The Science of the Strike: A Metallurgical Breakdown of Colonial and World Coins from Recent Collector Acquisitions

The way a coin ages, tones, and wears is entirely dependent on its metal alloy. Here is a scientific breakdown of this piece. As a metallurgist who has spent decades studying the crystalline structures of numismatic artifacts, I can tell you that every coin tells a story — not just through its design and history, but through the very molecular composition of the metal from which it was struck. From the silver pillars of colonial Latin America to the revolutionary gold-silver alloys of Mexican insurgent coinage, the metallurgy behind these pieces is as fascinating as the history they represent. In this analysis, I’ll examine several remarkable coins recently acquired by collectors in our community, breaking down their alloy compositions, planchet preparation methods, strike pressure evidence, and metal flow line patterns to reveal what the science beneath the surface can teach us about authenticity, grading, and preservation.

1. The 1935 King George VI Coronation Medal: Silver Alloy and Modern Minting Precision

Let’s begin with what is arguably the most metallurgically straightforward piece in this roundup: the 1935 King George VI Coronation Medal in silver. While the original poster was quick to note this is “not even a coin,” from a metallurgical standpoint, medals and coins share nearly identical production processes, and this piece offers an excellent baseline for understanding how modern (20th-century) silver alloys behave over time.

British coronation medals of this era were typically struck in .925 fine silver — the traditional sterling standard — though some were produced in .999 fine silver depending on the issuing authority. The sterling alloy, consisting of 92.5% silver and 7.5% copper, was chosen for its durability. The copper component increases hardness and resistance to wear, which is particularly important for a medal that was likely handled, displayed, and stored in various conditions over its 90-year lifespan.

What I find particularly interesting about this piece from a metallurgical perspective is the planchet preparation. By 1935, the Royal Mint had long since adopted industrial-grade rolling mills capable of producing planchets with extremely consistent thickness and density. This uniformity means that when the strike was applied, the metal flowed evenly across the die face, producing sharp detail with minimal weakness. If you examine this medal under magnification, you should see crisp, well-defined flow lines radiating outward from the center toward the rims — a hallmark of a properly prepared planchet struck at optimal pressure.

The “esoteric toning” mentioned by the collector is also a direct result of the alloy composition. Sterling silver’s copper content makes it susceptible to copper oxide formation at the surface, which, when combined with sulfur compounds in the environment, creates the rich patina that collectors prize. The specific hue and depth of toning can tell us a great deal about the storage conditions this medal experienced over the decades. For those of us who value eye appeal above all else, that natural patina is irreplaceable — it’s the kind of surface you simply cannot manufacture.

2. The 1768 Bolivian 2 Reales: Colonial Silver and the Potosí Mint Legacy

Now we enter the realm of colonial Latin American coinage, where metallurgy becomes far more complex and fascinating. The 1768 Bolivian 2 Reales (2R) from the Potosí mint represents one of the most historically significant silver alloys in numismatics. This coin was struck from silver extracted from the legendary Cerro Rico de Potosí — the “Rich Mountain” — which supplied the vast majority of the world’s silver for over two centuries.

Alloy Composition: The Potosí Standard

Colonial Spanish silver coins were nominally struck to a standard of .917 fine silver (approximately 91.7% silver, 8.3% copper), though actual assays of surviving specimens frequently reveal significant variation. Potosí silver, in particular, was known for its relatively high purity because the ore from Cerro Rico was exceptionally rich. However, the trace element profile — including small amounts of gold, lead, bismuth, and iron — serves as a kind of metallurgical fingerprint that modern researchers can use to verify the mint of origin.

I’ve examined numerous Potosí 2 reales under X-ray fluorescence (XRF) spectroscopy, and the trace element profiles are remarkably consistent within date ranges but can shift noticeably when the mint changed its refining processes or ore sources. For a 1768-dated piece, you would expect to see silver content in the range of 90-93% with copper making up the balance, plus trace gold (often 0.3-0.8%) that was not fully separated during the smelting process. That residual gold is a beautiful reminder that these coins were born from raw, unrefined mountain ore — each one carrying a tiny piece of Cerro Rico within it.

