By: Leo Moonen

The “members of the iron smelting group” at the “Hunebed Centrum Borger” are already for some year’s now extracting iron from ore, following a method dated back to the ”iron age“. That is to say, as far as we can trace back this method.

The “iron period” starts (western Europe) around 800 BC and ends around the Roman period.

Iron ore can be found in Drenthe at many spots in the near subsurface. The ore we use is found in the banks of a near tiny river/stream at Westerwolde.
One method of extracting iron from the ore goes as follows:

First the ore is roasted on a “good” fire. All organic material is broken down and the ore gets brittle.

Roasting iron ore
Roasting iron ore

After roasting for a few hours and cooling down, the ore is broken in pieces of an inch (more or less) and together with charcoal (layer by layer) heated in a furnace.

The iron smelting furnace is build of loam (in Drenthe made/crunched under a thick ice
pack during the Ice Age). A typical model has a height of 60 – 90 cm, a width of 30 a 40 cm at the bottom and 20 – 25 cm at the top. To reach the desired high temperature of 1100 °C at which the minerals start melting but not yet the iron, bellows are used for forced air intake.

Often a set of two, manual operated. The airflow has to be regular, sometimes a little bit more, sometimes less air is needed. A typical smelting session takes about 6 to 9 hours, depending on the amount of ore and the time the airflow gets blocked. Working with bellows is one of the difficult parts of the whole smelting process. Last year we started with operating one large “double action” bellow: one chamber is directly operated using a lever and pushes air into a second chamber. This second one acts as a buffer and pushes under constant pressure air into the smelting furnace.

Last year in august, during the “Iron Smelting Days” at Solms, Grube Fortuna, in Germany, a group from Poland used a hollow tree as an iron smelting furnace.

Wood Core Furnace
“Wood Core Furnace” by the “Hutnia” group from Poland.

This “hollow tree furnace” attracted attention by its height: over 2 meters. At the bottom of the furnace a venturi is made (inside, not visible on the picture) directly above a lower chamber. Burning wood in this chamber pre-heats the whole furnace. The upper part of the hollow tree serves as a chimney. The whole construction is covered with loam. Most of the wood inside burns up, except the lower part which gives good thermal insulation. This furnace was operated without using a bellow. Everything works with natural draft of air needed for combustion. Through the two holes in the upper furnace wall the colour of the hot smelt gives an indication of the inside temperature.

This “hollow tree method” is so interesting that it was tested at the “Hunebed Centrum” on May 24, 2014

How we did it:
As no hollow tree was available, use was made of willow branches to make a copy of a tree, 30 cm diameter, 170 cm height. First the lower chamber was constructed (60 cm height, lower internal diameter 60 cm) with a slag pool of 25 cm deep and wood was burned in the chamber to pre-heat the furnace pipe standing on top of the chamber. Through a hole in the upper part of the lower chamber, already during preheating, carbon enters the furnace tube and may reduce the ore. (*) Through this hole, also all the needed air enters the furnace. Because this hole has a smaller diameter than the furnace tube (12 cm against 40 cm, say 40%), the hole in the construction works as a venturi and the airflow has a much higher velocity directly above this hole than higher up in the furnace tube. The effect is more or less the same as using a bellow but now completely “natural driven” – not mechanical.

(*) Iron envelopes sand grains as rust (red) and taking away the oxygen part of the rust by the binding carbon is called “reducing” . This process already starts around 700 °C.
To start, we need carbon monoxide (from coal): 2 C + O2 → 2 CO,
to reduce the iron ore: Fe2O3 (rust) + 3 CO → 2 Fe + 3 CO2 (“blank”, “soft” iron and “bubbles”) )
But also possible is: 2 Fe2O3 (rust) + 3 C → 4 Fe + 3 CO2

The furnace was build using loam, stones and willow branches.

After the lower air chamber was built and dry heated, the construction of a fake tree trunk started: a tube (30 cm x 170 cm) of straight willow branches around three wooden circular shapes, tightened with rope. The tube was then covered with a sheet to prevent loam falling into the tube. In the upper part of the lower chamber a venturi was built with loam and bricks, using a tin can of 12 cm as initial support. Above the venturi the bottom inclines with 15 ° to the hole in the middle. Four extra air channels allow some airflow regulation through the venturi.

Venturi beneath furnace tube. Bottom 15 degrees dip
Venturi beneath furnace tube. Bottom 15 degrees dip

Around the tube of willow branches (standing upright on the venturi) the final furnace tube was built using bricks and loam. Loam was “smashed” on the tube, between and on the stones.

Furnace tube constructed from willow branches
Furnace tube constructed from willow branches
Furnace tube, mounted on top of lower chamber.
Furnace tube, mounted on top of lower chamber.

One week later (May 24):
On Saturday morning we started burning wood blocks in the lower chamber to heat the whole furnace, for several hours.
The iron ore was “roasted” two days earlier on a “good” fire for a few hours. A part of the ore was crumbled (to pieces of 2 a 3 cm) , the remaining was done this morning, sorted with a magnet and a little bit of household soda (Na2CO3) was added. (a total of 30 kg ore was prepared for the smelting process)

After a few hours – early in the afternoon – the furnace tube itself (where the actual smelting takes place) was topped with bits of wood. Again a few hours were needed to lighten up the wood in the tube over the whole height and for the first two hours the smoke was grey (vapour).

Smelting furnace, loaded with wood – burning.
Smelting furnace, loaded with wood – burning.

Then the time was reached to put the iron ore into the furnace, cover it with some charcoal, and leave it to smelt. Also this process takes a few hours. As soon as the colour of the hot ore in the tube (visible through three inspection holes in the tube wall) turned from bright yellow to hot white light yellow, to painful for the eyes to look into, the temperature was hot enough for the minerals to melt and it started to drip out of the ore and flowed slowly down through the venturi into the lower chamber. A very nice example of slag was formed of around 15 cm length (photo).

All steps we did so far took some more time then expected – all labour and the whole process needs some more time than working with bellows – and only late in the evening did the furnace reach the desired high temperature where ore could be added in the tube for smelting. Decided was to start with only 6 kg (of 30 kg waiting) to see what would happen and wait some time before adding the remaining – still to be decided as one volume or layered with charcoal.

The first “enthusiastic” flames above the furnace however diminished after some time… and the temperature dropped slowly to a 900 °C. The remaining ore was not used.

Smelting furnace, operating at 1100 °C, by natural ventilation only. (~midnight)
Smelting furnace, operating at 1100 °C, by natural ventilation only. (~midnight)

Decided was (around midnight) not to add an extra layer of charcoal followed by ore but allow the process to end slowly. One personal goal was reached: could the furnace get to the high melting temperature of minerals (around 1100 °C) without using bellows or other mechanical means, but only by natural draft : yes! The maximum temperature measured was even 1200C for a short period.

(temperatures are measured by three probes, inserted through the wall, reaching just inside the furnace and the output displayed on a pc)

The amount of 6 kg ore was brought into the furnace by simply turning a bucket at the top of the furnace, at a height 170 cm. It did not reach the smelting area as “one compact volume” but was the next morning – after opening the furnace tube – found to have spread all over the bottom. No “loupe” was found, just here and there some bits of light magnetic material. It would have been better to put the ore (gently) into the tube as one compact volume by putting it for example in a cloth.

A nice shaped piece of slag.
A nice shaped piece of slag.
Drawing of the smelting furnace
Drawing of the smelting furnace

This smelting furnace was based on “ancient wood core furnace”, Holy Cross Mountains, Poland.

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