In this article on traditional sahti brewing, the author mentions that some very traditional “sahti masters” do all their mashing in wooden tubs, and use heated stones added directly to the mash to raise it’s temperature rather than heating the water.  Hmmmm.  Given the typical sizes of the vessels found in a Viking context, this makes a lot more sense than trying to heat even a fairly large “cauldron” (which still isn’t very big) over the fire and adding it to the mash. 

Experimentation is required. 🙂