'Iron in the Viking Age' is a set of point form notes that accompanied a 2002 lecture that presented an overview of the iron smelting process. In spring of 2001, I participated in a project that added a new reconstruction to the L'Anse aux Meadows HNS - an iron smelter. One of the important features of the site is that it marks the first production of iron from bog ore in North America. Along with archaeologist Dr. Birgitta Wallace and historic metallurgist Arne Espelund, I consulted as the 'practical' member of the team. One of our achievements was the operation of a 'replica' of the same furnace that would have been used 1000 years before. (With many thanks to Arne for the chemistry!)

2008 Note: Looking back, this article shows how much was yet to learn about the physical process of actually smelting iron!

Iron Processing in the Viking Age

Definitions and Chemical Contents

Bog Ore: FeOOH
non magnetic
as much as 70% Fe (dry)
gathered as pea to walnut lumps at stream margins

Roasted Ore: Fe2O3
hematite or magnetite
magnetic
ideally a 'bright red' - black colour indicates high Manganese Oxide
heated to + 1425 F / 780 C ('cherry red')
done as a separate step, over wood fire

Bloomery Slag FeO at 60% / SiO2 at 25% / other oxides (varies)
forms at + 2100 F / 1150 C (medium yellow)

Bloomery Iron Fe with slag (SiO2) inclusions
other impurities such as Manganese, Phosphorus
for the Viking Age typically about 6 kg
flattened disk, often sliced on edge

Wrought Iron 'iron' from the Celtic
'wrought' from the hammering process used to squeeze out slag inside a bloom
little to no carbon content
cannot be hardened for tool making
very flexible in forge process
re-oxidizes (rusts) slowly

Steel Fe + C (typically .05 - 1%)
can be hardened for tool making, varies with C content
Modern steels made using Bessemer process

Cast Iron Fe + C (typically 4%)
addition of large amount of C reduces the melting point to about 2100 F /1150 C (like salt on ice)
this is below that of 'fast oxidation' (burning) allowing for mould pouring
cannot be forged (shatters) considered 'spoiled' by Norse

Charcoal wood converted by high temperature in the absence of free O2 to pure
carbon
typically produced by the 'shallow pit' method in the Viking Age
typical size is 2m dia by .5 - 1m deep.
start pieces at uniform 2" / 5 cm dia stacked in alternate layers covered with green twigs/ sand / sod to control O2
typical 'firing' takes about 30 hours, with constant attendance
will produce burn temperatures from 1850 F / 1000 C to 2500 F / 1400 C
consumption related to density / species type, but details unclear

Finding Bog Ore

Process 1) Iron rich underlying bed rock

1. shallow covering of water logged, rotting vegetation (bog) to provide
   acidic, oxygen reduced environment to leach out Fe into solution.
   
2. Fe rich water then needs to to interact with free O2 to create FeO0H
   
3. presence of 'anaerobic' bacteria fixes atomic FeOOH to nodules
   
   

1. peat bog with shallow pools, relatively shallow cover with poor drainage. Expect to see 'red / brown' colour to water from tannic acid and dissolved iron. Surface film with a 'blue / oil slick' appearance.
   
2. bog / pools drain off through small brooks over considerable elevation change. Should have seasonal variation in volume with ability to ' wander'.
   
3. bacteria found at boundary layer between sterile soil and vegetation / 'top soil'. Nodules will be exposed along stream margins as stream bed shifts.
   
   

Smelting Process

Furnace : There is a direct relationship between furnace size and construction / bellows size and rate / charcoal consumption / ore charge into effectiveness of smelting process and bloom production.

From LAM sample:
Furnace chamber 14" wide x 12" deep x 20" high
Construction stone slabs set on edge, backed with earth
bottom surface and seems finished with local clay?
Ore charge about 8 kg (roughly 60% FeO2)
Boom produced estimated at about 3 kg (poor results)
Charcoal required estimated at about 12 - 16 kg *
Bellows flow estimated at about 10 -12 litres per second *
Burn time estimated at about 6- 8 hours *

* These amounts extended from Spring 2001 experimental firing.

A) Reduction combustion of charcoal to produce CO gas at 2000 F / 1100 C
converts Fe2O3 + CO / C = Fe / FeC + CO2
process takes place in middle of furnace
problem is controlling absorption of free C, expect content of 1.5 - 2%
problem with particle size - typically 'dust' (see next step)

B) Sintering requires: high temperature (close to 'welding' at 2500 / 1400 C)
reduction atmosphere (balance of C / O2 = CO2)
available slag as a 'wetting agent'
larger particles condense to bloom, settle to bottom of furnace
slag surrounds and covers mass
(the exact mechanism poorly researched)

C) Carbon Control addition of further Fe2O3 into slag mass
creates 'free' FeO to absorb extra C
problem is control of temperature and content

D) Decant Slag requires slag to be fluid - temperature about 2100 F /1150 C
problem is temperature / content balance after step above
liquid is allowed to run off lower area of furnace to leave exposed bloom

Results: In any Bog Ore, SiO2 will also be present - if over 25%, unlikely ANY metallic iron will result from the smelting process.
'Perfect' yield would be a bloom at about 15 % of the starting ore weight, with an equal weight of slag (this assumes an ore content of about 70% Fe)