Forge Slope?

[Quarry Dog]

  Okay guys, weird question.  Why does it seem that all bottom blast forges I've seen have a slope that is graded in all directions to the firepot?  I've always been told not to put my piece pointing down into the pot, but make sure it has coal over it.  This gets annoying when drawing down long pieces less than half inch, as you really have to pile it up, and I know that half inch really doesn't need that much coal., since the slope is just pushing you away from an otherwise perfectly good fire.  Even the forges from the turn of last century have it, the cutouts on the edge sit higher than the firepot.  Literally every bottom blast forge I've used has this little quirk.  I don't recall seeing this on gassers or on side-blast, so why bottom blast?

  It almost seems like some knuckle-headed apprentice was pouring way too much (char)coal into the bed and it was all rolling off of the edge, and instead of telling the kid to knock it off, the master smith decided to make the whole forge slope inward.  I can see why either side of the firepot would slope down a little to hold coal for bigger jobs, but why not make a level strip the width of the firepot all the way across?  It sure seems like it would be easier to clay the bed of the forge this way too.

  Seriously though, maybe some of the guys that have been around for awhile can field why this is the way it is.  Pardon the rant, I guess you could lump this in with old world problems.

[HWooldridge]

The different lip heights found on most pots are designed to allow the user to set them in either a steel or brick framework.

With regard to piling fuel in the hearth, it's pretty common to put a bag or two in the general area when starting from scratch so it can be raked in from the side. A deep fire is preferred to reduce scale so sticking the work down into the pot usually makes for burnt or scaly results. I normally have a valley running lengthwise in the fire (parallel to the long side of the pot) and rake coke and green coal in from the side. It seems counterintuitive but trying to scrimp on fuel usually makes for a shallow, wide fire. Piling up fuel and using a water can will concentrate the fire in the pot under the work, where it does the most good. Charcoal and straight coke don't react to water like a coal fire will but they can still be controlled.

[ThomasPowers]

Perhaps you are trying to do small work in a forge designed for large work---a self inflicted difficulty

[arkie]

It's my understanding that you shouldn't jamb your work down deep into the fire, since that's where the air blast (oxygen) is greatest and results in an oxidizing fire rather than merely heating your work...detrimental to the steel being worked.  The pros probably can expound more on this.

[ThorsHammer82]

the slope in a bottom blast forge is to allow the ash to fall into the pipe, and thus out the ash dump. It keeps it from building up in the fire pot. It also forces a deeper fire which, as mentioned, reduces scale. Basically, the deeper pot uses up all the oxygen before  it can reach the metal you're working on.  along with making the pot deeper it also shapes the fire. different shaped pots = different shaped fire. There is no one forge that is perfect for EVERY job. so working small material in a large forge wastes fuel and working large material in a small forge wastes time. etc.

 

It sounds like you're more concerned about using fuel to it's fullest rather than being concerned about keeping the material in the best shape possible. Fuel is cheap, material typically isn't if you compare the two.

[Glenn]

Your metal should be placed in the fire as indicated in the drawing.

 

 

 

The amount of fuel does not control the heat of the fire. You can over fuel a forge and it will not get hotter.

 

The amount of AIR determines the heat of the fire. Control the air and you control the fire.

[arkie]

Great illustration!!  In color as well :)

[Quarry Dog]

Perhaps you are trying to do small work in a forge designed for large work---a self inflicted difficulty

  So, basically most of the forges I've used, all built in the last 20 of so years, are built based on an old template where heavier stock and heavier work was more common and you really needed to stack on the fuel?  That actually makes alot of sense.  Kinda goes with the old say "If it ain't broke..."

 

the slope in a bottom blast forge is to allow the ash to fall into the pipe, and thus out the ash dump. It keeps it from building up in the fire pot. It also forces a deeper fire which, as mentioned, reduces scale. Basically, the deeper pot uses up all the oxygen before  it can reach the metal you're working on.  along with making the pot deeper it also shapes the fire. different shaped pots = different shaped fire. There is no one forge that is perfect for EVERY job. so working small material in a large forge wastes fuel and working large material in a small forge wastes time. etc.

 

It sounds like you're more concerned about using fuel to it's fullest rather than being concerned about keeping the material in the best shape possible. Fuel is cheap, material typically isn't if you compare the two.

