A long long time ago, I played a pencil-and-paper Fantasy Role-Playing Game called Rolemaster. Like all FRPGs of the time, it was set in a world based loosely on an agglomeration of Tolkien's Middle Earth, Howard's Hyborian Age, Moorcock's Multiverse, etc.. And, like all FRPGs, it had a standard monetary system to allow player characters and non-player characters to trade with one another.
The monetary system in Rolemaster looked like this:
|10 iron pieces (ip)||= 1 tin piece|
|10 tin pieces (tp)||= 1 copper piece|
|10 copper pieces (cp)||= 1 bronze piece|
|10 bronze pieces (bp)||= 1 silver piece|
|10 silver pieces (sp)||= 1 gold piece|
|100 gold pieces (gp)||= 1 mithril piece|
Each of these different coins is roughly the same size and weight as all the others. (Thankfully, the Rolemaster rules didn't specify this size or weight, they merely suggested that each coin be ½ ounce or ¼ ounce. 1st Edition Advanced D&D was notorious for making all coins the diameter of a U.S. half dollar coin and weigh a whopping 1/10 of a pound avoirdupois each. Fun fact: a 1st Edition D&D copper piece would have to be 7 millimeters thick; a real half dollar coin is only 2.15 millimeters thick.)
From a playability standpoint, this system is very easy to work with. There are seven denominations of coin, each of which is worth 10 (or in the case of mithril pieces, 100) times the value of the coin below it. This allows easy conversion from one type of coinage to another in your head. And having so many different value tiers of coin allows for transactions consisting of just a few coins, across a wide variety of price magnitudes; e.g. you could negotiate the price of a night's stay at a cheap inn in tin pieces, or the price of a full sized castle in mithril pieces, and still end up with a price you could count on the fingers of both hands.
But then, I got to looking at it a bit closer, and a problem dawned on me. That problem was the tin piece that occupied this monetary system's second coinage tier.
Fictional economies, although fictional, are still supposed to reflect the internal realities of the fictional world in which they're set. For example, in the Rolemaster system given above, there's a coin called the "mithril piece." There is no such thing as a mithril piece here on real-life Earth, because in real life mithril doesn't exist. In the world of Rolemaster, mithril does exist, but it's very rare. It makes perfectly good, internal sense in such a world that coins made out of mithril would be extremely valuable. And they are: A single mithril piece in Rolemaster is worth as much as a hundred gold pieces, and gold is quite valuable to begin with.
This is consistent with the notion of specie money, a concept that was in use throughout most of human history: The currency units are made out of a material that has an intrinsic rarity, and cannot be synthesized. In a specie money system, the value of (for instance) a gold coin is about equal to the value of the actual gold metal in the coin. The money has an inherent value that's completely independent of the agency that produced it. The design on the coin is little more than a government stamp which indicates "Yes, this coin contains the amount of gold we say it does."
(I say specie money was in use throughout most of human history, because as of the mid 20th century most modern nations have abandoned it. You, the reader, probably grew up in a nation that didn't use specie money, but instead used currency that had value only due to governmental fiat. But that's a discussion for a later section of this document.)
The reason specie money was so common is obvious. It doesn't take a lot of incentive to convince a population that silver and gold are valuable. People already know they're valuable, and have a decent appreciation for how much they're worth. Putting a government stamp on little silver and gold discs of specific weights just helps reinforce the value they already have.
Gold is very valuable because it's rare. Silver is less valuable because it's not as rare as gold. Copper is less valuable still, because it's more common than silver.
And that brings us to the problem of tin.
In the real world — the Earth on which we live — here are the relative abundances of some metallic elements:
|Element||abundance in Earth's crust|
(parts per million)
Yes, iron is the most abundant, copper is less common, silver is less common still, and gold is the rarest. But look at where tin fits in on that list. It is rarer than copper by more than an order of magnitude!
It doesn't seem to make economic sense, in any system of specie money on Earth, that a tin piece would be less valuable than a copper piece.
