Gas Forges
There are two common choices in forges today, coal or gas. Although there is a
surplus of people that will say that one is absolutely superior to the other,
both have their pros and cons. Coal can dirty the air, floor and possibly your
workpiece if you are not careful. But if you have easy access to the fuel supply
I still believe it can be cheaper. It is very simple to construct and run,
with less things to go wrong or go "boom." Heating can be very precise and
controllable. It is the "traditional" and romantic way of doing things and thus
more appealing to the public.
Gas is very clean and easy to maintain and use. You have the ability to see all
of your workpiece at all times. Although you can burn steel in a gas forge you
have to be much more careless to do so than with coal. On a properly made gas
forge the atmosphere can be infinitely adjustable and very easily controlled.
Gas can be dangerous (BOOM) if you do not know what you are doing. The fuel can
be expensive unless you work with larger volumes. It is very difficult to heat
just one small area on a workpiece without heating the whole thing, which can
result in overheating of the tip and edges. The size of your workpiece is
limited to the size of your doorway into the forge shell.
I still use both a coal and a gas forge for all of the reasons mentioned above.
But with the increasing demand for my pattern welded steel I find myself using
gas more and more. Gas forges can make life easier and for some tasks they are
just plain superior to coal. But this only holds true if you go with a good
quality forge. Undeniably the most important tool in the smiths shop is his
fire. This comes in the form of the forge. This tool is no different than any
other, you get out of it what you put into it. A person who's business is clean,
tight machining would never settle with a cheap sloppy import but would spring
for a well built, precise mill. Far too many do not get this same concept with
forges.
There is a false economy of "spend a dollar to save a dime" in bladesmithing
these days. You can spend a bit more on a good, durable and efficient gas forge
or you can cut corners and time with a cheap patchworked forge that you will
replace often while your efficiency suffers.
The big trend in home made forges is the superific handy dandy coffee can type
ceramic wool forge that you can quickly whip up in one afternoon, and it is a
good thing too because you will be remaking it virtually every afternoon. I have
never subscribed to the notion that just because a large number of people are
doing something makes it undeniably correct. In centuries past, blood letting was the unanimously accepted way to cure any ailment. All medical scholars agreed on this so it had to be right, right?
Ceramic wool (a.k.a. KAOwool) is the insulation for the outside of a proper
forge shell, not a forge shell itself. It is an insulative, heat reflective
material that is far too delicate for any long-term exposure to the wear and
tear of a serious forge. Have you ever poured water on cotton candy? This is
exactly the same effect that flux has on ceramic wool. Making forge lining out
of it is the equivalent of eliminating your homes interior walls and trying to
hang pictures in the pink fiberglass insulation. I have yet to work in one of
these forges, that was more than a day old, when I didn't spend more time fixing,
patching and adjusting than working. Just read some of the hazards of the
stuff and you will know to cover the wool lining with some sort of hard-surface
refractory or the blast of the burner will fill your shop with particles that
have asbestos-like qualities. Since their walls reflect heat,
these forges do heat up very quickly, but do not hold thermal mass like solid
refractory. And if you lack the patience to wait for a forge to come up to a
good heat, bladesmithing could be very taxing for you. In my opinion, for the
aggravation, one is much better off just working with a coal forge.
I may seem a bit opinionated about certain tools used for bladesmithing,
I must admit that many of my views are from the full-time hard duty point of view.
From this perspective consistent quality by the most
efficient means is the key to success if not survival. To put it plainly -
I can't afford to waste time messing with substandard or ill-suited equipment.
It is worth mentioning that most of my opinions come from the standpoint of
the majority of work being pattern welded, the more fragile wool liners could
function fine for simple forging operations.
A gas forge is comprised of two parts, the combustion chamber and the burner
assembly. The combustion chamber is surrounded by the forge shell for holding
and concentrating the thermal energy. For this I insist on a good solid refractory
lining. Castible refractories are easily formed like cement into any shape and
are very durable ( I typically get more than 5 years of outright abuse out of
such a shell) . A castible refractory lining will take longer to get up to heat
because it must absorb enough energy to achieve the thermal mass necessary for
heating steel. But once this mass is achieved it will loose it just as slowly.
When the shell comes up to heat the workpiece is heated primarily from radiated
energy stored in the refractory, this is very even and thorough. The burner
assembly is made up of an incoming line for gas and either a blower for forced
air or a venturi for the "atmospheric" style forges. Down stream from these
there should be some kind of area for gas and air to mix and a nozzle or port
for injecting or entering the combustion chamber.
I prefer the forced air burner over venturi. Using fuel flow to pull in the air
mixture, venturis seem to eat up more fuel for the heat provided, and I have seen
my share that get hot and sputter or burn in the inlet tube. Forced air can give
you much more blast velocity and heat with complete control over fuel/air
mixture. For just forging alone I could work with veturi, but for welding,
forced air is always my choice.