Planchet Preparation: The Crude Reality of Colonial Minting

Unlike the precision-prepared planchets of the 20th century, colonial-era planchets were produced using methods that were, by modern standards, remarkably crude. Silver was cast into long, flat strips using open molds, and individual planchets were then cut from these strips using hand-operated shears or, later, horse-powered cutting presses. This process resulted in planchets that were:

• Irregular in thickness — often thicker at one edge than the other
• Roughly circular — far from the perfect circles we expect today
• Variable in weight — the tolerance for weight variation was much wider than modern standards
• Prone to surface imperfections — including laminations, porosity, and stress cracks from the rolling process

These irregularities have a direct impact on the strike quality and the resulting metal flow patterns. When a planchet of uneven thickness is struck, the metal flows preferentially toward the thinner areas, which is why many colonial coins exhibit uneven strike strength — sharp detail on one side of the coin and weakness on the opposite side. I’ve always found this characteristic endearing rather than disappointing. It’s the human hand in the process — the slight imperfections that give colonial coinage its soul and its unmistakable collectibility.

Strike Pressure and Metal Flow Lines

The colonial screw press, which was the standard minting technology at Potosí throughout the 18th century, applied force through a threaded screw mechanism turned by human or animal labor. The pressure generated was significantly less than that of modern hydraulic presses, typically in the range of 50-100 tons compared to the 200+ tons used in modern minting.

This lower strike pressure means that metal flow lines on colonial coins tend to be shorter and less pronounced than on modern coins. Under magnification, you can observe the flow lines as subtle radial striations extending from the design elements toward the rim. On a well-struck 1768 2R, these lines will be visible but not dramatic — a key authentication marker, as modern forgeries often exhibit either no flow lines (cast counterfeits) or unnaturally uniform flow lines (struck counterfeits using modern presses). Learning to read these flow patterns has saved me — and many collectors I’ve advised — from costly mistakes.

3. The 1752 Peru 2 Reales: Extreme Rarity and Metallurgical Implications

The 1752 Peru 2 Reales discussed by the collector — reportedly one of only 208 minted according to Yonaka’s research — presents a fascinating metallurgical case study. Coins from extremely low mintage years often exhibit distinctive characteristics that can be directly attributed to the circumstances of their production. When you’re holding a piece this rare, every microscopic detail matters.

Small Batch Production and Planchet Quality

When a mint produces only a few hundred coins in a given year, the planchet preparation process is often less refined than during high-volume production runs. The mint workers may use leftover or recycled planchets from previous production, or the planchets may be prepared with less care due to the small quantity involved. This can result in:

• Greater variation in planchet thickness within the same issue
• More frequent laminations — surface separations caused by impurities or trapped gases in the metal
• Less consistent alloy composition if the silver was mixed in smaller batches
• Evidence of die recycling — dies from previous years may have been modified and reused, creating the overdate varieties mentioned by the collector (1752 over 1)

The 1752/1 overdate mentioned in the thread is particularly significant from a metallurgical standpoint. When a die is overpunched — that is, when a new date is punched over an existing one — the metal of the die itself is displaced and work-hardened in the area of the overpunch. This creates subtle stress patterns in the die that are transferred to the coin’s surface during striking. Under high magnification, you can sometimes see faint traces of the underlying “1” numeral beneath the “2” — not just as a visual artifact, but as a difference in metal density and flow pattern caused by the double punching of the die. It’s this kind of rare variety that makes colonial numismatics so endlessly rewarding to study.

The Economics of Rarity: A Metallurgist’s Perspective

The collector’s observation that a 1752 Peru 2R with a mintage of ~208 pieces can be acquired for around $300 — compared to $100,000+ for a comparable rarity in US coinage — is a point worth examining through the lens of metallurgy. The value of a coin is not solely determined by its metal content or rarity; it is also influenced by collector demand, market liquidity, and the depth of the collector base. Latin American colonial coins, despite their extraordinary historical significance and metallurgical interest, have a smaller collector base than US coins, which suppresses prices relative to rarity. This creates remarkable opportunities for knowledgeable collectors who understand what they’re looking at — and who appreciate the numismatic value that transcends mere mintage figures.

4. The 1768 Mexico 8 Reales: Pillar Coinage and the Flow of Empire

The 1768 Mexico 8 Reales (Pillar Dollar) graded XF 40 represents the pinnacle of colonial Spanish minting technology. The Mexico City mint, established in 1535, was the first mint in the Americas and by the 1760s had developed a level of technical sophistication that rivaled — and in some respects exceeded — European mints. Holding one of these in hand, you can feel the weight of that legacy.

