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10.1.4 Smelting Copper - The
Second
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Let's
Smelt! |
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I am now going to demonstrate the
working of a "large-scale" copper smelter to you. That is the kind of
apparatus that allowed to start the "bronze-age" around 3 500 BC,
about 1500 years after the first small-scale smelting of copper. From today's
point of view, those smelters are pitifully small affairs. However, compared to
what was around before, they were complex and large "machines" that
also needed a certain infrastructure for successful operation.
It also doesn't matter at this level of generalization what you smelt. Here we
look at copper, but doing iron would also be possible, not to mention lead and
tin.
The first thing we need is a "container" that, well, contains the
fuel, ore and flux in the top part of the smelter, and the product in the
bottom part. It also - most important - must contain
the heat produced by the energy source in the bottom-near region.
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That you must somehow mechanically contain the ore, charcoal, and copper
produced is a bit trivial at first sight. I promise that it will stop to be
trivial when you look at it a bit more closely.
That you must contain the heat is also trivial. If you don't, it just won't get
hot enough where it's needed. Your heating apparatus in your room will not be
able to sustain the temperature in the room if you open all windows and make
sure that there is a draft. |
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So let's make a shaft furnace, a tube
with an inner diameter of roughly 30 cm, and up to a meter tall. We make it
from a material that can take the heat and that has a low heat conductivity, so
heat doesn't leak through the walls. Mud or clay, perhaps re-enforced with some
straw, might do the trick. |
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Our furnace tube must not contain the gases blown through it or produced
during smelting. They must be allowed to leave through the top or "flue". Since our construction
is open on the top, gases can not only leave there, we can also use that
opening to replenish fuel, ore and flux from time to time.
The bottom part is set somewhat into the earth, after we lined the very bottom
with some suitable stones as shown here: |
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| Bottom part of a shaft furnace |
| Source: Photographed in the Goslar museum
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Now we need a few
more openings. Definitely one for running the air in that is needed to keep a
fire going. Through that opening we stick a pipe, called a "tuyere", through which we will blow in the air,
somehow.
Maybe we add another hole lower down for tapping liquid slag every once in a
while. The whole contraption (without a slag tapping hole) now looks
schematically like this: |
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| Schematics of a real smelter |
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A first construction
problem becomes obvious. Everything would be far easier and more elegant if the
solids (charcoals, ore, flux, ...) would
sit on a grate, with the air flowing uniformly through
the load which we will now call by its proper name: "burden".
Sorry. Can't be done. There was (and more or less is) no material that can take
the heat and the weight of the burden for very long. We have no choice. We need
to blow in the air with a tuyere from the side as shown. In doing this we must
make sure that: |
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- It is the right amount of air (always a
lot), and
- It flows out of the business end of the tuyere with the right speed that makes it go nicely through the
whole diameter of the furnace as shown. If it comes in too slowly, it will not
reach the other side. Too fast and it will leave a "dead" pocket
above and below the tuyere.
The diameter of the tuyere is essential here. It is easier to move a lot of air
into the furnace if the diameter is large - but then the air will flow slowly.
The two demands are contradictory, and in order to meet them the (inner)
diameter of the tuyere must be just right
for the kind of pressure you can apply to
your blowing apparatus. |
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Even with the right tuyere diameter
it is not so easy to meet the two easy-to-state requirements above. It is
actually very difficult as we shall see. Read up the science module if you want
to know why this is so right now. Otherwise
we just assume that we somehow can do it right.
Now we fill our smelter with charcoals only, and start the initial fire. |
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No, No NO!! You don't use your
barbecue charcoals! That's one of the many reasons why
your smelting didn't work. You need
to use a carefully adjusted mix of high-energy
charcoals produced from special
hardwood trees, and highly reactive charcoals made from soft wood. For best
results the charcoal should have been made from branches and not from the trunk
of the tree, and at rather high temperatures. In other words: you use special
and expensive charcoal.
Not to forget: You pay a lot of attention on making sure that your charcoal
lumps have all about the same size, and that must be the right size. You need to meet two contradicting
requirements once more:
- The surface-to-volume ratio of your
charcoal bed should be large since the energy production (the
"burning") takes place at the surface. That calls for a small lump
size. Not too small though, otherwise too many surface regions touch and cancel
each other.
