Seppo Renfors wrote in message ...
I can understand that during a melting process where molecules are at
their most active, some reaction to air and a certain amount of mixing
can occur. What I find difficult is that an annealing process causes
bubbles -UNLESS it is overheated to a melting point locally. How else
does something get INTO the metal to cause bubbles when it is pure to
begin with?


Simply over annealing one time should not cause bubbles where no flaw
existed.
However, over annealing can play havoc with the grain size, subsequent
forging can start and then enlarge a separation caused either the weakened
grain structure or possibly a preexisting flaw. If gas or moisture is able
to permeate this separation, heating will will cause that gas to expand,
forming a blister.

The piece in question was the blade that had one single blister clearly
visible on the surface as well as in the radiograph.
If this piece were able to be radiographed from the side, I would bet that
the blister would be lentil shaped, showing clearily that it was caused by
the expansion of what had been a flat flaw.

This is not to say that the piece could not have been melted and the flaw
flattened by forging, but it also doesn't conclusively prove that it was
cast.


The cast piece was annealed 8 to 10 times in the course of it's forging.
That is a lot, considering it's length was only increased by 50%.
But the large crack on right the appeared in the first round of hammering
and I didn't want the piece to fall apart.

So it would appear that implied in the last statement is - the more
frequent annealing the better the outcome (for shape). Of course, in
this case you haven't trimmed the piece which might have been done in
that sort of situation of a severe crack by the ancients.


Yes, with a lot of annealing you can coax a lot out of even bad metal.
but the closer the anneal is to the finished product, the softer the
finished product would be.



This is true, but as cast copper is as soft as it can be and needs a
significant amount of hammering to harden it. For any edged tools or fish
hooks or awls, this would surely display the tendency to fracture. Sheet
goods also would require a lot of hammering.

Arrow and spear point are relatively small, and would want to be of a
medium hardness only - so the tips of retrieved arrows/spears would be
beneficial to NOT be too hard - so they will not break when missing
the target - and so they can be restored again when bent.


Even at it's hardest, copper is pretty darn soft.

Heavier decorative items or
ceremonial pieces would require much less work and cast preforms would
work
fine.



Certainly in Mexico casting was used even for very small items.

I have no idea of what purity of copper they were using.
Native copper essentialy is crystalized out of a superheated and
supersaturated geothermal stew.
The longer it takes this stew to cool the bigger and purer the crystals are.
The native copper in the UP tends to be of abnormaly high purity.
This is were the casting problems come from. Were they to melt a piece of
half breed with sufficient silver, there would be few problems casting it.

Paul K. Dickman