How to Make a Mini Forge

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.

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.

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.

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

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.

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.

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.

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.

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.

Per the instructions I applied the cement to the joints.

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.

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.

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.

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.

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.