- The air resistance of your
charcoal bed should be just right and not too large. Air resistance measures
how hard it is to blow air through your burden. For very fine lump sizes
(essentially a powder), air resistance is very large and blowing air through a
bed of powder would take a very high pressure. You might just blow your burden
right out of your furnace in this case. Making the lump size large, or using
just a pile of wood, makes it easy to move air trough the pile but now your
energy production goes down, and a lot of the oxygen in the air isn't even used
for burning but passes just through, preventing carbon monoxide production.
Your lumps size needs to be
just right. As a
rule of thumb, start with a size of about about 10 % of your inner furnace
diameter and then optimize. |
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Now set the charcoals on fire from
below, establish proper air supply, and keep it going until your furnace is
"at temperature". It will take a while because during this period you
heat up and "fire" the clay walls of your furnace, too.
You will observe that only the first five or so layers of charcoal are really
burning, with a temperature of (1200 - 1300) oC
((2192 - 2372) oF). Actually, you will not observe that
because there is no way to look inside a working smelter. You infer that from
indirect observations.
Why is that? Why shouldn't the whole stack be on fire? Because the oxygen
needed for burning comes from below, and at optimal conditions there is no more
oxygen left after the air traveled through the first few layers of your
charcoal bed. Instead of the normal oxygen - nitrogen mix that constitutes air,
you have now a carbon-dioxide - nitrogen
mix a few layers above the burning charcoals. That is good - it enables
smelting!
The charcoals above the ones burning and thus taking out the oxygen from the
air are quite hot, of course, since they are close to a very hot fire and
exposed to very hot gases streaming through. Those gases are the carbon-dioxide
(CO2) formed by the burning reaction that also produced all the
energy, and the nitrogen of the air that does not do anything except getting
hot. If you provide enough of that energy (by reacting the right charcoals
having the right size with the right amount of air) the charcoals above the burning layer should acquire a temperature
of at least around 800 oC (1472) oF); more is better.
That will allow the reaction of the carbon in the charcoals with carbon dioxide
from below, producing deadly carbon monoxide (CO) according to:
CO2 + C Û2 CO.
This reaction actually consumes energy and
thus cools things to some extent. So if you want the reduction zone to be around 1000
oC, the oxidation zone,
where the burning takes place, must be considerably hotter.
The carbon monoxide produced will move up the stack with the nitrogen and the
carbon dioxide that might have been left over. |
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After a while of burning just
charcoals, your smelter is ready for use. You can check that by hanging over
it. If you are dead quite quickly, the carbon monoxide production is taking
place satisfactorily. A better way is to watch how fast your charcoal moves
down. Charcoal disappears almost completely by either turning into
CO2 or CO, and the whole burden therefore moves down the shaft at a
certain rate or speed. As soon as this consumption rate looks right, you start
to add ore and flux to the charcoal you feed to your smelter. |
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Now we can finally start to do a bit
of smelting! And it goes without saying that you better take care to:
- have the right ore or mix of ores.
- The right average size of your ore
pieces.
- The right kind of flux or mixes of
fluxes,
- with right sizes.
- And in particular the right amounts of
charcoal, ore and flux!
This link gives an idea of
how that was done not so long ago in the case of iron smelting. |
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As your burden moves down, a given
layer in the burden will get hotter and hotter while it is exposed to the
carbon monoxide and nitrogen streaming up. The nitrogen still doesn't do a
thing except heating the burden and funneling out energy. There is nothing you
can do about that.
The carbon monoxide, however, will start to react with your ore, reducing it,
and it does so typically already at low to medium temperatures already present
high up in your furnace. The product of the reaction is the metal, and
typically carbon dioxide once more. Possibly some more stuff is produced,
depending on what kind of ore you use, either escaping as gas or adding to slag
production. That will turn out to be a serious problem; I'll ge to that.
In either case, you now need to fully understand an often overlooked but
serious fact of smelting: |
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Metal production by reduction is
always a solid state process!
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In a way that is
clear. You can't melt a metal before you
have it.
Very schematically what is going on looks like this: |
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| CO production and ore reduction in an
atomistic model |
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This is what happens on an atomistic
level. Hot carbon (upper left) reacts with one molecule of carbon dioxide
(CO2; blue ellipse) to form two
molecules of carbon monoxide (CO; pink ellipse) The CO takes out the oxygen
atoms from the surface of the metal oxide (center right). This leaves surface
vacancies behind. The left-behind metal atoms roam along the surface until the
find a metal crystal into which they become incorporated. Alternatively they
start to form a new one. The arrows just indicate that all the gas molecules
run around with a very high speed that defines the temperature.
