About 350 years after Nanni's legendary complaint on receiving poor copper from Ea-Nasir, another, perhaps more historically important complaint was sent from King Burraburiash of Babylon, 19th to the Kassite throne, to Akhenaten, Pharaoh of Egypt.

Initial contact between the two went well, as a marriage alliance was planned, and an exchange of gifts was arranged (we have all this recorded in the so-called Amarna letters). However, things went sour, as one of the King's messengers was "detained for two years" and, notably for our purposes, a gift of gold was found to be of poor quality.

What's helpful for us is that it records why it was known to be poor quality. It underwent an ancient process called "fire assaying": for gold, lead is added, and the combination is melted in a crucible. This causes alloying portions to be oxidized (and absorbed in the crucible) leaving silver and gold; a further process convers the silver to silver chloride so the gold can be recovered.

In the case of poor King Burraburiash, only a quarter of the original gold survived the process. In modern terms: the original was 6 karat gold, which isn't even legally considered "gold" in most countries.

So, one way to tell the original purity of a substance is to separate out any alloys and compare the weight of what's left with what it started as. This can be done with copper as well as gold (although our records are on gold). Fire assay is still a modern practice (although generally with samples, not taking an entire chunk of a substance).

Note, of course, weight is required for this to work; there were certainly weights at this time, with a general error (just based on tests of archaeological finds) of about 10%.

This couldn't have been the test done in Ea-Nasir's situation. Nanni didn't accept the copper at all, so couldn't melt it. This meant he must have used the other form of ancient assaying: rubbing.

The exact version of the process involves using a smooth black stone called a "touchstone" to create a streak and look at the colors. These colors are compared again streaks of known purity. This can be much more accurate than it sounds: Theophrastus in his On Stones (probably late 4th century BC) claimed 1-to-144 part accuracy. Pliny in the 1st century CE wrote that touchstones could be accurate for detecting silver or copper "to a difference of a scruple" (probably about 2%). (Note that the general idea is to not want copper infused in one's gold or silver merchandise, but the same process can be used to find copper purity.)

Unfortunately, archaeological evidence is scant for the use of touchstones in the time period we're referring to; however, it's not out of the question. The earliest confirmed evidence dates to the 8th century BC, although some tools have been speculated to be touchstones elsewhere, including the Indus Valley Civilization in the Harappah period (2600-1900 BC) and in the 12th century BC on Egyptian sites.

(The "Archimedes method" involving volume wasn't in use at this time, and the specific gravity of copper makes it not a good candidate for assays; gold's specific gravity, on the other hand, is more than double and easier to differentiate from contaminating metals.)

Given the quality must have been determined somehow, the best guess is that while touchstones weren't fully developed, a method for checking involved scraping and comparing colors with known samples was likely in use.

....

Kenny, S. L., & Venable, S. L. (2011). Gold: a cultural encyclopedia. ABC-CLIO.

Moorey, P. R. S. (1999). Ancient Mesopotamian Materials and Industries: The Archaeological Evidence. United Kingdom: Pennsylvania State University Press.

Oddy, A. (1983). Assaying in antiquity. Gold Bulletin, 16(2), 52-59.

Roaf, M. (1982). Weights on the Dilmun standard. Iraq, 44(2), 137-141.

Thavapalan, S. (2019). The Meaning of Color in Ancient Mesopotamia. Netherlands: Brill.

All right. Metal trading is not my particular specialty, but this seems to have been unanswered for a long time, and I have the sources to tackle it. Tragically, however, I do not read Akkadian and cannot give a detailed commentary on the words used in the original language. It’s a shame because the actual Akkadian words might give some more nuance about what was wrong with the ingots (or they might well not and the ingots might be described in words that just mean generic evilness). I can’t say!

But in either case, in the translation published by the UChicago Oriental Institute (see Further Reading below), the words chosen are generic: fine quality copper ingots were promised; the ingots which were actually offered were ingots which were not good. There is no information about what specifically about them was not good, although we can notice that Nanni does not accuse Ea-Nasir of attempting to pass off bad ingots as good ones. Instead, he accuses Ea-Nasir of producing bad ingots and arrogantly telling the representative that he can be satisfied with the bad ingots or else he can go home with no ingots at all. Not trickery but high-handedness. From the tone of the translation, it seems to be this attitude, that Nanni ought to be grateful for the scraps Ea‑Nasir is willing to throw him, that has offended Nanni at least as much as the quality issue itself.

This is relevant because we do have some evidence that Ancient Near Eastern (ANE) ingot-mongers would occasionally attempt to cheat. For example, we have found ingots that are high-quality copper on the outside, concealing a core full of worthless slag (something that could not have happened by accident). One such ingot has even been found at an ancient copper production site in Bahla, Oman (as noted by Dercksen, The Old Assyrian Copper Trade in Anatolia, p. 41), which is neat because Nanni’s letter mentions that Ea-Nasir does his trading in Telmun, which is another name for Dilmun, which most Mesopotamian historians believe was located in southern Arabia near what archaeology shows us was a significant copper-producing region in the ANE. Now I don’t want to hang too much weight on a lone dodgy ingot we’ve actually dug up near where Ea-Nasir likely sourced his copper, but I do think it’s a cool coincidence.

