One of the problem those of us looking at historical bloomery furnaces run into is that there is no clear typography established to describe various furnace designs. As the primary archaeological remains are almost always at best a slag bowl in place inside the vary base of some enclosing walls, much of what is described is based only on slag. Bad news for us attempting to actually * make * iron, knowing about only the base construction hardly gives you enough to build a correctly * working * furnace.
Most of the current technical knowledge of effective bloomery iron furnaces comes from the pioneering work of Lee Sauder & Skip Williams, along with an additional decade of trial and errors by the 'Early Iron Group' (via Lee's 'Smeltfest' workshops).
(Feel free to discount the following, but this is more or less how I see it. Note that this is for European historic designs only. I have never worked with a Japanese Tatara system, and only once with one of the African based furnaces)
The basic European model is a Slag Taping furnace. These furnaces are set on ground level.
This may be set up to allow for :
'Incontinence' (self taping)
Tap Arch (periodic larger taps via an opening in the base)
Continuous Taping (ongoing via a slot cut upwards from the base)
In Denmark, many of the bloomery furnaces I saw in use were what they described as Slag Room furnaces. The furnace is set at ground level, but with the tuyere elevated to leave extra room for a chamber for slag to collect into.
|Michael Nissen's Furnace - Heltborg, 2008|
Culduthel furnaces are this layout.)
Ancient Danish and also some Anglo Saxon furnaces are full Slag Pit furnaces. There the furnace is at ground level, but underneath is dug a pit of roughly furnace diameter for the slag to drain into.
This pit is filled with some kind of vegetation - * the pieces set on end like a bundle of drinking straws * (may be a sheaf of grass or small twigs). Initially un-burned charcoal (below tuyere level) rests on the tops of the supporting vegetation. As the slag forms, it can drip down between the individual pieces. Eventually the heat chars and burns away the tops of the supports, and so the whole slag bowl can sag downwards as it enlarges.
We ran two tests of a full Anglo Saxon type Slag Pit furnace here at Wareham:
This is an image of an 'Iron Age' (c BC/AD) slag mass from a Danish Slag Pit furnace, the mass is about the size of a bushel basket, and is upside down. (from a location near the base of the Jutland, Denmark)