination

• Tarnish: A thin layer of metal sulfide (silver), oxide (copper, nickel), or other compounds.
• Patina: A chemically stable layer of corrosion products that can protect the underlying metal (copper alloys).
• Dirt and Grime: Accumulated dust, oil, and other environmental pollutants.
• Corrosion Products: Unstable metal oxides, chlorides, or other compounds resulting from environmental interaction.
• Adhesive Residues: Remnants of tapes, glues, or other materials.

Principles of Surface Stabilization

The primary goal of artifact management is long-term preservation. Interventions should be minimal, reversible, and well-documented. Any process that alters the original surface, even minimally, can impact numismatic value and historical significance.

Mechanical Interventions

• Dry Brushing: Use of soft brushes (e.g., sable or microfiber) to remove loose surface dirt.
• Pick Removal: Employing wooden or plastic picks to dislodge adhered material. (Use under magnification)
• Compressed Air: Gentle blowing to remove loose particles.

Chemical Interventions

Chemical treatments should be used with extreme caution, and only when absolutely necessary. Always test the solution on an inconspicuous area first. The selection of chemicals should be based on the metal composition and the nature of the surface contamination.

• Solvent Immersion: Use of distilled water, acetone, or isopropyl alcohol to dissolve surface oils and grime.
• Chelating Agents: Substances that bind to metal ions and facilitate their removal (e.g., EDTA, citric acid).
• Acidic Solutions: Very dilute acids (e.g., acetic acid) can dissolve some corrosion products, but can also damage the underlying metal.
• Alkaline Solutions: May be useful for removing certain organic residues, but can also damage some metals.

Post-Treatment Considerations

• Rinsing: Thoroughly rinse all treated pieces with distilled water to remove any residual chemicals.
• Drying: Air dry in a dust-free environment, or use a gentle heat source.
• Storage: Store objects in inert materials (e.g., archival-quality sleeves or boxes) to prevent future degradation. Consider desiccants for humidity control.

Professional Consultation

For valuable or historically significant pieces, consulting with a professional conservator is highly recommended. Improper actions can cause irreversible damage and significantly reduce the value of the artifact.