Anyway, back on topic. We have no evidence of a formal metallurgists’ board tasked with promulgating quality regulations or publishing testing methodologies after the matter of the modern ASTM (and no reason to expect such an institution at this time in human history); however, short of clever density-testing methods (think Archimedes and his bath), the most practical way to detect the sort of trickery I just mentioned would have been to break an ingot or two open. That’s not something you need a board of experts to do, and we actually do have archaeological evidence for ingots sometimes being broken open in the course of trade and exchange. Namely, we’ve found broken ingots and ingot fragments in archaeological contexts that are clearly mercantile, rather than end-user, and although there are many possible reasons to want to break an ingot, it is a good way to double-check that the inside matches the outside.

There are other ways to guard against outright fraud. Scraping the metal and observing the color produced is one. Trading with known, trusted parties is another (though again, not in a formal, administrative, “board-certified trader” way, more in an informal, I have a good history with this trader, or I’ve heard of this trader’s good reputation, way). Metal trade routes in the ANE were often long-distance, international routes (note that Nanni also complains that his messengers to Ea-Nasir had to travel through enemy territory!) and likely often involved several intermediaries, making it difficult for an end buyer to know exactly whose hands an object had passed through before reaching its final market, but some of the ancient ingots we have found in shipwrecks or other archaeological sites have markings on them that might have served as maker’s marks to facilitate this sort of reputation‑advertising.

So that’s a bit about disguised or dodgy ingots and how a wary buyer might have guarded against such fraud. But what was Nanni’s messenger likely faced with? Quite possibly nothing tricksy at all, since as noted, Nanni didn’t complain about that. What sort of copper ingot can a messenger look at and straightaway conclude is “not good?”

Probably one that is visibly not highly refined, highly pure copper. So let’s talk about how copper was made, because depending on the source ore, it could take an awful lot of processing to get from raw ore to high-purity metal.

In ancient copper smelting, crushed copper ore is melted in charcoal-fueled furnaces, and impurities either sublimate into gas or form (perhaps with the aid of an added flux) into slag that is less dense than molten copper and can be skimmed off to leave purer copper behind.

Copper sulfide ores, however – and these are typically the most abundant copper ores – need extensive presmelting preparation to remove impurities, notably sulfur and iron, before they can be smelted. This preparation takes the form of crushing and roasting, and removing enough of the sulfur and other impurities, especially with ancient roasting methods, often took multiple rounds of roasting – especially if the ore had not been very finely crushed beforehand, the reactions that removed sulfur might not thoroughly penetrate in only one roast.

The most difficult to process ores, such as the abundant chalcopyrite exploited on ancient Cyprus that came to dominate Near Eastern copper supplies, might require multiple alternating rounds of smelting and roasting to successively purify the metal, and copper could be traded that had undergone some but not all of these steps and required further effort before being suitable for end use: in English, the most incomplete product is called matte copper, and might be significantly (50% or more) impure, containing various compounds of copper, sulfur, and iron. Visually, it is instantly obvious that matte is matte rather than refined copper, but though it was considerably less valuable, was still sometimes traded regionally, with further smelting and refining to be done elsewhere.

Other coppers with considerably higher copper content – such as black copper, which could have copper content well into the upper 90s% – are still visibly distinctive. Black copper is darker than pure copper due to the presence of iron impurities. Black copper could have been used directly for some ancient applications, but more delicate (and much more valuable) work, such as producing sheet copper for decorations or prestige objects, required more refined copper, so black copper would also not have been as valuable.

Experimental archaeology published by J.F. Merkel (see Further Reading) found that black copper required four additional melts to get the iron content down from about 5% to trace levels. That’s considerable additional work, even allowing for the fact that an ancient metallurgist with a lifetime of practice might have been better at the job than a modern archaeologist.

So, given the lack of complaints about trickery, we might guess that Nanni’s messenger just looked at the offerings laid out and saw that they were perhaps something visibly less desirable, like black copper, that would have required time-consuming and expensive further processing before being able to be used in the most valuable applications. Further processing that would have been especially expensive in arid southern Mesopotamia, where the necessary fuels, wood and charcoal, were not abundant.

Then Ea-Nasir was a giant jerk about it.

Good enough reason to complain!

Further Reading

https://oi.uchicago.edu/sites/oi.uchicago.edu/files/uploads/shared/docs/Publications/Misc/misc_letters_from_mesopotamia.pdf The Oriental Institute translation of the letter we’re discussing. Pages 82 and 83.

Dercksen, The Old Assyrian Copper Trade in Anatolia. 1996.

Oxhide Ingots, Copper Production, And The Mediterranean Trade In Copper And Other Metals In The Bronze Age, a dissertation by M. R. Jones, available at https://nautarch.tamu.edu/pdf-files/JonesM-MA2007.pdf – you might be particularly interested in Table 1 on pages 31 and 32, which collects a list of Akkadian words for good and bad copper attested in the records of Assyrian merchants at Kultepe, together with some speculation on what some of the words might specifically mean.

Rothenberg, (ed.) The Ancient Metallurgy of Copper, which has a good deal of articles by various scholars besides the one I cited (Merkel, Experimental Reconstruction of Bronze Age Copper Smelting Based on Archaeological Evidence). 1990.

Giardino, Magan, The Land of Copper: Prehistoric Metallurgy of Oman, for an overview of archaeology of ancient Oman with a focus on copper production in the region where Ea-Nasir's copper likely came from.