Here is an interesting article from the 1893 Columbia Exposition. I
wonder why we have not heard of this 'lost art' and advanced
technology of hardened/tempered copper tools used by pre-historic
THE BOOK OF THE FAIR: Chapter the Sixteenth:
Mines, Mining, and Metallurgy
..."But copper is the main feature in the Michigan section....
Among the more curious exhibits are prehistoric tools found in the
mines, fashioned of native copper, and in the form of knives, spear
and arrow heads, adzes, and hammers. These are among the state
contributions; and no one can tell how they were made, for the metal
is hardened and tempered by a process which modern scientists and
mechanics have failed as yet to discover. As to this process it can
only be said that thousands of inventors have tried in vain to
reproduce it, and that to the aborigines of the Lake Superior region
was known what is now a lost art, whereby weapons and tools were made
such as cannot be duplicated by the most improved of modern methods.
Says the official in charge of this exhibit: "It is claimed by
several that tempered copper is now being placed on the market; but
if the art is ever to be discovered, it has not been achieved so far.
I have heard of men who have seen weapons or tools of the aborigines
that would turn the edge of a steel chisel or dull a file."
full web page: http://columbus.iit.edu/bookfair/ch16.html