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Let's say this for the years 1900 - 1950, before we had modern analysis tools. When trade payments were made in gold, how was the gold inspected for quality? How did they assess the purity of gold, such that they knew a particular ingot weighed 3 kg but was only 0.9 pure and thus they were really getting 2.7 kg of actual gold?

I'm imagining this for trade agreements. At the port, someone exchanges gold for the goods. Surely that gold was inspected somehow.

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    Wasn't gold primarily traded through securities already by then, with physical delivery of gold the exception rather than the rule? – Denis de Bernardy Jul 29 '17 at 17:11
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    I think you're forgetting Archimedes :-) A gold object is going to have a certain density, which will be less if it's alloyed with other metals. So if you weigh & measure a coin or gold bar, you can see whether the weight varies from its expected value. – jamesqf Jul 29 '17 at 19:10
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    Also, unless you're going to claim that literally no gold trading went on at any port on Earth during this time, let's please stop getting sidetracked. My question is about how gold was verified on the spot, not about how common or uncommon this was. – DrZ214 Jul 29 '17 at 19:33
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    Just an observation, but gold bullion certainly was being transported on a fairly regular basis in the early 20th century. An example I happen to know about is the Japanese steamer Futami Maru. She went down of the Australian coast in August 1900. A consignment of gold bullion that she was transporting was salvaged at the time. – sempaiscuba Jul 29 '17 at 20:33
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    here is a relevant link: physics.stackexchange.com/questions/71629/… – Jeff Jul 30 '17 at 18:35
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As Denis observed in the comments above, for many countries, gold would have been traded through securities in the first half of the twentieth century. In those cases the physical transfer of the gold would indeed have been the exception, rather than the rule. That said, there would undoubtedly have been a great many cases where gold was transferred as payment, and some form of metallurgical assay to ensure its purity would have been required.

Apart from a few modern techniques like X-ray fluorescence (XRF), most of the techniques used to assay gold were in use long before the twentieth century. (In any event, XRF - although non-detructive - can be fooled by surface treatments like gold-plating). The most likely techniques, however, would be using a "touchstone" or conducting a "Fire Assay".

In the example you give, where the gold was to be exchanged for the goods at the port, testing would probably have been carried out with a touchstone. The use of touchstones dates back to antiquity, and they remain in use to this day. These could easily distinguish between, for example, 14 carat and 24 carat gold (I've done it myself), and I read that experienced testers can achieve accuracies of better than 5%.

If a more accurate assessment was required, a Fire Assay may have been carried out on a sample from one or more of the ingots. This technique is destructive, but has been available since the sixteenth century (a version is mentioned in Agricola's De Re Metallica). I would very much doubt that a Fire Assay could have been carried out at the port though (unless specialist facilities were available there expressly for that purpose).

[One of the reasons that national mints introduced mint-marks on bullion from about the eighteenth century was to assure its quality and so eliminate the heed for such testing].

Today, a fire assay on gold can achieve an accuracy of better than 0.05%. My reading of the text of A Manual of Assaying, by Arthur Stanley Miller, seems to suggest that similar accuracies would have been achievable when it was published in 1905.

  • Thanks, the touchstone thing seems quick and easy, but I wonder if it can assay the exact purity of the gold instead of just distinguishing between 14c and 24c? I have this impression from hollywood of some guy looking thru a monocule or microscope at the gold, but I'm not sure how accurate this would be back then. I look forward to whatever else you can find later. – DrZ214 Jul 29 '17 at 17:39
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    To clarify: A fire assay is "destructive", in that it, usually, destroys what's being assayed as anything other than a hunk of precious metal. It does not actually destroy the gold. For instance, if you are having an item of jewelry fire assayed it won't be usable as jewelry, but will still be a valuable piece of precious metal. In a fire assay, normally only a representative sample of the material is actually assayed, but obtaining a sample that is actually representative may require melting the entire amount and mixing. The assayed gold is recovered as part of the normal fire assay process. – Makyen Jul 29 '17 at 22:37
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    @DrZ214 - "I'm going to research the periodic table of elements for density near gold (lead is off by a factor of 2)" One of the primary benefits of using gold as an exchange medium is that there are no heavier elements that aren't actually even more expensive. The only elements you could use that wouldn't be detected by weight are basically tungsten, platinum, iridium, and a wide variety of radioactive elements (which can clearly be detected by other means anyway). All of these are more valuable than gold is, as well as being denser. – Jules Jul 30 '17 at 0:26
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    @Jules Tungsten is more valuable than gold? Are you talking in 1925 like my OP, or modern times? – DrZ214 Jul 30 '17 at 4:00
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    @Jules Seems to me, if the gold you're using as an exchange medium isn't pure gold, then a crook doesn't need something heavier than gold, just something heavier than whatever the gold is supposed to be alloyed with. – bof Jul 30 '17 at 11:16
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For smaller transactions, early 20th century literature has a lot of references of people biting into the gold. See more detail in this answer to a closely related question in the history section of Skeptics.SE.

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A classical test that does not seem to have yet been mentioned was a literal "acid test": the purported golden item is rubbed onto a stone, after which the mark is treated first with aqua fortis (nitric acid). True gold should not dissolve in it; a follow-up treatment with aqua regia should then dissolve the mark made, if the golden article is genuine. (See e.g. this.)

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