But of course, the relative abundance of these metals in the Earth's crust isn't the only thing that determines their rarity as actual commodities. They also have to be mined out of the ground and smelted into their pure metallic form. Perhaps the extraction and smelting process is more difficult for copper than it is for tin. And also, the value of a commodity depends not only on its supply, but on the demand for that commodity as well. Copper may be useful for making some kinds of things that tin would be a poor choice for. It's not enough to know the relative crustal abundances of these two metals; we need to know their market value.
Well, data exist for that in the modern world, too. Here are the prices for copper and tin, in United States dollars per metric ton, as they were on a few arbitrary dates:
|Date||Copper price per tonne||Tin price per tonne|
|October 1983||US$ 1435.21||US$ 12,747.11|
|August 1991||US$ 2236.19||US$ 5647.72|
|February 2002||US$ 1561.37||US$ 3724.61|
|December 2012||US$ 7966.49||US$ 22,880.89|
The prices of both metals fluctuates every month, but at no time in the last three decades was the price of tin ever less than twice the price of copper.
If this problem were limited to just the Rolemaster FRPG, I could chalk it up to bad planning on the part of that game's designers and move on. But the problem is, I've started to see tin pieces in other fantasy settings. On a discussion board devoted to 1st Edition Advanced Dungeons & Dragons, I've heard from more than one Dungeon Master who's established his own "house rule" monetary system in which the Tin Piece is less valuable than the Copper Piece. A search for Castles & Crusades house rules likewise turns up a formalized system in which the exchange rate is 10 tin pieces to the copper piece. Somehow, somewhere along the line, the notion of "tin pieces" occupying the super-cheap end of the monetary spectrum caught on among the gamers.
So how, in these fictional worlds, can you justify having tin pieces be less valuable than copper pieces? Offhand, I can only think of four different ways to accomplish this, and all of them are frought with their own problems.
Fantasy role-playing games don't take place on the real Earth. They take place in their own fictional world. This fictional world has to have some properties in common with the real Earth, otherwise you won't be able to have people on it. It has to have a breatheable atmosphere. It probably has a 24 hour rotation period, a surface gravity of 32 feet per second per second, continental land masses, salty oceans, and freshwater lakes and rivers. Its axis of rotation is probably tilted 20 or 30 degrees, so that the northern and southern latitudes experience seasons. And, given the gleeful abundance of them in fantasy literature, it almost certainly has at least one moon in its sky.
Let us refer to this fictional Earth, this Fantasy Earth on which an FRPG takes place, as "Fearth." (Pronounced like Colin Firth.)
On Fearth, we're not limited to conditions precisely like those on Earth. Maybe there are orcs running around. Maybe magic really works. And maybe, just maybe, tin is easier to come by than copper.
If that's the case, and tin is more commonplace than copper, it makes perfectly good sense that tin pieces are cheaper than copper pieces. Perhaps they're even ten times cheaper than copper pieces. Now the monetary system outlined at the beginning of this page actually works! Boom! Problem solved, right?
Not so fast there, buckaroo. Changing the relative abundances of tin and copper has implications far, far beyond its use as coinage.
On Earth, humans learned how to smelt and make tools out of copper starting around 7500 B.C.E.. It has a moderate melting point (only 1084°C), a density of nearly 9 g/cc, and rates a 3.0 on the Mohs hardness scale. It was useful for making cutting tools, cookware, and yes, even weapons. The Copper Age saw the flourishing of metalworking technology, and lasted for nearly four thousand years.
