In artifacts catalogs from 19th-century and 20th-century sites, I
usually encounter three categories of combustion-related objects:
1. Clinker: Rocky to ashy material left from burning coal, especially
Pennsylvania anthracite coal. This material is usually friable and
multicolored, with grays, whites, blacks to dark brown and even
grey-pink colors, sometimes within the same piece.
2. Slag: Harder than clinker, with visible bubbles--rounded pockets
within the object that look to be the remnants of burst gas bubbles,
solidified as the object solidified. More solid than clinker, and the
colors are more muted. However, I've also seen pieces that appear to be
clinker fused to slag. I have heard that slag is evidence of forging or
smelting metal, especially iron in, for example, a blacksmith's fire.
But, I've seen slag of this description on domestic sites and maple
syrup processing sites, where coal was burnt along with wood, and where
no intentional metal forging was taking place. Possible that heat
stress and damage to iron objects (stoves, evaporating pans) exposed to
the fire may have produced this slag, but not in the medium-to-large
quantities seen, without wholesale consumption of these iron stoves,
etc. I tend more toward the opinion that there were metallic (most
probably, ferrous) impurities in the coal, these were partially and
poorly smelted by the coal fire, and so create this slag.
3. Coal: Unburnt coal, especially anthracite. This could be spilled
coal from fueling the fire, or coal that never caught, in a poor fire.
Drawing the line between these three categories is problematic, as they
will intersect with one another--how much must a piece of anthracite be
burned, before it is clinker, and how "bubbly" must clinker be, before
it is slag? I also think this three-part "typology" is a little
reductive, since there are definitely more intermediate categories than
the scheme will admit. But, if these items are present at all, they are
present in great quantity, and dealing with them can consume great
amounts of analysis time and budget. In most cases, I've seen these
artifacts counted and weighed in the field, with only representative
samples brought back to the lab, sometimes with even smaller samples
added to permanent collections. Can't say I'm comfortable with this
procedure, as I have heard that interesting work is being done to source
coal from unburnt coal or clinker, relating it to specific coal fields,
even mines. This has obvious potential to allow trade analysis, even
dating, since the dates that mines were in operation are fairly well
known, and the time ranges for exploitation of certain coal fields are
also known. But, still, this stuff is dirty, heavy, bulky, ugly, and a
harder sell to a project manager as a diagnostic than rusty nails will
ever be.
D. Babson.
-----Original Message-----
From: HISTORICAL ARCHAEOLOGY [mailto:[log in to unmask]] On Behalf Of
Marty Pickands
Sent: Thursday, October 13, 2005 3:23 PM
To: [log in to unmask]
Subject: clinkers, slag, etc.
I have always thought of the term "slag" as referring simply to
something that has melted and congealed. Thus, it could mean the liquid
waste resulting from some reductive process such as smelting, the bubbly
looking cinder from burning coal, the discarded residue from glass
production, the glassy material coating the inside of a stoneware kiln,
or even melted glass or metal from a house fire.
Going over the Histarch archive references to slag, however, forced me
to realize that my definition was not the accepted one, so I looked up
the word in several online dictionaries and in Webster's Unabridged. The
definitions all agreed that the term applied to the liquid byproducts of
smelting metals, and a couple also applied it to the non-combustible
residue from the incomplete combustion of coal. One mentioned extremely
burned brick under the term. "Clinker" appears to be a partial synonym
referring to the glosssy, bubbly form. "Cinder" is given as referring to
the products of incomplete combustion, including coal clinker.
However, it still seems to me that the thick glassy stuff from the
inside of a kiln, glass manufacture waste and melted glass and metal
should all come under the term "slag" as well. With the exception of the
melted glass and metal, I am not sure what other term can be used to
describe them. I decided to see what the rest of you think about the
definitions of these words in an archaeological context.
Marty Pickands
New York State Museum Cultural Resource Survey
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