Diagnostic Controls & Presets Numismatic Chemical Forensics Lab
Diagnostic Forensic Readings
Forensic Attribution Diagnostic
Real-time metallurgical and die variety cross-examination verified against certified populations.
Technical Breakdown: Physics, Metallurgy & Die Diagnostics
Coin toning is an optical thin-film interference phenomenon caused by microscopic layers of silver sulfide (Ag2S) reacting with ambient atmospheric hydrogen sulfide. Conversely, polyvinyl chloride (PVC) damage is an active chemical breakdown caused by acidic plasticizers that eat permanently into coin metal.
2 × n × d × cos(θr) = (m - ½) × λ
Where:
• n = Refractive index of silver sulfide nanocrystal layer (~2.2)
• d = Thickness of the toning layer in nanometers (nm)
• λ = Wavelength of reflected constructive light:
• 25–40 nm: Light Gold / Russet (~580 nm light reflected)
• 50–75 nm: Magenta / Violet (~420 nm light reflected)
• 80–120 nm: Peacock Blue / Emerald Green (~500 nm light reflected)
• > 200 nm: Terminal Black (Complete destructive oxidation, zero light return).
Soft vinyl flips contain bis(2-ethylhexyl) phthalate (DEHP) plasticizers. Over time, heat and UV radiation cause dehydrochlorination, releasing free hydrochloric acid (HCl):
PVC breakdown: -(CH2-CHCl)n- → -(CH=CH)n- + HCl
The HCl reacts with copper and silver, forming insoluble green cupric phthalate/chloride goo that pits planchet metal.
Safe Conservation Protocol: Immersion in 99.9% pure ACS Reagent Grade Acetone [(CH3)2CO] dissolves the organic polymer chains and plasticizers in under 3 minutes without oxidizing or reacting with the elemental gold, silver, or copper coin matrix.
Documented Benchmark Auction Records
Common date Morgan dollar (normally worth $1,000 in MS-67) achieved a staggering 38x price multiplier due to jaw-dropping natural textile bag rainbow toning.
Collector purchased a raw key-date cent covered in green flip residue. A 5-minute pure acetone dip eliminated the PVC before metal etching began; certified PCGS MS-64RB.
Submitted by dealer seeking monster toning premium. PCGS rejected the coin with code '91 - Questionable Color' due to unnatural pigment pooling in letters.
Frequently Asked Questions
What causes rainbow toning on silver coins?
Natural rainbow toning is caused by thin-film wave interference. Trace amounts of atmospheric sulfur react with the coin's surface over decades, forming a microscopic layer of silver sulfide (Ag2S) between 20 and 150 nanometers thick. Light reflecting off the top of this thin film interferes with light reflecting off the silver beneath it, producing brilliant prismatic colors.
What is PVC damage and why is it so destructive?
PVC (polyvinyl chloride) damage occurs when coins are stored in soft, flexible plastic flips that contain chemical plasticizers (phthalates). Over time, these chemicals break down and release hydrochloric acid, which actively eats into the coin's metal, leaving sticky green goo and permanent microscopic pits.
Is it safe to clean coins with acetone?
YES, but ONLY with 100% pure ACS Reagent Grade or pure acetone—never fingernail polish remover (which contains oils and scents). Acetone is an organic solvent that dissolves oils, grease, tape residue, and PVC without chemically reacting with silver, gold, or copper. Coins should never be wiped or rubbed; only soaked and air-dried.
How do third-party grading services distinguish natural from artificial toning?
Authenticators look for: (1) Natural progression of colors (following Newton's thin-film color sequence: gold → amber → magenta → blue → cyan); (2) Pull-away effects around stars and letters where the die polished the field; (3) Absence of artificial pigment pooling in recessed fields or across post-strike scratches.
What are 'milk spots' on modern silver bullion coins?
Milk spots are cloudy, milky white blemishes frequently appearing on modern silver coins (like American Silver Eagles and Canadian Maple Leafs). They are caused by trace chemical detergents used to clean planchets that were not fully rinsed off before being struck in the annealing furnace.
Does natural rainbow toning increase the value of a coin?
Dramatically so! Eye-appealing original rainbow toning on Morgan dollars, classic commemoratives, and Franklin half dollars can add a premium of 200% to over 3,000% above the value of an untoned white coin in the exact same grade.
Can coin dip (acid thiourea) remove toning safely?
Commercial coin dips contain sulfuric acid and thiourea, which strip toning by permanently dissolving the top microscopic layer of silver metal. Repeated dipping destroys original cartwheel mint luster, leaving surfaces dull, lifeless, and graded as 'Cleaned' by PCGS and NGC.