Iron ore comes as Hematite, Goethite, Limonite, etc, and Magnetite.
Basically Fe2O3 for the others and Fe3O4 for Magnetite. Plus contaminants.
Fe3O4 contains more iron per molecule than Fe2O3. Purified it comes to about 67% iron and 33% oxygen. Fe2O3 comes in at around 63% iron.
But, as dug out of the ground, Hematite etc. can be pretty close, with little ...
In a word, globalization.
Since the colonization of North America, and even before the inception of the "United States," labor has been the scarce factor of production, compared to land, or even "capital." So labor has always been highly compensated in the United States, compared to foreign countries. That's why the United States has had ...
Here are some rough economic facts for a start. The North had four times the GDP of the South. The North had 22 million people. The South had 9.5 million people, of which just under 4.0 million were slaves, so 5.5 million white people. If you divide the white populations, you get a four to one North/South ratio, the same as the GDP ratio. This means that the ...
Weight for weight probably never happened. Ocean water is 3.5% salt. If you could get an ounce of gold by boiling 10L of sea water a lot of people would be doing it.
Now as total value of trade, quite possible. Ancient nations used a lot of salt for conserving food, especially meat/fish.
A different question but related Slavery economically advantageous?
If cotton is taken to be a significant driver of the practice of slavery it can be argued that slavery was not in decline leading up to the Civil War. Cotton exports summary:
1820 America Exported 250,000 bales worth $22 million
1840 America Exported 1.5 million bales worth $64 million