Smelting Bog Iron in a Bloomery Furnace — From Ore to Wrought Iron

Choose flat, dry ground at least 5 meters from any structure or combustible material. The furnace radiates intense heat and produces thick smoke for hours. Clear a 3-meter radius down to bare earth. Avoid low-lying areas where wind is blocked — the natural draft supplements the bellows.

Excavate a bowl-shaped pit 30 cm deep and 40 cm in diameter. This hearth collects slag during the smelt and supports the furnace shaft above. Pack the pit walls firmly. If the soil is sandy, line the pit with a 2 cm layer of clay to prevent collapse.

Combine refractory clay with fine sand and chopped straw in a ratio of roughly 3:1:0.5 by volume. The sand adds thermal resistance and reduces shrinkage cracking. The straw burns out during firing, leaving pores that absorb thermal shock — without it, the furnace wall cracks when heated. Knead the mixture thoroughly until uniform, adding water to reach a stiff, workable consistency similar to bread dough.

Construct a cylindrical shaft centered on the hearth pit. Build upward using thick coils or slabs of the clay mixture, each layer 5-8 cm thick. Target dimensions: 30 cm internal diameter, 60-80 cm tall, with walls 5-8 cm thick. Keep the interior smooth and the walls as even as possible. Leave a gap near the base (10 cm above ground level) for the tuyere opening — do not seal this hole.

The tuyere is the air inlet where the bellows nozzle enters the furnace. Form a circular opening 5-7 cm in diameter, positioned 10 cm above the base of the shaft. Angle it slightly downward (about 15 degrees) so the air blast hits the center of the hearth. The tuyere can be a hole through the wall, or a separate clay tube inserted into the wall. Ensure the bellows nozzle fits snugly but can be removed.

Allow the furnace to air-dry for at least 24 hours. If time permits, 2-3 days is better. Rapid drying causes cracking. Once surface-dry, light a small fire inside (kindling and a few sticks) and maintain it for 2-3 hours to drive moisture from the walls. Increase the fire size gradually. Steam escaping from the walls is normal — cracks can be patched with fresh clay. The furnace must be completely dry before the full smelt or it risks explosive spalling from trapped steam.

Before smelting, the ore must be roasted in an open fire. Spread the dried bog iron nodules on a bed of burning firewood and maintain a strong fire for 1-2 hours. Roasting drives off chemically bound water from the limonite (FeOOH → Fe₂O₃ + H₂O), converting the hydrated ore to anhydrous iron oxide. The ore changes color from ochre-brown to dark red or purple-black. Roasted ore is more porous and reduces faster in the furnace.

Once cooled, break the roasted ore into pieces 1-2 cm across using a hammer on a flat stone surface. Smaller fragments expose more surface area to the reducing gases inside the furnace, dramatically improving iron yield. Discard any powdery dust — it clogs airflow. Sort and set aside approximately 5 kg of crushed, roasted ore.

Break hardwood lump charcoal into uniform pieces 2-3 cm across. Consistent sizing ensures even airflow through the furnace column — oversized chunks create cold spots, while dust chokes the draft. Prepare approximately 15 kg of sized charcoal. The charcoal-to-ore ratio for bog iron is roughly 3:1 by weight — you need far more fuel than ore.

Fill the furnace shaft with firewood and charcoal and light from the top. Allow the furnace to burn freely for 30-60 minutes until the interior walls glow orange. This step brings the refractory walls up to operating temperature gradually, preventing thermal shock cracks. The furnace is ready for the smelt when you can see orange-red heat radiating from the tuyere opening.

Fit the bellows nozzle into the tuyere opening. Seal any gaps around the nozzle with wet clay to prevent air leaks. Begin pumping the bellows at a steady rate of 60-80 strokes per minute. The forced air blast supercharges combustion inside the furnace, raising the temperature from roughly 800°C (natural draft) to 1100-1300°C (forced draft). You will hear the furnace roar as the air blast takes effect.

With the bellows pumping and the furnace at full heat, top up with charcoal to fill the shaft. Then add the first ore charge — approximately 500 g of crushed roasted ore spread evenly on top of the charcoal. Cover immediately with another 5 cm layer of charcoal. The ore sinks slowly through the charcoal column, passing through the reduction zone where carbon monoxide strips the oxygen: CO + Fe₂O₃ → Fe + CO₂.

Every 10-15 minutes, add another 500 g charge of ore followed by charcoal to keep the shaft full. The charcoal burns down constantly, so the column settles. Maintain a consistent rhythm: charge, pump, charge, pump. The smelt requires 3-5 hours of continuous charging depending on the ore quantity. Keep the bellows running without interruption — even a brief pause drops the temperature below the reduction threshold.

After 1-2 hours of charging, liquid slag begins forming in the hearth. Slag is a glassy mixture of iron silicates (fayalite, Fe₂SiO₄) that separates from the metallic iron. It may flow from the base of the furnace as a glowing orange-yellow liquid, or pool in the hearth pit. If slag blocks the tuyere, use tongs or a long stick to clear it. Flowing slag is a good sign — it means the furnace is hot enough and the reduction is working.

After all ore charges have been added, continue pumping the bellows for another 30-60 minutes. This final burn consolidates the iron particles into a single bloom at the bottom of the furnace. The bloom forms just above the slag pool, where reduced iron particles weld together at high temperature without fully melting. Stop the bellows when the last charcoal charge has burned down to the tuyere level.

Break open the front of the furnace at the base using tongs and a hammer. Bloomery furnaces are typically single-use — the shaft is sacrificed to access the bloom. Reach in with long-handled tongs and extract the incandescent bloom. It will be a rough, spongy, irregular mass glowing orange-white, riddled with charcoal fragments and trapped slag. Handle with extreme care — the bloom is 1000°C+ and slag can splatter.

Transfer the glowing bloom immediately to a flat stone slab or anvil surface. Strike it with a sledgehammer using firm, even blows. Each strike squeezes out trapped liquid slag (which sprays as hot sparks — wear protection). The iron particles weld together under the combined effect of heat and pressure. Rotate the bloom between strikes to work all sides. If it cools below orange heat, reheat in a charcoal fire before continuing. Repeat until the bloom compacts into a dense, roughly bar-shaped billet.

Once the bloom is consolidated into a solid bar, quench it in a bucket of water. The rapid cooling locks the iron in its worked state. Examine the surface — good bloom iron shows a silvery-grey metallic color with a fibrous grain visible on fractured edges. A typical 5 kg ore charge yields 500-1500 g of usable bloom iron (10-30% yield is normal for bog iron). This wrought iron can now be forged into tools, weapons, nails, or any other iron implements.