# What would be an ideal alloying metal for gold coinage?

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 Consider a setting in the modern world, where a large country e.g. the U.S. has collapsed and been replaced by a sort of modernized version of a feudal monarchy.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.

• Silver.. the resulting metal is called 'electrum' – Richard Oct 17 '18 at 18:50
• 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. – Alexander Oct 17 '18 at 19:07
• 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. – Gary Walker Oct 17 '18 at 19:39
• 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. – elemtilas Oct 18 '18 at 6:43 • 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. – 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 :

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.