The New Forge-Building Picture Book



(08/04/2002)
Every once in a while I get into a situation where my forge is just not big enough. On my last project, the iris, I had problems fitting the fully bent flower for final touch-ups. So now it's time to build a new forge. Coal would probably be a way to go, but that's out of the question for now. There's no way I could fit a coal forge in the shop and I don't really want to move my shop outside, especially in the winter. So I'm sticking with propane. This time around, I'm going try to keep a more step-by-step record of my plans, thoughts, and progress.

But first, I'll give you the finished product. This ain't a novel where you save the ending to be surprised. If you like how it turned out and want to see how it got there, then keep scrolling down. It's all there somewhere.


A while ago one of my smithing friends, Ralph, mentioned he was looking into a new gas forge design to handle bigger stuff. It involved a "roof" that can be raised, with firebricks inserted where needed for walls. Ron Reil mentioned this design when talking about his clamshell forge, and I got to see several in action at another blacksmith friend's, Ian's, shop. So I decided to try to combine the best of that design with the old standard pipe forge.

  

  

Here you see the basic concept. The first picture shows the "normal" operating mode. But if I need a somewhat taller opening, I can open up the front and back. I use a regular insulating firebrick as a sliding door.

If I need to work something wide, I can raise the top and re-arrange the bricks. The last picture shows the roof raised. You can also see the sliding rear wall in this view. Usually I only have the front burner on and keep the rear wall in just enough for what I'm working on.

The rear picture shows some clothes-dryer tubing. I've got a blower attached to the other end and a re-director that you can see in the front. This sends an "air curtain" up the fron of the forge that works very nicely taming the dragon's breath. I can hold my hand (or any other body part, for that matter) about 6" - 8" away comfortably. Any closer and the infrared radiation takes over.

Another couple things, it keeps the handle ends of the work cooler longer and the door raising handles never get too warm to hold if I need to lift it during the forging. I run the air along the bottom to keep the forge from burning the tool cabinet and heating the tools in the drawers.

  
Now you see an example of using the convertibility. This is a leg to a candle-stand. I've also got a helper attached to the forge base that folds out of the way when not needed.


Here's a closeup of the valving. My previous forge used a gas-saver manifold. This was an arrangement of a ball valve in parallel with a needle valve. Close the ball valve to reduce the gas flow when hammering.

I decided to just use a needle valve for this. It's really not that difficult to just twist about a 3/4 turn to idle the burner and simplifies the plumbing. The bright yellow tubing that feeds the burners is a stainless propane flexible tube. I wanted flexibility when raising and lowering the roof. I've also got a needle valve to control the back burner, located up near the front burner. A ball valve would work, but this way I get to do a little adjusting if I need to.


Here's the roof raising mechanism. It's just a $1 flea-market scissor jack. It's not attached to the angle iron that's attached to the raising rods. And those rods just slide into a socket on the roof, so I can get them out of the way completely, if needed. I cut off the bottom, but the jack's bottom is attached to a leg that goes directly to the floor. I just didn't trust attaching it to the sheet metal of the tool cabinet.

Now for some other details:

The roof insulation is insulating castable refractory, coated with ITC-100. This refractory is lightweight and insulates very well. The forge gets to forging heat in maybe five minutes and the roof takes about 1 1/2 hours to get too hot to touch.

The walls and floor are insulating firebrick. My previous forge's floor, made of the same material, lasted close to three years before I moved up to this one. And except for a divot from some spilled flux, it was still going strong.

The burners are built from the instructions for the side-arm burners on Ron Reil's site. The front burner uses a 1 1/4" cross fitting that I could only find at McMaster-Carr. The rear burner uses an asymetrical 3/4" X 1 1/2" X 1 1/2" T, that I did get at a local plumbing supply. The cross works better than the T. The T doesn't seem to let in enough air and unburnt propane leaks out. But for the rear burner, it's good enough.

The burner mixing tubes are 3/4" 90deg sweeps of rigid conduit. These took some experimenting to get them to work right. All the gory details are in the blow-by-blow description below. I angled these tubes to point inwards by maybe 10 degrees. This helped control the dragon's breath in my previous forge by directing the flame into the forge. The flame also is angled 10 deg. from top left to bottom right, looking from the front. This seems to help an internal swirl while giving me a nice "hot-spot" to the left of center on the floor. I think the hot-spot's more efficient by quickly heating only the spot I'll be hammering on. It's not as selective as spot-heating in a coal fire, but better than a wide heat. If I want a larger, even, heat for twisting or whatever, then I just move the piece to the left or right of the spot.