In all that time, through all four millennia of the copper age, nobody smelted tin. Sure, tin has an even lower melting poing than copper (a paltry 232°C), and a density of only 7.4 g/cc. But it only rates 1.5 on the Mohs hardness scale, making it about as soft as lead (which, you may remember, is over 6 times as abundant in the Earth's crust as tin). Its metallic form can, at low temperatures, deteriorate into the brittle "grey tin" allotrope, which eventually crumbles into powder, a trait known as "tin pest." But worst of all, tin was scarce. Remember that table of relative abundances in the Earth's crust? Tin could only be found in any appreciable quantity in a few places on Earth:
Since tin was relatively hard to come by, when compared with copper, it had no industrial uses. That is, until that lucky day in 3700 B.C. or so, when an ancient metalworker accidentally discovered that copper with a little impurity of tin in it was even harder than pure copper. This new alloy is what we now call bronze, and its discovery heralded the beginning of the Bronze Age.
Throughout the bronze age, tin was a valuable, hotly contested commodity. Any nation could get its hands on copper, but if you wanted tin you had to trade for it. (And if you didn't obtain tin, you couldn't make bronze, which would put your technology base hopelessly behind that of your bronze-using neighbors.) Thankfully, only a little tin has to be added to copper to turn it into bronze. Ancient bronze artifacts typically contain less than 10% tin. A few bronze artifacts found in Phoenecia, Ceylon, Korea, Crete, and Machu Picchu contain up to 20% tin, but they're the exceptions that prove the rule — and it's noteworthy that both Korea and Machu Picchu are situated very close to known ancient tin mines. Ultimately, that little extra tin needed to turn copper into bronze could have a huge impact on your materials cost — and on your international relations with the traders that sold tin to you.
Bronze's primary quality was its hardness, when compared with copper. A nation that could equip its soldiers with bronze swords, spearheads, and arrowheads would have an edge — figuratively and literally — over a nation that had to use copper or other primitive materials for its armaments.
But ... what if it had been different?
On Fearth, let's say that it was tin, not copper that was hopelessly abundant, and that copper was relatively rare. With this great abundance of tin, the inhabitants of Fearth would have started smelting tin long before they started smelting copper. Tin's very low melting point means that the kilns required to smelt it don't even need to be very advanced. The Fearthlings would likely have discovered tinmaking hundreds, or even thousands, of years earlier than the Earthlings discovered coppermaking.
But the utility of smelted tin would have been very limited. It's so soft that a piece of tin can barely hold a cutting edge. (Those of you who've slashed their fingers on an open tin can lid would do wise to remember that a "tin can" is actually made of steel plated with tin. Nobody ever produced canned food in cans made exclusively of tin.) It would not have been used for cutting tools, or chisels, or weapons designed to cut or pierce. For that, the Fearthlings would have had to continue using flint or other stone tools.
However, tin could have been used for weapons designed to bludgeon. Tin is nearly three times as dense as granite, and a mace with a tin head would be that much more deadly than a mace with a stone head. On Earth, bludgeoning weapons like the mace only came about as a counter to metal armor. But on ancient Fearth, where metal spearheads and axeheads were still centuries or millenia in the future, maces might have appeared first! (The soldiers who owned these maces would have to take care of them, though. Any time "grey tin" started to appear on the surface of a macehead, it would have to be scrubbed off. Grey tin has a nasty tendency to accelerate the formation of more grey tin, much like rust begets more rust.)
Imagine that. On Fearth, it was the mace, and not the sword or the spear, that became the symbol of warfare. Imagine primitive armies charging toward each other across the battlefield, swinging tin maces against each others' wooden shields as their front lines clash. Imagine noble knights whose badge of station was their gleaming, polished macehead, kept 100% free of grey tin, instead of the gleaming polished swords of Earth's middle ages.
I'll bet that image is nothing at all like what the authors of Rolemaster had in mind for their world's ancient history. But it's what would likely happen in a world whose Tin Pieces were ten times as abundant as its Copper Pieces.
Eventually, maybe some millenia later, the denizens of Fearth would have discovered how to smelt copper. But again, copper is rare on Fearth. Sure, you can use it to make ornamental objects for kings and princes, or weapons for the field generals, but everyday items? Weapons for the rank-and-file foot soldier? Forget it.
