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I Forge Iron

Forges and Fires


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There have been several questions on forges and fires.
Many of the questions have been answered several times on the site already. 
A forge is something to hold the fire. An anvil is something to hit upon, and a hammer something to hit with. The rest are details that make life easier for the smith.

>What do I need to get started in blacksmithing is a good review.
How to build a fire
First if you have a chimney, loosely wad up a couple sheets of newspaper, set them on fire and place them into the chimney so the heat will get a draft going. Next build a fire that would make a boy scout proud. Gather a bunch of sticks, chop up some kindling, or even break up the stalks of weeds so you have a bundle about 4 inches in diameter. If none of that is available cut some cardboard into 2 inch or 3 inch strips and roll it into a circle.  Wad up a couple of sheets of newspaper and set it on fire, and put it in the fire pot of the forge. Put the sticks, kindling, or roll of cardboard on top of the burning newspaper and fill in around the sides with coal or coke to hold it in place. Add just a small bit of air to get things started.
Once you have a fire started, add a double hand full of coal to the top, and a bit more air. The idea is to establish a bed of coals from the sticks, kindling, etc with enough heat being generated to set the coal on fire. When you see the coal start to catch fire, add another double hand full of coal.
The amount of air needed is surprisingly small. You do not need a F5 hurricane force wind to forge with. You can not get fuel hot if the fuel is being blown out of the fire pot, or is dancing in the fire pot from the force of the air.  You need only the amount of air to produce the heat you need from the fuel.
Be sure to keep about a 1 inch opening in the top of the fuel  forming a volcano so the fire can escape. As that fuel catches fire add more and more fuel always keeping the opening in the top clear so the fire can escape.  As the fire heats up the fuel will give off volatiles (smoke) and the hole in the top will allow these volatiles to escape and burn, as well as burn the remaining volatiles coming up through the coal. If you burn the smoke, the neighbor can not see it, or at least see a lot less of it. 
 The size of the fire is determined by the size of the stock. The shape of the forge determines the shape of the fire ball and the shape of the heat. A small round fire can be designed to heat no more than a 4 inch section of metal, where as a brake drum or pan forge full of fire can heat 12 inches or so of metal, and a slot forge can heat 2 feet or more at a time. 
 If you can work 4 inches of metal with your hammer (during one heat) then it does you no good to heat 24 inches of the metal at a time. No matter how much metal you get hot, you can only work so much under your hammer in one heat. Build your forge and fire to match the work you are doing.
The 55 Forge is a good, quick to build, low cost, starter forge. Build it first and play in the fire. It will not be perfect but you will have a working forge and time to design and build your second forge. >Reference The 55 Forge
Side blast 55 Forge
Bottom blast 55 Forge
Forges are designed so that the work can be inserted into >the sweet spot of the fire, that is about midway to 2/3 of the way up the fire ball. This is where the oxygen has been consumed by the fire and only heat reaches the metal. Without oxygen you get little or no scale. If you have a shallow fire and lots of air, you can actually cool the metal by the air blast, and it will not get to forging temperatures.
Now that you have a forge, the fire and heat is determined by the air being pushed into the fire, NOT be the amount of solid fuel in or around the forge. Most folks try to save the fuel and do not use enough fuel to have a proper size fire. Reference cross section of a forge
The amount of fuel does not make the fire hot.
The amount of air going to that fuel makes the fire hot.
The yellow is the burning coke, the fire ball. The red is the coal being transformed into coke and the black is the coal.
Notice the size of the fire ball and where the stock is placed. You can put more coal on the top to insulate the fire and hold the heat down into the fire ball. 
You want to restrict the air produced but not restrict the air going to the fuel. Keep the air tube open and just enough grate to hold the fuel in the forge. For the 55 Forge I use a 2 or 2-1/4 inch auto exhaust pipe and one or two pieces of 1/4 in mild steel rods as a great. This leaves a LOT of room for air to pass into the fire.
After several fires notice the cone the ash has formed. The bricks were placed in the forge to make the fire deeper  and smaller for the project at hand.
There are two of  my forge pots, The one on the left has an air tube of 2-1/4 inches with one piece of mild steel welded in for a grate.  The fire pot on the right is a 3 inch air tube with one 3/8 inch bolt welded in for a grate.
No one ever said it was going to be right the first time. This was too much opening and I was loosing fuel down the air tube. So I put 2 each 3/8 inch bolts across the opening and it now works very well. I am even able to use fines or coal dust as a fuel in this fire pot.  
This is actually a deep brake rotor and not a brake drum. I like it better as once you cut a hole in a forge table it will just drop into the hold and hang on the rotor disk. The disk has a low profile and you can easily rake coal into the fire pot when you need to.
A second 55 gallon drum with both heads removed was then placed on top of the 55 Forge and tack welded into place.The supercharger!!  This creates a 24 inch diameter chimney and very good draft. The two support drums are only for the photo, as the forge will be used on a stand I will construct later.
Do not feel you have to have a steel drum, a hole in the ground will work just as well. To make things easy and so the smith does not have to been over, put the mud or a stack of bricks on a table. There is no right way, and the metal does not care how it gets hot. The metal only wants to BE HOT in order to move under the hammer.
And no one said you had to forge with coal, coke, or charcoal. You can use any fuel that burns with enough heat to get the metal hot. Some have used wood, and corn as a forge fuel. You may have to modify your forge a little to make these alternative fuels work best.
Additional References
BP0048 How to Build a Coal Fire
BP0046 How to Build a Coal Fire
BP0045 How to Build a Coal Fire
BP0042 How to Build a Coal Fire
BP0037 How to Build a Coal Fire
BP0036 How to build a Coal Fire
BP0137 Fire Starters
BP0051 Good Coal
BP0131 Coal, Coke, and Rocks
I may add more information to this post. Please check back.


