With the slag cake now cool enough, we took it out of the furnace and started documenting its weight and taking samples. Finally, after a quick photoshoot of the cake it was finally time to crush it and prepare it for further steps of the experiment next week.
T. Chabert proudly presenting: “Big Pit”
Since further experiments were planned for tomorrow but also rain was forecasted, we decided to dig another pit beneath a shelter so we wouldn’t be affected by the weather. After lining the freshly dug pit with clay and after 4 days of work finishing the grinding of the matte, we had the opportunity to do what every four-year-old would envy us for. With the freshly ground matte and waterbuffalo manure as fuel we formed small pellets which were roasted overnight to get rid of the remaining sulfur.
Since the furnace was still too hot, and the wind too strong for further experiments, we decided to take a fieldtrip to Maria Laach. After a few hours of intensive Cloister-sightseeing and a short Boat trip (during which Thomas showed us that he is not only talented in taking charge of experiments but also of a boat) we finally went back to LEA to continue our work.
With new found energy, and the furnace now cool enough we proceeded documenting the remains and tearing down the front wall to reveal the products inside. Even though the slagcake itself was still too hot to be taken out of the furnace, we were able to clear it from the overlying charcoal leftovers and retrieve some of the loose parts of the slag and the matte.
Getting the groove: P. Süßer and T. Chabert are working on synchronising their movements.
Author: Philip Süßer
The wheather Gods have been kind to us. Due to the calm wind and generous irrigation of the area surrounding the furnace using the facility-built in rainwater cistern, we were able to proceed firing up the experiment. Before heating up the Mitterberg-inspired furnace everyone got the chance to familiarize themselves with operating the bellows and working in pairs to get a constant air flow into the fire. As soon as the furnace reached a temperature of about 1000 °C practice time was over and the bellows and the newly achieved skills were put into action.
After reaching 1200°C using the bellows, the furnace was filled to the top with firewood and a short while after, the first batch of ore was introduced into the fire and covered again with wood. Subesquently more wood and ore were introduced accompanied by regularly compressing the material already in the furnace. To make sure that the last batch of ore could sufficiently smelt, the bellows were operated for two more hours before finally letting the fire burn down.
Furnace operator E. Hanning clears the nozzels from the slag of molten furnace lining blocking the air flow. Bellow operators: K. Mittelstädt (back) and T.Chabert (front)
In constrast to our previous experiment, we managed to reach temperatures exceeding 1500 °C. Consequently, parts of the inner furnace lining started to melt and caused blockages in the air supply. Keeping the tuyeres free meant extra work for our furnace operator E. Hanning from a relatively early stage of the smelt on.
But not only E. Hanning was running on high temperatures. With a cosy warmth of 40 °C outside, also the bellow operators were getting their share of a good roasting. But plenty to drink and good sunprotection, as well as frequent exchange of the operators were the first priority, the effects of the heat were kept at bay.
To see the fruits of our labor, patience is needed. The furnace is still very hot and will take at least a day to be cooled down enough before it is possible to tear down the front wall and have our freshly produced slag-cake brought to the surface.
P. Süßer refreshing the clay-lining of the furnace walls
Author: Philip Süßer
Yesterday the big day had come, the first group of volunteers arrived and were ready to fire up some ovens. After the obligatory introduction and health and saftey instructions, the preparations could begin. Wood chopping, ore grinding and furnace lining – all of us were fully engaged.
With all that said and done, we are not yet sure whether we will be able to take our freshly built furnace for a ride tomorrow. The Heat and the absence of rain might force us to postpone the experiment….
E. Hanning places a batch in the ofen while S. da Zilva and T. Rose operate the bellows.
In the last week of June, the great moment had finally come: One of the furnaces was operated for the first time! A batch of roasted ore was little by little placed in the preheated furnace, alternated with layers of wood. We operated the bellows about 3.5 hours and reached more than 1400 °C in front of the tuyeres. During this furnace campaign, nearly 240 kg of wood were consumed. After the bellows stopped, it took one day until the remaining charcoal had cooled down enough to remove it. The slag cake took another day to cool down. And even then the furnace walls had the cozy temperature for an oven in cool winter nights. We crushed the slag cake and separated the matte from the slag. Although the temperature distribution within the furnace was less than ideal, separating the matte from the slag was easier than expected.
S. da Zilva carefully watching on the crucibles in the fire pit while E. Hanning and T. Rose are on the bellows.
During cooling of the furnace we took two hours to dig a fire pit and lined it with clay to use for the experiments with malachite ore. We heated the pit slowly to dry the clay. During this time we prepared crucibles of different shape and placed them on the margin of the fire pit to dry. After one or two hours we established a bed of charcoal in the pit and filled the crucibles with ore and charcoal. The crucibles were placed in the center of the fire pit and special care was taken that they were permanently surrounded by glowing charcoal. This was important because otherwise the oxygen from the air will oxidize the melted ore immediately and we won’t be able to produce copper. After about 20 min the ore in the crucible was molten. We quenched the crucibles in a pot of water revealing the shiny red of the copper in some of them, indicating our successful conversion of the malachite.
E. Hanning working on the pebble layer in front of the furnaces.
In June, we enhanced the surrounding area of the furnaces for safer working conditions. We installed a working platform behind the furnaces to allow a secure stand during the smelting. Additionally, we widened the trench before the furnaces to give the bellow operators more space. The bellows were made from leather, cut from a complete hide and sewn with an awl. We also prepared the clay tuyeres and burned them in an electric furnace.
We stacked the crushed ore with wood to a roasting pile. After lighting the pile we added multiple new layers of wood and let it burn nearly two days. From the temperature in the center of the roasting pile we saw that the sulfur kept the roasting reaction going by itself. We were utterly surprised that we even had to stop the reaction due to upcoming holidays. Nobody of us thought we would be able to build the pile a way that the desulfurization kepp going on by itself for that long.
Sulfur efflorescence on charcoal.
The strong smell of the sulfur permanently accompanied us during this time and getting even wors during the documentation when some parts in the center of the roasting pile came in contact with oxygen and started to react again.
The result were beautifully zoned roasted ore pieces with an unroasted core and a clear rim of reacted, reddish iron oxides.
Preparations for the smelting experiments began in early April and were scheduled to take about two months. During the first days, we installed the roasting bed and dug the trenches of the two furnaces under the watchful eye of Sam. We used clay as mortar for the furnace walls and also lined the insides with more clay. Surprisingly it took only three days to build the roasting bed and both furnaces although it was a hard task to dig the trenches into the debris of the basalt quarry.
Sam is critically looking on T. Rose recording the furnace pits.
Subsequently, we started to crush the sufide ore, a superb way to reduce aggression, especially the ones arising from the incredibly hard ore pieces, where ore and host rock are intimately grown. In total, about 140 kg of ore were crushed.