That is, unless you can figure out a use for mixing a small amount of copper with your (overly abundant) tin. An alloy consisting of a lot of copper and a little tin is called bronze. An alloy consisting of a lot of tin and a little copper has a name, too. We call it ... pewter.
And lo and behold, pewter does have some useful properties. Pewter made with copper is considerably harder than tin. In fact, it can be almost as hard as pure copper. Now, at last, Fearthian civilization has a not-too-expensive metal alloy hard enough to make cutting tools (and cutting weapons) out of. Instead of having a Bronze Age, like we did on Earth, Fearth would have a Pewter Age. The rare copper mines would take on roughly the same role on ancient Fearth that the tin mines did on ancient Earth.
Eventually, of course, Fearth would discover ironworking, and from there the technology would start to play out pretty much like it did on Earth. But Fearth at the dawn of its Iron Age would be a very different place than Earth at the dawn of its Iron Age. One of the things that made ancient Greece such a major European power on ancient Earth was the invention of heavy bronze swords, that could cleave a man's arm off with a single swing. Without bronze swords, Fearth's ancient Greece would never have become the power center it was on Earth. Sparta would never have emerged as a nation to contend with. With no Sparta on Fearth, the other nations — e.g. Rome — would have had no prototype for the Full-Time Soldier or the phalanx formation that Earth's Sparta had pioneered. Fearthian Rome would be a backwater footnote in its history as other minor empires contended for control of the Mediterranean.
With no ancient Rome, Fearth wouldn't experience the collapse of ancient Rome, or the Dark Ages that followed it. No Dark Ages means no Feudal period. No Feudal period means no knights on horseback dominating the battlefield and inventing a code of chivalry.
And that means ... NO PALADINS!
And, really, what kind of a fantasy role-playing game would you have without paladins?
Okay. So, changing the relative abundances of tin and copper is bad. It turns Fearth into an unrecognizable world. Instead of inspiring awe and wonder in the players, it simply becomes hard to relate to. If that option is off the table, we have to give Fearth the same relative abundances of tin and copper worldwide that the Earth has. So how do you explain why tin pieces are so much cheaper than copper pieces?
Well ... what if the relative abundances are only turned upside-down locally? Maybe worldwide, tin is just as rare on Fearth as it is on Earth, but our Fearthian adventures happen to take place near one of those few major tin mines that supply the rest of the world?
In an area where tin was an export rather than an import, surely the local economy would treat Tin Pieces as being only worth a tenth as much as copper pieces, right?
Examples of such upside-down economies do exist, both in the real world and in the more well-thought-out worlds of fiction.
In Frank Herbert's Dune, for example, a spice called Melange is the most important substance throughout all civilization. It is only produced in one place, Arakkis, and everywhere but Arrakis it is horrifically expensive. But within Arrakis, Melange is everywhere. It takes effort to keep Melange out of your food. Since Arrakis is a desert, water is actually more valuable there than Melange.
Could a similar economy spring up around a tin mine? Well, yes and no.
In most cases, the economy of a mining town is going to center around the ability to sell whatever it is they're mining to outsiders. The more tin they can export, the more they can trade for other commodities they need. At first, they might need to import everything but tin. At that time, of course, tin will indeed be cheaper than copper.
But does that mean that tin pieces will exist, and be worth less than copper pieces? Maybe, if the town is big enough, they might set up a local mint to stamp out standard-sized tin rounds. But you can forget about local merchants exchanging these locally-minted tin pieces with visiting traders. In the traders' economies, tin pieces — if they even use tin coins — will be more valuable than copper pieces, not less. And the likelihood of tin pieces even existing outside would be slim. Any tin in an outsider's economy would get snatched up pretty quickly to make bronze.
