"It is now known to all who have studied the subject that the cause of copper, yellow metal, zinc, and other metallic alloys placed on a ship's bottom, keeping clean and free from fouling, is the exfoliation of the metal and the constant renewal of the surface caused thereby, through which the adherent matter is, as it were, sent adrift, by the friction of the water against the metal sheathing washing off the exfoliated parts or films."
The Royal Navy took some time to realize this. John Bingeman, who has made an extensive study of the copper sheathing applied to HMS "Victory" and other ships of the era, notes that uncertainty over the ideal composition led to the practice of stamping each plate so that its origin and date of application could be compared with its rate of wear (see photo). This, it was hoped, would help identify the "good" copper, which sloughed off marine residue at just the right rate, from "bad" copper which wore away too quickly (thus being expensive), or did not wear away (thus allowing marine life to accumulate and foul the plating):
"I believe the reason for dating was an attempt to discover why copper varied from good to bad. Coppering ships served two purposes. It prevented worm attack, especially important in the West Indies where new hulls could be destroyed in under two years. The second need was for the copper to erode slowly preventing excessive fouling. This was known as "good" copper and relied on small quantities of impurities to achieve this effect, since completely pure copper eroded quickly and neededreplacing in less than two years. Really bad copper had too many inclusions and did not erode at all; fouling was then just as bad as plain wooden hulls. In an attempt to recognise good from bad, the Dockyards recorded the plate's life by dating each sheet. I would stress that these copper marks are not easily discernable when hidden by an oxide coating."
So if "good" copper relied upon small amounts of impurities, how is it that Robert Grenier can say with such confidence that Royal Navy copper plating was "100% copper"? He may simply be rounding things off, as the impurities in RN copper were relatively slight. Merchant vessels, in contrast, tended to use an alloy known as "Muntz's Metal," which was only 60% copper alloyed with 40% zinc and a trace of iron. This material, in fact, was used for the sheathing of the Cutty Sark, one of the most famous vessels of its day, or ours.
There has been some uncertainty in the past as to whether "Erebus" and "Terror" were in fact copper-sheathed. I talked with Dave Woodman about this, and he notes that they had been copper sheathed during their Antarctic service just prior to being re-outfitted for Franklin, and that he has seen work orders for the removal of some of their copper sheets. This may have been a prelude to re-sheathing, or because copper had to be removed from the parts of the ship that were to be sheathed in iron (copper and iron could not be allowed to have direct contact, as this created a "galvanic effect" -- essentially turning the plating, and the sea-water around it, into an electrical cell which resulted in rapid corrosion of the metals).
Woodman believes that the copper pieces that Grenier has been discussing were not in fact found in 2008, but rather as part of the the original Project Utjulik in 1997. He describes the location and significance of this copper as follows:
"These copper sheets and other artifacts were not found on the beach but associated with Inuit tent circles on one of the islets to the north of O'Reilly Island, so they were not primarily associated with a ship at all. They could have been transported there by either drifting wreckage from the north or Inuit travel (as could the relics recovered by the 1967 Project Franklin group) but since some of the testimony indicates a wreck nearby they could also be corroborative. Even if from the ship it may not be external sheathing but 'trade copper' or the remnants of copper sheeting carried by the expedition for making pots etc."
You can see an image of this copper here in the original report. One of the sheets had a tarry substance adhering to it, with traces of what may have been oakum (a mixture of tar and hemp used by carpenters to fill cracks and irregularities in a ship's planking), which suggested it may have been attached to a ship or boat, but tests at the time showed it had not been immersed for a long period in salt water.
What I would suggest is that Grenier, and others hoping to follow this trail of copper, get a hold of some of the bits of copper recovered by earlier searchers, such as Charles Francis Hall. Hall's bits were marked with the broad arrow; their metallic composition and thickness could be readily compared with the Utjulik finds. I believe that even the best Royal Navy copper probably had some trace impurities, and these could be used to help make a definitive match. What's more, if any additional copper is found next summer, there would be a ready way to evaluate it and determine if it resembled material known to have been recovered from Franklin sources. We have the technology, after all.