Your gangue plus the added fluxes also start to react, forming glassy stuff
that is, however, neither fully formed nor liquid at the medium temperatures
where reduction is already happening. This is not shown above since my artistic
talents are limited. |
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Once more: Reduction starts early and
turns the ore into the metal plus, depending on what kind of ore you use,
something else that either escapes as gas or participates in slag making.
Nothing is liquid while that happens. Eventually, the process is finished. The
newly smelted tiny metal particles move down with everyhing else, getting
hotter, but for a while they are still in the carbon-monoxide rich area we call
reduction zone, together with charcoals not
yet consumed and the slag that meanwhile also has formed from the gangue and
the flux added to the burden.
But after a while you run into a big a problem. You must get your metal now
through the very hot and oxygen containing zone (the oxidizing zone) where the charcoal burns, all
the way to the bottom. You must do that without
oxidizing your metal again! |
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If you ever watched a smith putting
a piece of iron in the really hot part of the fire on his hearth, you saw that
his solid piece of iron oxidizes very quickly. Your solid small copper or iron
particles don't stand a chance! They will never make it through the oxidation
zone if the go down there in a leisurely pace. They will get completely
reoxidized! You must move them through rapidly, and that is best done if they are liquid! Liquids can move much faster down the bed of
charcoals than solids that must move with the charcoals. Liquids just trickle
through.
The problem of course is that we do not necessarily have our metals liquid. The
melting point for copper may not yet have been reached at the edge of the
oxidation zone, and for iron it will never be reached. Iron stays solid all the
time. So nothing helps but enclosing the metal particles in liquid slag
Liquid slag can easily take small solid particles along when it trickles down.
And it not only transports them fast through the dangerous oxidation zone, it
also protects them from oxidation since it coats the particles.
If a liquid metal trickles down by itself, it will become partially oxidized
but some metal might make it. The metal oxides formed will end up in the slag.
I'm talking efficiency here. How much
of the copper contained in the ore ends up as the elemental copper you are
after, how much ends up in the slag as copper ore? It is not difficult to have
a very high percentage for the latter. You fill in a lot of ore and charcoal
but you get just a little bit of metal; see the picture below. |
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Now you have one very good reason why
slag is good for you - if its the right
kind, and so on. There are more very good reasons why you want to produce slag
- read about them
here or just believe me!
All I wanted to do here is to demonstrate one thing. Smelting adheres strictly
to the Goldilocks
principle: |
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Smelting is a complex process!
A lot of things need to be JUST
RIGHT!
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A lot can go wrong. If you don't
start with everything being just about
right, there is very little you can do after you set your smelter on
fire.
Nowadays we understand most everything and can model what is going on inside a
smelter on a large computer. But a few details are still not all that clear.
The reason is that it is impossible to "look" inside a running
smelter, watching what is going on. |
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Next let's look quickly at the key
problem of smelting: the air supply! |
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The
Overbearing Importance of Air
Supply |
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Very simple calculations or quickly
gained experience will teach you that you need a lot
of air to run your smelter from above. How are you going to supply
that air. Let's quickly go through the options: |
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1.
Blowpipe
The picture shows the use of blowpipes around 2 400 BC in Egypt. The other half
of that picture is here. You supply air to your smelter by
exhaling into a blowpipe that leads right into the furnace or crucible. |
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| Use of blowpipes around 2.400 BC |
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Blowpipes are laughably inefficient.
It takes six guys in that picture to keep a tiny little crucible at
temperature! There is no way to run the smelter above with blowpipes. There
would not be enough space for assembling the necessary throng of guys around
your smelter. And no, you can't just make the blowpipes longer; they loose
efficiency with length. |
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While smelting probably did start
with with blowpipes, you didn't get very far this way and it went out of
custom. It took only a few thousand years for the guys to figure out something
more effective.
That's right! Smelting in the very beginning, before it became more common
place around 3 500 BC, was done with blowpipes or with one of the methods
coming next. It was still used after 3 500 BC as the picture proves. However,
blowpipes still would have been your instruments if you just wanted to melt small amounts of a precious metal like gold in
the privacy of your home. Maybe that's what those old Egyptians were doing.
I'll get back to that further down but first I like to introduce a somewhat
better if still not-so-good method for supplying air. |
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2.
Natural Draft and
Wind
Potters could (and still can) heat a large kiln to very high temperatures by
burning wood in an mostly enclosed
fire box connected to the
"hot box". The whole contraption was constructed in such a way that
it produced sufficient natural draft (via the
chimney
effect) to keep a lively fire going.
