After seeing the Japanese forge running on used engine oil I was very keen to try my version of it on this little forge. The principle was very simple. If you spray oil into a hot forge it will burn very well without any smoke or smell. I found that a litre of diesel in four litres of used engine oil made it run much better than just straight engine oil (Image A).
The oil spray is made using two 0.9 mm MIG-welding tips (B). The fixed bottom one is the oil supply, which goes into a four-litre container of used engine oil. The top tip blows compressed air across the oil tip creating a vacuum that sucks the oil up from the container. It is adjusted by the screw so that the distance from the bottom tip can be varied. By raising or lowering the tip, more or less oil is put into the furnace and this controls the heat. I ran it on about 20psi of air pressure but it will take more if required.
The secret to getting a clean burn from the start is to ensure that the furnace is hot before introducing the oil spray, so I mounted a small home-made LPG burner on the side of the oil burner. I used the same jet assembly as I used in the forge in a much smaller burner tube. The burner tube is 12mm ID with a sheetmetal flare on the end (C–E). Easyflo was used to fix it in place. Four 6mm holes were drilled in the pipe for air to mix with the gas.
When the furnace is hot the air to the oil burner is turned on and the gas burner can be turned off.
It is a little bit tricky to get the air and oil nozzles in the correct position for good suction but once everything is adjusted it works well. You can watch a video of it operating on The Shed website. A friend in Australia has the forge now. He is using it to heat up railroad spikes to make into knives. Prior to getting the oil-burning forge he was using LPG but was finding it just too expensive.
flare taper in the brick. The gap around the burner should be packed with some Kaowool to prevent air entering (40).
Open up the front and rear forge doors. Important: Always do this before lighting!
Close up the choke till it is just level with the top of the intake reducer, turn on the gas slightly, and light the furnace using a barbecue spark igniter.
The burner should now be running with a very soft yellow/blue flame. Carefully adjust the choke to admit more air until the flame is a strong blue colour. To minimize scale formation on steel it is important to have a neutral flame running in the forge (41). We have measured the internal temperature of the forge after running for 10 minutes at 1300°C (42). I mounted the forge on a mobile stand made out of some old Dexion angle framing that I had lying about.
Cylindrical forge running on used engine oil
The cylindrical forge that I made uses the interior steel casing from an old mains pressure water cylinder. The steel casing is 3mm steel plate with a glazed lining. The first job is to remove the outer sheet-metal skin off the cylinder to expose the insulation (43).
If you are lucky you may strike one that has loose glass-fibre insulation, which is easy to remove. If not you may get one that has expanded polyurethane foam insulation, which will take a lot more work to get off. In any case a dust mask should be worn while removing it. The cylinder I had had a 300mm diameter internal steel casing. Using a cutting disc in an angle grinder, I removed the top and bottom to give me a 400mm long cylinder (44).
I cut and welded four 50x50x6mm angle-iron feet to the cylinder and also welded four lengths of ½-inch water pipe internally. These function as guides for the extendable 12mm rebar work rest (45).
In the side of the cylinder, about 100mm down from the centre line, I cut an 80x80mm square hole. This is where the gas or used-engine-oil burner will enter.
The cylinder is now ready for installing the insulation. I used 25mm thick Kaowool and 25mm thick hightemperature lightweight insulating board. ‘Kaowool’ is a ceramic fibre insulation capable of withstanding more than 1200°C. Kaowool is not considered to be hazardous but a dust mask should be used when handling it.
I first cut some of the insulation board to fit between the water pipe. The purpose of that was to give support to the insulating board that was going to serve as the floor of the forge. The spaces between the supports were filled with Kaowool (46).
When that was done a larger piece of insulating board for the floor was cut and fitted. I secured it by welding four tabs on to the forge wall (47).
With the base in place the Kaowool could then be installed. By carefully cutting it to length it was selfsupporting when pushed into place against the floor. I put in two layers to give 50mm of insulation, which left
me with approximately 200mm internal diameter (48).
A 60mm diameter hole was cut in the Kaowool for the gas/oil burner port.
If Kaowool is exposed to the direct flame of a burner it will erode away, releasing fibres into the atmosphere, so it must be coated with a refractory to prevent that happening. I got what is known as a ‘rigidizer’ from the supplier of the Kaowool. This is a colloidal silica compound that you paint onto the Kaowool. When it dries and is hardened with the heat of the furnace, it puts a thin, eggshell-like coating on the surface.
A 12mm rebar handle was made and welded to the body of the forge. The end walls were made from the lightweight insulating board fixed to 12mm rebar. These were made adjustable so that the right gap could be determined to prevent the burners from being restricted (49).
The work rest was bent up from 12mm rebar. I did not install a locking screw because friction between the bar and pipe was sufficient to hold it in place.
A 100mm long x 12mm rebar rod was welded to the side of the forge for mounting the burners.
At the Halswell Menzshed we have a number of enthusiastic knife makers. Ian has made a forge that uses two LPG gas burners, and we have also experimented with using diesel to fire a forge. It ran very hot but the noise level was over 100dBA (50).
Another member, Ross, has produced a number of knives and his furnace is a very simple pile of bricks that he quickly assembles and dismantles as required. He uses an LPG weed burner for heat. He has produced knives from old files beaten into shape and carefully ground while others are made from old hand shears (51).
With the new brick forge up and running, I suspect that we will see a lot more creative knife-making in the shed.
‘Kaowool’ is a ceramic fibre insulation capable of withstanding more than 1200°C
A I found that a litre of diesel in 4 litres of used engine oil made it run much better than just straight engine oil
C–E: The secret to getting a clean burn from the start is to ensure that the furnace is hot before introducing the oil spray, so I mount a small homemade LPG burner on the side of the oil burner E
B The oil spray is made using two 0.9 mm MIG-welding tips
42 We measure the internal temperature of the forge at 1300°C after running it for 10 minutes
40 The gap around the burner should be packed with some Kaowool to prevent air entering
41 To minimize scale formation on steel it is important to have a neutral flame running in the forge
43 The first job is to remove the outer sheet-metal skin of the cylinder to expose the insulation
44 I removed the top and bottom to give me a 400mm long cylinder
45 These function as guides for the extendable 12mm rebar work rest
49 The end walls are made from the lightweight insulating board fixed to 12mm rebar
46 The spaces between the supports are filled with Kaowool
47 I secure it by welding four tabs onto the forge wall
48 Two layers in place to give 50mm of insulation
50 This diesel-fired forge runs very hot with its noise level over 100dBA! Ear muffs will be required
Knives made from old hand shears and files 51