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I Forge Iron

Natural gas powered forge


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I attended a one day blacksmithing course a while back and got enthused enough to build a natural gas powered forge over a couple of weeks.

We live one a small inner city block about 1 mile in a straight line from city centre so I have a few limitations, like noise and space but I do have a compact metal/wood shop with a fume hood that I can use the forge in.

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Internal dimensions of the opens space are 8"  in diameter x 14" long

Doorway uses a 6.5 lb lead counterweight that balances the silica firebricks.

The other frame in front of the doorway is an removable air curtin.

The frame also can hold a piece of pyrex glass from an old door oven so one can watch what is going on with the doorway up without getting cooked.

That frame is on a hinge and can be rotated right out of the way as required.

The air supply is a variable speed 1800W vacuum cleaner that runs at about 1/3rd of full power.

I got it up to 1200ºC before I melted the thermocouple.

It uses 2 cubic metres or 3 lbs of gas an hour.


So far it runs off the NG bayonet on our back veranda but this has me excited enough to extend the gas line to the workshop. 

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  • 2 months later...

I finally got my natural gas line out to my shop. The line is 72 ft long and had to go under brick paving, sewage, stormwater and mains water pipes, and 3 garden reticulation line, I also managed to hit about 6ft of old builders rubble and glass plus tree roots everywhere.


Anyway it's done, and it's 3/4"diam pipe all the way from the meter box to the shed.

Previously it was 3/4" then 1/2", then 3/8", back to 1/2", then down to 5/16 and then back up to 1/2" - none of which helped the gas flow.

The gas pressures are very low in our suburb so it was important to use 3/4" diam pipe.


While I was at it I upgraded the plumbing on the forge to be 3/4" all the way to the T- piece where it divides into two 1/2" lines to the two torches.

Not much different to see.

I removed the idler line as it was not working well and I need to reconfigure it.

The box of electronics is a new starter from an old gas Hot water System (HWS) we had recently replaced



Here is a close up of the starter


It uses 2 D size 1.5V batteries. The HV unit from the HWS has a number of sensor inputs which are basically relays that can be short circuited except for one that is wired to a switch.

Throwing the switch sends a fat spark across the spark unit with a frequency of about 2 sparks per second so it lights a bit more assuredly than the push button BBQ starter.


Previously I was gas flow limited but that does not seem to be the case now. It's 8:30 at night here but I fired it up briefly and got to 1100ºC with just the front torch running with the valve opened about 3/4 of the way, and lots more air to play with too, so still have a lot to experiment with.

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Very cool! Nicely done. You're just running on a regular residential sized line?

Correct. It's a real challenge because the pressure of our gas supply in our area is very low (0.2 psi)

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  • 3 months later...

One of the reasons I have not done anything for a while with my forge is because I realized it badly needed some safety features.

The major problem was that being a blown mains gas forge, if for whatever reason the forced air supply was to stop, the small blue blown flame would immediately turn into a very large yellow fireball.
The fireball would be so big it would engulf the forge and I could not hope to get near the manual gas taps either on the forge itself or the stopcock on the wall behind the forge. Then I would have to run some 20m to the next stopcock back up near the house.

The air flow could stop for any number of reasons e.g. blower failure, mains power failure or I could just put a red hot piece of metal through one of the plastic air supply hoses.

After much mucking about, this is what I came up with to automatically turn off the gas on air flow failure.

It all starts with the psuedo Pitot tube.
One copper tube points into the air stream and the other is perpendicular to the air stream which generates a small but measurable pressure differential under air flow

The pitot tube is mounted in a 3/4" T fitting so that the ends of the copper tubes are in the middle of the air stream passing through the forge torch

The pitot tubes are connected to this air flow switch ($20 from fleabay) using 2 pieces of black irrigation tubing - sorry its so hard to seeing being all black
The two blue taps are air pressure bypass monitors that I can attach to an air pressure meter to set the switch triggering point.
The switch is adjustable from 0.1 to 10" of WC.

I have set it so that when the air pressure falls below about 0.2" of WC the air flow switch opens removing power from this 24V solenoid valve.
Interestingly got the solenoid valve out of a dumpster at work as they were having a major cleanup. The valve was part of a water cooling control panel for a vacuum furnace. It's indeed amazing what is thrown out.
The fancy looking ex-CPU fan and heat sink have been added to cool the solenoid which gets quite hot as it really meant for use use with water which would carry the solenoid heat away. 

Unfortunately nothing is quite as simple as what I have described above as lighting the flame requires no forced air but some gas, so the air flow sensor needs to be temporarily bypassed while the flame is being lit. 
So I made up this control box (grey box in middle of photo) that has a few bells and whistles to enable a safety aware startup to take place
A small switch temporarily bypasses the the airflow sensor. At the same time the bypass displays a red LED and sounds a piezo buzzer to warn the operator that the airflow switch has been bypassed.
The operator can then manually open the gas line just a whisker to light the gas (this generates a yellow smokey flame about 150 mm long)
The the air supply is turned on just enough to get the flame approaching a green colour.
Then the gas is turned up some more, the air some more, and this is repeated until a satisfactory flame is generated.
At this point the air flow is sufficient to keep the air flow switch closed so the bypass switch can be turned off.
This also switches off the red LED and the buzzer, and also switches on an orange LED
If the power fails at anytime during start up the solenoid will still active and turn the gas off and the orange. 

The control box contains the 240/24V AC transformer and the rectification circuits for the LEDs and Buzzer.


I can't use the forge for blacksmithing for another 4 or so weeks due to some surgery I had a recently but I will at least fire it up tomorrow and do some testing with small flames eek.gif.

