Saving Corroded Coins: Conservation Techniques That Work

Corrosion threatens coins of all compositions, from ancient bronze to modern clad issues. Understanding conservation techniques helps collectors preserve their coins while avoiding interventions that cause more harm than good.

Understanding Corrosion

Corrosion results from chemical reactions between metal and environmental factors:

Types of Corrosion

Oxidation: The most common form, creating tarnish on silver and patina on copper. Natural oxidation can be attractive and protective.
Verdigris: Green corrosion on copper and bronze caused by moisture and acidic conditions. Active verdigris continues spreading and must be stabilized.
Bronze disease: Aggressive corrosion appearing as powdery green spots on bronze. This active condition destroys coins if untreated.
PVC damage: Soft plastic holders containing PVC damage coins through chemical reaction, causing green, oily residue.

Causes

Environmental factors promote corrosion:

Humidity accelerates chemical reactions
Fingerprint oils contain acids that attack metal
Improper storage materials off-gas damaging chemicals
Pollutants in air react with coin surfaces

When to Conserve

Not all corrosion requires intervention:

Leave Alone

Stable, attractive toning on silver
Smooth, even patina on copper and bronze
Ancient coins with stable surfaces
Any coin where intervention risks greater harm

Consider Treatment

Active corrosion spreading across surfaces
PVC residue requiring removal
Encrustation obscuring design details
Bronze disease requiring stabilization

Conservative Approaches

Always start with the least invasive option:

Distilled Water Rinse

Simple rinsing removes loose surface contamination without chemical intervention. Use only distilled water to avoid mineral deposits. Pat dry with soft, lint-free material.

Acetone Soak

Pure acetone removes organic residue including PVC damage without affecting metal. Use only pure acetone (not nail polish remover, which contains additives). Brief soaking followed by air drying is usually sufficient.

Mechanical Removal

For localized encrustations, careful mechanical removal using wooden toothpicks or similar non-metal tools may be appropriate. Work under magnification and proceed slowly.

Professional Conservation

Valuable coins deserve professional attention:

NGC Conservation

NGC offers professional conservation services. Trained conservators evaluate each coin and apply appropriate treatments. Conserved coins can then be graded and encapsulated.

Independent Conservators

Specialized numismatic conservators work outside grading services. These professionals often handle ancient coins and specialized conservation needs.

When to Use Professionals

Coins of significant value where amateur treatment risks loss
Active bronze disease requiring stabilization
Complex conservation needs beyond simple cleaning
Coins intended for grading service submission

What to Avoid

Some common “cleaning” methods damage coins:

Never Use

Abrasives: Baking soda, toothpaste, pencil erasers, and polishing compounds remove metal and destroy surfaces.
Acids: Acidic solutions (vinegar, lemon juice, commercial dips) strip patina and can etch surfaces.
Wire brushes: Metal brushes create scratches that are always visible.
Ultrasonic cleaners: These can damage surfaces and drive contaminants into metal.

Understand the Consequences

Improper cleaning permanently reduces coin value. A problem-free coin in lower grade is almost always preferable to a cleaned coin in theoretically higher grade. Grading services note cleaning on holders, and the market significantly discounts cleaned coins.

Prevention

The best conservation is prevention:

Proper Storage

Use only archival-quality holders free from PVC
Store in stable, low-humidity environments
Keep coins away from environmental pollutants
Consider silica gel packets in storage containers

Handling Protocols

Always hold coins by edges
Wear cotton gloves for valuable pieces
Work over soft surfaces to prevent damage from drops
Minimize handling of raw coins

Regular Inspection

Periodically examine stored coins for developing problems. Early detection of active corrosion allows intervention before serious damage occurs.

Special Cases

Ancient Coins

Ancient coins often require specialized conservation. Their surfaces may be fragile, and inappropriate treatment can destroy irreplaceable patina developed over millennia.

Shipwreck Coins

Coins recovered from saltwater environments need immediate conservation to prevent chloride-induced corrosion. Specialized desalination processes are essential.

Archaeological Finds

Coins from archaeological contexts carry information beyond the coin itself. Conservation must preserve contextual evidence alongside the object.

Making Decisions

When facing conservation questions:

Assess whether intervention is truly necessary
Research appropriate methods for the specific problem
Consider the coin’s value and whether professional treatment is warranted
When in doubt, do nothing until you’re certain of the proper approach

Conservation done properly preserves coins for future generations. Conservation done poorly destroys value and historical evidence. The distinction lies in knowledge, patience, and appropriate humility about one’s own expertise.