"Paul K. Dickman" wrote:

Seppo Renfors wrote in message ...


"Paul K. Dickman" wrote:

In order to illustrate the nature of the porosity in melted copper I put
a
few
pictures up.

http://tinyurl.com/3cw7p

Very interesting!

The first labeled Casting is a small ingot cast of ca110 copper It
started
out
.......
The size of this is indicative of how much gas was dissolved in the
metal.
It is approximately 10% of the original volume of the ingot.


Essentially what you are saying is that a lot of the gases escaped
before it solidified.

Possibly, probably.
But more importantly, there is still an volume of gas bubbles trapped inside
the ingot, that is equivalent to the volume of the blob sticking out of the
upper left hand side.

Yes I understood as much.

The third picture, labeled forgings, shows it's workability.
The lower shot is from the pure copper ingot, You can see it is full of
fractures and tears.

I assume this is from the melted copper described above?

Yes

Are you suggesting the "tears" are the result of (A) the pure copper having
been melted (B) because it is pure copper?

They are a result of the small air bubbles trapped throughout the metal
caused by melting it in less than controlled conditions.

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?

Pure copper, direct from the mill, melted in a vacuum furnace, or a void and
inclusion free piece of native copper, would forge out as well as the upper
alloyed sample.

Indeed, and that is because it has no inherent flaws with in it, to
cause fracture points. Only it would appear, as I read various
replies, than in certain circumstances "welding/forging" etc occurs
without much temperature - Eric is even suggesting room temperature in
one reply. Only when copper has been melted and has these gas bubbles
occur, it can't be done - apparently.

I do understand the idea of oxidization, but that would result in
embedded impurities more than wholly prevent welding to occur. Granted
it wouldn't be as good a copper as modern melting techniques provides,
but then such techniques and quality wasn't known about then and
ignorance is bliss. They would have been happy with what they could
do.

If (A) how does it compare to not melted copper of equal grade? Would it
not also depend on the
handling of the material if it tears or not - eg more frequent
annealing - hot working etc?

I have added a 4th picture to the above mentioned url to address this.
It is of a forging made from manufactured bar. In fact it is made from the
same bar that made up most of the cast piece.

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.

The piece made from manufactured copper was annealed twice. I probably could
have done the forging with out annealing at all, but it was a small piece
being held in my fingers. As the hammering hardens the metal, more
vibrations travel up the metal and into the hand. It can be quite painful.

That one I'm also familiar with - and it doesn't only apply to forging
:-)

If it is from the melted ingot - what has happened to the "myriad of
amorphous blobs" - they are no longer visible.


For the sake of orientation, the sectioned surface of the ingot is the small
end of the forging.

The outer surface of the casting solidifies very rapidly, and is usually
bubble free, the bubbles tend to be trapped inside.
Subsequent forging would mash these shut (although not bond them together)
making them difficult to see in a radiograph as they would either be smaller
than the resolution or look like regular forging flaws.

This is why radiographs are inconclusive an any piece that may have been
forged.

It is something I have been thinking as well.

The two metals are visually identical, but one casts like crap and the
other
doesn't.

If something is acceptable or not, depends on the use and views of
acceptability on the day. If something was melted to get a single
lump, to be later beaten into a sheet for further working to jewellery
for instance, then perhaps it really doesn't matter - perhaps it may
no longer even be obvious to have been cast.


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.

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.

[..]

If you want to prove casting, stop stroking around with radiographs and
look
for alloying.

Well..... there is the problem. This information hasn't been gathered
of either kind to any extent to my knowledge. The radiography (or
analysis) of two items... or even a dozen items, is not representative
of the tens of thousands of artefacts found.


Yes, that appears to be the rub.
And is an area in which someone should undertake some real hands on
research.

Otherwise, it is like the old saw about the man who lost his keys late one
dark night.
He spent the rest of the night looking for them beneath the only street
light that he passed,
because if he lost them anywhere else, he wouldn't be able to find them in
the dark.

Exactly :-)

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