I also got myself a CO and mains gas sensor/alarm so that I don't gas myself in the process.

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My first gas forge was a natural gas forge I bought from Ron Reil (he of the gas burner page).  Other than being a commercial type forge, I'm not sure what safety features it had, but I commend you for doing a fabulous job of setting in safety features on your forge.


On this side of the pond, I believe most places run house side pressures of about 4-7" of WC. 

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Thanks Torin.


I finally fired up the forge with the new safety features and the upgraded plumbing.


The safety cut outs all work fine which I am very happy about.


Previously my gas plumbing was a bit of a mess (see post #2) with some section of the plumbing as narrow as 5/16" in diam.

Now I have 3/4" pipe all the way from the main line to where the gas line splits into 2 x 1/2" pipes to each of the torches.

I only measured the gas consumption for the front torch and it consumes nearly the same as the two torches together did previously.


I'm still working out optimum settings but it got a piece of metal up to 1200ºC using just the front torch and then my pyrometer started playing up so I couldn't measure any higher than this

It must have got quite a bot hotter because the forge doorway surround started to glowed hot.


I had some minor surgery a couple of weeks ago but I'm not allowed to lift anything heavier than 5 lb or engage in anything strenuous for another 3 weeks!

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Tagged for reference.


 Nice job. Eventually I'd like to do a natural gas forge in the garage since my gas line is right on the other side of the wall for the kitchen, and if I enlarge the garage like I want, the house connection would be right next to the new back wall. I never thought much about what to do if the blower died. A remote gas cut off will definitely be in the game plan so I can shut the gas off inside the garage, but away from the forge location, and I'll have to study your pitot tube design in greater detail when we get to that point.

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Thanks Phily.


Another approach (or even addition to what I have done) to safety switching, is to power the solenoid through an RF remote  switch, which consist of a small RF receiver/switch, which plugs into a mains socket and the solenoid power supply plugs into the RF unit. The operator would then need to carry around with them the remote RF sender. While this is a much simpler arrangement and RF switches are relatively cheap, you would need to carry the RF unit with you at all times and the last thing I would want to be doing whilst holding a red hot piece of metal is be fumbling around, or looking, for the RF remote. I also don't like to rely on things that use batteries.

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Outstanding work all around. The safety shutoff is a great feature but the fireball from losing the air blast is typically not that bad (unless you are in a shop with low ceilings). I've had it happen several times with propane and it tends to reach a certain size then stops expanding because the fuel is burning in atmosphere. If you haven't already done so, consider letting the forge to reach forging temps with the NG at normal line pressure and purposely turn off the air so you can see what it will do under those circumstances.

I also like your avatar - is that your dog?

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Unfortunately my shop is very crowded and I do only have 8ft ceilings at the end where the forge is located. The low ceiling does spread the flame further than I'd like. Everything at that end of my shop is relatively flame resistant. Walls/roof are all metal and insulation is rock wool and I have very good ventilation but I don't especially want to be caught in the flame.


Yep that is one of our dawgs.

They are Border Collies.

The bigger one is 2 and the other one is 11 months.



This pic was taken when the younger one was only 3 months old.

They most conveniently love sleeping around the base of my anvil.


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Love those Border Collies - yours are gorgeous. We've also owned several Blue Heelers - all great dogs...

Good luck with the forge and stay safe.

Thanks HW, Blue heelers are great dogs.


The bigger one was not so loveable this morning when I took them for a run down by the river and she managed to find and roll in a large deposit of human excrement and got it all over her back and head and even in her eyes!!! I gave her the cold fire hose wash when we got home - she doesn't like that but it won't stop her rolling in smelly stuff again. She's also partial to rolling and eating fish that fishermen leave behind on the river bank. Several times they were almost certainly toxic blowfish  which required expensive visits to the vet for a stomach pumping.

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Gotta love dogs, Deb's a big border collie fan, I'm more partial to Aussies but love dogs in general. Right now we have a few, the first registered pedigreed dog ever, Falki is Icelandic sheep dog, then there's Abby, half Icy and probably/maybe something else, then there's Baxter our first doxy. Pocket our old girl, (16 next spring) half mini doxy half chihuahua and last but not least journey a Great Pyrenese mountain dog.


Everybody but Pocket and Falkiare are rescues. We don't let them run free so they don't get to roll in aromatics. Poor deprived dogs. <sniff>


Are you going to do any sport training? Border collies are super competitors at agility and such. Falki loves agility and free style while Baxter is the nose work rock star at the facility. I think it's because he has such a high nose to body ratio.


Frosty The Lucky.

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Are you going to do any sport training? Border collies are super competitors at agility and such. Falki loves agility and free style while Baxter is the nose work rock star at the facility. I think it's because he has such a high nose to body ratio.





We did two terms of puppy level agility with the bigger dog (Willow) but then we switched to sheep herding which she loves and is a natural at. We live less than a mile from city centre and it takes us about 50 minutes to get to the sheep herding farm so it is a bit of a commitment. The younger dog (Skye) is a pedigree sheep herder with champion sheep herding parents but she has shown no interest in sheep so far and is much more interested in the other dogs than the sheep although it's been 3 months since we have tried her out and now that she is a bit older we will give her another try. Even though she is more than a year younger Skye is the dominant dog but she is also more obedient and really wants to hang around with us. She is also very agile, she can leap into the air and catch a full size Aussie rules football or soccer ball with her paws and teeth. Willow knows a lot more tricks because we have spent more time training her, she is also more gentle and submissive and great with kids, but less obedient and sneakier than Skye, and tends to wander off and get up to mischief - and of course rolling in smelly things. 

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