  I understand why a firepot is sloped to make it easier to clean and promote a clean fire while in use.  I'm referring to the bed of the forge around the firepot though.  I 've never had much problem with ash outside the firepot.

  I do keep the scale build up in mind and under control when working.  Am I worried about fuel usage?  Yes!  I'm absolutely trying to get the most bang for my buck.  Good coal costs at least $25+ a bag with shipping here in California, and that's if you buy it in tons.  I've only found one place in 50 miles that sells coal, and they sell it for $50 a 50lb bag.  With that said, I don't like using more coal than Is necessary, for minimal gains in scale reduction.  I would go with a gasser, but I'm not very comfortable with something that could literally blow up in my face If I don't do it right, that and I have no idea how to regulate a reducing/oxidizing environment with a venturi.

 

  The firepots I'm used to are the Centaur Forge coal pots that are about 3.5 to 4 inches deep, 8-10" long, and about 6" wide. The biggest short (12" or less) pieces I've heated in them was 1" round or 3/4"x3" flat, and i don't get much scale, even while hammering and bending.  I start by laying my work across the lip of the pot, making sure that the work is well covered by coke, so that I can just see the color of the metal, heating it up quickly but thoroughly, and once I'm up to temperature I kill the blast for a few seconds to ensure a reducing atmosphere before I pull the metal out.

  When I'm trying to heat the middle of long pieces (3 foot or more) I have to stack about 1.5" to 2" higher of a pile, force more air through to get the heat up that far from the tuyere, and I end up burning through more coal, which is really kind of sad when it's something thin, like trying to draw out reins from both ends of a piece of steel withough using tongs.  More coal, more mass and radiating surface area, which requires more air and fuel, for more heat to keep lt all hot.  Meanwhile the smaller fire does just as well with the scale.

  I started this post because I couldn't find a definitive reason anywhere and I'm thinking about finally making a forge of my own instead of being limited to driving to the next town over for 3-6 hours and a lunch break once a week with my local group.  I just wanted to make sure that it wasn't something I was overlooking, like maybe magically helping the draft of the hood, or maybe I was underestimating how much coal likes to fall of the edge.  It's seeming more like it was primarily for keeping bigger work up higher to prevent scale, which I can solve by propping up the metal on something, like a piece of scrap.  It could also be for coal storage, but a 1" rim around the bed with a 6" pass through  it on either side of the pot should fix that.

  Thank you for the info.  I momentarily forgot about the days of the industrial grade blacksmith that could swind a 16 pound sledge like it weighed 1 oz.

[arkie]

Jerome,

 

For holding your work, you might try a "goal post" like I made for my brake drum forge, also to hold longer pieces in the fire.

 

I made an "H" shaped contraption out of 1/2" rod.  I tack welded a couple of U-shaped holders to the side of the forge that the goal post can slide up and down in.  I then drilled several 1/4" holes in the lower portion of the uprights so as to allow vertical adjustment by using 1/4" pins through the upright holes.  You can put the cross piece of the H-brace lower so as to allow a more horizontal lay for your work.

 

Posting a picture for the first time...hope it works.

[Glenn]

If there is a question about fire pot shapes, make the same shape out of clay and build a fire in it.  Forge for a while and then remove the coke and coals without disturbing the ashes. The ash will tell you what works best for that fire pot and the forging you were doing. Different designs will leave very different ash walls and deposits. Besides clay is cheap.

 

 

This is a brake drum or rotor in a 55 Forge. Notice the cone shaped sides.

 

The depth of the fire was increased by adding bricks to the in side of the drum or rotor. Looking to the 10 o'clock position of the photo you can see just how high the side walls of the fire pot were built up. The cone actually continued out of the brake drum or rotor and up the wall. 

[Frosty]

Jerome: Now I'm understanding your questions better.

 

No, I don't think there're any special reasons for the forge pan's shape and the shapes of fire pots have been pretty well covered. First so you know I'm a propane forge guy but I've been messing with fire since I was a kid. I have a couple coal forges ad one I made, the commercial forges are a cast iron buffalo rivet forge and the other is a small 12v powered farrier's forge. Both have pretty flat bottoms but the rivet forge once clayed can be almost ay shape.

