Curse of Oak Island archaeology update: Did they find an ancient rectangular copper trade token?

Episodes 2 and 3 of The Curse of Oak Island (COOI) devote some time to the finding and analysis of a rectangular piece of copper. Having dabbled over the years in the early history and archaeology of metallurgy, this one was immediately intriguing to me.

The COOI team initially hoped it was some kind of coin. After showing it to a numismatics expert, they have settled for the weaker possibility that it might be some kind of trade token.

The copper object, from The Curse of Oak Island season 10

When I first saw it, I was hoping it might be something like that. It wasn’t unusual for indigenous populations in northeastern North America, starting in the late 1500s, to trade for copper and bronze kettles and pots, and to cut them up and flatten them into sheets, which could then be either traded, or shaped into tools, or rolled into tinkling cones, for example, for ceremonial dance outfits. The resulting material was widely traded.

Let’s review what we know about his object, in archaeological terms.

Composition

The team, encouraged by the numismatics expert, are hoping that the object is old, perhaps dating back to the fifteen hundreds. The composition shown on screen in the episode is about 83% copper, 7% tin, 3% lead, 1.5% zinc, and 0.45% arsenic.

Given that they are analyzing the surface with an energy dispersive spectrometer, and that the surface is corroded, uneven, and somewhat contaminated, all of which affect readings, these numbers have to be considered approximate, but they are still pretty good.

One of the experts calls this arsenical bronze, and says that it stopped being made a long time ago, suggesting that the metal may be very old. In fact, concentrations under 0.5% arsenic in copper alloys are usually considered to be from the ore itself, and not added intentionally. 

Some sources of ore around the world can contain as much as 1% or more arsenic naturally. So this particular alloy is right on the cusp, and I would tend to consider the arsenic in this one as naturally occurring.

This impression is reinforced by the fact that the rest of the composition is remarkably close to what is called admiralty gunmetal, or sometimes red brass. The nominal composition of gunmetal is 88% copper, 10% tin, and 2% zinc. This is very, very close to the composition of our object.

Modern admiralty gunmetal started being made in the mid-eighteen hundreds, and has been used ever since because of both its strength and its resistance to corrosion in marine environments. In fact its zinc content makes it into a kind of low zinc brass. 

The intriguing part of this one is the 3% lead, which is usually added to make metals more malleable, so that is a mystery to be solved here. Either the lead is an unintentional contaminant in the manufacture of the gunmetal, or it contaminated the surface later during the use life of the object, or after it was discarded. The lead could even be a phantom peak on the graph, caused by conditions of observation. I’ve seen it happen.

Gunmetal has typically been used in ship building, boilers, gears, or anything that requires both strength and corrosion resistance.

Morphology

The shape of the object is interesting. As the numismatics expert notes, it is tapered. I also note that it has notches along two sides, almost like serrations. These are sometimes added to edges to increase their effective length, and therefore their cutting potential.

If this was a stone flake, for example, I would have no trouble concluding that it was retouched to create a cutting edge. In this case, it is entirely possible that a piece of a broken boiler or something was reshaped and reused for other purposes, and then discarded when no longer needed, or simply lost.

The team makes a big deal of the fact that the object weighs 4 grams “on the nose”. It suggests to them that it might have been some kind of standardized trade token. This is interesting. But if the object is a trade token from the fifteen hundreds, the fact that it weighs precisely 4 grams would be a meaningless coincidence, since grams were designed by the French Academy in the 1790s. 

Then there is the fact that it would have been lying there for hundreds of years, losing mass over time through corrosion and erosion, meaning that it would likely have originally been over four grams.

If the object is a more recent fragment of gunmetal, then it is almost certainly not a trade token.

Still, the lead bothers me just the tiniest bit, and I would love to find out more about this object.