How to Make a Mini Forge




Introduction: How to Make a Mini Forge

About: I'm not an expert in anything. I just enjoy making things sometimes for the process sometimes for the end product.

In this Instructable I show you how I made a two brick mini forge for heat treating knives. I recently started delving in to knife making and part of the process requires the ability to heat treat the metal. I have seen other tutorials were they use a hole saw bit to make the hole. I chose to go a simpler and less expensive route. Instead I used a hacksaw, a flat head screw driver and a rasp to make and define the hole. I also used furnace cement to adhere the two halves together. From what I understand this isn't absolutely necessary but for me it was worth the extra $5.

Step 1:

You need to buy three fire bricks (I'll speak to why you need three later) they sort of remind me of pumice stones they are very light and brittle. It doesn't take much to break or crumble these bricks which makes them very easy to work with.

Step 2:

The first thing I did was place one brick on top of the other and found the center. I then looked for different circular objects that I could use to draw a circle on the bricks. I ended up using the inside diameter of a roll of blue tape and ironically a 2-1/4" hole saw bit. I know I said you don't need a hole saw bit for this project which you don't this is just what I used to draw the circle. You can use a smaller roll of tape or a drinking glass. Anything that is a circle about this size will work. What you end up with is a larger hole on one end and a slightly smaller hole on the other end. This will result in a slight taper to the hole. Don't frit this doesn't have to be perfect close enough will work.

Step 3:

Here I am connecting the front hole to the back hole using a ruler. You can see the resulting taper in the last pic.

Step 4:

Now using my hacksaw I start to make the cuts along the length of the brick I made them about 1/4" apart but I was just eyeballing it. This material cuts very easily this took me less than 5 minutes to cut. And it only took me that long because I was cutting carefully to the radius of the lines. I would start the cut and then check how close I was to touching the marker lines on the front and back. Here again I would like to stress that this doesn't have to be perfect you can over shoot the line slightly just pay attention as you cut. You just don't want to cut all the way through. Also I would like to mention that I did were safety glasses, gloves and a respirator as I read that this stuff is not very healthy so wear proper protective equipment. Cutting the brick produces a very fine powder.

Step 5:

Once youve made all your long cuts take your flat head screwdriver and wedge it in between the cuts. Use it as a mini pry bar to break apart the pieces. The brick offers no resistance and the pieces break out very easily. This is also another reason I decided to use a hacksaw it seemed a lot less messy.

Step 6:

Once all the big chunks are removed I used my rasp to shape and refine the hole. You could probably use a regular rounded file for this part. You don't want to be too aggressive with the rasp just use light even pressure. I used the marker lines as a guide for the shaping I tried to remove material up to the marked line.

Step 7:

Next I turned the bricks over and lined them up to see what areas required more refinement. I was trying to get as close to a circle as possible. It doesn't have to be absolutely perfect just a roundish shape is good.

Step 8:

I used furnace cement to bond the two bricks. I will be using a propane torch as the heat source which is not capable of reaching the 2700 Fahrenheit degree limit of this cement. Read the instructions on the container for application and curing as other brands may be different.

Step 9:

Per the instructions I applied the cement to the joints.

Step 10:

I also applied some to the sides of the joints as well. Once I was done I put a paint can on top of the bricks while it cured. Per the instructions on the furnace cement I waited one hour for the cement to cure.

Step 11:

Per the instructions on the furnace cement to finish curing you have to gradually bring up the bricks to 500 degrees Fahrenheit. So I took them out to my gas grill and turned on one burner waited for it to get to temperature then turned on another burner and again waited for it to get to temperature then turned on the third burner. The temp gauge on the grill was displaying 500 degrees Fahrenheit but the temperature of the bricks was actually closer to 300 degrees Fahrenheit. So I placed the bricks directly on top of the 3 burners and using my laser thermometer checked the temperature. Once it reached 500 degrees Fahrenheit I turned off the burners and closed the lid and let the bricks cool to room temperature.

Step 12:

Next I used a drill bit that matched the diameter (3/8 inch) of the propane torch nozzle I would be using and drilled an angled hole about 1 inch from the front of the forge mouth. The drill bit will go right through the brick so you don't have to use a lot of force, these fire bricks are really easy to work. The torch tip I used is angled itself so take that in to consideration when you drill your hole you don't want the angle to be too severe or make for an awkward position. I laid my torch nozzle on top of the brick to get an idea of the angle I should shoot for and then drilled the hole.

Step 13:

Now it was time to test it out. This is were the third brick comes in to the picture. I used the third brick standing up to close off the backside of the mini forge. It can act as a sort of regulator. I still need to experiment more with this but basically if you keep the exit hole closed off the mini forge won't get as hot. If you angle the third brick so that air can get in (refer to the 3 aerial shots for clarification) the forge will heat up more, like I stated I still need to experiment more with this to find the sweet spot. I have found that opening and closing changes the sound of the forge. Closed its not impressive at all but with the third brick slightly open it sort of sounds like a rocket engine, at least to me it does. The cooler the sound the hotter the forge.

Step 14:

I used a piece of mild steel to see if I could get it red hot. You can see in the pic I got red hot, I don't know how long it took since I was trying different angles with the third brick but it did happen quicker than I thought it would. The last pic shows the after glow once I turned off the propane torch its not relevant I just thought it was cool. I also wanted to mention that the torch head can make a difference in how hot the forge will get or at least that's what I have been told. I have only tried one torch the Bernzomatic ts3000 and it worked for my purposes. I have successfully annealed a piece of steel and heat treated a knife with this mini forgre and torch combo. I have also experimented with making mokume gane were I forge welded 8 quarters together. Overall I am very pleased with what this mini forge is capable of and in the future I will probably find a way to wrap it with metal so that it will last longer. The bricks are very fragile and are prone to chipping so a makeshift bracket of some sort may be in order. I also wanted to mention that when in use the outside temperature of the bricks gets to the range of 250-375 degrees Fahrenheit so make sure its not sitting on top of any combustible material. I place mine on top of my old BBQ grill doors which dissipate the heat. I also keep a fire extinguisher near by just in case there are any mishaps.