 

I really prefer a "duck's nest" rather than a "proper" fire pot. A duck's nest is a shallow depression around the air grate, mine is about 4.5" and about 3/4" deep. I made it by laying a Tupperware bowl over the air grate when I rammed the clay in. When I use it I lay fire brick around the nest in whatever shape, size, depth, whatever I want. I can make a fire virtually any size or shape I need up to a point, maybe 12" across. I can make a fire smaller than a tea cup, virtually a coal powered torch flame.

 

The forge I built is again a duck's nest but the deck is 3' x 4' with a 2" rim all round it. The table is fire brick and the nest is half a brick. My basic nest is 4.5" x 4.5" x 2.25" deep. This isn't the max size nest though, most of my air grate is covered by a coarse of bricks, I can open it up to 9" sq. if I want a major conflagration.

 

The tables are flat, no slope of any consequence, the rivet forge has a very slight dish down to the center but once clayed it's flat as a fritter.

 

If you're going to build a forge just don't get locked into what you THINK is what you need. It's no accident I have so many forges, I just kept picking them up expecting THAT one to be THE right forge. There is no RIGHT forge, all have advantages and disadvantages. You'll end up with several of most variable dependent tools before you get settled in. don't sweat it, we all do.

 

Frosty The Lucky.

[Quarry Dog]

Good deal guys.

 

Arkie: I like the goal post idea, although if I go with a portable forge I'll probably make a Y-stand to hold the end of my stock.  I like Y-stands better for long pieces because you aren't limited to a certain angle of attack or a certain length of stock, although if I'm bending 20 footers I might need a few of them.  The forge mounted stand seems nice for smaller awkward stuff like if I'm trying to heat up a big curve and I can't really lay it sideways over a forge.

 

Glenn: I kind of expected something like that with brake drums.  The leftovers of the fire pretty much burned down and then slipped down the hole until what was left settled into the angle of repose.  I guess you would start with normal old potter style clay with sand before you commit to a shape in fire clay?  Or depending on the variety of work expected maybe just stick with the clay? Say if I need really long heats for a bunch of long scrolls and twists, and then concentrated heats for forge-welding them all together.  I imagine the desired result would be a shape that lets most of the ash escape out the bottom, concentrates the clinkers away from the grate, isn't so shallow that it's hard to get the fuel at the edges going, and doesn't burn up the wall of your firepot.  I also imagine that shape changes not only based on the type of work, but your fuel, as coal, industrial coke, and charcoal all burn different.

 

Frosty: One of the guys in my local group has a couple of old cast iron duck's nest tuyeres attached to some really deep narrow brake drums.  I didn't know it was such a flexible system, I thought they were just a quick way to convert one of the old bellows side-blast rigs into a bottom blast.  I've never seen one in use, that I know of.  Maybe you could post some pics?

[Glenn]

The clay is fresh from the creek bank, made into the shape of the day. 

 

Nothing wrong with making the forge large and long if that is what you need. Remember that you can only hammer on a certain length of metal before it cools and must be reheated. This means that if you can only hammer 5 inches of hot metal then it is of little benefit to heat a 24 inch section of material. Now if you are bending or scrolling you can actually work 24 inches of hot metal so you will need the full 24 inches (or more) of metal up to working temperature. If so build the forge to be able to heat that length of metal. You will use more coal or other fuel and at a much faster rate because of the length of metal being heated. It is the nature of the process.

 

No matter what the size or shape of the forge clinkers are the stuff in the fuel that does not burn. They will collect above the air hole as that is where they form. Just let the fire cool a bit and you can hook them out. If not they can block off the air or redirect it in a unwanted pattern. 

[Quarry Dog]

  As far as long heats go, I've made something like this out of 6 foot sections of 1" round, although not galvanized and shaped differently:

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I wish I had actual pictures to post of that. 

  And that leads me into clinkers.  It was actually while we were making these when we started the months long process of slowly burning a sizeable hole into the side of the firepot in our brick demo forge.  We had a bad batch of coal that had so much clinker that it actually coated the bottom of the firepot and redirected a good bit of the flame straight into the side, and we didn't realize it until it was too late.  I make sure to check for those little buggers at least on an hourly basis now.