And the local coin's value would suffer severe fluctuations. Every time the miners struck a new lode, the local supply of tin would skyrocket and its value would plummet. If representatives of a major world power came by and agreed to buy thousands of tons of tin, the value of the local tin pieces would likewise shoot up. For a metal to come into widespread use as specie money, even locally, it must have a comparatively stable value. Both its supply and its demand must be relatively constant. Gold and silver tend to fulfil these requirements because there's not much of the stuff being mined, and their utility is based around ornamentation, for which the demand is realtively inelastic. But tin, in a tin mining town? The commodity you're mining out of the ground and basing your entire economy around is the last thing you'd want to turn into coinage.
If fiddling with the relative abundances of tin and copper either globally or locally is off the table, you need another answer as to why tin pieces are less valuable than copper pieces. One possibility is commodity-backed currency.
You might be familiar with commodity-backed currency if you are a paper money collector, or if you were alive in the U.S. prior to 1963. Back then, U.S. Federal Reserve Notes (paper money) had the following language in a blurb on the front:
This note is legal tender for all debts, public and private, and is redeemable in lawful money at the United States Treasury or at any Federal Reserve bank.
"Lawful money," while never actually defined by Congress, meant gold, silver, or treasury securities. Private ownership of bullion gold was outlawed throughout the U.S. between 1933 and 1963, so in practice lawful money redemption was limited to silver and treasury securities. The U.S. also issued other kinds of paper money called "gold certificates" and "silver certificates," which could be redeemed on demand at the Treasury Department for their face value in gold or silver coins, and which served as evidence of deposit of said coins.
So ... perhaps the reason tin pieces are worth so much less than copper pieces on Fearth is that copper pieces — and maybe other coins too — are commodity-backed money. In other words, the copper piece is a token whose metal content is worth far less than 10 tin pieces. If it's the same size and weight as a tin piece, it maybe contains only half a tin piece's worth of copper. But it's backed by the full faith and credit of some government, which agrees to redeem your copper pieces for 10 tin pieces each (or to give you 1 silver piece for every 100 copper pieces you turn in to them).
This notion brings its own set of headaches with it. Whereas the problems with saying "tin is more abundant than copper on Fearth" are technological, the problems with saying "copper pieces are commodity-backed currency" are political.
First and foremost, only governments that are both powerful and stable can get away with issuing commodity-backed currencies. If the government is weak (and can thus be overrun easily), or if the government could implode at any moment, no one is going to trust that the tin or silver they "could" redeem their Copper Pieces for will actually be available when they need it.
The Confederacy during the U.S. civil war provided an example of a failed commodity-backed currency. A confederate $100 bill, for example, carried the following language:
Six months after the ratification of a treaty of peace between the Confederate States and the United States, the Confederate States of America will pay to the bearer on demand One Hundred Dollars with interest at two cents per day.(One could argue that such currency was not in fact commodity-backed at all, but was a kind of treasury bill or fiat currency, since it wouldn't become redeemable on demand until after the war.)
For over a century after the Civil War, Confederate money was little more than the punchline of some jokes. Nobody in the South was willing to accept Confederate scrip as payment for anything, preferring to do business instead in the gold dollar coins being minted by their enemies up North. The citizens of Fearth will likewise refuse to accept commodity-backed Copper Pieces issued by any government they fear might go belly-up.
So, now you have to explain how the issuing government on Fearth got so strong and so stable that its citizens are willing to accept its commodity-backed Copper Pieces. If your Fearth resembles Robert E. Howard's Hyborian Age, in which the world had devolved into barbarism with only a few flimsy nation-states that can hold themselves together, you're going to have an awfully tough time convincing any of your players that commodity-backed currencies would be accepted by all the people of Fearth.
But okay, let's say Fearth does have a government that's big enough and stable enough for its citizens to trust. They issue commodity-backed Copper Pieces, and they're a huge success. Now all the merchants use those "representative" Copper Pieces for their buying and selling. Although the Copper Pieces aren't worth anything by themselves, the merchants know they can go to the central government at any time and turn them in 100-at-a-time for silver pieces.
