On Mon, 28 Jun 2004 21:45:11 -0400, Gary Coffman
wrote:
On Tue, 29 Jun 2004 09:49:39 +1200, Eric Stevens wrote:
On Mon, 28 Jun 2004 13:07:35 -0400, Gary Coffman
wrote:
On Mon, 28 Jun 2004 08:52:10 +1200, Eric Stevens wrote:
On Sun, 27 Jun 2004 02:58:26 -0400, Gary Coffman
wrote:
No trick to melting copper. Doing something intelligent with the molten
metal in an atmospheric environment is a different matter. As I noted
previously, casting pure copper is difficult, even today.
But the question is, how pure was the copper.
The native copper we've been discussing is very high purity.
The halfbreed ore does contain silver, but the silver isn't in
solid solution with the copper (copper-silver alloys are difficult
to produce). Instead it is in the form of distinct crystal inclusions
which would melt out and separate before the copper would melt.
In any case, copper can mostly by prevented from oxidising by melting
it under a layer of crushed coal or charcoal. In fact this method was
used for the production of largely deoxised (tough-pitch) copper in
recent time.
A graphite cover was used to prevent oxidation while melting (coal
won't work because of the large fraction of volatiles, charcoal might
be useable). But you also have to deal with the air entrained when
pouring.
Here is a quote from 'Metallurgy for Engineers' Rollason, 2nd Edition,
first published 1939:
Begin quote:
---------------------------------
Production of Tough Pitch Copper. In fire-refining copper the
impurities are removed by oxidising the metal until about 4 per cent
copper oxide (Cu20) is absorbed. During this stage the impurities form
oxides more readily than the copper and are removed as a slag or
evolved as gas. The last impurity so removed is sulphur which is not
completely driven off as sulphur dioxide by mere oxidation, but to
remove the last traces the metal has to be violently agitated by
poling, i.e. introducing an unseasoned piece of wood under the
surface. This causes a miniature fountain of molten copper, and allows
the air to come into contact with the spraying metal. Small test
castings or button castings are taken to indicate the state of the
metal. With sulphur present the ingot spurts just as it goes solid due
to the evolution of gas (SO2), but as the sulphur is reduced in amount
the surface of the ingot sinks in the manner normal to most metals. If
a micro-examination is made of this metal it will be found to contain
globules of copper oxide in the form of a eutectic (Cu-Cu2O). A layer
of crushed coal is then placed on the molten copper, and as poling
continues the copper oxide is reduced and when a content of about 0.04
to 0.08 per cent oxygen is reached the surface of the button remains
level and the properties of the metal are good, in other words
"tough." The lower the oxygen, the higher the so-called "pitch" and
vice versa, hence the name "Tough Pitch." As poling continues past
this point the copper absorbs hydrogen from the furnace gases and when
cast the metal rises on solidification.
These changes in behaviour, micro-structure and mechanical properties
are due to the influence of hydrogen and oxygen on the copper.
----------------------------------------
End quote
The above confirms not only the use of crushed coal but also the
primitive nature of the processes by means of which relatively pure
copper was produced even in the 20th century. Stirring with a piece of
unseasoned wood is a practice which may have roots going back for
millenia.
The quote is a procedure for smelting chalcopyrite ore.
I don't know where you get that from. The opening sentence says very
clearly "In fire-refining copper ... ".
That's a very
different procedure from what is required to process pure native copper.
Apples and oranges.
But then that's not why I quoted the article. I did so to deal with
your rebuttal of the use of a layer of coal to prevent oxidisation.
A bottom pour furnace is helpful, but you really need deoxidizers in
the alloy to prevent severe porosity problems. Tin and zinc are the
preferred deoxidizers. Arsenic also works, but the fumes are deadly.
Lead makes the metal more fluid, and assists in filling out the mold.
None of those are naturally present in the native copper we're
discussing.
Also, as a side note, where is the evidence for coal mining or large
scale charcoal production in the area? You don't get to copper
melting temperatures with a simple wood fire. You need a forced
draft fire with a high carbon fuel.
A good bed of well ventilated charcoal will suffice. One often finds
melted copper in the remains of burned out buildings.
A fully engulfed large building, or a forest fire, can produce sufficient
natural draft to reach copper melting temperature, but you'd need a
forced draft for a simple bed of charcoal. For doing very small amounts
of metal, such as small silver jewelry items, blowpipes would suffice, but
for doing anything on the order of the size of the artifacts we've been
examining, a bellows or blower would be required, and a *lot* of charcoal.
Let me propose that you conduct an experiment. Go to your local "high
end" audio shop and purchase some oxygen free copper "monster" wire
(similar properties to native copper). Now try to melt it in your backyard
barbeque. The insulation will burn off, but I'll be very surprised if you can
get the wire to melt without a forced air draft and *several* loads of
charcoal.
Actually I have carried out that very experiment to replicate damage
seen to 'Monster cable' in a domestic fire. Just for the heck of it I
through some into the base of a Jotul Alpha wood stove. The monster
cable variously melted or sintered into a solid bar of copper. FYI,
the Jotul Alpha is an 'air-tight' stove with the only air entry being
down the face of the front door glass from the top.
Making charcoal is an industrial enterprise in itself. I'm asking is there
any evidence of such activity in the area under discussion? So far I
have seen no reference to such activities. Nor have I seen any reference
to coal mining activity in the area. All that has been reported is mining
of native copper deposits.
That's a very different question from the use of coal to prevent
oxidisation.
I believe we are agreed that only atmospheric casting was within
reach of the ancient Native Americans (or ancient Old World
founders for that matter), so we *should* see characteristic
porosity in any pure copper items they attempted to cast.
Only if they used the relatively pure meteoric copper of Michigan. It
was laikely to be naturally alloyed if it was smelted.
Meteoric copper? Perhaps you're thinking of iron.
Its a term used to describe the copper deposited by contact with
meteoric water. Meteoric water is ground water formed by
precipitation. See
http://www.minsocam.org/MSA/collecto...r/vft/mi2c.htm
The copper we're
discussing is native copper. Native copper is the result of a natural
geochemical leaching process in certain types of rock formations.
It results in extremely high purity copper.
Only in some places.
Now
of course the Old Worlders had the advantage of ores which
did contain suitable deoxidizers. They weren't actually casting
pure copper. But the Michigan copper was essentially pure
native copper.
But it wasn't the only source of copper.
True, there are impure ores present in the region as well. But
there is absolutely no evidence that any of it was mined or
processed prior to the latter part of the 19th century. Further,
the impure ores which are present contain iron and sulphur
as their major contaminants. Those impurities are extremely
undesireable in copper that is to be cast. The ore has to be
smelted to remove those impurities.
No significant amounts of tin, zinc, arsenic, or lead, which
would improve casting qualities, are present in the ores of
the region. So even if the ancients had adulterated their
native copper with these ores, the result would not be an
improvement in the ability to cast objects from the resultant
mixture.
The ancients lacked a scientific understanding of metallurgy,
but they weren't stupid. They proceded by a sequence of trial
and error steps. If they added something, and the result was
worse, they'd quickly understand not to do that again. Since
the Native Americans in Michigan already had access to very
high purity native copper, and any local adulterant they added
would only make its properties worse, I'd suggest that they
quickly learned not to add any adulterants.
Now the situation was different in the Old World. The metalworkers
there had access to adulterants which *would* improve the casting
properties of copper, and they fairly quickly learned to add such
materials to their copper. That's not because they were brighter,
it is simply because they had materials at hand which weren't
available to the ancients of Michigan.
Gary
Eric Stevens