Old coin which fluoresces under short wave UV.
- By sjastro
- Physical & Life Sciences
- 46 Replies
(3) Why does copper especially migrate to the edges.
The next post is about I theory I had at the time about how electrolysis removed florescence from a Japanese Yen coin.
Okay it has a LaTex related problem in expressing the reaction Cu -> Cu² + 2e⁻, I will have to do more research on this (on the mechanism not LaTex).Copper does not “choose” the edge in a conscious sense — the edge lights up because physics, metallurgy, and corrosion kinetics all bias copper toward rims and boundaries over long timescales. On an ancient silver coin, the edge is where every relevant process overlaps.
Below is the clean, mechanism-by-mechanism explanation.
1. Metallurgical reason: grain boundaries intersect the edge
Ancient silver coins were:
This leaves a microstructure with:
- Cast or hot-worked
- Cold struck between dies
- Never homogenised by modern heat treatment
Grain boundaries:
- Silver-rich grains
- Copper preferentially segregated to grain boundaries
➡ The edge is where the most grain boundaries are exposed, so copper is already enriched there from the start.
- Have higher free energy
- Are fast diffusion pathways
- Terminate at the surface, especially at the edge
2. Mechanical reason: striking concentrates defects at the rim
When a coin is struck:
These defects:
- Metal flows radially outward
- The rim experiences:
- Highest plastic strain
- Highest dislocation density
- Microcracking and work hardening
➡ Copper migrates faster where the lattice is already damaged — the rim.
- Lower activation energy for diffusion
- Act as pipelines for copper atoms
3. Electrochemical reason: copper is sacrificed first
In a silver–copper alloy:
In moisture, soil water, or air films:
- Copper is anodic
- Silver is cathodic
Edges:
- Copper preferentially oxidises:
[
\text{Cu} \rightarrow \text{Cu}^{2+} + 2e^-
]- Silver is protected
➡ Copper at the rim is continuously drawn to the surface and removed as corrosion products, pulling more copper from below.
- Retain moisture longer
- Are thinner and more exposed
- Have higher oxygen access
This is dealloying, not simple diffusion.
4. Chemical reason: silver enrichment pushes copper outward
As copper is removed:
Edges have:
- The surface becomes silver-rich
- This creates a chemical potential gradient
- Copper from deeper regions migrates outward to re-establish equilibrium
➡ Copper supply is preferentially drained from the rim.
- Shortest diffusion distance
- Highest corrosion rate
5. Geometric reason: curvature accelerates corrosion
The rim has:
Corrosion rates scale with:
- Higher curvature
- More exposed surface per unit volume
- More frequent micro-damage
➡ Even tiny differences become dominant over centuries.
- Surface energy
- Curvature
- Defect density
6. Why this takes centuries (and why modern coins don’t show it)
This process is:
It requires:
- Thermodynamically slow
- Diffusion-limited
- Corrosion-driven
Modern silver coins:
- Repeated wet/dry cycles
- Oxygen access
- Long burial or atmospheric exposure
- Are homogenised
- Have cleaner alloys
- Have fewer defects
- Haven’t had 1,900 years
7. Why this produces fluorescence specifically at the edge
Because the edge ends up with:
✔ Copper oxides
✔ Copper carbonates
✔ Defect-rich corrosion phases
✔ Minimal silver quenching
These phases fluoresce under short-wave UV, giving the yellow rim.
One-sentence summary
If you want, I can also:Copper accumulates and expresses itself at coin edges because grain boundaries, strike-induced defects, electrochemical dealloying, and geometric exposure all peak at the rim, making it the fastest pathway for copper migration and oxidation over centuries.
Just say the word.
- Sketch a cross-section model of a Roman silver coin
- Explain why cracks glow before fields
- Compare this with surface-silvered fourrées
The next post is about I theory I had at the time about how electrolysis removed florescence from a Japanese Yen coin.
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