"Paul K. Dickman" wrote:

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.

..... or even more than one time "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.

Yep, that is quite possible too - only then one would expect, under
close inspection, it would show a crack to the outside. Still one more
thing is required - the metal be hot enough to be quite "plastic" for
it to expand and form the bubble. Which isn't a long way away from it
being fully melted to a liquid form


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.

As you have demonstrated the bubble's shape isn't a governing factor
of its cause.

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 problem being two fold. One the radiographed picture showing the
bubble isn't large enough to see if it has any associated porosity
with it. Again forging can have reduced the visibility of such as well
as any telltale shape of the bubble. The only other "clue" is the text
claiming "proof" of "cast". Until proven to the contrary, one would
have to assume they had a better view of the radiograph and physical
inspection of the artefact than we have from the web page - secondly
that they have the knowledge of the effect of casting - as well as
forging of cast billets.

I for one am not prepared to reject their view on the basis of the
information able to be seen on the web page. Instead I would say even
IF it is cast, it is insufficient evidence to claim any common
practise of casting from those few artefacts without further
investigation.

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.

Yes, I though it was probably the case - here it is worth noting that
the "bad" is a relative term and to the ancients it probably wasn't as
"bad" as it would be to modern craftsmen.

but the closer the anneal is to the finished product, the softer the
finished product would be.

Yep. Here again is information that isn't known or at least readily
available.

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.

But they wouldn't know that. What they would note is that the tips
would break after having been bent and unbent several times. Sharpness
to the point and sides of a projectile tip would probably be more
important to them than the hardness.

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.

It was mined in West Mexico, and exported to where it was used.

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.

So I believe. I have no idea of the purity of copper from Mexico, but
this is stated:

http://www.nmnh.si.edu/anthro/outrea...004_winter.pdf
"Bells of native copper were among the exotic goods excavated in
Chihuahua, Mexico ......" As I understand it "native copper" is a
reference to rather pure copper.

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

--
SIR - Philosopher unauthorised
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The one who is educated from the wrong books is not educated, he is
misled.
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