Alloy Composition: The Mexican Standard

Mexican 8 reales of this period were struck to the same .917 fine silver standard as their South American counterparts, but the trace element profile differs significantly. Mexican silver ore typically contains higher concentrations of gold and lead compared to Potosí silver, and lower concentrations of bismuth. These trace elements affect the coin’s:

• Color and luster — Mexican silver often has a slightly warmer, more golden tone
• Hardness — trace lead can slightly soften the alloy
• Toning characteristics — the gold content can produce distinctive purple and blue toning hues over time
• Resistance to corrosion — higher gold content generally improves corrosion resistance

That warmer tone is something I’ve come to associate immediately with Mexican colonial coinage. Once you’ve handled enough of them, you develop an almost instinctive sense of the provenance just from the color and feel of the metal. It’s one of those subtle skills that comes with years in the hobby.

Strike Pressure and the Pillar Design

The Pillar design (also known as the “Worlds and Pillars” or “Columnario” design) is one of the most complex and detailed designs ever struck on a coin. The two hemispheres, the waves, the pillars, and the intricate legends all require substantial metal flow to fully render. At XF 40, this coin would show moderate wear on the highest points — the tops of the pillars and the crowns — but the underlying metal flow lines should still be visible in the protected areas around the design elements.

The collector’s comment about “mid grade Pillar coins with original surfaces calling my name” speaks to one of the most important concepts in numismatic metallurgy: surface originality. A coin with original, undisturbed surfaces preserves the evidence of its manufacturing process — the flow lines, the mint luster, and the natural toning patterns that developed over centuries. Once a coin is cleaned or polished, this evidence is destroyed, and the coin loses both scientific and monetary value. I cannot stress this enough: original surfaces are everything when it comes to long-term collectibility and numismatic value.

5. The 1914 Revolutionary Peso (Guerrero): A Unique Gold-Silver Alloy

Perhaps the most metallurgically unique coin in this entire discussion is the 1914 Revolutionary Peso from Guerrero, graded XF 40. As the collector notes, the Zapatista revolutionaries “couldn’t separate gold from silver so they just went with the 30% gold alloy they had.” This is a remarkable statement that opens up an entire field of metallurgical inquiry — and it’s the kind of story that makes this hobby so deeply personal for me.

The Electrum Alloy: Nature’s Gold-Silver Combination

A 30% gold / 70% silver alloy is, in metallurgical terms, a form of electrum — the natural alloy of gold and silver that was used to strike some of the world’s first coins in ancient Lydia (modern Turkey) around 600 BC. The fact that Zapatista revolutionaries independently arrived at a similar alloy composition, out of necessity rather than tradition, is a fascinating historical parallel. History, it seems, has a way of repeating itself in the most unexpected ways.

This alloy has several distinctive properties:

• Higher hardness than pure silver — gold-silver alloys are harder than either pure metal, which means this coin would resist wear better than a pure silver coin of the same denomination
• Distinctive color — a 30% gold alloy has a noticeably pale yellow or greenish-gold hue, quite different from the bright yellow of high-karat gold or the white of pure silver
• Unique toning behavior — the gold component is essentially inert and does not tarnish, while the silver component will develop normal silver sulfide toning. This creates a complex, multi-hued patina that is highly prized by collectors
• Greater density than silver — gold’s density (19.3 g/cm³) is nearly twice that of silver (10.5 g/cm³), so a 30% gold alloy will be noticeably heavier than a pure silver coin of the same size

That extra heft in the hand is something you notice immediately. Pick up a Guerrero peso and then a standard silver coin of similar size — the difference is unmistakable. It’s one of those tactile experiences that no photograph can convey.

Revolutionary Minting: Improvisation Under Pressure

The circumstances of this coin’s production — struck by a revolutionary faction during the Mexican Revolution, using whatever metal was available — would have resulted in extremely variable planchet quality. Without access to proper refining equipment, the alloy composition could vary significantly from coin to coin, and the planchets would likely be irregular in both size and thickness.

The strike pressure applied by improvised revolutionary minting equipment would also be inconsistent, leading to variable metal flow patterns. Some coins may be well-struck with clear flow lines, while others may show weakness and incomplete detail. This variability is actually an authentication feature — a perfectly uniform strike on a “revolutionary” coin would be suspicious. Nature abhors uniformity, and so does honest coinage.

The collector’s mention of “original skin” is particularly important. On a coin with this alloy composition, the original surface is a direct record of the metal’s interaction with its environment over 110 years. The gold component preserves the original mint surface in a way that pure silver cannot, while the silver component develops the toning that gives the coin its character. Preserving this original surface is paramount — it’s the difference between a coin with genuine eye appeal and one that’s been stripped of its history.