That kind of construction culminated in the "reverberatory furnace" that even
allowed to melt iron in the 19th century. |
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Fine - but we are talking 6 000 BC or so here.
Those guys might have mastered
kinky sex by then, but
they couldn't even get close to a reverberatory furnace or an advanced kiln.
Nevertheless, more simple "natural draft" furnaces might have been
used. Unfortunately, not much remains after 8000 years or so, and what one
finds often does not allow an unambiguous identification. Here is an
example.
Some natural draft furnaces for smelting iron have been used in Africa in more
modern times, so it can work - provided you do everything exactly right, of
course. |
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That is also true for the next kind: Wind-blown furnaces. Besides doing everything just
right, you now must also have the right wind from the right direction.
Wind-blown furnaces have been used at least in
Sri Lanka
around 500 BC for smelting iron, so you can do it.
What we can reasonably assume for neolithic times is that natural draft and
wind-blown furnaces were used and that they were an improvement compared to
blowing into a salad-bowl sized crucible with your blowpipe. Natural draft and
wind furnaces might have been instrumental for starting larger-scale copper
smelting arond 3500 BC. They are still rather inefficient and messy, though.
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Both types - natural draft and wind -
have one huge advantage: if it works, it is very simple and cheap. You save the
costs for food and whips, needed to keep your slaves going, and so on.
However, they also produce a major problem: |
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Natural draft and wind furnaces
can't be controlled!
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The only major control you have while running a
smelter is the air supply! You can blow into your blowpipe vigorously or
sluggishly. You can move its business end from here to there. You can get a
second guy to help blowing. You have some
control!
With simple natural draft / wind furnaces you must take it as it comes.
Not so good. If we want to do some serious large-scale smelting now, we need to
invent... |
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3.
Bellows
Bellows were the major invention for getting serious smelting going. We call
everything "bellows" that turns mechanical energy into a stream of
air, not just the accordion type you're visualizing right now. Your bicycle
pump qualifies as bellows, for example. Here is a picture: |
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Business end of the big bellows shown
here plus a
piece of modern junk for blowing up air mattresses |
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Why bellows are of the utmost importance for
smelting is simple to see. One well-trained guy working a well-designed bellows
replaces at least 40 if not 80 guys with blowpipes! Your muscle power is just
that much larger than your breathing power.
Attach one decent bellows to the tuyere of our smelter
from above and you are in business. Of course,
the diameter and length of your tuyere must be just right, not to mention the
"angle of attack" (the 15 o shown are about right), and
you should have no leaks in your set-up.
It is really simple: no bellows - no large-scale smelting. American cultures
never invented bellows and they had no large-scale smelting of copper either.
Moreover, they never got around to smelting iron. More about the topic can be
found here. |
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Bellows offered one more big
advantage: you can also work them with powerful machines like water wheels or
windmills, increasing power relative to an human another 50 or 100 fold.
Besides the Chinese,
however, nobody did that before the Middle Ages, even so the infrastructure was
in place. The Romans, for example, did use waterwheels extensively for other
jobs.
The reason for not using large water-powered bellows and matching large
smelters was simple. It made no sense to smelt iron in very large
"bloomeries" since you couldn't handle massive blooms, the solid iron produced in there. How would you get it
out, for example, without destroying your smelter? That would be an
insurmountable problem even today. But bloomeries went out of style around AD
1500, in exchange for blast furnaces that
made liquid cast iron. Doing that you can
go for large-size smelters since your product is easily removed by tapping it.
A blast furnace, by the way, is exactly the same thing as your old bloomery,
except that it is bigger and you blast a lot of air inside continuously by
mechanical means. |
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Who was the
genius who invented bellows, one the most important pieces of hardware needed
for the progress of civilization? When and
where?
I don't know, neither does anybody else. The Hittites certainly knew bellows
around 1700 BC since they brought that technology to Egypt when they invaded
it. It is likely, however, that bellows are considerably older than that. It is
tempting to identify their invention with the start of
"industrial-size" copper smelting around 2 500 BC, somewhere in
Mesopotamia, Asia Minor or the Balkan. |
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Primitive
Smelting |
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It remains to do a bit of primitive,
inefficient and messy smelting, like much of humankind before around 3 500 BC.
The most primitive smelting you can do in your living room, using a salad-bowl
size crucible. In fact, your ceramic salad bowl might be good enough for the
job.
