Charcoal was the natural fuel for creating the necessary temperature in the hearth. The consumption was huge. Wood was cut and dried in the Summer, processed into charcoal in the Autumn and brought to the forge in the Winter on special sledges.
There were more people engaged in the production and transportation of charcoal than at the forge.
Around 1900 when production was on top, Karmansbo used 3 charcoal stacks per week.

Setting up a charcoal stack.

One stack  about 75000 - 90000 liters