This situation itself is inherently unstable. All it's going to take is one economic downturn — a stock market crash, a trade blockade by a neighboring nation, an attack by a firebreathing dragon — and everybody's going to panic and try to redeem their government-issued Copper Pieces all at once. There's no way the central government's going to have enough silver or gold or tin on hand to cover all those copper pieces. You're going to have a run on the bank. When the dust finally settles, a hell of a lot of merchants are going to be left with sackfuls of worthless Copper Pieces they can't redeem. This is going to leave a hugely bad taste in a hell of a lot of people's mouths. The recession it creates might last for years. And when the economy finally turns around and the recession ends, NO ONE is going to trust government-issued Copper Pieces, or any other government-issued currency, ever again for the rest of their lives.
Non-specie currencies have another problem, too: Ease of counterfeiting. If a copper piece contains a full 10 tin pieces worth of copper, there's not much point in making a counterfeit copper piece out of copper. Sure, you could use some cheaper material like wood ("Don't take any wooden nickels!"), but such forgeries are easy for all but the most gullible merchant to see through. Even taking tin and painting it to look like copper isn't going to fool many people for long. But if copper pieces aren't specie currency, and contain far less than 1 copper piece worth of copper, you can make counterfeit copper pieces out of copper and still make a profit. And since the metal in the counterfeit coin is the same as the metal used in the real coin, it's going to be a lot harder to tell the fake from the real McCoy.
Ease of counterfeiting still plagues the currency of the United States. The U.S. Bureau of Printing and Engraving has had to come up with all sorts of counterfeiting countermeasures, in an ever-escalating arms race: Color-changing ink, plastic strips embedded inside the paper, watermarks, microprinting, and on and on. Such anti-counterfeiting measures were never needed for gold and silver specie coins; their only anti-fraud feature was a reeded edge to prevent "coin shaving."
How does the government of Fearth deal with counterfeit copper pieces? A convincing forgery might not be found until the coins are redeemed at the central bank, and might not be found even then. If enough counterfeit copper pieces are in circulation, they'll even start to form an important part of the economy. Crack down on counterfeit coppers, and your marketplace could collapse. Don't crack down on counterfeit coppers, and the bloated money supply could result in price inflation, which also hurts your marketplace. It's a no-win situation.
The only other possible answer is fiat currency. You're already familiar with fiat currency if you live in the modern world. Take a dollar bill out of your wallet, and look at it. What do you see? It's a piece of paper, made out of special linen, on which are printed some designs and the words "One Dollar." It doesn't say it's "redeemable" for anything. The paper and ink it's made of are worth, maybe, a fraction of a cent. So what gives it value?
What gives it its value is the faith and credit of the governing agency that created it. You know that, if you have to pay a tax to the government, they'll accept payment in the form of dollar bills. When the government buys something, it pays for it in dollar bills. It's legal tender, which means that if you owe somebody money, they are legally required to accept payment in the form of dollar bills. (They're allowed to accept payment in other forms, but are not required to.) And, finally, the government has a track record of not printing up more of these dollar bills recklessly.
A dollar bill is fiat money. It is money because the government, by fiat, says it's money. And unless you're living in the hills of Montana, sitting on a five year stockpile of food and ammo and trading with your neighbors only in gold and silver, you're okay with using fiat money.
The coins in your pocket are fiat money too. Starting in 1965, the United States stopped minting dimes and quarters made out of silver. Since that time, American dimes and quarters have been made out of a kind of nickel-copper sandwich, whose metal content is worth far, far less than the coin's face value. In fact, in 1981 the U.S. faced a crisis with the one-cent piece. What was the crisis? That the copper in the one-cent coin was now worth as much as the face value of the coin itself! Now think about that. America had become so entrenched with the notion of fiat currency, that the fact that the one-cent coin had become specie money was considered a crisis of epic proportions. Not only are we used to the idea of fiat money, we're actually afraid of the consequences of going back to specie!
