I made 3 furnaces from bricks using different configurations to test their effectiveness. The benefit of using bricks to make a furnace is that it’s quicker, easier, re-useable and portable relative to a furnace constructed in-situ from clay.This is because the old furnaces crack and need repairs where as the brick ones can be assembled and re-assembled with any broken bricks being swapped out as necessary.
image: Primitive Technology
Also, the shape and height of the furnace is easily changed by adding/subtracting or re-arranging bricks.The first furnace used a square plan with the bricks laid on their sides with a inner cross section of 18 cm square. A 2.5 cm diameter tuyere was used and about 15 g of iron was made. The second furnace was the same but used a 5 cm diameter tuyere.
It made about the same amount of iron. The 3rd furnace was a haexagonal plan furnace with a diameter of about 22 cm. The bricks were laid on their ends for this type. The smelt yielded the same as the square ones for the same input of ore, charcoal and effort.
I’ll probably stick to using the square design made from bricks in future. Things I may try later would be to increase the height of the furnace by adding more brick layers. I’d put in all the charcoal and ore for the smelt after preheating. Then I’d be able to operate the smelt without the interruption of needing to charge the furnace every 5 minutes with ore and charcoal.