Coal Fire Management by Russ Jennings

New England Blacksmiths
Summer 2018

Last year I took a class at the Adirondack Folk School, called "forging transitions and setups", it was taught by Bob Valentine. I had been blacksmithing as an amateur for about 3 years at that point. Bob Menard told me this class would help me take my smithing to the next level, and reminded me that the scholarship program would cover the majority of the cost.

I started as a smith watching You Tube videos, and then built a small coal forge in my garage. I used it for a summer, in the evenings aver work and on weekends. I soon built a propane forge and didn't use coal again. Unless I was taking a class somewhere, and I always tried to get some time in the green coal tent at NEB meets. Often with some cranky old smith telling me I was doing it wrong.

He was right, but I didn't realize it at the time. After all, when I used a coal forge, it got hot when I cranked it or turned the blower up. I may burn a piece here or there, but that's just life as a blacksmith, right?

My class at the folk school was a five day, 8 hour a day class, using a hand cranked coal forge. There was only one other student. You couldn't ask for better, more personal instruction than I got; not unless you get to intern or apprentice somewhere.

The first day of the class I did alright and kept up, but I struggled with the fire. By the middle of the second day, I had burned several pieces and was gettng frustrated, and tired. My usual go to plan of "crank the blower faster" meant I was exhausted, and I wasn't learning much since I kept having to start over. Mr. Valentine took the time to go over the finer details of coal fire management with me, and by the morning of day three I had the fire under control. Now I could focus on the actual class material because I had a clean, hot fire to go to every time.

The class ran until Friday. On Saturday there was a tool making day, where the experienced smiths and instructors had been invited to make tools. Those 9 teaching stations all need punches, fullers, and other tools. I had the opportunity to stay that Saturday and make tools with them. The skills I learned all week paid off in spades as I was able to keep a hot, clean fire all day and was able to forge weld with ease.

This class did live up to my expectation as something that took my skills to the next level. In addition to everything I learned about swinging a hammer, I learned how to control a coal fire. and what I learned on that topic is as valuable as anything else I learned that weekend.

First, let's talk about how I used to manage my fire. I thought I was doing decent on this topic, but that's mainly because I was using a coal fire for short periods. I would get home from work around 6, fire up the forge, and if I was lucky I got two solid hours in before it was late.

I would clean out my forge and look for good pieces of coke. Then I would take some wood shavings and kindling, light it, and add the coke around it. Once the flames are going well I'll turn up the blower and after a minute or two of that I would start to put my remaining coke around the edge of the fire, and begin moving raw coal to the edge and turning it into coke.

I'll pull my piece out, hammer away for a bit, then slide it right back where it was. Wait a minute or two, maybe take a sip of water while my electric blower does the work, and then pull the piece out. As the fire burns I'll keep adding coal to the edges and bringing coke into the fire itself, but otherwise I'm not doing much maintenance. Maybe every once in a while I'll look for clinkers or use the ash dump. After an hour or so of this, my fire isn't doing great. I'm lucky if I get a dull red heat every time I pull the piece out. If I want it hotter, I have to jab it down deeper into the fire in‐ stead of laying it flat. This sure does get them hotter; this is when they might start to burn. Maybe I crank up the blower higher, but this too begins to diminish in returns. I'm not getting the big clean clumps of coke I used to, and sometimes it's even a struggle to get the fire to grow.

But, it's now getting late, so it doesn't matter. I turn everything off, stop forging, and do other things while I wait to ensure it's all safe and cold before I call it a night. The next time, I go through the same process of cleaning up. The big chunks of clinker at the bottom are easy to find. My fire burns well for a while, but by the end of my forging ses‐ sion it's back to the same dull, smoky, dirty fire.

That's not how I manage my fire anymore. Before we talk about the right way to do it, let's figure out what's going on when we do it the wrong way.

When we put a piece in the fire, we want a clean, hot heat. The ideal would be something akin to a Bunsen burner in a laboratory; a pure flame with no byproducts or contamination. And we CAN get something like that out of our dirty chunks of coal. In the beginning of the fire, those big pieces of coke I put in to get it started, they burn pretty clean and hot. As I start to bring in more coal and turn it into coke, that starts to change. Two factors are happening. First, the coke and coal is breaking down into smaller pieces. These smaller pieces fall down and pack themselves together more tightly than bigger pieces do, so they slowly start to block airflow from your tuyere. Second, as I bring in more coal and it turns to coke, I'm also generating slag and clinker.

In small quantities, this stuff isn't too big a deal. It's not great, but it's one of those unavoidable byproducts of burn‐ ing coal. When that raw coal starts to emit blue and green sulfurish smoke, it's also emittng hot pieces of solid con‐ taminants into the fire as well. They trickle to the bottom and stay hot, and they somehow find each other. Just like the small pieces of coke do, these ever growing chunks of slag block the airflow. Even worse, they contaminate it. The clinker literally sucks the heat out of your fire, so even though you may add more fuel and crank the air more, you don't see the same heat you used to. Your piece builds up more scale and takes more work to clean up when you're done. Not to mention how much harder it is to forge when the iron doesn't get hot!

Those big pieces of clinker might be easy to find, but that's the only upside. In reality they are a sign that the fire isn't being cleaned or managed well enough. Take a look at some of the behemoths we find at green coal during a meet; they didn't magically appear after a single shovelful of coal. Small bits of slag began to fall down and congeal togeth‐ er over time, leading to a solid plug of glass at the bottom of the firepot. No wonder the fire burns better when that stuff is taken out.

