THE BASICS OF BLOOMERY SMELTING
This is the unedited version of the article
that Lee wrote for The Anvil's Ring, the quarterly of
the Artist Blacksmith Association of North America, in July of
THE BASICS OF BLOOMERY SMELTING
Deep in my January reading binge of 1998, inspired by accounts
of African iron smelting and an account of the 19th century iron
industry in my home county, I hatched the notion to smelt my
own iron from local ore, and make a sculpture with it. Until
I read about the African bloomery furnace, I had never considered
the possibility of iron being smelted on a small scale. It was
one of those deadly ideas that once surfaced, would not leave,
so I enlisted the help of my fellow eccentric Skip Williams.
We thought a little ironmaking would be a fun way to spend a
few chilly weekends. Fourteen months later, after many attempts
and building two furnaces, we finally succeeded in smelting a
42-lb chunk of iron.
So what Im going to do here is try to tell you how I
made it work, trying mightily not to spin too deeply into Digression
Land. But first I must warn you here: one lesson I learned in
this project was not to put too much stock in what I have read.
So if youre crazy enough to try this dont blindly
trust what anyone tells you. Trust your eyes and your experience
and your instinct.
WHAT IS A BLOOMERY?
For those who dont know, the bloomery process (also
referred to as direct reduction) is the original method of producing
iron. Operating on a small scale and at relatively low temperatures,
it produced a sponge of malleable iron and slag that was forged
directly into a wrought iron bar or billet. The furnaces varied
greatly in form from shallow hearths to tall shafts. Beginning
in medieval times, the bloomery was gradually supplanted by the
blast furnace process, also referred to as indirect reduction.
The blast furnace operated on a larger scale and at higher temperatures,
and produced molten cast iron, which had to be further refined
by eliminating carbon and reintroducing slag to create wrought
iron. The blast furnaces advantages of producing large
quantities and removing a higher percentage of iron from the
ore outweighed the difficulty of this extra step.
A BRIEF THEORY OF THE PROCESS
Heres the basic theory of how the bloomery works. As
we all know from our lifelong battles with rust, iron and oxygen
really love each other, and are constantly trying to unite. Iron
oxide is the dominant form in which iron occurs out here on the
skin of the planet, and thats what most iron ore is composed
of. Luckily for us, at elevated temperatures, the oxygen prefers
other partnerships, most handily with carbon. So simply put,
we want to put iron oxide in a charcoal fire above 900°
C with lots of carbon monoxide. The carbon monoxide will grab
oxygen from the iron oxide to create carbon dioxide, leaving
behind bits of pure iron. These particles of iron will fall into
a bath of liquid slag in the bottom of the furnace, and float
around until they stick together into a bloom. The slag is largely
composed of iron, silicon (the major impurity in most iron ore)
and oxygen combined into a substance called fayalite. This slag
has a relatively low melting point of 1200ºC, and a nice
liquid slag is necessary to keep everything moving and to protect
the reduced iron from burning. It also has a chemical function,
as we shall see later. If our slag has too low an iron content,
its melting point will rise and the furnace will freeze
Many things you might read would lead you to believe thats
about it. But a problem rapidly presents itself. If you have
enough carbon fuel to produce enough heat and carbon monoxide
to reduce much iron from the ore, you tend to have enough available
carbon to combine with the iron to make it unforgeable. Most
of the European experimenters Ive read about, such as Tylecote
and Crew, have dealt with this by reducing the amount of charcoal,
so there is little carbon beyond that required for reduction,
and blowing the fire very gently, so the ore spends a long time
in the reducing area of the fire and the hot zone remains down
near 1200ºC (carbon absorption increases with temperature).
The problem with that is theyd spend 12 hrs to smelt a
4 lb. bloom that forged down to a 1 ½ lb. bar.
We discovered a better way when we went ahead and tried
to make cast iron. By throwing in plenty of charcoal and
running the furnace hotter, we get optimal reducing conditions
in the stack of the furnace. Yes, the carbon particles that fall
to the hearth are cast iron, but the slag bath decarburizes them
(at least this is what I think happens). Besides fayalite,
the slag also contains wustite (FeO). When the wustite comes
in contact with the carburized iron, the carbon and oxygen combine,
simultaneously decarburizing the existing metal and contributing
more pure iron to the bloom. In this way weve progressed
from 5 lb. slaggy blooms to 20-40 lb. dense blooms.
