Cast vs Struck Counterfeit Coin Forensic Diagnostics & Acoustic Ping Lab
Analyze metallurgical grain boundaries, gas escape porosity, and Archimedean specific gravity displacement. Synthesize live acoustic resonant ping frequencies via Web Audio API to immediately identify deceptive cast and electrotype fakes.
Coin Specimen Under Test
100% Genuine StruckAcoustic Resonant Frequency Mechanics
The fundamental acoustic resonant frequency \(f\) of a thin circular disc depends directly upon its speed of sound \(v_s\), Young's elastic modulus \(E\), and density \(\rho\):
Where \(h\) is planchet thickness, \(R\) is radius, and \(\nu\) is Poisson's ratio. Because silver possesses high elasticity and uniform grain structure, sound propagates at \(3,650\text{ m/s}\), producing a sharp, clean sine wave that fakes cannot imitate.
Specific Gravity Hydrostatic Formula
Using Archimedes' displacement law, the specific gravity (\(SG\)) isolates fraudulent pot-metal castings from true precious metals without destructive assays:
If an alleged 1893-S Morgan Dollar weighs 26.7 grams in air but displaces volume yielding an \(SG\) of 8.9 instead of 10.34, it is definitively a copper-nickel counterfeit regardless of appearance.
Infamous Numismatic Counterfeit Case Studies
Centrifugal Cast Illusion
Possessed convincing artificial wear to mimic a VF-20 coin. Acoustic ping produced a dull 1,900 Hz thud lasting only 0.4 seconds; specific gravity tested at 8.14.
Solder Seam Micro-Surgery
A genuine 1916 Philadelphia dime with an embossed 'D' mintmark soldered from a donor 1924-D coin. Loupe revealed telltale discoloration along the mintmark perimeter.
The Repeating Depressions
Struck in genuine .900 gold alloy to match weight and SG, but microscopic examination revealed three identical contact depressions repeated across 12 distinct auction lots.
Frequently Asked Questions: Counterfeit Forensics
1. How can you tell a cast counterfeit coin from an authentic die-struck coin?
Die-struck coins are pressed under 50 to 150 tons of hydraulic pressure, forcing the metal into crystalline radial flow lines that produce the famous cartwheel luster. In contrast, cast counterfeits are poured into molten molds, creating diagnostic microscopic gas escape bubbles (porosity pitting), a soft greasy texture, lack of sharp letter serifs, and seam ridges along the rim edge.
2. What is the acoustic ping test and how does it detect fake silver or gold coins?
Every metal alloy possesses a unique speed of sound, Young's elastic modulus, and density. When struck gently, authentic .900 fine silver rings with a high, sustained harmonic chime typically oscillating between 5,800 Hz and 6,200 Hz with a decay time over 3 seconds. Base-metal lead, tin, or brass fakes produce a low, muffled, quickly decaying thud below 2,500 Hz.
3. How does specific gravity hydro-static testing prove coin metal authenticity?
Specific gravity measures Archimedes' buoyancy displacement: \(SG = W_{\text{air}} / (W_{\text{air}} - W_{\text{water}})\). For a 90% silver / 10% copper US coin, SG must measure exactly \(10.34 \pm 0.05\). Counterfeits fabricated from lead-tin pewter or pot metals display distinct specific gravities (e.g. 7.3 to 9.2), immediately exposing the fraud without damaging the coin.
4. What are transfer die counterfeits and how do they differ from simple cast fakes?
Transfer die fakes are struck using forged steel dies spark-eroded or pantographically copied from a genuine host coin. While they possess struck luster, they replicate every microscopic scratch, bag mark, or depression of the host coin onto hundreds of counterfeit duplicates ("repeating depressions").
5. Can an electronic magnetic slide test detect cast counterfeit silver dollars?
Yes. Silver is diamagnetic, meaning it creates an opposing electromagnetic eddy current when exposed to a moving neodymium magnet. A genuine silver dollar will slide slowly and smoothly down a 45-degree neodymium magnet slide, whereas non-silver cast fakes either stick instantly (if ferrous) or slide down unrestrained with zero magnetic drag.
6. Why do cast fakes often have altered or filed rims?
Molten metal must be injected through a sprue gate, leaving a physical stub where the metal entered the two-part mold. Counterfeiters must file down this sprue stub and the mold parting seam, leaving telltale horizontal file marks across the rim reeding.
7. What handheld instruments do professional numismatists use for counterfeit detection?
Top numismatic authenticators rely on 10x-20x triplet aplanatic loupes, digital analytical scales measuring to 0.001 grams, ultrasonic thickness gauges, X-ray Fluorescence (XRF) elemental spectrometers, and acoustic resonance frequency analyzers.