I just have some 1" straight pipe for the burner nozzles. The flame works very nicely from near-zero psi to the 25 psi limit of the propane supply. We'll see how long it lasts, but these burners are easily removable and disassembled, so putting on another 2" of pipe in year or two is not much of an issue. I tried casting the flared nozzles in the refractory, but that didn't work. All the reasons, plus way too much detail is explained below for the insomniacs among us.

The forge body is made from a 30-lb tank of propane. The bottom was cut in half to make the ends. The cylinder was cut in half length-wise and welded to make a longer half-pipe. All the painting is high-temp BBQ paint. Maybe I went overboard painting the bricks, but this whole project was over-kill anyway. Why stop here?

With the 2.5" thick bricks, the forge internal floor area is 8" X 18". The center height is 8.5". And the opening with the end closed is 8" X 4.5". The ends latch closed, so I can let the roof sit on the sliding doors. This gives me an 18" X 4.5" opening from the side, if I need it.

And, as you can probably see, I forged most of the hardware and had some fun with it.I suppose if I were trying to pay the rent with my smithing, I might have just bought a forge from Ralph. But getting there is half the fun - so I had some fun. Sue me.

I think the air curtain's blower is a 60 cfm squirrel-cage. Well, I know it's a squirrel-cage, but can't remember the cfm. It was an eBay find at maybe $40. Where would I be without eBay?



OK, here goes the complete story. If you're reading this far you're either interested in the design you've gone over just about every other blacksmithing page on the Internet and need this for a fix. Whatever your reason, this may get a little drawn out, but I wanted to cover all my design decisions so that something may make sense to another adventurer. I've also got the wrong turns I made along the way. Maybe there's something there that would keep you off those same paths.

The following was done as I went along, so you can see my random thought process. You've already seen the finished project, so you'll know the false paths when you see them. The start date is a little misleading. Yes, I started this over a year ago, but there were a few large breaks in between. Life happens. Anyway, here we go.

(08/04/2002)

A lot of my design is for aesthetic reasons. I like the arched top look. I think it also is more efficient in that the hot gas will swirl better without the square corners.

I also would like to cast the flared nozzle in the refractory wall instead of using the stainless steel ones. My previous nozzle lasted barely a year. Not really a bad expense, but a pain to sawp out. And finally, I'm still a cheap son-of-a-gun, so I looked into whatever scrap and supermarket science would work.

Yet another smithing friend, Dick, had gotten several 12" pipes and cut them to 24" length. He offered one of these and I accepted. This is heavy pipe, with 5/16" walls, but I felt it would do fine. As you'll soon seet, the heavy gauge walls actually turn into a necessity.

Here's what I started with:



The holes are 1 7/8" and were cut by yet another blacksmith, Roger. You'll probably notice that it's got cuts along the top and sides. Those are my cuts. I forgot to take a "before" picture.

And here's the cut tube.



First - Why cut it in half? That's part of the new design. I'm going to have an arched top that will be able to be raised when needed. The walls and floor are flat. If I desire, or need, the "swirl effect" I can put in some diagonally cut bricks in the corners.

I cut one end to bring the burner closer to the front, like my current forge. I'll angle it slightly to the back to help the "dragon's breath" syndrome. You'll also notice that the burner holes are a little to the left of center. I'll be aiming the burners straight down, so that brings the hot spot(s) to the left, again like my current forge. I really like that burner placement, especially for small stuff. It lets me partially close the door to keep heat in. One note here. The pipe was evidently under some tension. As soon as I finished the cut, it sprang open by 1". The 12" pipe is now effectively a 12 3/8" pipe.

Cutting the pipe was a piece of work at first. I tried my sabersaw, but that heated up to quickly. Then I tried a hacksaw. I inserted the blade in the cut started by the sabersaw and mounted the blade at 90 deg to normal. This was to cut the front back about 4", and was actually faster than the sabersaw, but not workable to cut the pipe in half. What worked wonderfully was using an abrasive disk, like for a chopsaw, in my circular saw.

So why all this work to get an arched ceiling? Since I was casting the top, an arch will hold up better than a flat roof. Some of my firebricks have cracked and a wider ceiling probably won't hold up.

Speaking of the ceiling, I added a couple angle iron supports to keep it in place. These angles are recessed 1/4" up into the roof. This gives a 1/4" lip to fit around the side walls. The following picture shows the finished roof structure, along with the plywood "jig" I used to make sure the angle iron supports were square.



