Making a wood ash-insulated furnace for smelting iron

John Plant of the popular Primitive Technology channel continues his advance into the iron age.

In this video he builds a new smelting furnace with insulated walls of wood ash to try and improve his efficiency in producing iron.

Several takeaways: This guy has built a dizzying number of different fire pits, furnaces, and kilns, and smelting iron is time-consuming and hard.