So much for theories. Heres how to do it. There are
lots of variables to fool with in this process; remember to treat
the following as a starting point.
To build our furnaces, Skip & I used old hot water heaters
lined with castable refractory. Besides being a convenient form,
the steel skin is handy for welding on legs, tuyeres, handles
and such. Inside the shell is a layer of insulation, and then
the 3200ºF castable inside that. Leave an opening to insert
the tuyere 8 or 10" above the bottom of the furnace and
another opening at the level of the furnace floor to form a slag
tapping arch. The interior diameter of our first furnace was
12", and of the second 14". The first shaft was cast
in a single piece, but removing the hot bloom through the top
proved an ugly chore. We built the second furnace in sections
that could be lifted of with an overhead hoist (tip o the
hat to Wally Yater for this idea) so the contents could be more
easily removed, to allow further experimental changes, and to
We tried several sorts of tuyeres. The first tuyere was cast
iron, and melted immediately (Duhh!). Then we cast refractory
tubes, which worked OK but had to be replaced every other smelt
or so. We finally built a water-cooled tuyere, which works great.
The hollow tuyere opens directly into a water reservoir. Weld
all those seams carefully to avoid nasty steam explosions! Weve
had best luck so far with an interior diameter of 1 ½".
This dimension is one of the primary determinants of air flow,
especially if you use a low pressure air source like a squirrel
cage blower. Our current tuyere is 14º from horizontal.
A removable peephole in line with the tuyere is entertaining,
as well as vital for monitoring temperature and cleaning occasional
We are currently using a 150 CFM squirrel cage blower, which
seems about right with this furnace. You also need an air gate
to adjust flow. Bellows should work well if youve got lots
of free labor. Probably the optimal set-up would be a variable
speed/stroke piston. If youre really ambitious, some provision
for pre-heating your blast will save some charcoal.
First we need iron ore. This planet is mostly made of iron,
so you should be able to find some. We have gotten ours
by picking through the tailings of old iron mines in our area.
You can find these places by asking local geologists and rockhounds.
Our state Division of Mineral Resources was also helpful. If
youre not fortunate enough to live in an old mining district,
you may be able to find bog ore (iron oxide precipitated out
of water in current or ancient swamps). I guess if worse came
to worst, you could buy some.
Before smelting, the ore should be roasted. You can roast
ore in a wood fire or a gas forge. Bring the ore to a nice red
or low orange heat, and try to keep it there a while. If you
get it hot enough to start melting, youve screwed up. You
can cool it down with water to help the ore to shatter into smaller
pieces, or just let it cool. The ore will now be more friable.
Bust it up so that most of the pieces are fine to pea size. Youll
see that the ore is now shot through with fissures that will
be accessible to the furnace gases. It will probably also have
changed color to red and hammer-scale gray, making any other
bits of sandstone etc. easier to see and remove. If you got the
heat just right, non-magnetic ores may convert to magnetite,
allowing you to pick out the good stuff with a strong magnet.
Obviously, the better your ore, the better your results will
be. If youre not getting iron in your furnace, look to
your ore first. The fayalite slag contains two iron molecules
for every silicon molecule, so if theres too much silicon
there wont be any iron leftover for the bloom.
Next we need charcoal. This is really the biggest part of
the whole job if youre making it yourself. In my furnace,
Im burning 10- 12 feed sacks of charcoal for every smelt.
Ive tried lots of ways of making charcoal, and Im
telling you now, dont bother with any thing but the retort
method. Your charcoal will be better, and so will your relations
with your neighbors, since the retort burns some of the noxious
fumes. For info on charcoal making in a retort get Making
Charcoal and Coke by Barrie Howard (available from Norm Larsen).
You can buy charcoal also, but its not quite as good as
you can make, as it tends to be a bit damp and under-cooked.
You need real chunk charcoal, not briquettes, which will not
work because of their fillers and their lousy structural characteristics.
Bust the charcoal up so the average piece is 1-2" across,
and sift out the fines.
A flux is optional. About ten percent of the iron in the slag
can be replaced with calcium without altering the slags
melting temperature, so you might as well get that 10%. Limestone
or oyster shells are the traditional sources of calcium. Manganese
can also replace iron in fayalite, if you found any manganese
ore in your travels.
