This is something of a practical question more than anything else. Consider a setting in the modern world, where a large country e.g. the U.S. has collapsed and been replaced by a sort of modernized version of a feudal monarchy. Let's call it "federal monarchy". In this new Kingdom of America, the benevolent Philosopher-King has decided he doesn't want to have to deal with fiat money. Maybe he's concerned that the Royal Bank of America will manipulate the money supply to the detrement of the People, or whatever, I'm not really concerned with the rationale.

Now, the King has decided that his biggest denomination will be one ounce (not a troy ounce, he wants to keep things simple) of 20 carat gold. The idea here is that, roughly speaking, the purchasing power of that amount of gold will be more or less the same as 5lbs of sterling silver (which he's going to make the next denomination down). Ignore the practical concerns of lugging this much metal around for the moment.

He wants to make his gold "Crowns" tough, but keep the weight relatively manageable at an ounce and still be worth about 5lbs sterling silver. What should the remaining 4 carats' worth of alloying metal be, if he wants to maximize the durability of the alloy? I've heard that titanium really strengthens the metal significantly, but I don't know if there would be enough in there to strengthen the alloy enough to justify the expense.

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    $\begingroup$ Silver.. the resulting metal is called 'electrum' $\endgroup$ – Richard Oct 17 '18 at 18:50
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    $\begingroup$ Silver and copper. This is how it's been done in history, and this is how it's done today. 20 carat, however, would be uncommon for gold coins. $\endgroup$ – Alexander Oct 17 '18 at 19:07
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    $\begingroup$ This is a complex subject, resulting crystal structure affects hardness, and it is likely that the ideal solution would not be Au + Ti, rather Au + Ag + Cu + Ti or suchlike. Ti is likely not a good choice though, according to a study of 18 carat gold alloys. The platinum group metals seemed likely to be better choices (adding to Au+Cu+Ag baseline) though the results could be different for 20 carat than the 18 carat alloys in the study. $\endgroup$ – Gary Walker Oct 17 '18 at 19:39
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    $\begingroup$ 22kt was used in the UK for the sovereign (20/-) and in the US for the eagle ($10.oo) -- plenty tough enough for banking, commerce, train robberies & any assorted economic activities you could imagine. $\endgroup$ – elemtilas Oct 18 '18 at 6:43
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    $\begingroup$ Your main issue isn't going to be whether the gold crown (essentially a double eagle, or $20.oo) is tough enough for use, but rather the delicate balancing act of what is essentially a true bimetallic system. A study of bimetallism across world economies (particularly in the 19th century) will be very helpful to you. $\endgroup$ – elemtilas Oct 18 '18 at 6:46

Edit : I forgot to adress the weight question. Considering the following densities :

  • Au : 19,3

  • Ag : 10,5

  • Cu : 8,9

You won't have a drastic change of weight by simply changing less than a quarter of the alloy. But an ounce is quite a lot, just make small coins.

As a goldsmith, I can only tell you what alloy we used nowadays in jewelry :

750‰ gold / 18 carats :

The six remaining carats are copper and silver in equal amounts. You can put more copper if you want rose gold, or a bit of nickel (now forbidden) or palladium (not in use in medieval times, to my knowledge) if you want white gold.

This is the most you can alter gold without loosing its most interesting properties :

Malleability, ductility and low chemical reactivity.

And from this you get several advantages :

  • Better durability
  • Better elasticity
  • Less gold in your alloy (gold is heavy, and gold is expensive)

Money however, isn't required to be as shiny and stainless as jewelry. It will be worked with a press, and won't require an extraordinary ductility or malleability. We often prefer it to be durable. That' why money alloys were often lower in gold than jewelry alloys.

But I'm no expert in money and could not give you precise values about that.

Daniel is right in saying that it is copper that significantly increases the durability of the alloy, as silver is quite a soft metal. You could totally replace all of the silver, at the risk of seing some stains on your coins.

  • $\begingroup$ A much more informed answer than mine. Thank you for your valuable insight. $\endgroup$ – Daniel B Oct 18 '18 at 19:58
  • $\begingroup$ Why is nickel forbidden? $\endgroup$ – Andon Oct 18 '18 at 20:36
  • $\begingroup$ Thanks very much for your detailed answer! Having done the math, it seems like the 18 carat alloy you mention would probably be the best one to use, since it sounds like it'll also be the most commonly available alloy. I wanted to have the 1 oz gold coin as the "base unit" since it's easy, but they'd offer half and quarter coins too, in addition to using a true "pound sterling", which would be divided equally between 12 sterling silver shillings; so a proper bimetallic currency. With 18 carat, I wind up with something like 5 pounds 6 shillings to a crown coin, which is workable. $\endgroup$ – Horik Oct 19 '18 at 2:01
  • $\begingroup$ Even jewelry isn't required to be stainless and shiny! Look at any actual antique jewelry and it just does not look like the gold & silver stuff you find in a modern jewelry shop. I guess that comes from buffing and polishing the articles to a mirror finish! $\endgroup$ – elemtilas Oct 19 '18 at 5:11
  • $\begingroup$ Well, the fascinating thing with gold is that, except for a little "tarnishment", it doesn't change in milleniums. Golden pieces of jewelry from the ancient Egypt are the proof of that. But, agreed, there are also many examples of jewels made of less "stable" materials, such as the bronze "torques" made by the celts, that didn't fare so well with time. $\endgroup$ – Zaa Oct 19 '18 at 8:16

What you want is Crown Gold.

It's what Henry VIII switched the gold sovereign to in the 1500s, going to 22 karat gold from 23, to reduce the occurrence of clipping and wear (both of which you are also attempting to avoid.) The remaining material is copper. Considering in 1887, the royal mint replaced some of that copper with silver to get a slightly softer coin (for a better impression of Queen Victoria's new portrait) it's safe to assume that the copper will make a tougher alloy than if you mix in silver.

Of course, 20 karat (with the rest as copper) would be tougher still, but I think if it worked for the UK for 300~ years, it'll work for you.

  • $\begingroup$ I think I'm going to go with Zaa's answer, since being a goldsmith I reckon he knows quite a bit about the qualities of the alloy in question. But I also like the historical perspective you gave, thank you. $\endgroup$ – Horik Oct 19 '18 at 2:09

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