DIY charcoal furnace

A DIY charcoal furnace that can make its own charcoal from waste wood, pre-fire and fully fire DIY ceramics.

I don’t mind a little ambiguity in a title. I can keep it short and have the reader question is this a device that makes charcoal or uses charcoal? In this case it is both and it might even cook my dinner or smoke my eels!

When it eventually reached the melting point of of bog-ordinary whit student clay I decided to call it my Nebuchadnezzar furnace on according to the biblical story where Nebuchadnezzar supposedly made the furnace “……..one seven times more than it was wont to be heated…….”. Don’t let the facts or physics get in the road of a good story, but in my case the furnace survived (and needed a little refractory patching) and I reached my target of melting white clay!

Mothy The Elder

Introduction to the DIY charcoal furnace

This post is under construction.

I play with fire and where possible like to use waste wood from fallen trees for my fire play. I have probably grown too many trees and now have excess wood that is not required to heat my home, and it would otherwise be burnt in the paddock.

Charcoal from such wood when blasted with forced air achieves the highest combustion temperatures from this modest fuel for firing DIY ceramic items.

The DIY charcoal furnace design

I hoped to make a furnace that could also be used to make the charcoal from waste wood. This would mean that the heat from its pyrolitic conversion to charcoal could produce modest temperatures that can be used for other purposes such as the long and steady preparation of the ceramics for their final firing.

Other heat using processes such as metal melting, slow cooking and even meat smoking may be useful byproducts. An additional bootstrap bonus would come from making the furnace from a refractory mix that is essentially soil. “It will be fun to see if the furnace can fire its own walls as in this little video?”

Prototype testing. I made a little prototype furnace out of four house bricks, soil and a sheet of metal and during the sustained charcoal combustion phase, the furnace reached 1250C. It had many leaks, poor insulation provided by a sheet metal top cover and limited forced air supplied by a small USB blower. I presumed that I could achieve much better temperatures with a well-designed furnace with minimal leaks, good insulation and the air injection of a powerful vacuum cleaner. Here is a little video of the prototype in action.

Furnace lining testing. During the test of the prototype furnace, I also put in a sample of my proposed DIY insulating furnace liner to see how it coped with the rigours of that roaring hell hole. It was a mix of perlite, finely ground loam soil (clay free) and sodium silicate. For more details please my post on DIY refractories from sodium silicate.

It survived, with a bit of cracking, but it had some bird wire mesh embedded in it. It became very light and developed a glassy texture. It may need another final coating to lessen the glassing if the furnace achieves considerably higher temperatures. I applied a talcum powder render to each side of the test sample. This seemed to prevent excessive glassing that I have observed in previous tests. It does this by raising the melting point of the surface.

Carbon dioxide curing. I often use sodium silicate to make refractories and the curing of items prior to full firing benefits from modest temperatures between ~200-300C to remove chemical bound water.

Also, a high concentration of carbon dioxide in the air accelerates the conversion of silicate back into silicon dioxide (in preparation for conversion into a glass-ceramic at high temperatures). Such heat and rich carbon dioxide vapours are made while wood is being pyrolysis to charcoal.

I subsequently found that the furnace could easily reach 200-300 C near the top of the burn chamber during natural draft burning of wood to make charcoal. This should provide sustained drying/curing conditions prior to the high temperature firing in the charcoal.

Weatherproof and mobile. It would be best if the furnace could be weatherproof so that it could be kept safely outside, maybe with a rain hat on it. Being mobile would mean that it could be shifted if it got in the way or could be taken to a place that was most appropriate for its potential uses ( For example, a smoky charcoal maker or a quiet sneaky eel smoker). Consequently, a solid round concrete base seemed like a good starting point on which to form the barrel-shaped refractory furnace wall.

The charcoal furnace base

Portland cement base. I made a cylindrical sheet metal former (bottom and topless) to cast the furnace base in. I welded ~40mm dia elbow air pipe that I cast into the middle of the base. The internal pipe exit was pointing upwards to a standoff with a small replaceable grill wire fitting that would prevent large charcoal chunks from falling down the pipe.

The plan was to use this air inlet as a restricted air entry device for charcoal making and carbon dioxide curing, using a natural stove pipe draft. Alternatively, it could be used for a modest forced air injection from a small USB fire blower (as shown in some photos below where refractory render was cured with flames and carbon dioxide). Lastly, for the final high-temperature ceramic fusion, it could have forced air from a mains-powered vacuum cleaner. “The type of vacuumed cleaners that are thrown out everywhere because they have been made redundant by the Dyson style revolution.”

The lower portion of the casting was done with portland cement and sand mix with some reinforcing wires and light mesh to protect against cataclysmic cracking of the base. I do not expect it to get so hot as to decay the Portland cement.