At this point, I feel compelled to follow the time honored
Anvils Ring tradition of The Safety Lecture. You will be
dealing here with a very large and hot fire. Im sure you
all know the attire required for such occasions. Take special
care of those eyes and hands! In addition, if you have hair you
feel strongly about keeping, youd better cover it up when
charging the furnace.
First, get the furnace hot. To save charcoal, I stick the
gas burner from my pipe forge into the slag tapping arch for
about an hour before starting the charcoal. You can also start
the preheat with wood. Then, leaving the tap arch open for draft,
add the charcoal slowly until the fire burns up to tuyere level.
Start the blast and add charcoal gradually until the furnace
is full, and keep it near full during the entire preheat. Our
preheat time has been 4-5 hours in our furnace, and uses the
majority of the days charcoal. When the smoke stops and
the gases at the top of the furnace start to burn, youre
ready for the first ore charge.
Fuel:ore ratio is one of the important variables, affecting
temperature, efficiency of reduction, rate of burn, and carburization.
A ratio of 1:1 (by weight) is a good place to start. As the furnace
burns down enough to allow a charge, add in the charcoal, followed
by the ore. We have been adding ore in 15 lb. charges. At this
early stage, we tend to use a fairly gentle blast, increasing
it during the course of the smelt. There is a limit to the total
ore you can charge in your bloomery. In ours it seems to be about
60-80 lbs. The charging sequence could last anywhere from two
to four hours. As your bloom and slag bath grow up to the level
of the tuyere, you can prolong things by tapping some slag out.
Spectators (and believe me youll have some) really like
this part, but try to refrain from doing it too early or too
often. Remember, slag is your friend. Of late, we have often
found it unnecessary to block the tap arch at all, simply poking
through the charcoal and cooled slag that accumulates there to
tap, but have a firebrick handy to stop it up if need be.
Happy slag is very black and very liquid at a bright orange
heat (seen outdoors). Other colors, especially olive green, and
high viscosity indicate low iron content.
At a certain point, you may find the furnaces temperature
will begin falling, and youll have difficulty keeping the
tuyere cleared. Youve reached the charge limit of the furnace.
Try to stop just short of that next time. In any case, at the
end of the charging sequence, add another bucket or so of charcoal,
and burn the furnace burden on down to the bloom, which we hope
has formed just below the tuyere. This post heat can take an
hour or two.
The bloom will be an irregular spongy mass firmly attached
to the furnace wall directly below the tuyere and extending most
of the way across the furnace. Pry it loose however you can and
pull it out. If Vulcan, Ogun, and their compadres feel you have
approached the project with the proper reverential attitude,
you will find your bloom is spongy iron with slag in it. Celebrate.
If not, it will be mostly spongy slag with little bits of iron
in it. Try again.
When weve examined our blooms by spark testing, we have
usually found a wide range of carbon content. The portion of
the bloom nearest the tuyere is denser and higher in carbon content,
receding to spongier, lower carbon iron towards the periphery.
If you want to make a knife or tool with this you may want to
try removing the high carbon section now so you can work it separately
(I have not tried this). Spark testing may lead you to believe
youve made cast iron, but dont panic yet. The cast
may only be skin deep, and the included slag and the more oxidizing
environment of forging will continue decarburizing the bloom
with each heat.
We often begin working up the bloom immediately after smelting,
using our bloomery with the top sections removed as a forge,
moving later to the gas or coal forge for welding. At this stage,
your object is compaction rather than forging. If you take a
welding heat now, all your slag will run out of the bloom, leaving
you with lots of separate pieces of sponge iron. Rather, take
a series of orange heats and compact the bloom, beginning with
fairly gentle blows. Watch out, slag will be squirting all over
the place. You will note that the spongy character of the bloom
causes it to neither conduct nor hold heat very well, requiring
many short heats to work it. When its compact enough to
heat like iron and return energy to the hammer like iron, take
a welding heat and go for it.
Now remember, all of the above is not gospel, its merely
what I think and see now. Ive also left out a lot in order
to write an article rather than a book. Youll find some
other information on our web site at http://iron.wlu.edu/,
and we certainly welcome questions or discussion by phone, letter
Of course, the big question remains: Why? I reckon that would
take a book. But I will say I see a lot of sculptural possibilities
in the grain and crystal structures of the iron, and the potential
for true escape from bar forms. And this process has definitely
deepened and intensified my understanding, appreciation and love
for this material Ive built my life around.