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  • 3 weeks later...

There have been some questions on how deep a fire, how much coal do you use, etc. I found this video which will explain some of that. Remember that these blacksmiths are being times to build a project, horse shoes. They do not have time to play, only to work, so they build the fire to fit the work at hand.


How much coal in your forge


Yes there is a good size fire ball to work in, and yes the fire pot does get hot......

And yes, they should be wearing safety glasses.

It is only 45 seconds long so you may have to watch it a couple of times.

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Yes, thank you, Glenn. I have always used gas but a dear friend gave me a solid fuel forge to compliment my asenal and I'm excited to have the edition. The images here give a great view of whats going on in the fire. One question: the fire cut-away shows the yellow as burning coke and the outer layer is green coal. What is the red layer called?




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  • 11 months later...



Remove the bottom blast pipe. Put the end of the tywere level with the bottom of the forge in the photo.

The side blast the tywere would be supported and the new bottom of the forge would be level with the old bottom blast grate.  That would be close to what is going on in the fire.




You can see in this side blast forge the red zone and below that the yellow zone. The fire needs more fuel at this point. The notch at 5 o'clock position is the side blast air pipe for the forge and is 2-3 inched off the bottom of the floor of the forge.  Any clinker would work its way to the bottom of the fire and collect usually below the side blast pipe..

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I respect the work by some who take time to showcase and post videos etc but I don't understand the reasoning behind a blanket statement; " your firepot only needs to be 1.5" deep and 7 x 9 rectangular etc. etc. Otherwise you are wasting fuel"

How does one know what I need?

What about a clinker breaker?  Why say they are not needed. Quite obviously several companies in the past thought enough so that they made, perhaps thousands. 

A shallow fire goes out when you stop long enough to eat a sandwich, make a phone call and grab the newspaper from the box. 

I have many years worth of punching the holes out of a firepot to clear the passageways that a clinker breaker seems like a good idea.

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  • 4 weeks later...
On 18-3-2016 at 4:03 AM, SReynolds said:

I respect the work by some who take time to showcase and post videos etc but I don't understand the reasoning behind a blanket statement; " your firepot only needs to be 1.5" deep and 7 x 9 rectangular etc. etc. Otherwise you are wasting fuel"

How does one know what I need?


That has to have reasoning behind it. A context there must be. There are many people (especially with many people posting on Facebook and YouTube) who post what they do, beginners as well. And there is far more than just IFI
Assuming it's aimed for the beginning smiths amongst us who don't often forge much larger stock than the 3/8" range, perhaps 1" stock once or twice.
If you have a medium sized fire pot, say 3-4 inches deep, 9 x 9 or 12 inches, and load this up with fuel to only forge small stuff, that is fuel wasting.