High-temperature insulating cement. Next, I added a generous layer of perlite mixed with high-temperature cement and water. “The high proportion of perlite in the cement makes it an odd mix as the perlite just floats around on the cement powder, like rice bubbles and sugar, until some small amount of water is added.” It poured and packed well and could be finished like normal cement with generous trowling. The perlite granule behaves like a small aggregate. I also have left a provision for the addition of an extra hot face render if this should ever be needed. However, my experience with very hot blower stoves is that heat does not willingly go downwards through an ever-present layer of ash. Time will tell.

The furnace wall. I don’t know what to call it, but the wall will do. It will be the insulated enclosure where the wood fuel will go, the charcoal will form and the ceramic creations will be bathed in wood gas flames and then radiated with the intense heat from forced-air combustion of charcoal.

Side port for a stove pipe. I hope to form a port for a stovepipe so that it can be used to create a natural draft for slow combustion when the highest temperatures are not required. This would mean that the dome could be capped with a simple round refractory lid. Alternatively, a slow-cooking pot or a smoker could be heated on the top.

Furnace wall. I made the wall quite tall as a gently tapering conical shape with a larger opening at the top. The capacity is much larger than anything that I am likely to fire. However, I hoped that it could be loaded with a big load of rough wood that could pyrolyse to charcoal and fall down over time to replenish the charcoal that would be nearest to the air blast nozzle.

The inside mold for the dome was is formed from a 22L cooking oil drum that I slit in four places. I put the drum back together so that it was tapered and it could easily be extracted after the dome was rendered with insulating refractory. A plastic sheet was wrapped around the cone so that it would release easily from the render when required.

The charcoal furnace dome wall. The innermost layer of the dome is a DIY refractory made from loam soil (essentially clay free), sodium silicate and perlite to make it insulating and quite lightweight (as described above. The layer of light galvanized wire will hopefully protect against cataclysmic collapse.

” Just as I had finished mixing the first batch of refractory render, the rain started, as it does. So I quickly tried out the portability of the charcoal furnace by slipping it onto a sack trolly and it was delightfully easy to shift undercover.”

The render applied very easily, but it started to slump if it was thicker than about 15-20mm. I decided that a second layer of the same refractory mix would be required for good insulation. It would also provide a convenient opportunity to add a second layer of reinforcing mesh in the form of a spiral of bird wire. Anyway, a considerable amount of drying/curing time will be needed to develop some strength before the next rendering. Or would it?

When to remove the dome forming mold?

I could remove the forming mold after the render had dry-cured but then it may shrink and crack badly or get stuck on the mold. On the other hand, if I removed the mold while the render was ‘green’ I would avoid the shrinkage and stuck on scenarios. Removal of the mold early would greatly increase the drying rate and the silicate render has the capacity to heal while it is green. I took a punt and removed the mold while the render was in this delicately firm green state. ‘It looks like that gamble paid off”

I was tempted to try to fill some little hollows that were left in the render by bulges in the plastic film. However, I poked my finger through the wall while doing so and gave that up as a bad idea at this stage.

To immediately fire to hasten the cure or not? Having given up on the cosmetic repair I felt that luck was still with me. I decided to rapidly hasten the cure of the green render by setting a small fire going inside it. “After all, that is what it’s for!” Such heating with fire accelerates the cure by drying it out and greatly increasing the carbon dioxide concentration that is involved in turning the silicate into a form of silicon dioxide in preparation for high-temperature conversion into glassy silica.

The firing of the render reached 550C (max temperature for the thermometer) near the top of the fire dome and the lower zone was over 550C. This temperature combined with the high carbon dioxide made the inside of the furnace solid and hard. Here is a little video of the firing.

These temperatures mean that it is a good preliminary cure that makes the render ready for full ceramic firing. “This little video shows the burning of the render and its transformation to a hard finish. This is just the ceramics conception, its real birth is at a much higher temperature when it becomes a glassy substance. That will happen while the furnace is put to use.

The next outer layer was going to be portland cement and sand mix with perlite. However, I thought that an extra layer of soil silicate perlite was needed for a better depth of insulation. “Anyway, it is dirt cheap!” The final outer layers of portland cement will provide extra strength and abrasion resistance and will be well protected from heat by the inner layers.

Refractory render coating the furnace hot face

I think I was a bit hasty with my firing of the first refractory layer while it was ‘green’ or not dry-cured. “It certainly did not have that stiff meringue texture of the test piece shown above.” It was a bit crumbly and would not be strong enough for rough wood fuel loading. Consequently, I made a soft fine render out of talc (baby powder) and sodium silicate (40g silicate: 40g talc: 80g water). It was deliberately silicate rich and quite ‘runny’ and I applied the render very gently with a brush to the cracked and crumbly surface. The moisture and dissolved silicate rapidly soaked into the original surface and the talc started to fill the cracks and holes. The surface that was rendered in this way rapidly became very stable and the render firmed up quickly.