What did I do differently, and what did I learn?

The fire needs maintenance every heat. Normally this is as simple as fluffing it up a little before returning your piece to the heat. Reach into the bottom of the fire pot with the poker and, with a lifting motion, pull the fire up and let the pieces re settle. Then slide your piece in, horizontally, and let it heat back up.

The ideal speed to turn your hand crank is between 30 and 60 rpm. It doesn't take much effort, and if it does seem to require a strong stream of air to keep your fire hot, then it's time for more cleaning.

As your fire burns, every 3rd or 4th heat you should twist the clinker breaker a few times and open the ash dump as many times as needed until it comes clean. This may result in hot embers and even unburned fuel to spill out, but it's also dumping out all of those small pieces of clinker you can't see yet.

To add fuel, start by adding raw coal to the edges of the fire. If your forge allows it, the base of the chimney is a great spot. The hot air from the fire will blow over the raw coal, turning it to coke faster.

Raw coal is small, black, and shiny. When you begin to heat it, it'll smoke up and turn to coke. Coke is puffy and light‐ weight, and burns without smoke. As the fire consumes fuel, you need a steady flow of new fuel. That means you will be adding raw coal to the edge, moving coke into the fire, and emptying the ashes out regularly. This is another reason not to add any more air than you need, because the more fuel you burn the more you have to add.

If you are proactive about maintaining your fire, you will fix problems before they become noticeable. You will find that it only takes a little airflow to keep things white hot, and you aren't gettng as much scale as you used to. Once you are comfortable with the basics of fire management, here are a few other things I learned. First and fore‐ most is the importance of keeping things flat in the fire. Make sure if you are working on a piece like a hook or a leaf, that the thin part is up out of the fire.

You can take this a step further, too. Now that you've got this clean, consistent fire, you can control the heat in your metal much more.

To get an even heat, be patient and let the piece soak in the fire. Let's say you've got a small fire, which creates a small heat, but you want to work a larger section of the piece. If you stop adding air to the fire and let it sit for a mi‐ nute in the hot coals, that large heat will spread out, and try to equalize itself. For bending this is invaluable; you can heat up the section you want to just shy of yellow, then stop the blower and just step back for a few seconds. The piece won't burn, and instead you'll end up with a very even orange heat that bends smoothly.

Another great tip to prevent overheating a piece. You need fuel on both sides of the piece to burn it. If you bury your metal in the coals and crank away, that's risky! So for a small piece, use your poker and scrape coals away so there is nothing on top of your piece. Let the metal sit atop the coals, and it's just about impossible to overheat it.

The most advanced use of your fire comes when you want to forge weld. Forge welding requires that the smith con‐ trol both the heat and the atmosphere of the fire within pretty tight guidelines, and doing it on two separate pieces. So far I have covered controlling the heat, so let's talk a little more about atmosphere. The coal needs oxygen to burn, but too much oxygen joins with our steel and creates scale. But if we stop adding air, then the fire stops getting hotter, so how do we control that?

More fuel is the answer, ideally in a deeper fire. When I forge weld, I start by getting a good clean fire going, and I create a depression or hollow. I want to be able to see the piece as it heats up. I also want all the hot fuel underneath my work to consume all of the oxygen before it can get to the steel.

Since it's impossible to create a 100% oxygen free environment, you'll still get scale on your work. The secret to gettng easy forge welds is watch your piece until it heats up enough, pull it out and brush it, flux it, then put it back in immediately and stop adding air. There will be enough residual heat left in the fire, and the piece will be so close to forge welding temperatures that you probably won't have to crank at all, and it will heat back up in a few seconds.

How do you know if it's hot enough? There are three ways I determine if I'm at forge welding temperature. The most difficult in a coal fire, is to watch the surface of the metal. When you reach the right temperature the steel becomes wet and glossy looking. This is much easier to do in a propane forge. In a coal forge, it's easier to look for the right color. It just so happens, hot burning coke is the right color! So once your piece disappears into the fire and you can't tell the difference between the hot steel and the burning fuel, you're very close to the right temperature. And finally, as soon as you start to see sparks, you're at the upper end of the forge welding temps. You can forge weld metal that has just started to spark, that's just barely white hot, but it's a sign that you went a little past the ideal heat.

If you have good control over your fire, you don't even need flux. Flux simply keeps scale from building up on the piece while it's heated, and I've been able to forge weld without flux using the technique I described above. Get the steel up to temperature, brush off the scale, and put it back in a fire with no airflow and things tend to stick!

The tools we use as smiths are all full of extra capabilities, and require skill and experience to learn how to use. The way I use my anvil now is very different than it was when I started out. The fire is just as subtle and complex a tool, and the better you can manage it, the better off you'll be. The coal fire may take more skill than a propane fire, but in exchange you get more control over the size and temperature of your fire. Propane forges have a hot zone of a spe‐ cific size and generally uniform temperature. A coal forge allows you to vary the size and heat which makes it flexible. I can create small red heats for bending, or heat a large and irregularly shaped object which would never fit through the doors of any practical propane forge.

The biggest hurdle for me was remembering how dynamic a coal fire is. A propane forge is set and forget; as long as you have pressure left in the tank, the fire burns the same way the whole time. A coal fire is changing every minute, and you as the smith have direct control over every single variable. Don't leave it to chance; decide what you want from the fire.

    Gee Mom, look what else I've found!

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