" I am not recommending this process to anyone ..it is dirty and a lot of work..and may only work with some ores. "
(Quote from a recent post on the Bladesmith Forum)
I wanted to pipe in here.
I've maybe worked with more ore types than most others reading here, primarily because there is no naturally occurring iron ore in my local region - due to geography.
Certainly, * any * natural ore will vary considerably in potential iron content, oxide type, silica combination, dynamic impurities, structural form.
One of the source 'ores' I had access to for a while was processed hematite as fine particle blasting grit. I have worked with this stuff at least 5 + times. I know both Antoine Marcel and Jesus Hernandez have used this material with success as well.
On a guess the original comment refers to using either hematite or 'iron sand' as the ore?
My own experience is that the small particle size of the hematite grit tends to create a metallic iron particle size that also is quite fine. Because of this, temperature control is critical. The iron will very rapidly absorb * too much * carbon. Unless you are very careful, you end up with cast iron.
As I understand it, this absorbtion is a surface area effect. Those small starting particles just have a lot of surface area compared to volume.
Your images of the results of this smelt certainly look like my own results sometimes - with this fine material at least.
The iron produced (if everything is going well) tends to be a crumbly texture, looking much like dampened dark brown sugar. This also tends to be a higher carbon content iron as well. The image you provided of the (potentially) forgeable iron bloom certainly looks the same to my eye as my own results ( http://www.warehamforge.ca/ironsmelting/smeltfest06/report03-06.html )
And yes - I certainly have gotten identical cast iron formed below the slag bowl.
One key in your own comment: "...may only work with some ores."
For any given ore - there will be * one * ideal design of furnace.
The problem here may be in attempting to utilize a single static furnace layout - then attempting * that * to dictate the process (and expectation of results).
My consideration of ancient furnaces has caused me to appreciate the basic wisdom of a clay construction for the furnace. This material functions well, especially when modifications of mix using various proportions of sand / addition of organic materials is undertaken. This construction is relatively fast, simple and cheap. The furnaces can be fairly durable under normal operations, especially if a little care is taken with the construction - and especially the extraction method used.
More importantly, it is relatively simple to modify the base design of the furnace (stack height) - or simple enough to just build a whole new furnace body (Two or three bags of powdered clay and a couple of hours!)
When you examine the archaeology, one thing is commonly seen at larger scale, long term use iron smelting sites. There is often two or three 'unique' furnaces, commonly that show often only single firings. Then there are a whole pile of escentially identical furnances, usually with more durable construction and showing multiple uses.
Iron smelting was located most commonly at the site of the ore body.
Furnaces were then adapted from a theoretical template to suite the exact combination of ore (and likely charcoal) available at that location.
It also needs to be constantly remembered that the historic objective of a direct process bloomery furnace was dead soft * easy to forge * iron. Meaning no or next to no carbon content.
As many of the 'old hands' will tell you - bloomery iron making is more an art than a science!
This is a piece I started working up before ICMS in early May - that got caught in draft form.
I have just posted it without further editing