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    52 Discussions

    Where did you get the fire brick? and how much is it?

    2 replies

    I got them off of Craigslist. You can order them online they are called "fire bricks" or "kiln bricks".


    1 year ago

    Would this be better called a mini forging "oven" or "furnace"?

    6 replies

    Not sure I looked online and there's no clear answer. They seem to be interchangleable.

    Thanks for replying. FWIW, I think people are beginning to use words wrongly, and it spreads easily. BTW, there is hot and cold forging. Things are heated up in a furnace for hot forging.

    Seriously, the only one using words incorrectly here is you. It is a forge, it has ALWAYS been a forge! This is very well documented throughout history for petes sake! You use a furnace to remove the ore metal from the ore befor e you get to the forging process which is from the FORGE. Simply do a real bit of research into the history of blacksmithing and ore processing.

    "Hot forging and cold forging are two different metal forming processes that deliver similar results. Forging is the process of deforming metal into a predetermined shape using certain tools and equipment—deformation is accomplished using hot,cold, or even warm forging processes."

    Oven: A chamber or compartment, as in a stove, for baking, roasting,heating, drying, etc.

    Furnace: a structure or apparatus in which heat may be generated,as for heating houses, smelting ores, or producing steam.

    So you can copy paste, thats good. Either way what you copied and pasted only proves my point. You heat the metal in a forge, and then the act of moving that metal into the shape you want it is called forging. I do not understand what your issue is in understanding this, unless English is not your first language. The oven is used to make Coke, which is the fuel used to smelt ore. The impurities in coal are driven out by heating it in an oven, see Beehive Coke Oven. This turns the raw coal into a higher grade fuel, known as Coke. Coke is then used in a furnace to drive the metal out of the ore, as well as to burn out any of the impurities in the base metals, see Blast Furnace for a reference. Finally, after the ore has been smelted into usable material for the forging process it is put into an actual FORGE where it is heated and handled while the final forging process is taking place. Seriously, none of this is hard to find or research, and your assumption of people miss using the term forge is very VERY incorrect. Educate yourself on the process if you are going to comment on it in such a way.

    There's no need to excited. Yes, copying and pasting is how information is relayed. My point is that the significance of forging is obtaining grain structure for strength which can be done in cold forging also. Reheating hardened steel in a "forge" doesn't make sense then. Does it to you? It's heated in an oven even though everybody calls it a forge. No, English isn't a second language, and this contention comes from the processes learned in a Bachelor's of Science in Mechanical Engineering and metallurgy. Seriously, you don't need to try and berate. Educate your too.

    The material you are using is also know as hebal because I use it to msake scuptors

    reminds me of something my dad did...

    1 reply

    Dude are you for real? Are you really John's son? Dude your dad is one of the many content creators that inspires me, I've been subbed to him for awhile. I mention him in my video description for inspiring the 4 brick forge. I made his version too but I cemented them together so that I could move it around easier. I didn't film it but I also ended up rounding out the hole of the forge so I could have a little more room. It also works great. Also his "Furious POV' vids are hilarious.

    Great instructable. Just what I was looking for to forge railroad spikes.

    I won't argue with the experts that equate MAPP gas and propane combustion temps, but my personal experience with both suggests that MAPP gas IN USE burns WAY hotter than propane. While it costs significantly more here in AK, I have completely abandoned using propane canisters unless I am concerned about overheating something. The time-to-melt (solder for sweating joints) or time to cherry red (annealing brass) is close to half that with propane when using the same tip. I have to be careful not to melt the brass with a MAPP gas canister, when I have to WORK at melting brass with a propane canister.

    5 replies

    Have you used MAPP gas recently? I was actually just thinking of building a tiny foundry using similar methods to this instructable and thought about using MAPP gas, but read that the original mixture is no longer sold for some reason and it's instead a substitute. I would like to know if the new stuff still burns hotter than propane, even though it would only be a few bucks wasted just to see.

    It does burn hotter, but only by 40•f. Just doesn't justify the difference in expense. But a plus is that it burns cleaner and it can be turned down to a much lower flame (and temp) than propane.

    The "new" MAPP gas, which ISN'T MAPP gas but costs as much as the OLD, GOOD, MAPP gas cost, does indeed not burn noticeably hotter than propane. Extra cost is NOT justified in my opinion, and is purely marketing (deceit) riding the coat-tails of the "good stuff" which is apparently no longer available.

    When you get as old as I am, you get tired of seeing GOOD, WORKING, products discontinued or replaced by INFERIOR products. Especially when the INFERIOR replacement ("upgrade" or "new and improved"), are sold at a premium price.

    To the point of this Instructable: Use propane. Do NOT be fooled into purchasing the "new" MAP/Pro. The old genuine MAPP gas, while a superior product, is sadly extinct.

    The last time I BOUGHT a canister of MAPP gas (as opposed to actually USING it) was a couple of years ago, so if the change occurred within the last couple of years, I can't help you. Sorry.. I hadn't heard about the mixture change. I'll have to look into that.

    I wasn't able to find this information before, but apparently MAPP Pro(i.e. fake MAPP gas) has a flame around 3,600 F, which is only 150 degrees hotter than propane. I guess if I had to choose one, I'd still go for MAPP Pro and maybe that extra 150 degrees may help with some metals.

    Thanks for your reply! And yeah, I totally get you about "upgrades". I avoid them whenever I can.