My work with this type of furnace started after a fellow bladesmith from Barcelona, Miquel Segura, sent me a book with a compilation of the information available on the Catalan forge. Known by the locals as “Farga Catalana,” this type of furnace was capable of continuously processing batches of 500 kg of raw ore together with a similar amount of charcoal in a space of about 50 x 60 x 90 cm and turning that ore into 150 kg of iron. Since the ore contained an average of 45% iron, the yield was a very impressive 70%.
My challenge was two-fold. I wanted to replicate the process in a smaller-size forge, and I wanted to use it to make steel and not iron. I also wanted the forge to be reusable with as few repairs as possible for the next run. As usual, I started at the drawing board. After some thinking, I settled on a set of dimensions of about half the length for all sides. That would give a final volume inside the forge of about an eighth the size of the original furnaces.
I built the forge out of bricks. Casting the bricks yourself allows to vary the composition of the refractory to suit your purposes. It is also cheaper than buying the commercial refractory and insulating bricks. It also allows me to make them in the size that I want. I used mostly fireclay, vermiculite, ash, and sand.
The Catalan process calls for a steel wall to be placed sideways inside the forge. This wall divides the forge into two sections. A smaller section further away from the air blast is used to charge the ore. The larger section is used mostly to charge charcoal. The wall remains during the first stages of warming up the forge and is later removed. My first run was meant to be a test of the concept. I did not want to use good quality ore, so I collected dirt from around the anvil (forging scale from W2 steel) until I had about 5 kg.
I set my steel divider inside the forge, charged 2 kg of ore together with some silica flux on the chamber furthest away from the tuyere, and a small amount of charcoal in the other chamber, and lit it.
I progressively filled the charcoal chamber to the top, and within half an hour of starting the fire the temperature measured in the ore chamber reached 1500° F. In the next half-hour the temperature went past 2400°. I was not paying attention, and the heat melted the temperature probe. I was using a small 50 CFM blower, and I did not expect it would produce enough air flow but the proof is in the new probe that I will have to buy for my pyrometer.
At that point I removed the steel plate that I used as a divider for the chambers. I continued to pour charges of about 500 g of ore and 1 kg of charcoal keeping the ore to the further wall from the tuyere. The charges went in at about 15 minute intervals. At the end of the next hour I had run out of junk, I mean scale, to feed the forge.
Within two hours I had used up 12 kg of charcoal and 5 kg of ore. It was time to let the fire burn down. As the charcoal level lowered, I noticed that the walls of the furnace held up well to the heat and because all the ore went in the forge further away from the tuyere, I did not run into any trouble with clogging of the airflow. I did not use enough ore in this run to create an overflow problem either.
The bloom, called “masser” by the Catalan smiths, weighed at 2.5 kg right out of the forge, but after some hammering and cleaning, actually only contained about 1.125 kg of metal.
I considered this first small run a successful proof of concept.