[TheForge] Water on your coal forge fire? A Novel

[info at reciprodyne.com]

Dear Dave and List,

Coal is a coarse mixture of various fuels: most importantly the carbon in
the coke, and the flammable gasses trapped in the coal. Sulfur, peat, oils,
and other combustible impurities in the coal can also act as fuels, but will
be disregarded for the moment. The coke (carbon) is the most important
component where heating metal for forging is concerned. Coals are classed
according to their carbon content, Anthracite (most carbon), Bituminous
(Most common for forging), Sub Bituminous, and Lignite (least carbon).

The gasses in the coal have a lower ignition temperature in air than the
coke (carbon). Some coal with a lot of volatile material will burn by itself
without forced draft, just from the combustion of the gasses. This type of
fire will not heat metal to a forgeable temperature, because the gasses are
burning off at a relatively low temperature. On the other hand, some coals
have so few volatiles that they will not burn at all without forced draft.

With the addition of atmospheric oxygen from the forge blower the carbon in
the coke becomes incandescent, and the radiant heat from the glowing coke
then heats the metal in the fire to a useable temperature. By the time the
coke in the coal is blown and heated to a sufficient temperature for
forging, the gasses are long gone as "steam", smoke, or flames. The steam,
smoke or flames do not heat the metal.

In the heap of green goal surrounding the fire itself, the radiant heat from
the fire (and some escaping heated air) begins to drive off, or "distill"
off the volatile gasses, which, if heated to combustion temperature, become
large romantic flames which heat the smith, but not the work.

Wetting the green coal does several things, but most importantly, it keeps
the carbon (coke) in the green coal below it's combustion temperature while
the more volatile gasses are being driven off. That way, the all important
coke is preserved from premature combustion while the distilling of gasses
is taking place. When the gasses are gone, the coke is ready to be raked or
pushed into the heating portion of the fire.

The presence of water in the green coal immediately surrounding the fire
also keeps some of the gasses from reaching combustion temperature, so that
they boil off in the steam, rather than creating unpleasant flames. Whether
you steam off or flame off the gasses is largely a matter of preference, but
you generally get more useable coke for heating your metal if you keep the
surrounding green coal below the combustion temperature of the coke. You get
less flames and smoke if you keep the rapidly departing gasses below their
combustion point. 

The cost of BTUs absorbed in the vaporization of the water are completely
insignificant compared to the BTUs preserved by preventing the premature
combustion of the coke. The water (and the gasses) in the green coal are
being boiled away by the stray radiant heat that escapes from the center of
the fire, heat that is not doing useful work anyway.

The amount of water necessary to produce the maximum quantity of usable coke
varies widely with the quality of the coal, and the size of the coal
particles. Some fine, high carbon coals burn well almost dry.

The more carbon (and less gasses) in the coal, the less water generally will
be needed to keep the coking process under control. Coal with more carbon
and less gasses is (in general) "better" for forging purposes, ignoring the
melting temperature of the slag for a moment. If you just buy coke already
made, you don't need any water at all, except perhaps to keep the fire from
spreading too much, but you have to blow it continuously, or it goes out
pretty fast.  Pure metallurgical grade coke is my forging fuel of choice,
and charcoal second.

Finer coal particles help keep the hot (burning coke) portion of the fire
contained, and less draft air escapes through the surrounding green coal, so
that the fire spreads less readily, and the available air blast is more
effective. If the coal is too coarse, enough blast air can escape through
the loose pile to make it difficult to reach forging temperature. The
coarser the coal, the more wetting and packing may be necessary to keep the
fire hot and contained.

Ok, let's talk about clinkers.

Coal comes from the ground, it is comprised from carbonaceous materials
(such as old trees and dinosaurs :-), and also contains a more or less
percentage of dirt (ash). This dirt (ash) is comprised of a fantastic
variety of materials, but largely silica, minerals, etc... you know, Dirt.

In the high temperature of the forge fire this dirt mixture forms a very
crude glass or slag know as clinker. Some rare forms of dirt (like pure
silica) may have such a high melting point that they are not a melting
problem at forging temperatures, but these are not often found in coal, so
the best way to prevent clinkers from forming is to obtain coal without so
much dirt (ash) in it in the first place.

All coals contain some ash (dirt), but whether it becomes problematic during
forging depends on the melting temperature, friability, etc. Some clinkers
are sticky, some are not. Some coals may be dusty grey, have 25% by weight
of ash, and only 50% of carbon, but forge very well because the clinkers do
not clump. Some coals are very shiny black, very little ash, a rating of
15,000 BTUs per pound, a high price tag, and immediately turn to a sticky
mess at forging temperature. Most coals are somewhere in between.

The free coal that comes the from the stoker bin in the old farmhouse may be
fantastic or horrid for forging, the only way I know to tell is by trying it
for a couple of days. Some coals may work OK at "forging" temperatures, then
become sticky when blown to "welding" temperature.

Clinkers are caused by dirt mixtures being exposed to temperature above
their melting point in a forge fire, not by the apprentice who doesn't hold
his jaw right, too much stirring, or by the phases of the Moon. You can't
"cause" clinkers by improper treatment of the coal. You CAN prevent them
entirely by buying "better" coal, or using coke or charcoal.

I have found that "clinkery" coal can sometimes be made to work better if
kept very wet, which tends to keep the clinkers solidified, and broken into
smaller pieces. 

Once, when I had some sticky coal, I put a wide, heavy ring of 1" thick
steel plate around the blast hole in my forge, and kept the coal (and the
plate) very wet around the fire. Instead of raking the newly formed coke
into the center of the fire, I pushed it in from the side with my hands. The
relatively cooler ring o'plate caused the clinker to form a solid "doughnut"
of glass around the hole, and kept most of it out of the tuyere. Every 45
minutes or so, I would pour a liberal amount of water around the opening,
and lift out a perfect ring of clinker with my poker. I would hang these on
a hook, just because I could. If I raked the top edge of the "volcano" into
the fire, the little free clinkers would melt together over the air hole and
shut me down. Getting some good forging coal solved the issue completely.

There are many different kinds of tuyeres, grates and air holes, and they
all have their place, experimentation is the best teacher.

Excuse the over "blown" response. I will ignore all "flames".


-- 
Tom Troszak
Reciprodyne
47 Panola St.
Asheville, NC 28801 U.S.A.

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