I made a furnace insulated with wood ash to smelt iron in. Furnace insulation stops heat being lost from the walls of a furnace and so increases the heat within the furnace. Dry wood ash makes an excellent insulator for heat as the fine particles separated by small air gaps don’t allow heat to conduct very well.
source.image: Primitive Technology
To make the furnace a double layered wall of clay was made and the space between was filled with wood ash from different fires. The clay needed to be completely dry before doing so however as it was important the the ash not get wet, which would cause it to loose its insulating properties.
The furnace was tested with an iron smelt and it produced 34g of iron from 10 charges of ore. It was going quickly to start with but then I had issues with the blower towards the end of the smelt (broken housing).
The furnace design is promising though as this produced the third best smelt I’ve done in the wild with natural tools and materials (the best were 39g and 41g over a year ago). I’ll try it again later when I’ve replaced the broken forge blower.