Wealden Iron Research Group’s Smelting Experiments in 2015
The three smelts in 2015 in an experimental bloomery furnace on the Pippingford Park estate produced three large blooms with two being successfully forged to produce billets of wrought iron that a blacksmith would use (Figs.1a & 1b) ; the third bloom awaits the final smithing (Fig.1c).
1.85kg billet of 0.1% carbon wrought iron.
From 43kg of ore & using 73kg of charcoal.
1.7kg billet of 0.43% carbon wrought iron.
From 34kg of ore & using 64kg of charcoal.
This did not happen without some problems: (1) the 2-hour-long time to pre-heat the furnace with charcoal to >750°C when smelting can start; (2) the temperamental generator (powering the blower), which is always stopping for some unknown reason. The latter has been solved by the purchase of a new, petrol, 4-stroke generator ready for next year’s smelts. The former problem of quickly pre-heating the furnace with charcoal must be resolved as it consumes excessive, expensive charcoal.
Partially consolidated bloom from 29kg of ore and using 72kg of charcoal. Further forging is required to remove most of the slag and all the charcoal and will reduce its volume and weight considerably.
Outside dimensions: 1.5m high, 1.2m outer diameter at bottom, 0.6m at the top.
The bloomery furnace being used (Fig.2), here shown being dried out after building, is a close copy of a Roman furnace discovered and excavated at Mayfield, Sussex. The furnace is supplied with locally dug iron ore, which, after roasting, is loaded into the top of the furnace along with an equal weight of charcoal, this is added in 2kg loads as the charcoal slowly burns down to make room. Molten slag is run out via the tapping arch; this is the non-metallic waste material from the impure ore. A “tuyere” is necessary whilst smelting, this is a pipe that extends into the furnace about 400mm above ground level, through which air is blown to increase the working temperature to between 800° and 1000°C. After 5 to 6 hours of smelting a solid bloom (the iron is never molten!) is dislodged from the furnace wall and removed via the tapping arch (Fig.3). The hot bloom is not usually weighed because its initial consolidation is better carried out straight from the furnace before the bloom cools. The larger the bloom the longer it takes to cool, with the hot iron centre glowing red through a crumbly surface of slag and charcoal, and is describe as “sponge-like” in ancient texts. The first gentle hammering to consolidate the bloom is carried out using a mallet on a wooden anvil (Fig.4).
Scaled bloom just extracted from the furnace.
Gently hammering the bloom to consolidate the surface.
Further work is necessary before the bloom really looks like iron, called a billet, and is suitable for the blacksmith. After its initial hammering, the bloom cools and must be reheated. Due to its size and weight it is easier to re-heat it in a ground level forging hearth (Fig.5). This can be built-up close to the furnace in a few minutes from fireproof bricks to form a horseshoe-shaped hearth ~700mm x ~1250mm, with the wall 300mm high through which a ~25mm diameter iron tuyere is fixed. The end of the hearth is open to give easy access for raking charcoal to and from the iron being forged. Nowadays blacksmiths use a horizontal tuyere in a hearth fuelled by coke but when traditional charcoal fuel is used it is so light that it is blown away. But if the tuyere angled down ~30 degrees and the air reflected up from the ground towards the iron the charcoal is held in place by the iron. It is necessary for the air to pass through 50-75mm of hot charcoal to remove the oxygen (by producing CO & CO2 gas) which would otherwise oxidize the iron.
Fig.5 The forging hearth.
By far the hardest task is hammering the bloom into a billet of iron suitable for the blacksmith. If a crack appears in the iron it becomes necessary to fold the metal at the crack to form a sandwich, and forge-weld it together; this is carried out by heating to an almost white heat and quickly hammering together, whereby it forms a seamless join, one of the many magic tricks that blacksmiths perform.
Brian Herbert
© Wealden Iron Research Group 2015