The picture tells it all:. |
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| Schematics of a working salad bowl
smelter |
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Heap up some charcoal, more on the side opposite
to your blowpipe. Ignite it, put your blowpipe into it, blow. Add some ore,
stir maybe, blow here, blow there, and with luck you will produce a mix of a
little slag and a few prills of copper, wherever you happened to keep an area
hot enough for a little while and produce some carbon monoxide.
It appears that something like that was done for millennia in some towns, here
and there. Interestingly, those towns were not close to the places where copper
ore was found or even mined. The ore obviously was "imported" to the
places where smelting was done, maybe just as much for its
"greenstone" quality in the beginning as for smelting a bit of copper
on occasion.
Here are two real crucibles. Note the hole in the base, where
you could insert a stick for handling. |
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Unused (left) and used (right) crucibles from
the early
fourth millennium BC; Ghabristan; Iran 1) |
| Source. Courtesy T. Stöllner and G,
Steffens, Deutsches Bergbau-Museum, Bochum, Germany |
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Tin smelting, on the other hand, appears to
have taken place close to the mines, for example in Göltepe (not to be confused with
Kültepe), a large
bronze-age town thriving from about 3300 BC to 1800 BC in what is now Turkey.
Remains of crucible smelting have been found, but also enormous deposits of
slag, indicating that the smelting method must have been more advanced than
what is shown in the picture above. |
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Now I can dispense with (simple)
melting in just a few sentences. Do more or
less the same thing as in simple crucible smelting. Use a smaller bowl /
crucible because you must be able to pick it up when it is very hot so you can
pour the metal out. Only put charcoal and copper in there. Blow directly on the
surface because you do not need to make carbon monoxide. Look up a few more
details here. |
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Just for being complete, there is yet
another way of smelting. Take a crucible formed more like a beer glass than a
salad bowl. Fill it with powdered charcoal
and ore plus some magic ingredients (powdered flux in other words). Immerse it
in a bed of burning charcoals or into a very hot kiln. In other words: heat it
from the outside. If everything is done right, you might get the so-called
direct carbothermic reaction inside
the crucible, reducing the ore first with solid carbon and not with carbon monoxide. This is rather
inefficient because the reaction can only take place at the points where the
carbon touches the ore and not all over the surface as with carbon monoxide.
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However, if your were lucky and had the right ore
for (iron) smelting, the carbothermic reaction produces its own carbon monoxide
according to, if you must know:
2 Fe2O3 + 5 C Þ 4 Fe + CO2 + 4 CO.
The process thus was self-amplifying. It allowed to reduce ore in a furnace or
kiln with an oxidizing atmosphere; you
didn't need to produce reducing conditions. This kind of crucible smelting
might also have worked for copper - I don't know.
More about that in
this link.
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Of course, in a
space as large as Europe plus parts of Asia, and during a time span of a few
thousand years, all kinds of smelting variants must have occurred. The
technology slowly improved from extremely simple and inefficient "salad
bowl" smelting, via highly organized blow-pipe smelting (like the
guys above) and natural draft
/ wind furnaces to bellows driven "large-scale" operations. Zillions
of variants must have been used, and the archeological records tend to show
this. Here is a rather illuminating statistic from two places of what is now
Jordan: |
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Percentage of copper found in slag as a
function of time.
Different color=different general areas |
| Source: Redrawn from
Haupmann's
book |
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Note the concentration scale - it's
logarithmic! Duringn the very early times of smelting, up to 100 % of the
copper ended up in the slag. The "pink" area (actually the Timna area
in Jordan, close to the gulf of Elat) was doing better than the "light
blue" area (Faynan, also in Jordan, further North) with only around 5 % Cu
lost. As time progresses, efficiencies get better. Around Roman times Faynan
was doing better than Timna, and losses were below 1 %
It took several 1000 years to get there! |
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Still, nothing helps, without bellows
plus getting everything about the furnace and its burden "just
right", copper smelting could not take off and become the large-scale
operation that used copper alloys and
started what we call the "early bronze age". |
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Maybe you noticed that I'm inching away from
copper smelting towards iron smelting? And that right now it looks that there
is hardly any difference between copper and iron smelting.
Be patient, please. We are almost there. We
only need to look at alloying copper first,
so we can make proper (tin) bronze and what is referred to as
"arsenic" copper. |
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© H. Föll (Iron, Steel and Swords script)
