The Mark Twain National Forest has been sponsoring a series of excavations on
a late 19th century (1880s) iron complex in the Ozarks for several years now.
 This was a hot blast iron furnace with a 100 ton/day capacity processing
specular hemitite into pigs to be shipped to St. Louis.  All of the slag that
I remember seeing generally ranges from a gray to a lime-green color and
there are acres of it, sometimes 20 feet deep.  A typical charge in the
furnace consisted of a mixture of ore, charcoal and limestone flux.  No one
had ever commented on the color of the slag.  I have absolutely no idea about
the cause of the color and our work, other than mapping, has focused on the
associated town.  James Price (Southeast Missouri Research Station,
University of Missouri) has directed the work and may have some ideas about
this and is an excellent source of info on this time period in general.  As
far as I know, all iron furnaces in Missouri used limestone flux and it may
be worth looking into whether they all have green slag.
 
James Wettstaed
JWettstaed @aol.com