Tom McDonald wrote in message ...
Eric,

In the context of this thread, at least its original context,
the copper was native copper in the upper Great Lakes area of
the US and Canada. That copper is typically well over 99% pure
out of the ground, and does not have to be smelted to remove
impurities. If another context is in evidence, then a
definition of the term 'pure' is needed.

In the cases Yuri noted (e.g.: Egypt, Harrapa, China), that
copper was apparently smelted from ore, and analysis of
individual artifacts would be necessary to describe the ratio of
copper to alloy materials. In one of Yuri's examples, 'pure'
copper artifacts were all below 98.8% copper.

I agree, therefore, that one cannot take a statement that some
artifact or artifact type was 'pure copper' at face value. It
needs to be quantified.

Tom McDonald



Much has been bandied about concerning the purity of the copper from the UP,
but you must realize that the same geological process that separates the
copper also separates several other metals at the same time. It does not
place them miles apart but leaves the next to each other, fractions of a
millimeter apart.
for some clarification we will define some vocabulary.

Native copper
This is copper that was left in it's metallic state by the process that
concentrated it. It can be loose, or they can be stuck in a hunk of matrix
exactly as they came out of the ground with other native metals in close
proximity.
Drift copper
This is native copper that has been pounded from its matrix by glacial
action.
Placer deposit
This is a deposit of native metal that has been removed from it's matrix
by erosion (glacial or otherwise) moved from it's original location (usually
by wind or water) and, by nature of its specific gravity and it's resistance
to the motive force has been concentrate with other bits of metal with like
characteristics.

The native copper of the UP is unusually pure. This does not, however, mean
that every piece of rock with copper in it contains only copper.

Below is snip from a site about gold mining in the UP.

..http://www.geo.msu.edu/geo333/gold.html


In June the following year(1846), Houghton’s younger brother Jacob, found a
vein of native copper on the Keweenaw Peninsula which held a small amount of
gold. An assay yielded 10.25 ounces of copper, 1.75 ounces of silver and 12
grains of gold from the 28-ounce specimen



You can see that this particular sample was nearly 15% silver!

Drift copper has had most of the other materials removed by mechanical
action and is usually very pure.

The specific gravities of silver, copper etc. are actually quite close when
compared to sand and placer deposits may contain these in any mix.

Now as to melting.

These native metals are melted for one of three basic reasons.

One, to change it's shape to a finished product
Even today, casting generally produces products that are inferior to
wrought. It is only used when the form cannot economically be produced any
other way,
It is fairly difficult with pure copper, and frankly, if you found a 3
lb hunk of drift copper you would be better off pounding it to shape.

Two, to amalgamate several smaller pieces into one or more larger ones.
The purpose of this is not ,necessarily, to produce a finished product,
but to produce an ingot . Despite copper's casting difficulties, we have
managed to pour ingots of it for almost as long as we have worked metals.
The beauty of the ingot is that if you make it big enough, you can cut
off the bad parts, melt them into the next ingot and pound the rest into
whatever you want.
However, since parent metal is no longer a single nugget of pure copper,
the purity of the casting can be anything.

Three, to separate the metals from the matrix.
This too produces a fine ingot and in the case of Mr. Houghton's sample,
one with 15% silver .


Paul K. DIckman