Hi folks, this sounds like a stupid question but can anyone tell me why fire pots are so deep? I read somewhere, (I think in Loreli Sims book), that you shouldn't bury your work in the coals because then you can't see colour temperature changes in your work.

 

With that in mind it seems inefficient and wasteful to me to have a mass of coals burning away with the hottest coals deep down at the bottom of the pot furthest away from your work. Why aren't fire pots shallow or even flat so that you don't need to burn so many coals before the heat reaches your work? I'm guessing fire pots are deep because a "pot" keeps the coals hot whilst your away from your work so you don't need a constant air supply?

In your country, the firepot is not as popular as it is in the U.S. The water-cooled side blast is most used. Here, we find that with a bottom-blast coke fire, a hot spot results about 4 inches to 6 inches or so above the tuyere. I liken it to a 3 inch sphere of intense heat; it can get to 3,000F, which can really be too hot for forging. I tell my people not to put their work in angled toward the tuyere, like a dip stick. Combustion is just beginning at that lower level, and the oxygen in the blast hasn't had a chance to super heat the coke until it reaches hearth level and above. This area of intense heat is termed "the heart of the fire" in the British book, "The Blacksmith's Craft," whether bottom or side blast. We call it the hot spot or sweet spot.

 

I do bury my work pieces out of sight in this sweet spot of coke. The coke on top of the work acts as a refractor allowing one to get a quicker heat than if the work were just laid visibly on top of the fire. For beginners, it is aggravating to bury the work and nevertheless try to recognize the proper working temperature.  With experience, you'll develop a mental alarm. You will also gauge the heat somewhat by the incandescent appearance of the sweet spot. You can also pull the pieces toward you and take a quick peek, looking for heat color, trying not to disturb the fire unduly.

 

To keep the fire going when you leave for lunch or a break, there are three things you can do to keep the coals alive. Open the ash-dump door to allow air to reach the fire from below. Cover the fire with about one inch of green coal or unburned breeze. Take a wooden stick about  one inch D by six inches and bury it vertically into the center of the fire.

You certainly SHOULD bury your work... otherwise it will be cooler than it ought to be and unevenly heated... plus you'll waste LOTS of fuel trying to get it hot!  The best way to put your fire OUT is to spread it out... so how is that a good working fire?  Even to roast marshmallows you need a good coal bed and that can't be a shallow flat fire!  The hottest coals are NOT (at least not supposed to be) near the bottom of the fire pot... the incoming air cools the bottom of the fire and would also tend to oxidize metal in that area.  Remember that heat rises and the hottest area of the fire is slightly above the center of the fireball.  Most newbies use too little fuel and end up wasting both the fuel and their time!  Make a good fireball, get the irons HOT, get the work done, and then put the fire OUT!  

A comment about wastage, I see many noobs cranking away on the air, which uses 2 to 3 times more fuel than needed,  the only air I use is just enopugu to keep the fire at the temp I need. any more increased fuel comsumption and scale creation. Once again where being able to learn from a good smith face to face pays off in the learning curve.

IMHO its not the depth of the firepot you work in, its the height of the fire you are working in..and where your work is positioned in that fire.

 

And the size of the fire appropriate to the material being worked.

Edited January 1, 201313 yr by John B

Thanks for the replies. Some more questions...

 Here, we find that with a bottom-blast coke fire, a hot spot results about 4 inches to 6 inches or so above the tuyere.

How do I raise up the "heart of the fire" so that as you say it is a few inches above the tuyere? I'm finding that the hottest coke in my forge is right at the bottom of the firepot.

 

I do bury my work pieces out of sight in this sweet spot of coke. The coke on top of the work acts as a refractor allowing one to get a quicker heat than if the work were just laid visibly on top of the fire.

I've been burying my work too, but only by about one or two layers of coals so that I can just about see the colour changes and easily move the piece about if needed. To avoid disturbing the coals too much I tend not to just stick it in, I rake back a few coals, put the steel in and then rake coals back over.

