John

All that Ned Heite said was true.   It is also the case that your
question has one of those "it depends"  answers.  The answer depends on
when your structure is from and where it is.  I don't feel even vaguely
qualified to comment on non-European iron making.  And I also am not
very knowledgeable on "early" European (pre-16th century).  But it is
probably safe to say that in general the larger the structure, the more
likely it is to be a blast furnace.  The average blast furnace in the
US, even the early ones were 20' square.  By the mid 18C they were
25-30' square.  The footprint of a bloomery is much smaller.  But, as
Ned points out later bloomeries (mid to late 19C) were fairly large (see
Percy).   The "hearth" area is probably not that much different in
size.  But because the blast furnace operates continuously it requires
much more massive construction to withstand the heat, and they are very
differently constructed because of what they are designed to handle
(molten iron in a BF, bloom in a bloomery).   You would also not see an
anvil base with a blast furnace.

In addition to the slag differences, at a bloomery you will usually find
skulls or mossers.  These are the slag filled casts of the hearth
floor.  Just as slag blankets a blast furnace site, mossers/skulls
blanket a bloomery site.  I have often seen them used as reinforcement
for banks, dams and even used in the building walls.  On other sites
they are just chucked out the door.

The presence of a casting floor might also indicate a foundry as well.
Can't help you with those.

If the foot print is small, it may be a finery forge, not a bloomery.
My impression is that archaeologically they look very similar.  And
historically the same hearth was often used for fining and blooming.  It
was not uncommon for a bloomery to be built to try the ore
(chemists/analytical geologists being in short supply) and initiate iron
production (small quantities of wrought iron).  Once the blast furnace
was blown in the former bloomery would be used as a finery.  A finery
will not have raw ore, a bloomery will.  Although to confuse the issue,
I seem to remember reading that some fineries used ore as a flux?  A
finery will also have cast iron (this has caused some to mistakenly
identify fineries as blast furnaces or foundries).  It was fairly common
in the US for early fineries to not only process pig iron, but also
scrap cast iron.  This can and did include all sorts of broken bits and
pieces, including stove plates.

The slag differences are fairly well known.  But two things I have never
seen discussed are

1-What does blast furnace slag look like prior to the use of lime flux?
If it was self fluxing, wouldn't' that result in a fayalitic slag?  If
so, how do you tell it from bloomery slag?

2- In the later "American" bloomery process where most of the iron is
reduced to metal, what does the slag look like?  Ned indicated that some
bloomery slags are very "glassy".  Would these include the late 19C
ones?  It sort of stands to reason that if you reduce more of the iron
you get less fayalite and more of something else.  Glass maybe?

Is there an archaeometallurgist in the house?

Jamie Brothers

Ned Heite wrote:

> John:
>
> I agree with the other statements, but size may be deceptive. Some
> nineteenth-century bloomeries were quite large.
>
> A bloomery will have no casting floor.
>
> In any form, the bloomery process will have a very different
> apparatus for removing slag and metal. That's because the metal is
> not in liquid form when it comes out.
>
> Bloomery slag can be mistaken for blast furnace slag by investigators
> who are not familiar with the process; some of it is deceptively
> glassy, but chemically distinct. I recently reviewed a report in
> which bloomery slag was described as blast furnace slag by a
> respected non-metallurgical chemist.
>
> There is a vast literature on slags, and several good survey works.
> Off list I can give you a few references.
>
> At 11:05 AM -0400 4/16/03, John White wrote:
> >?Query:
> >     Can one distinguish between a stone bloomery furnace a blast
> >furnace merely by reference to the physical remains of the
> >structure -  and only the structure -  itself.  I 'd be appreciative
> >of opinions.
> >JRW
> >
> >
>
> --
> [log in to unmask]
>
> Sitting here drinking diet mead
> from my plastic fake auroch's horn
> flagon, I wonder that my
> contemporaries look like a bunch of
> old geezers.