Although copper occasionally occurs as metal in nature, the great majority
of all copper in the copper and bronze age was made by smelting various
copper ores. These ores therefore first have to be processed into metal,
before they can be cast into useful tool, weapons or other metal artifacts.
The copper ore
photo by Hans Splinter
As ore I selected malachite, in a form which would have been mined during
the copper and bronze age (so not the nice polished banded jewellery grade).
Malachite is the first copper ore known to have been smelted into copper,
as early as 5000B.C. in Serbia. The malachite I used comes from Morocco,
though is similar as samples I have from Europe. The malachite is mixed
with some azurite and cuprite, other forms of copper ore. The purity of
the ore is very high, with some small amounts of iron ore mixed in. This
would make it very easy to smelt, creating little slag. Copper is a noble
metal, which means it has a high resistance to corrosion. Because of this,
it also makes it easy to remove any attached elements to convert it back
into copper. The ore was crushed to smaller pieces, in order to create
more surface area, speeding up the smelting process in the furnace.
The chemical principles of smelting copper ore
The chemical formula of malachite is Cu2CO3(OH)2. This means that it's a carbonate, where the copper is bonded with carbon and oxygen (plus chemically bonded water). In order to turn malachite into copper, it has to be reduced. Reduction is the opposite of burning, were the metal is releasing its oxygen. Reduction is a chemical reaction. To reduce malachite into copper, two things are needed: heat and carbonmonoxide (CO). This gives the following chemical reaction:
The fuel used is charcoal, which provides both the heat and the carbonmonoxide.
The malachite already starts to reduce at temperatures well below the melting
point of copper. This was visible by the malachite still on top of the
furnace already getting streaks of copper.
Spread through Europe, there are signs copper smelting from the copper and bronze age. Remains of furnaces are rare though. Therefore I opted to build a furnace based on my own understanding of the workings of charcoal fires, furnaces and shape and size that I'd expect to be required. I build the furnace out of a 50/50 mix of clay and horsedung, which provides a robust mixture to withstand the internal heat without a lot of cracking. The furnace was build loose from the floor, so it could be moved into place for the smelt. The furnace is slighly burried into the ground, so that the tuyere (tube at the back of the furnace) is at the ground level, allowing the bellows to be connected to the tuyere.
I use the furnace two times, each time smelting roughly 5kg malachite.
The first time I placed a large crucible on the bottom of the furnace (shown
left of the furnace in the photo above), based on an example found in Kition,
Cyprus. This crucible has a hole in the side near the bottom, in which
I placed a clay plug. The idea was that if the metal should be liquid,
I could unplug the hole, and let the copper flow straight into a mould.
The metal did not stay liquid though. So in the second smelt, I did not
include a crucible.
After the furnace was dried by firing it with wood for several hours,
it's filled up with charcoal. When the charcoal is fully burning, the first
charge of malachite is added. As the charcoal burns down, alternating layers
of charcoal and malachite are added from the top of the furnace. This continues
for about 2-3 hours, until sufficient ore has been processed. The furnace
is then topped up a few times with only charcoal, which is then burned
down until all of the ore has gone down.
View inside the furnace through the tuyere. It can get over 1300°C
Workings of the furnace
The image above shows a schematic drawing of the furnace in sideview. Air is blown in from the back using the bellows, entering the furnace through the tuyere (= clay tube). The air flow up to the top of the furnace. The hottest zone starts about 10cm away from the tuyere, and gets colder the further removed. The zones in the furnace depend on the size and quality of the charcoal, force and volume of the air blown in.
The image above shows the flow of material through the furnace. Alternating
layers of charcoal and ore are loaded from the top. As the combustion is
the fastest near the side of the tuyere, most of the ore moves to that
side as the charcoal below burns away. This results in the metal being
collected just below the tuyere. As cold air is blown over the metal, and
the metal leaks heat through the furnace wall and bottom, the metal cools
down and solidifies there.
When the smelt was completed, the furnace was turned over. Above you
can see the a large amount of copper stuck on the inside of the furnace.
One of the two large lumps of copper from the two smelts. This one weighs
about 1.2kg. It still as charcoal embedded, and a little amount of slag
(mostly molten furnace lining). But it's very pure copper, and ready to
be melted and cast.
Here is the total amount of copper made during two melts. From each
melt there is a large lump that was just below the tuyere, plus lots of
small pieces that were spread through the furnace. The total amount of
copper is 3-4kg. The total amount of charcoal used is about 8kg, so less
then the weight of the ore processed. So the fuel consumption is quite
low. Below you can see a video of the copper smelting: