Iron Smelting

Iron smelting extracts metal from ore heated with charcoal in a small clay furnace supplied with air by bellows. This bloomery normally produces a solid, low-carbon lump called a bloom, which the smith consolidates and forges into wrought iron.

What it is

Inside, a raw bloom is a sponge of iron and slag. Hours of forging turn it into a bar.

Iron is one of the most common elements in the Earth’s crust, yet it is almost never found as metal. It sits in ores as oxides: red haematite, black magnetite, brown bog ore dug out of marshes. Smelting pulls the oxygen out of these ores and keeps the metal.

For close to three thousand years the tool for this was the bloomery, a clay furnace about a metre tall in which charcoal burns in a blast of air from bellows. It is named after its product, the bloom, a spongy lump of iron mixed with slag. Hammered while hot, the bloom becomes wrought iron, tough and easy to forge. Ploughshares, nails and swords were made of it until the blast furnace took over in Europe at the end of the Middle Ages.

How it works

Burning in front of the air nozzle, the charcoal heats the furnace. Higher up the shaft, where oxygen is scarce, it forms carbon monoxide, and this gas takes oxygen away from the iron oxide as it rises through the charge. The ore gives up its oxygen in stages, and the reaction becomes fast above about 900 °C.

The bloom is first consolidated with light blows while hot; it can be reheated between rounds of hammering.

The furnace runs at around 1,100 to 1,200 °C and normally produces a solid, low-carbon bloom. Pure iron melts at 1,538 °C, but carbon-rich iron can melt at bloomery temperatures. Silica and other minerals in the ore combine with part of the iron oxide to form iron-rich slag, which can flow at about 1,180 °C depending on its composition. The slag trickles down past the tuyère, the clay air nozzle, while metal grains gather just below it into the bloom. The smelter drains the slag through an arch at the base, or the slag collects in a pit underneath.

The bloom leaves the furnace as a glowing, porous mass full of slag and charcoal. A fragile bloom may need gentle first blows, sometimes with a wooden mallet. It can also be cooled and reheated before consolidation; immediate hammering at white heat is not essential. Repeated heating and forging compact the metal and expel much of the slag. Weight loss varies with the bloom and the forging method. Carbon content is usually low and uneven, so soft iron and patches of steel may occur in the same bloom.

How to build

What follows is the outline of a small smelt, not a measured drawing. You need clay mixed with sand and straw, ore, a lot of charcoal, bellows or a blower, and a team for a full day.

Prepare the ore. Roast it in an open fire to drive off water and make it brittle, then break it to pieces of about a centimetre. Break the charcoal to about two centimetres and sieve out the dust so the gas can flow through the charge.

Build the shaft. On dry ground, raise a clay cylinder about a metre high and 25 to 30 cm across inside, with walls several fingers thick. Leave a hole near the bottom for the tuyère and an arch that can be opened and sealed again with clay. Let it dry slowly for several days, then finish with a small fire inside.

Preheat and charge. Fill the shaft with charcoal and supply air until it glows throughout. In WIRG experiments, charges of about 1 kg each of ore and charcoal are added at intervals of 10–15 minutes, with 15–20 kg of each reported per run. That is 15–20 paired charges, representing roughly 2.5–5 hours of charging at that cadence, rather than five continuous hours. The reported run takes about five hours including the final burn-down, with preheating separate. Allow extra charcoal for preheating and the final charcoal-only charge; the paired-charge total is not the full fuel requirement.

Control the blast. Monitor temperature and charge descent, and adjust the air supply to keep reduction and slag flow going. Carbon uptake depends on the charcoal-to-ore ratio, furnace atmosphere, residence time and slag chemistry. More air alone does not predict cast iron: some experiments produced lower-carbon iron at higher airflow. Tap accumulated slag as needed; WIRG reports a first tapping after about three hours.

Extract and consolidate. After the last charge burns down, open the front and remove the bloom with tongs. Begin gently if it is fragile, for example with a wooden mallet, then use stronger blows as it holds together. Cooling and reheating before consolidation are also possible. Reheat in a forge between passes until the metal forms a coherent bar.

Safety. A bloomery runs above 1,100 °C. Liquid slag causes deep burns and starts fires. Work outdoors on bare, dry ground, away from dry grass, with dry sand ready. Keep tools and the slag-tapping ground dry. Never let water touch hot or molten slag or enter the hot furnace: sudden steam formation can cause an explosion. Wear leather boots, gloves, an apron and eye protection, and never stand in front of the tapping arch. Carbon monoxide leaves the top: keep your face away from the stack and never smelt indoors. Damp clay or ore can burst when heated.

History and evolution

People worked iron before they could smelt it. Some of the oldest iron objects were hammered from meteorites, which contain nickel. Evidence points to early smelting in Anatolia, in present-day Turkey, though the place and date of its invention remain uncertain. Fragments from Kaman-Kalehöyük dated to about 1800 BCE include some carbon steel, and their uneven carbon suggests smiths who were still experimenting. Around 1250 BCE the Hittite king Hattusili III wrote a letter about making “good iron”.

Around 1200 BCE marks a phase of wider iron adoption in the Near East, with different regions adopting it at different times. Smelting itself was already known earlier. By about 1100 BCE it was common in Cyprus and the Levant, most of Europe used it from around 800 BCE, and by 500 BCE bloomeries were known from the Mediterranean to Scandinavia. Unlike the copper and tin for bronze, iron ore could be dug almost anywhere.

Pot bellows: pumping the leather covers in turn keeps the air flowing without a break.

Africa had its own traditions, with furnaces in Nubia by the seventh century BCE and in West Africa by the sixth at the latest. China probably learned bloomery smelting from the west around the eighth century BCE, then moved early to furnaces hot enough to make liquid cast iron.

European blooms weighed about two kilograms for centuries, small enough to forge by hand. Roman furnaces, up to two metres tall, gave blooms of 10 to 15 kg, and around 1500 water-driven hammers could work blooms of 100 kg or more. The blast furnace at Lapphyttan in Sweden was already operating from the mid-twelfth century. Blast furnaces spread more widely in later medieval centuries and were producing cast iron in quantity by around 1500; bloomeries gradually declined. Catalan forges in Spain and southern France were still working in the middle of the nineteenth century.