While we are on the subject, fuel consumption versus heat output is a far too
often overlooked quality of a gas forge. I have seen very few out there that I
thought was as fuel-efficient as they should be. This doesn't just result in
higher fuel costs; it can also effect your work and the finished product. Some
forges need to cook like a fighter jet engine just to reach proper temperatures,
while some purr up to welding heat with little effort at all. One thing to
remember is that the answer is not always adding more fuel. Over a certain
amount, fuel is only wasted and can even create a cooler burn. I personally would
not use any forge that required over 4lbs of gas to run efficiently. Mine,
while working with volume, requires only 1.5-2lbs of propane for a serious welding
heating. If you are frosting up your bottles to the point that fuel use is
affected you must either get a more efficient forge or get larger volumes of
propane.
For the type of gas forge that I prefer I must recommend the forge offered by
Tim Zowada. While I did build my own forge to suit my particular needs, the
majority of my concept and inspiration came from the Zowada design. After much
research, I found Tim's basic forge principles to be the most sound and effective.
My combustion chamber consists of a 12" diameter steel tube into which has been
cast a 1 ¼" lining of refractory. Flux at elevated temperatures is very corrosive
to refractories. One way to combat this is to use a high alumina or other
refractory that is resistant to the action of the flux, the drawback to this that
these refractories can be 4 times more expensive than regular refractories.
Since the flux drops mainly on the floor and only really effects this area, I
solved the cost dilemma by casting only a portion of the bottom out of the more
expensive high alumina and the remainder out of traditional refractory. The
steel encased shell is then wrapped with an insulating layer of ceramic wool
and sheathed in a thin metal skin to hold everything in place. There is 4"x 3"
door at either end so that a longer piece can be passed all the way through.
The rear door is covered with a flap of ceramic wool when not in use.
With gas forges, evenness of heat equals efficiency and squared chambers or chambers
with corners defeat even heating. I work with cylindrical interiors because
they can use vortexes to achieve even heating quite well. This vortex can only
be taken advantage properly if the blast enters near the outer wall in line with
the curvature of the arch so that the cylinder will capture it and guide it
into a vortex around its circumference. A blast entering in the center, aimed
at the center will result in a very hot spot on the opposite wall and lots of
heat energy being deflected out the doors. Not only can this waste energy
but it can make the work area in front of the door quite uncomfortable.
The placement of the workpiece in the vortex can also effect the efficiency
and evenness of heating of the steel. If the workpiece passes lengthways
through the center of the vortex the swirling flame will heat an even 360
degrees around the steel. If the Steel is passed perpendicular to the
vortex so that it has to pass through one wall of the vortex, the center and
then the opposite wall, you will have a hot spot on your steel just inside
the door a cool spot in the eye of the vortex and another hot spot at the
other side. There is no evenness in this at all; making welding a real
challenge, and controlling an atmosphere in such a configuration would be
tricky at best.
My burner assembly has a small inexpensive gate valve for controlling gas flow
and a light gauge to measure the adjustment. On the other side is my electric
blower for the air supply. When choosing a blower I like one that pushes enough
air to cope with the use of a large gate valve downstream for controlling airflow.
Experience has shown me well that the flimsy silly plates that slide over the
intake are worthless for proper airflow control. The control of air is very
important since propane is relatively harmless by itself; it is the addition
of oxygen that makes it explosive or even flammable for that matter. Close one
of those sheet metal things entirely sometime and see how much air still comes
out of the blower, and how far do you slide it for a given amount of air?
And the vibration of the motor will continually act to loosen the setscrew
making it impossible to reliably set it in one position. The use of a gate
valve for the air as well as the fuel essentially turns your forge burner
into an oxy-propane torch that will allow you complete control over
infinite adjustments of blast intensity and atmospheres. For forging
operations you can save a bit on the fuel by cutting it back a bit until
short spikes of flame jets just peek out of the forge shell door. This
is a neutral flame. For welding, increasing the fuel a bit, or
decreasing the air, will result in a reducing atmosphere which will be
evident by upward curving flames coming out of the door as unburned
fuel mixes with the oxygen outside the forge. But this means that
there no oxygen left inside the combustion chamber to interfere with
your welding. Turning the airflow way up or the fuel way down will
result in no flame at the door, and all combustion occurring deeper
within the shell. This would be an oxidizing atmosphere.
Although it doesn't look like much, an important feature of the burner tip is
the expanded mixing chamber. As the unmixed gas and air enter this space
there is a drop in pressure resulting in a swirling and mixing of the gas
before accelerating through the high-pressure area created by the bottleneck
of the nozzle tip. I played with other configurations and ran into trouble
when the lack of this high-pressure injection allowed combustion within the
burner tip, resulting in the telltale popping and backfiring. This
confusion for where the combustion should occur also results in inefficient
burn in the combustion chamber.
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