The EconoNorse Iron Smelter

The EconoNorse small test smelter takes its rough size and general operating characteristics from a blend of Viking Age archaeology and modern practical experience. The rough form was developed in Fall of 2004 by members of the Dark Ages Re-Creation Company, with further refinements made in February of 2005 with guidance from Sauder / Williams / McCarthy.

Past experience has shown that the smelter team ideally should consist of at least THREE individuals, with an additional person as record keeper. Charcoal and ore should be prepared before the actual smelt.
The basic structure is made up of three circles of standard fire bricks set above one another, with eight fire bricks stood on end making each layer (for a total of 24). This gives an internal size roughly 10" / 25 cm in diameter and about 24" / 60 cm high. The bricks are contained inside a support structure made of a cylinder of sheet metal (or leaning stone slabs) about 24" / 60 cm in diameter. The gap between these two is filled with a loose mix of wood ash and sand (or other insulating material).
The internal floor of the smelter should be built up to a depth at least 2" / 5 cm deep with a tamped down mixture of wood ash and charcoal dust. A gap in the first layer of bricks should be constructed about the size of a single brick laying horizontal to form at tap arch. If the whole structure is constructed on plinth of bricks containing a built up layer of sand / ash / charcoal fines, the slag tapping process is made easier.
The tuyere (air inlet) can be made of a simple length of 1 or 1.25" OD / 2.5 to 3 cm standard schedule 40 (or standard black threaded) steel pipe.It is placed on top of the first layer of bricks, in a small gap in the second layer - at 90 degrees to the tap arch. Above the tuyere, this gap should be filled with part bricks and sealed with clay. The tuyere should protrude about 2" / 5 cm inside the smelter wall (B). It should be positioned on a slight down angle of roughly 20 - 25 degrees (C), and stabilized with a spare brick or wood block. The shape and quality of the bloom is greatly effected by the position of the tuyere.
Air flow can be provided by a old vacuum cleaner blower, great bellows or hand rotary blower. If an electric blower is used, it should be equipped with either a sliding plate air valve or adjustable motor speed control. The air volumes required are considerable, and must be delivered both constantly and consistently over the entire smelt. Air flow is a variable related to smelter diameter, and for this size will be approximately 600 - 800 l/min.
At Smeltfest 05
A huge quantity of regular hardwood charcoal will be consumed during the smelt, normally from 150 to 175 lbs / 70 - 75 kg. This should be broken up and then screened so that no pieces are larger than 1" / 2.5 cm dia. (walnut) and smaller than .5" / 1 cm (pea). The fines are used as insulating material.
Iron ore may be natural rock, bog ore or prepared taconite. In any case the iron oxide ore must be pre roasted by heating to glowing (it will become magnetic after this step). The roasted ore must be crushed when cooled so that bulk of the particles run between rice grain and no larger than pea sized. It has been fund that quenching the hot ore (rock or taconite) assists in this process.
The smelter should be preheated, starting with softwood splits and no air blast. The fire size plus air blast will be increased over about a one hour period. At this point the first load of unsorted charcoal is made, with graded fuel used after. As temperature inside the smelter increases, both the consumption rate will increase, and the position of the observable top to the burning zone will rise. The smelter will be approaching operating temperature when the time to consume a standard galvanized pail (at roughly 4 lbs / 1.75 kg) is about 10 minutes and the burning zone approaches the top of the fire bricks. Fuel should heap up over the top of the smelter and added whenever the level drops to even with the top edge. Note that this consumption of fuel should remain relatively constant throughout the remainder of the smelt. This is the method used to determine changes in air flow or rate of adding ore charges.
Operating temperature should be arrived at after about 30 minutes and the first 'seed charge' of roughly .5 lbs /.22 kg is added by sprinkling evenly over top. This should be repeated with every fuel charge for about the next four. It is suggested that a standard sized long handled scoop be used for this. Note that various ore types will have different volumes to achieve the weight.
About one hour into the smelt, the smelter should 'take off' with consumption rates rapidly increasing. The consumption rate can be stabilized by increasing the amount of ore added per fuel charge to maintain the consumption times, typically increasing by .5 lb / .22 kg amounts. Normally the consumption rate will peak with ore charges in the range of 3 to 3.5 lbs / 1.5 - 1.75 kg.

Pre-heat Phase
Adding Charcoal Charge

About four hours into the smelt, The process of tapping off slag typically will begin about four hours into the smelt. The rate and volume of slag will vary due to ore type, but a balance must be made between maintaining a bowl of liquid slag - and not 'drowning' the tuyere. Tapping is done by removing the brick blocking the tap arch, then digging away the loose material to expose the bottom of the slag bowl. Typically a rod is used to pierce the solid bowl through to the liquid slag above. Returning tapped slag to the top of the smelter effectively recycles iron still embedded in the slag into the growing bloom. It also is a way to lower the position of the developing slag bowl to keep the tuyere clear. If there is a large volume of slag, but it is too viscous to easily run, a small amount of forge scale can be added with the charges.
Just how long the sequence of adding ore charges should continue will depend on just how large a metallic bloom is desired. The EconoNorse Smelter has successfully produced blooms as small as 3 kg / 7 lbs, with 7 - 8 kg / 15 - 18 lbs being common. Generally, at a point ranging from 5.5 to 6 hours into the smelt, addition of ore charges should be stopped. A further 3 - 4 fuel charges should be added, then air flow should be somewhat reduced and the remaining charcoal be allowed to burn down. At this point the bloom mass can be extracted via the tap arch, out of the top of the smelter, or (easiest with brick construction) by tearing the walls down. Be aware that the heat is intense!
Primary consolidation of the bloom can be undertaken using the residual heat of smelting. If care is taken with dismantling the smelter, it now can be operated as a large forge for further compression of the bloom mass. Typically the original foot ball shape of the bloom is hammered to a hockey puck - a shape found with artifact blooms.

Tapping Slag
Smelter Walls Dismantled

Total charcoal consumed +/- : 70 - 75 kg / 150 to 175 lbs
Total ore added +/- : 13 - 25 kg / 30 - 60 lbs
Bloom expected +/- : 3 - 7 kg / 7 - 15 lbs

Exact yield dependent on ore purity (from 25 - 35%)
Exact carbon content of the metal is related to a large number of variables over the smelting sequence!

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Text, Illustrations and Photographs © Darrell Markewitz - 2005
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