The way all smelters currently work is to have a % chance return for smelting any particular ore. This might work when you get a 50% return, but for the precious metals like gold and silver, it's pretty absurd to only have a 1/400 chance of return.
Instead, these rare metals should simply produce smaller "bars" and in addition the hard coding work should be taken to make smelters actually produce fractional metal from each piece of ore.
By smaller bars, I mean that a piece of gold ore might produce a gold ingot that can be made into only a handful of coins.
Let me stress that I certainly don't have any reasonable access to gold, but I just don't see anyone wasting hours mining gold for a 97% chance that a full load of ore won't produce anything.
So, Fractional bars, even invisible ones inside of smelters, and perhaps precious metals aren't normally made into bars, but smaller ingots.