Fiat currencies don't have the "run on the banks" problem that commodity-backed currencies do, but they have all the other problems — often to an even greater degree. It's one thing for a government to convince its people to start using a cheap token Copper Piece if they promise to redeem it in tin or silver on demand. It's quite another to convince the people to start using a cheap token Copper Piece just because. Indeed, the only reason modern Americans use fiat currency is that they eased into it by using commodity-backed currency for several decades.
And there's one additional problem that fiat currencies can have, which commodity-backed currencies don't (usually) have to worry about: hyperinflation. There's no physical law restraining the government from cranking out extra Copper Pieces and injecting them into the money supply. Indeed, if the government is deeply in debt, the temptation to pay off its debts with new fiat money can be quite strong. This is what happened to Weimar Germany in the 1920s.
Tin pieces being worth less than copper pieces is bad. Bronze pieces being worth more than either of them is worse. The copper-to-bronze exchange rate in games like Rolemaster raises economic conundrums that make the tin-to-copper exchange problems pale in comparison.
Let's say, for the moment, that despite our better judgement we go with the first model above: Fearth has an abundance of tin and a scarcity of copper. Now, we try to introduce a Bronze Piece that's ten times as valuable as an equal-sized copper piece.
Bronze is just an alloy of about 90% copper and 10% tin. How do we explain why bronze is 10 times rarer than copper?
Your first though might be: Maybe making bronze is a complicated process, requiring great artisan craftmanship, and thus would fetch a high premium. The problem with this excuse is, in the real world at least, it ain't true. Bronzemaking isn't the tough, involved process that steelmaking is. If you can make copper, and you have tin lying around, you can make bronze, period. All you have to do to is add a little bit of metallic tin to your copper before you melt it, then stir 'em together. Tin has a much lower melting point than copper; you can even drop cold tin ingots into your molten copper and they'll melt fast enough that they won't even slow you down. You do need to keep track of how much tin you've added, but that's a simple matter of counting your ingots as you throw 'em in.
Well, okay, you say. That's how bronzemaking works on Earth. Maybe on Fearth things are different, somehow, and bronzemaking is a lot more difficult. Maybe the laws of metallurgy in the Fearth universe are different, or maybe there are magic forces in the environment that prevent copper and tin from mixing easily. After all, this is a universe where people can wiggle their fingers and speak a few syllables, and a fireball will appear out of nowhere. Right?
If you make that excuse, though, you've done more than just tweak the relative abundances of the metallic elements. You've changed some fundamental property of the universe. And the repercussions of that are enough to make anyone's head spin. If the basic laws of metallurgy are different, you have to explain why all the other properties of copper, tin, and bronze — their melting points, their ductility, their conductivity, their density, their hardness, their appearance — are the same as they are on Earth. If instead some "metallurgical suppression field" is generated from all those wizards and warlocks on Fearth casting spells willy-nilly, and this field makes it harder to make bronze, then you have to ask why none of those wizards have concocted an "anti-suppression field" spell to counteract it. Bronzeworkers would pay a pretty penny (or a pretty bronze piece) to a wizard who could allow them to make bronze more quickly and easily.
And even if you accept this explanation, or come up with some other excuse as to why bronze is so much rarer and more expensive than either copper or tin, you're still stuck with an economy that should be swimming in items made of copper and tin (and maybe pewter, depending on the excuse you come up with), but nearly devoid of items made of bronze. Bronze tools should be rare and valuable implements, with copper (or pewter) as the norm. Ancient bronze weapons should be prized posessions. Both contemporary armies, and armies in antiquity, should be equipped with copper spearheads and arrowheads, rather than bronze ones — at least until the advent of iron weapons, of course. Unlike 1st Edition AD&D, in which bronze plate mail cost only a quarter as much as regular iron/steel plate mail, on Fearth bronze plate mail should be more expensive than its iron/steel equivalent.
Does your campaign treat bronze items as though they were made of semi-precious metal? No? Then you have no business saying that bronze is 10 times rarer than copper and 100 times rarer than tin. Period.
