Best place to start is Robert Gordon's American Iron (Johns Hopkins
1996). It has a good chronology of how things were made and the
results. If you don't own a copy, you should. Bob has also produced a
number of papers and a couple of other books on various aspects of
the iron industry (including slag analysis). If you want something a
bit smaller, and I think quite good (I wrote it), look at "Blast
Furnace, Forge, and Foundry" (pp 335-353) in Uplands Archaeology in
the East: Symposia VII & IX (available from the Archeological Society
of Virginia). I also have a 70 page bibliography I'd be happy to send
you (as I wrote my MA on the Virginia colonial industry it is a bit
heavy on that, but covers the US/UK up to 1900). If you want to do
research your problem will be getting references. Very few libraries
have anything, expect to use inter library loan a lot.
There is not a great deal on slag. Unfortunately many archaeologists
don't even collect it. And if they do they seldom ever either
describe it or have it analyzed. It is commonly mischaracterized as
"clinker".
Almost all blast furnace slag is glassy. But it can take on different
textures depending on how it was cooled (the Us Govt produced a book
on how to do this in the 40s). This is because it is, for the most
part, a silica-lime glass. Lime is used as a fluxing agent. Generally
it is green or gray green, but other colors are possible including
blue, white and red (it all depends on what is in the ore).
Regardless of whether or not it is hot or cold blast, charcoal or
coke, it will still be glass and glassy. If it is tapped out onto a
surface containing a lot of moisture it will be full of airholes. If
it is cooled quickly it will tend to look stoney. But regardless it
is still chemically glass. Batso, after it stopped making iron, used
the slag pile to make bottles.
Finery and bloomery slag are not glass, they are fayalitic (Fe2SiO4).
And may contain up to 70% iron. The bloomery process is "self
fluxing" and it is the iron that makes the slag liquid. Bloomery slag
will often contain chunks of charcoal and unreacted ore. Finery slag
may contain chunks of charcoal. Both are usually dark brown to almost
black and have a ropy texture.
Foundry slag is also different, but I really haven't seen much. As I
remember it may have an almost metallic look to it.
On Apr 23, 2006, at 22:49, Ben Ruset wrote:
> Hi All:
>
> I'm in the process of writing an article on the bog iron industry
> in the New Jersey Pine Barrens for my website, and had a few
> generic questions related to blast furnaces, forges, and slag. If
> someone can point me to some good source material, or even answer
> my questions, I'd be much indebted.
>
> Firstly, there is definitely a difference in the type of slag that
> is produced by a blast furnace and a forge. Within the scope of
> furnace slag, it appears that the slag is different depending on
> whether the furnace used the cold blast method, or the hot blast
> method. From my understanding, cold blast furnaces tended to
> produce very rocky, metallic slag that in some ways resembled
> pyrite in texture and form. Hot blast furnaces produced slag that
> is almost glass-like. Is my assumption correct?
>
> Forge slag seems to look just like you would expect a pool of
> cooled iron to look like. It is smooth in texture, without a lot of
> visible impurities (ie: no unburnt charcoal, etc.) Are there other
> ways of telling if forge slag is in fact forge slag?
>
> I have seen varieties of cold blast slag and forge slag, and what I
> assume to be hot blast slag, although it was at a site that I was
> unaware used hot blast technology.
>
> Secondly, when it comes to forges, it seems that many had both a
> stamping mill as well as a tilt hammer. The thing that I am unclear
> on is what the main difference in the finished product of both
> were. I have scoured "Early Forges and Furnaces in New Jersey" but
> it is pretty unclear to me what the main differences are. If
> anybody can shed some light on the subject, I'd really appreciate it.
>
> Thanks!
> Ben Ruset
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