In any size fire pot there is a certain amount of volume of fuel consumed per hour. The larger and higher you pile up, the more fuel gets consumed.
It all releases its heat, but if you don't have stock in there to absorb it, it's wasted. A certain % of heat is always lost. But you don't have to waste more than needed. 
Or do you always shovel a ton of fuel on your fire and let it go up in smoke, hahaha. I assume not.
Even with a crank blower or shutting off air inbetween heats, you have a fuel volume that can heat many more pieces of stock than what one puts in.

Say with the medium fire pot, you have capacity to easily heat 3" square stock. Now only put in 1 bar of 3/8" round.
You still have all that fuel getting consumed. You can minimize it by carefully regulating air. Making the fire smaller, but large enough for the stock at hand is more fuel efficient.
Or go the other way around, load up your forge with more bits of stock. A saying comes from this "Too many irons in the fire".
For the same reason, one would perferably load up a gas forge with as much stock as you can. Because that burning gas is giving off heat.
Whether you put it one piece or load it up, it will release the same amount of heat. Of course some regulation of heat and gas consumption is possible.
But that is not the point I am trying to make.

Still assuming it's aimed for the beginner smiths amongst us. Who of them can work more than one piece in the fire without burning or messing it up?
Especially taking into account many don't have a teacher to watch over their shoulder. Or can instruct them and correct any info. they accidently took on differently than what whas said.


I'd bet that whatever video you say, was totally not aimed at more seasoned smiths such as yourself.
So that bit of info. doesn't at all apply to you.
Context is important. If I could look at the video, I would look for the stock sizes used.
If it's all small stock, a shallow fire makes sense.

Many smiths (clearly not all of them) in Europe mention stoking a fire appropriate for the stock size you are using - And to not load up your fire pot with as much fuel as you can.
It makes no sense to make a big fire suitable for forging an anvil if you're only going to forge nails.


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  • 2 weeks later...

This site is full of good advice how to get heat in a forge. However, I have not found as many answer to the question Why? as to how? The following is an attempt to answer some of the why? questions. It is my belief that if I understand 'why' it is easier to control 'How'.

Why is it that I do not get a zillion degrees in the forge? The main answer is exhaust losses.

We add air which is a mix of 1/5 oxygen and 4/5 of mostly nitrogen. This mix has room temperature. Leaving the burning zone is a mix of nitrogen and carbon dioxide that has the same temperature as the burning zone so it carries away a lot of the heat that is generated. Thus if I add more air than is necessary for a complete combustion, the extra air will remove more heat from the burning zone. I will get a cooler burning zone not a hotter if I increase the air flow.

If there is a pile of coal above the burning zone as in Glenn's picture, the exhaust losses are not a complete loss since the heat is used to coke the coal.

It is a mistake to believe that a large pile of fuel will consume more fuel than a small pile. If this happens we are using too much air.   

All oxygen is of course not instantly consumed. Close to the air inlet, there is more oxygen than is needed to burn the carbon (and thus also more nitrogen). This not only means that steel put there will oxidise (=scale) excessively; the extra air will cool the area, which is why the bottom of our forges does not melt.

What will happen if we increase the air, (with moderation) is that the burning zone will increase in size but while that happens, the fire is really cooler.

There are of course also other losses even if  these usually are less important. There is conduction and there is radiation. At the temperature in a forge, the radiation heat loss can be considerable but if the fire is covered, as in Glenn's pic, that heat is contained by the surrounding coal and helps coking. I think we all have experienced the intense radiation when we rake the fire apart to close down.

The heat in the burning zone is generated by volume. So twice the size of the zone generates eight times the heat. The heat loss by condution and radiation is on the surface of the zone so twice the zone gives four times the heat loss. Thus relative heat loss by conduction and radiation goes down by a factor of two if the burning zone size goes up by a factor of two.

The heat is generated on the surface of the carbon. The surface per volume goes up by a factor of two when the mean size of the coal goes down by a factor of two. This is why it is difficult to get high heat if the fuel size is too large. Eventually the carbon pieces will of course burn off and become smaller and the temperature will go up. If the fuel is half the size, the heat per volume will go up by a factor of two provided that there is enough air. The amount needed for a fire ball of the same size will go up by a factor two also and the pressure needed to force the air through will increase. This means that bellows - including the Japanese - will be more independent upon the size of the fuel in the supply of air. Most blowers will have no problem, however.    

