Faraday's Law of Electrolysis:
Mass Reduced (g) = (I × t × M) / (z × F)
Where: I = Amperage, t = Time (sec), M = 107.87 g/mol (Silver), F = 96,485 C/mol
When precious metals sit on the ocean floor for centuries, they undergo complex bio-geochemical alterations.
Silver ions slowly react with ubiquitous chloride ions ($Cl^-$) in seawater, creating thick mineral shells of
cerargyrite (AgCl, 'horn silver'). Simultaneously, iron fasteners and cannons nearby oxidize,
generating acidic micro-environments that fuse sea sand, coral, and calcium carbonate into rock-solid marine concretions.
Detail Revealed: Complete Jerusalem cross with lions and castles, full date '1714', and Mexico City 'oM' mintmark.
Market Realization: NGC certifies the specimen as 1715 Fleet 8 Reales (Ch VF). Market value jumps from $150 (unidentified lump) to $2,200.00!
Three Famous Shipwreck Discoveries
Artifact
Condition Upon Recovery
Conservation Process
Auction Realization
1. Atocha 8 Reales Cob (Mel Fisher)
Pitch black with thick horn silver
18-hour electrolytic reduction
$1,500 – $3,500
2. SS Central America 1857-S $20
Pristine uncirculated gold in vault
Distilled water rinse; no acid
$15,000 – $65,000+
3. El Cazador 8 Reales (1783)
Concreted in silver roll cluster
Separated in ultrasonic sodium bath
$350 – $750
Frequently Asked Questions (FAQs)
How does electrolytic reduction restore silver shipwreck coins without destroying details?
Unlike acid dips that dissolve metal, electrolytic reduction uses low-voltage direct current (typically 6 to 12 volts, 0.5 to 2.0 amperes) in an alkaline sodium carbonate (washing soda) electrolyte. The silver coin serves as the cathode (-) and a sacrificial stainless steel or graphite plate acts as the anode (+). The electrical current forces electrons into the brittle silver chloride (AgCl 'horn silver'), converting chloride ions back into soluble solution while reducing metallic silver back onto the coin's surface.
Why are gold coins recovered from shipwrecks usually pristine while silver coins are heavily corroded?
Gold is a noble metal with high electronegativity that cannot react with saltwater, dissolved chlorides, or marine acids. Gold coins recovered from wrecks like the SS Central America (1857) emerge with original blazing mint luster. Silver, conversely, readily reacts with chloride ions in seawater (NaCl) over centuries to form thick layers of waxy, purplish-gray cerargyrite (AgCl) and concreted marine sandstone shells.
What was the 1715 Spanish Treasure Fleet disaster?
On July 31, 1715, a fleet of eleven Spanish galleons laden with millions of silver Reales (Pieces of Eight) and gold Escudos struck a catastrophic hurricane off the coast of Vero Beach and Fort Pierce, Florida. Over a thousand sailors perished, and the sunken treasure remained buried beneath shifting ocean sands until modern salvage operations began in the 1960s.
What is a 'Cob' coin in shipwreck numismatics?
A 'Cob' (from the Spanish 'cabo de barra', meaning cut from the end of a bar) was a crudely made coin produced in colonial Spanish mints (Potosí, Lima, Mexico City) between the 1500s and 1700s. Silversmiths cut rough planchet blanks from cast bullion bars, clipped them to legal weight, and hand-hammered them between dies, resulting in irregular, organic shapes.
How does shipwreck provenance affect certified coin market value?
Coins certified by PCGS or NGC with official shipwreck pedigrees (such as 'SS Central America', '1715 Fleet', 'SS Republic', or 'El Cazador') command dramatic value premiums—frequently 200% to 500% higher than unpedigreed coins of identical date and grade—due to their romantic provenance and museum historical pedigree.
What is marine concretion on treasure coins?
Marine concretion is a hard, rock-like crust formed when corroding iron artifacts (cannons, nails, hull fittings) release ferrous oxides that bind together ocean sand, shells, sea floor debris, and nearby silver coins into a dense conglomerate clump. Conservators use pneumatic air scribes and gentle vibratory tools followed by chemical baths to free individual coins from concretion.