I made more of the render and was able to apply a second coat immediately and this left a smooth, strong and dense surface. “This time I would not be in any rush to fire the ceramic coating.”

Another refractory furnace hot face coating

Given the surface improvement of the talc/silicate render, I thought that an extra thicker hot face coating would be good. I was running low on baby powder, so I used some garnet powder (waste grits from the sump of a water jet cutting machine). “From experience, I knew that it could make a refractory as hard as a knife sharpening stone.” So the refractory mix was (40g silicate: 40g talc: 400g garnet powder: 100g water).

This was a very thick mix that I applied generously with a brush so that the white colour of the talc render was completely covered. This mix became stuck in the brush bristles as the liquid phase was drawn into the surface while brushing Consequently, I used two fills ~100g of water in a second pot to soften and remove the render from the brush. Steadily, these extra pots of water became like the render and I used it to wet or prime the surface before quickly applying the final render. In this way, no component of the render was wasted, it all went into or onto the hot face.

The charcoal furnace outer surface

To make the outside of the furnace strong I applied a thick render of sand and Portland cement. This render was applied through a layer of light gauge galvanized bird wire. The last render will be white cement with ‘damp-course’ waterproofing added to the mix. “This last render would be used just for the sake of good looks and vanity and because I had some leftover in the workshop and it makes ugly cement look great just like the houses on Greek islands do.”

Who forgot to build in the ports for the furnace thermocouple probe?

The rendering went so quickly that I forgot to add the thermocouple ports. I had made three little stainless steel tubes that I was going to embed through the wall of the furnace so that I could insert a cheap thermocouple probe to monitor temperatures. One up high where I might do cooking and smoking, one in the middle in the pyrolysis zone and one in the high-temperature charcoal combustion zone. Never mind, this refractor can be drilled very easily when cured, so I can still add the ports and continue to render around them.

Here is a photo of the retrofitted thermocouple ports. After drilling the holes with a masonry drill, I packed the stainless steel port tubes for the thermocouple into the wall with my miracle aluminium foil and sodium silicate expanding refractory. “It is magic stuff as it bubbles and foams and expands all by itself when stuffed into a small place and becomes a fire-resistant refractory the just laughs at fire.”

Functional temperatures for making refractories

In the end, I settled for only two temperature monitoring ports. I found that the furnace achieved 200-300C up near the top with a natural draft where new refractories could be gently dried and cured while making the charcoal for the final firing. The zone at the location of the middle port (shown in the above photo) reached about 1050C which according to the above Aprovecho post should provide adequate firing for stove making refractories. I aborted the idea of a port low down in the charcoal bed, as I expected the temperature to go way beyond the 1300C limit of my K-type thermocouple.

The photos below are of ceramic test pucks that have been fired in the furnace. The pucks are almost invisible at the highest temperature in the first photo, just after lifting the furnace lid. As the furnace cools a little, the pucks start to be visible.

Here is a little video of the thermocouple readout while the furnace was running and here is another at night time.

After looking at this furnace hell hole, I think it is time to think of our brave brothers and sister of Ukraine:

Amazing, to act up with anger and frustration,
When you trick your youth to invade another nation,
The Goliaths army undone by its own belligerence,
Against Davids tenacious homeland defence,
Also, remember the cost to your nation of the eventual war reparation!

Who forgot the ash removal port?

For someone who plays with fire so much, I should not have been surprised to find that it is a bit tricky removing the ash from such a furnace. I probably will cut an ash-out port and make a ceramic plug for it in the fullness of time.

Previously the ash accumulation happened when I used the furnace with a natural draft or with a tiny USB fire blower. Luckily, when I fired up the furnace with its full forced air blaster made from an old ‘double-ender’ Electrolux vacuum cleaner. “Try doing that with your Dyson!” The blast was so strong that there was no significant ash to be removed as it just left a little volcanic-style cone of ash around the air entry port. “A nice insulative coating for the next use.” Here is a little video of the blast furnace at full power.

Conclusion

This DIY refractory project has shown that a charcoal furnace can be made with soil and sodium silicate. With patience, it can be used to fire itself to become an insulating and heat-resistant refractory. It then will be able to conveniently cure and fire other refractory items at high temperatures using waste wood from fallen branches.

Tim

Addendum

After a little bit of trial and error, I have managed to get the furnace up to a sustained temperature that can fire ‘white student clay’ to make it fuse properly and even start to melt and slump a little.