 

 I see many noobs cranking away on the air, which uses 2 to 3 times more fuel than needed

I'm sure I am using too much air. I chatted with a pro blacksmith a while back and told him I was having problems with clinker whilst burning coke, he said it sounded like I was using too much air. It goes back to what I said earlier though with my hottest coals being at the bottom of the pot at an unworkable height - I'm having to use a lot of air to get the coals at the top hot enough to work with, which is killing the coals at the bottom.

 

 Once again where being able to learn from a good smith face to face pays off in the learning curve.

 

I'd love to but I have health problems which are a real pain to work around.

We have an old Farm Blacksmithing book which states that the fire should be deep, clean, and compact. Deep means to use lots of coal and coke. The coal or coke from hearth level, should be coned upward around the firepot, volcano style, except in front where the work enters and exits. Don't try to skimp on fuel. Build a high, deep fire for best heating results.

Cheers, I'll experiment with using more coke and see if it raises the heart of fire. You hear so often that novices waste coke with inefficiency and that has made me cautious about piling coke on, but from the sounds of it maybe my inefficiency comes from using too little coke.

 

At the minute I start the fire at the bottom of the firepot with charcoal, I encircle the charcoal with coke and gradually rake the coke from the sides over the top into the middle, I then add more coke to the sides and repeat the process. In the past I've had problems with moist coke spitting which has made me wary of putting coke in the middle too soon so I've been letting it dry out on the hearth, (disc of the brake disc), before raking it into the middle.

 

To help raise the heart of the fire off the bottom of the firepot should I be doing anything different? Is it worth starting with a layer of coke on the bottom of the firepot with the charcoal ontop of it?

With this discussion in mind, the question , how deep is to deep? So the real question is depth of the coals as much as the depth of the fire pot. I currently use a gasser, and would like to build a coal forge. I was thinking of building a table with a brake drum in the table. I have access to all sizes from compact cars to bus brake drums. Considering the depth you are talking about, would I be better off using the bus brake drum? What is the best depth for a fire pot? I could even plasma cut the cast brake drum to the optimum height but I don't know what that is.

I use a brake drum for my firepot.  About 13" across and 3" deep, set into a forge table.  Seems plenty deep to me - I can build a pretty good mound of coke over the top of firepot/table and get up to welding temps, assuming I've managed my fire well up to that point (mostly not let it burn hollow or let ash build up to much around the tuyere).

If you need to raise bottom of fire pot a layer of clay (clay and Portland cement mix) or refractory fire brick would do it....

Dale

It's not the bottom of the brake disc that needs to the be raised up, (It's about 3inches deep), it's the heart of the fire which needs to be raised. Going by what Frank says that means the heart of the fire in my firepot should be a couple of inches above the hearth, (disc of the brake disc), but I'm finding the heart of the fire is at the bottom of the firepot.

If you watch that BBC Mastercrafts production on the blacksmith, you'll see that his forge is absolutely heaped over with fuel.  It is this mountain of coal that allows you to create a hot zone that's high enough to get away from the air coming into the forge.

 

He also talks about wasting fuel but it's not by using too much or too little.  Novices waste fuel by not striking while the iron is hot.  If you bring the hot steel to the anvil and then have to think and plan your strikes, the steel is cooling and you have to put it back into the fire.  Lots of coal/coke burned up in the process.

Cheers Vaughn. I'd always mistakenly assumed that the blacksmiths that had big mounds of coke did so because they were working with thick stock, but it seems a decent pile is required regardless. I haven't seen that programme but I found it on youtube so I'll watch that later. Last night I re-read a few pages of book by Charles Mcraven I was given for Christmas, he says the coke should be a minimum of 6 inches deep!

Cheers for all the advice, I rebuilt my forge to allow for a bigger coke mound and it's running well with a lot less clinker. It's now basically a 55 galon forge, (horizontal like a BBQ, not veritcal), with a 5" clay firepot and clay hearth. Today I annealed a meaty hunk of scrap my Mum found in her garden, it's about 3" in diameter and looks like a chisel bit of a pnuematic drill...so I guess the forge is running well to be able to heat that up!

Congratulations!  Sounds like you're well on your way and will be doing great things soon.  I'll look forward to seeing your work.