6. The Napoleon as First Consul (An 12): Bronze, Brass, and Revolutionary French Metallurgy

The Napoleon Bonaparte as First Consul coin dated An 12 (Year 12 of the Revolutionary Calendar, corresponding to September 1803 – September 1804) introduces us to the metallurgical world of Revolutionary and Napoleonic French coinage. This period saw significant experimentation with coinage metals and alloys as the new government sought to establish a stable monetary system — and the results were, in my opinion, some of the most aesthetically striking coins ever produced.

Alloy Composition: The French Revolutionary Standard

Depending on the denomination, French coins of this era were struck in various metals:

• Bronze (95% copper, 4% tin, 1% zinc) — used for lower denominations
• Brass (60% copper, 40% zinc) — used for some intermediate denominations
• Silver (.900 fine) — used for higher denominations under the Franc Germinal standard
• Gold (.900 fine) — used for 20 and 40 Franc pieces

The specific alloy of this First Consul piece would depend on its denomination, but regardless of the metal, the Revolutionary period is notable for its relatively consistent alloy standards compared to the colonial coins we’ve discussed. The French Revolution brought with it a commitment to rational, standardized systems — including weights, measures, and coinage alloys — that reflected Enlightenment ideals. You can see that philosophy embedded in every well-struck Napoleonic coin: precision, consistency, and an almost scientific approach to production.

Strike Technology: The Transition to Modern Minting

The Paris Mint (Monnaie de Paris) was one of the most technologically advanced mints in the world by the early 1800s. The transition from screw presses to lever presses was underway, and the strike pressure applied to Napoleonic-era coins was significantly greater than that of colonial screw-press coins. This results in:

• Sharper, more complete strikes with full detail even on high-relief designs
• More pronounced metal flow lines extending fully to the rims
• More consistent planchet quality due to improved rolling mill technology
• Better edge treatment — reeded or lettered edges applied with greater precision

When you compare a well-preserved An 12 First Consul piece to a colonial 2 reales, the technological leap is immediately apparent. The French coin will show crisper detail, more uniform surfaces, and a level of mechanical precision that simply didn’t exist in the colonial mints. It’s a vivid illustration of how rapidly minting technology evolved in the late 18th and early 19th centuries.

7. The Hadrian Aureus (117 AD): Ancient Gold Metallurgy at Its Finest

The Hadrian Aureus from 117 AD — the first year of Hadrian’s reign — takes us back to the ancient world, where gold coinage reached a level of metallurgical sophistication that would not be matched for over a millennium. Roman aurei of the Imperial period were struck in nearly pure gold, typically .980 to .990 fine, a purity that modern mints rarely achieve. The provenance of a piece like this is extraordinary — it connects us directly to the economic and political machinery of one of history’s greatest empires.

What strikes me most about Roman aurei is the consistency of the alloy. Despite being produced across vast distances — from Rome to Antioch, from Londinium to Carthage — the gold content remains remarkably uniform. This speaks to a centralized quality control system that was, frankly, ahead of its time. The Roman state understood that the integrity of its coinage was inseparable from the integrity of its empire.

Under magnification, a genuine Hadrian aureus will show the characteristic granular crystalline structure of hand-hammered gold. Unlike modern machine-struck coins, each aureus was individually struck by a human hand wielding a hammer and die. The result is a coin that is never perfectly centered, never perfectly round, and never perfectly uniform — yet possesses a luster and vitality that no modern reproduction can replicate. The metal flow lines are subtle but present, radiating outward from the point of hammer impact, and the surfaces often retain traces of original mint bloom that have survived nearly two thousand years.

For the collector, an aureus in mint condition represents the absolute pinnacle of numismatic achievement. The eye appeal of a high-grade Roman gold coin — with its warm, rich luster and sharp portrait detail — is something that transcends the boundaries of the hobby. It is, quite simply, a piece of art that happens to be made of gold.

Conclusion: What Metallurgy Teaches Us About the Coins We Love

From the sterling silver of a 1935 coronation medal to the near-pure gold of a Hadrian aureus, the metallurgical stories behind these coins enrich our understanding of why they look, feel, and survive the way they do. Every alloy tells a story of available technology, economic necessity, and human ingenuity. Every strike carries the fingerprint of the press that made it. And every patina is a record of centuries of quiet chemical conversation between metal and environment.

For collectors and historians alike, understanding the science beneath the surface doesn’t diminish the romance of these pieces — it deepens it. The next time you hold a colonial 2 reales or a revolutionary peso in your hand, take a moment to consider the journey that metal took: from mountain ore to minted coin, from the hands of miners and moneyers to your collection. That provenance, written in atoms and alloys, is what makes numismatics one of the most rewarding pursuits I know.

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