We cannot burn more carbon than the air supply allows. Thus the size of the fire ball depends upon the air supply. Usually we do not need a very big fire ball so if we supply more air than is needed for it and have a pile of coal above, the fire ball will increase in size and we burn fuel for the crows as we say in Sweden.

It is possible to use fuels like wood in the forge but not immediately. The wood must first turn into charcoal. When we heat wood there is moisture that will be driven off and this consumes heat and lowers the temperature. Then when the temperature increases the complex molecules in the wood will desintegrate and form combustible gases. When these burn, we get flames and the heat from the flames is obviously not generated where we want it. This desintegration is also heat consuming and lowers the temperature. Thus we cannot get forging temperatures until the wood has turned into charcoal.

It is the same with coal only that the amount of combustible gases is much smaller and coal cannot absorb so much water as there is in "dry" wood. 


S. Reynolds,

 If you have a relatively high air pressure and small tuyere opening, the clinker will not impede air flow. In my forge I occasionally have to fish out a doughnut of clinker but I never had it impede air flow.

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Fuel choice is a factor, as mentioned blanket statements with out context are worse than meaningless. 

Main example being fuel consumption and fire characteristics between coal and charcoal. While due to the density of the fuel, fire spread and fuel consumption with out forced air is a very minor issue with coal it is a whole different ball game with charcoal, as charcoal will happily burn in free air, spreading to and consuming a fuel pile like commonly used with coal. Furthermore for efficiency and effectiveness charcoal like a smaller fire, a 6" heat zone being about sweet spot with a single tuyere, wile a coal fire (side blast) can beneficial from larger tuyere and dealer fires when heating large pieces, wile one uses multiple tuyere (and a pipe with more than one hole drilled in it is in effect multiple tuyere in this case) to make a larger fire.

Bottom blast vs side blast ads another venerable, as one always hears one needs a deeper fire with charcoal, and in the case of a bolton blast this is true, while in the case of a side blast the opposite is the case. 

Context is important. 

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  • 1 month later...

I have a very large bellows and five mall holes (just under 3/8")  in the tuyere plate and they plug off frequently from clinker. So that statement concerning small tuyere holes and high air pressure won't apply to my forge. The fact that I opened up the tuyere plate and it functions much better, is a great relief  to my operation.

zoar blacksmithing 8-21 023 sm.JPG

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The fan allows me to have both hands free and I can do other things when the stock is heating (Yes I also burn things if I am not careful). The design of my tuyere is such that I regulate the flow just there. Thus I have the highest airspeed in the tuyere and nothing falls down in spite of it being a bottom blast. Advantage: no tuyere block, disadvantage: clinker must be fished out.

I have high regard for Technicus Joe and his videos but he is wrong when he states that more fuel means more fuel burnt. More air means more fuel burnt - at least when using coal or coke. I always pile up so much fuel that I hide the fire ball just as in Glenn's pics. It preheats/cokes the fuel and keeps the heat where it belongs also I am not tempted to gaze into the fire.

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Fuel does not make the fire hot, air makes the fire hot.

You can over fuel a coal fire and it will not get hotter than the amount of air being pumped into the fire. This is discounting the natural draft of the system that may want to keep a fire burning while the blown air is not being used.

Fuel consumption has a direct relation to the amount of heat needed plus some consumption to keep the fire lit. Think of it this way, if you are heating 1/4 inch stock you need a smallish fire. If you then heat 1/2 inch stock you will need 4 times the heat as you have 4 times the volume of metal.  If you heat 1 inch stock you will need 16 times the heat, and for 2 inch stock you will need 64 times the heat as you have 64 times the volume.  This does not account for the longer time needed to get the stock at the same temperature all the way through, both inside and outside, or the larger size fire to heat the larger stock.

Fuel is a consumable item. You MUST use what is needed to heat the stock you are using. You will waste some heat during the process. This waste can be seen in the heat of the water in the slack tub, the anvil being too hot to lean on, and the chimney draft removing the smoke from the work area.

At the end of the day you can say that X amount of fuel was used to do Y amount of work. If you are trying to figure out exactly how much fuel is being used, your not blacksmithing.


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