So what to cast with? Castable refractory is not all that expensive, but I couldn't find any local distributors. The shipping of a 50 lb. bag makes castable a lot more expensive. I'm getting pretty good at scrounging the web and ranacross a few recipes for DIY refractories. One involved furnace cement and perlite. Perlite is a crushed volcanic product normally used to aerate soil for gardening. It's got a high melting temp, 2000 degF, and is extremely light. It's also used in refractory insulation. Another recipe used fire clay, portland cement, and sand. Still another added sawdust to the mix. The sawdust burns out, levaing thousands of small air pockets to make the mix lighter and add insulation.

From an answer to one of my questions on a maiing list I found out that fire clay is the heat-resistant (refractory) material and portland cement makes the wet mixture sticky. These alone would be a good refractory lining, but a thick layer will tend to crack from the curing (shrink-checking), and then from the high temperature (heat-checking). Sand lets the mixture expand and reduces checking. The perlite also gives a check "cushion", even more than sand.

I settled on a two-layer solution. The insulation layer is :
1 part fire clay
1 part portland cement
8 parts perlite.

There's just enough cement/clay to stick the perlite together, resulting in the lightest, although weakest, refractory. When mixing the two, get the perlite damp. I used 1 part water. Too wet a mixture will take longer to dry and could result in incomplete drying, which then could result in spalling at forge temps. This is when the internal water turns to steam, with explosive force. Anyway, I then mixed in the cement and then the fire clay. This mix should be sticky enough to hold onto a shape when squeezed, but not enough to squeeze out water.

To figure out how many cups in a "part", I poured the dry perlite into the mold to get an idea of how much I would need. After mixing and tamping, it loses about half its volume, so do the dry pour and double the result.

Since this is a fragile layer and would melt at welding temps, I wanted to add a tougher inner liner. I'm trying the cement/sand/fire clay/sawdust mix. I used:
1 part fire clay
1 part sand
1 part portland cement
4 parts sawdust

(05/15/03)

OK, by now I've done some more experimenting. First, the weight of the roof is about 68 lbs. When placed on top of loose firebricks on their side, which is my intended configuration, the whole thing is pretty wobbly. I'm going to abandon the heavy pipe. That alone contributes 35 lbs.

Next, I fired up a sample of the cement/sand/clay/sawdust in my old forge. The result was disappointing. This mixture also melts. It looks like Portland Cement can't stand the heat. I was unable to find any real melting temperatures, but I did see a reference to this in some of the glass-shaping websites. Just blurbs, but they say that Portland cement doesn't hold up to the heat. So it looks like I'm back to the drawing board.

As of today, 5/15/03, I'm planning to go with a much lighter shell for the roof. My options for this, so far, are:
1) Buy a rolled half-pipe: $35 from Jay Hayes (best price I could find, even with shipping)
2) Roll my own. This would involve either making a roller, making a brake and do some "step-bending", or
3) Find a scrap-yard thing that fits.

And for the liner, I found a local refractory contractor who sells insulating castable refractory. So I'm leaving my home-brew behind and going with the pro stuff. In reality, the castable is less expensive than home-brew. I'll save the fire clay, sand, and cement for my Gingery foundry (stay tuned for that probable misadventure).

(6/15/03)

I found a stash of gas cylinders at the town transfer station. For some reason, I'm not allowed to pick through the scrap metal, but I can dive headfirst into used, flammable, containers. The 30lb tanks, a little taller than your standard 20lb BBQ tank, looked to be the right size for the forge body, if I cut it in half lengthwise and weld the two half-pipes end-to-end. And the rounded ends might make it a pretty cool-looking forge.

So now you're saying, "What? Cut up a propane tank? Are you nuts?". Well, I don't thjink so, in this case. I actually did a lot of reading and it's not so crazy. Please remember my disclaimer -

PLAYING WITH PROPANE CAN BE RISKY. DO THIS AT YOUR OWN RISK, AND DO LOTS OF READING ON THE SUBJECT. IF YOU FEEL UNSAFE, DON'T DO IT. AND I REFUSE TO BE RESPONSIBLE FOR WHATEVER DAMAGE YOU CAUSE TO YOURSELF. IT WORKED FOR ME - THAT'S ALL I CAN CLAIM

Basically, filling the cylinder with water and draining removes most, if not all, of the gas. I used some dish soap to help remove some of the smelly chemical. For what it's worth, that chemical, mercaptan, is added to a normally odorless gas to help detect leaks. The smell doesn't necessarily mean the gas is still present, though, but it can be unsettling. I then did all my cutting with a reciprocating saw.