The only other option is to go the way of non-specie money. If copper pieces are only worth 10 times as much as tin pieces because the government says so, then it's an easy next step to say that the government also mandates that bronze pieces are worth 10 times as much as copper pieces. The actual bronze content of these coins might only be worth half a tin piece if you melted it down, but with the government's official stamp saying "One Bronze Piece", it is now worth 100 tin pieces.
Clearly, fiat Bronze Pieces carry with them all the same problems that fiat Copper Pieces do. They are relatively easy to make convincing counterfeits of, they rely on the trust of a fickle populus, etc.. Only now, the problems are compounded by a factor of 10. It's one thing to have a fiat Copper Piece, whose value is only 10 tin pieces; but a fiat Bronze Piece is valued at 100 tin pieces, and just ten of them are worth an entire silver piece.
If Copper Pieces are your only commodity-backed currency, your government might be able to weather a run on the banks. But if both Copper Pieces and Bronze Pieces are commodity-backed currency, there's going to be a hell of a lot more non-specie money in circulation, and the danger of a government default is that much higher. It would take a lot fewer people turning in their Bronze Pieces, and demanding tin or silver in payment, to run the government's reserves of tin and silver dry.
Counterfeiters, likewise, would have 10 times the incentive. Just as a modern counterfeiter would make more money by making counterfeit $100 bills than by making counterfeit $10 bills, so a counterfeiter on Fearth stands to make a lot more by making counterfeit Bronze Pieces than by making counterfeit Copper Pieces. Fearth's currency police would be under that much greater strain. The amount of counterfeit money snuck into circulation would be that much higher, and the price-inflation caused by the swollen money supply would be that much worse.
(Hmmm . . . maybe that's what happened in Dungeons & Dragons. In D&D, a simple dagger costs 2 entire gold pieces, and a hooded lantern costs 7 gold pieces. Perhaps the D&D economy suffered rampant inflation because of counterfeiters. Then again, D&D doesn't have tin pieces or bronze pieces. . . .)
In short, the more (and larger denomination) non-specie currency Fearth has in its economy, the more unstable that economy will be. You have to explain why your player's characters happen to have been born during one of those brief periods when commodity-backed or fiat money was in common use — and be ready to pull the economic rug out from under them should the campaign last for more than a couple of in-game years.
As you can tell, coming up with a justification for tin pieces being cheaper than copper pieces can be quite daunting. So one is left to wonder: What was their motivation in the first place? If tin is more valuable in the real world than copper, why did designers of Rolemaster — and the other games that may have drawn inspiration from it — decide to make Tin Pieces worth only a tenth as much as Copper Pieces?
I suspect some of that may have to do with how tin is treated in the modern world. Because tin is soft, it can be worked into shape just by pounding it with a hammer or rolling it between steel rollers. There's no need to melt it down and pour it into molds, nor is there a need to heat it up before you start pounding it into shape. This lent it well to making inexpensive trinkets with minimal labor.
Thus, tin carries with it a connotation of cheapness. A tin whistle is a cheap, toy imitation of a "real" flute. Model railroad enthusiasts who enjoy O-gauge scale look down their noses at "O-gauge tinplate" train sets, which are viewed as mere toys. A blind beggar will hold out a tin cup to ask for spare change. Cheap, low quality speakers or earphones are said to sound "tinny." Tin cans are the refuse left behind after getting out the food inside. Tinfoil was a cheap, disposable food wrapping material. An old, cheap car that's falling apart is a "useless hunk of tin." Everywhere you see tin used in an obvious manner today, it's as something so cheap as to be practically worthless.
The game designers may have grabbed that notion and run with it. Tin is cheap? Then let's make tin pieces into a lower denomination coin than copper! If copper pieces are pennies, then tin pieces can be ... oh ... mills, let's say.
But sadly, this flies in the face of reality, both today (in terms of the commodity spot price of tin on modern exchanges) and in the putatively medieval-ish period in which most of these games are set.
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