Slag as a historical resource
As well as being the waste removed in the smithing process described above (smithing slag), slag is the waste
material produced in smelting. This material has a pivotal importance in the field of Archaeometallurgy, the
study of the Archaeology of metal production. In archaeometallurgy the interest has primarily been in to Iron
and Copper (Bronze) production (althought lead, tin and zinc smelting is also the subject of research) due
to their pivotal importance in human development.
The basis of metal smelting is to take a metal rich ore and chemically reduce this at high
temperatures, leading to the seperation of a metallic phase and a slag phase. The slag is formed of all the
components of the ore/furnace system that are rejected by the metal. Thus the metallic phase will consist
almost exclusively of the primary metal and some siderophile elements, leaving the slag with a virtual
encyclopedia of chemical information about the smelting process in the form of silicates (Assuming that
the initial ore, like most of the planet, is rich in silicon). The slag is therefor a liquid record of each
individual metal producing smelt that is then rejected as waste.
Techniques you can apply to slags
The development of the blast furnace forms the end of the main period of interest to archaeometallurgy, since
from this point on the slag produced is formed in a glass phase. Previously slags typically formed a crystalline
material that can be viewed as an artificial lava, allowing all the techniques of Geology to be applied to it.
There are huge advantages to using slags in archaeological study, since they avoid all the problems of destructive
analysis that usually plague artefact science (fundamentally no-one cares what you do with slag, you can crush it
and stick it in machines that go ping) and further, it is almost always completely untouched since the day it was
The process of metal smelting
An archaeological furnace typically consists of a clay furnace, ore, charcoal, bellows tubes(clay
tuyeres) and smelting additives (our ancestors were not necessarily sensible and added all kinds of oddities).
Basically an iron furnace consists of a closed bowl of clay in which you funnel charocal and ore while continuously
adding air. The air increases the rate of charcoal combustion resulting in an increase in temperature and highly
reducing chemical conditions (the charcoal sucks the oxygen out of the ore and converts it to carbon dioxide). The
reduction converts the ore to metal and slag, which then mixes with the molten charcoal (typically rich in calcium)
and clay (typically a complex silicate).
Most iron manufacturing process have a mechanism for releasing the liquid slag mid-smelt, termed tapping, to
improve the rate of metal production, this liquid slag then crystalises as it leaves the furnace forming an
artificual rock that looks very similar to hawaiian lava. The main crystal component in Copper and Iron
slags is Fayalite (Iron olivine, Fe2SiO4), and in the case of Iron slags this is accompanied
by various iron oxides (FeO, Fe2O3 and Fe3O4), By judging the
slag mineralogically (in terms of the iron content and oxidation state of the crystals) we can get an estimate of
smelting efficency (how much iron was left behind in the slag), temperature of smelting and the oxidation conditions
in the furnace.
Thus if you want to know how the Romans built the modern world, go find a slag pit in Italy. If you stick
that waste in a machine that goes ping it will tell you far more than a fat tome by Plutarch ever will.