And here's the new forge tube kit, with the roof welded together. My cut wasn't perfect, so I couldn't do a decent butt-weld. I lapped it about 1/2" and welded that way. Eventually I cut the ring stand off and cut the bottom in half to make the ends. But with the stand, the bottom looks a like it would make a decent wok. But it still smelled like propane (mercaptan), and I wasn't ready for a rotten-egg-fried chicken dinner. Lastly, I removed the paint with one of my experimental burners.

1 1/4" holes were cut for the burners. I played with 3/4" burner tubes, but couldn't get a good flame out of them, so I'm going with 1". These give a good flame for the entire range of idle to about 30 psi. The 1 1/4" just fit the O.D. of the burner tube, and I'll be flaring the refractory.


Now here's a dry fit with all the hardware in place. All the hardware is screwed into nuts welded on the inside. I wanted to keep the aesthetics, so didn't want to weld them in place. All the hardware is left uncoated, and I cooked the screws in my old forge to get the same color.

I don't have any pics of the floor in progress, but the shell is a cut-down drawer from a file cabinet. That turned out to be a perfect size, and I love the idea of building from stuff from the scrap yard. It's about a 1/2" deeper than the firebricks that make up the floor, so I put in a piece of expanded metal that was 1/2" thick. Then I poured some perlite in between the spaces in the expanded metal to better insulate whatever's going to lie underneath. The expanded metal is very stiff and will keep the firebricks nice and flat.


Here's the roof waiting to be poured, showing how I formed the burner nozzle. The nozzle molds are really straight 1" pipe. I'll rasp the 1:12 flaring when the refractory is dry. The bits of tape cover the welded nuts to keep the refractory from gumming them up.


Now the refractory. The inside diameter is formed with a section of 8" sono-tube. The plastic wrap keeps the refractory from drying too fast. The notches are to fit the clamps to keep the tube from floating.

Check out my previous attempt:


See how the top of the roof is fatter than the sides? That's because I didn't use an inner mold. Instead, I formed it by hand. Even though that refractory mix felt like it was pretty stiff, upon drying, it settled to the bottom, which was really the top of the roof. That was another lesson learned.

(11/25/03)

Now for the burners. I learned the most in these experiments. I desperately wanted the "sweep-over" concept. I like the look, plus it moves the heaviest part of the burners closer to the lifting hardware, keeping the droop when raising the roof to a minimum.

As you may recall, I chose 1" sweeps of rigid conduit because they burned the best. But once inside the forge, the low end crapped out. And as the forge got warmer, my "low-end" now required about 10psi to burn right. Any less and the flame moved all the way to the air intake. It seems as if back pressure was doing me in. I tried different ways to slow things down, including changing the shape to add a "trap", but nothing helped. I was essentially forging at a welding heat.

Going back to the 3/4" tube let gave me a low end all the way to a candle flame, but I still had a problem where I didn't get enough air and unburnt propane would drift out both ends of the forge. I figure the bend in the pipe slowed the flow too much. But instead of giving in and going with a straight tube, I tried something else.

I drilled some 1/4" holes just above the nozzle, thinking that good old Bernoulli would suck in the air needed. This actually worked fairly well. But now I lost some low end. As the pressure dropped, it would now burn out these new holes. While that was pretty nifty-looking, it wasn't desirable, so I added a sleeve that I would rotate to cover the holes when idling. That did a pretty good job.

All was looking OK, and then I built the back burner. While building this one, I noticed that the galvanizing was pretty thick and rough on the inside. Maybe this extra friction could be a problem??? So I took my front burner tube and removed the zinc. If you try this, read about the methods somewhere else. I felt a little worried when I did this, and would not want to advise. Anyway, after removing all the zinc, and with a nice, smooth, interior, I put the front burner back on. Eureka!!! I didn't exactly run naked down the street, as people are a little less tolerant of that stuff nowadays, but that smooth inside did the trick. The flame is nice and hot, with enough extra air that I need to choke the intake, and I keep that sleeve closed all the time. And now my low end is back to candle flame.


Here's the burner up close and personal, complete with simple sheet metal intake choke. If I ever feel like it, I'll replace the tube with a new one, sans little holes. But for now, I go by, "If it ain't broke, don't fix it".


(12/16/03)

So that's about it, except for things I would do differently.

The only thing I can think of is, since I now use a piece of pipe for the nozzle, I would probably go with Kaowool insulation for the top. It's lighter, not much more expensive, if you can get small quantities, and would be much easier to fit in.

So that's it. I hope you got something out of this latest adventure. One thing I noticed is how much "collateral knowledge" I've gained since starting this new hobby. I now know not only something about forging steel, but a little machining, metallurgy, lots more welding, masonry, and burner technology. Have fun. Stay safe.

-Marc