Iron Prills

I smelted iron bacteria in a short furnace and produced a small quantity of iron prills (small iron spheres). In my ongoing quest to reach the iron age, further experiments were conducted concerning furnace design and the treatment of ore. I began by making a very short furnace. A pit 25 cm wide and 25 cm deep was dug and the tuyere of the forge blower placed in a 15 degree downward angle into the pit. Onto this, a furnace stack made of mud and grass was built 25 cm above ground level. The furnace was fired at various stages to help dry it. It took less than a day to build.
Eucalyptus wood was collected dead off the ground and stacked into a re-useable charcoal mound I had made previously. The top was sealed with mud and the mound lit. It took about 2 hours 30 minutes for fire to reach the air entries, at which time the holes were sealed and the top closed with mud.
Iron bacteria from the creek was gathered and brought to the smelting hut for processing. Charcoal was ground into a powder and mixed with the ore and water in the proportions of 1:1 char to ore by volume. This mixture was formed into 59 pellets 2.5 cm in diameter and then dried on top of the furnace.
To make the smelt, a wood fire was made in the furnace and allowed to burn for about an hour by natural draft and blowing. When the wood burnt down to the tuyere the furnace was filled with charcoal and 10 pellets were added to the top and the blower was engaged. Three handfuls of charcoal and 10 pellets were added at about 7 minute intervals totaling about 42 minutes. Charcoal was then continuously added after the last charge until the basket was empty. It took a total of about 3 hours working the blower until the operation ended.
The mass of slag and iron prills was prized out of the furnace using a log and wooden tongs. It was hammered flat while hot but no large bloom was made. Instead many small iron prills were found. These mostly seemed to be cast iron.
So far this is the largest amount of iron I’ve made in the wild and it used less charcoal than previous attempts, so I consider it a success of sorts. The ore must be mixed with carbon to ensure the correct reduction chemistry normally provided by carbon monoxide in a taller bloomery furnace. The fact that cast iron was produced suggests that next time less charcoal powder be added to the ore pellets or perhaps none at all considering that dead iron bacteria may also contribute some carbon to the ore. Alternatively, cast iron can be re-melted in a “finery” furnace, a small highly oxidizing furnace, to remove excess carbon, producing steel or iron. Alternatively cast iron can be converted into malleable cast iron by heating it in an enclosed container at 800-1000 c for long periods. Further experiments will be conducted.