Primitive Technology: Natural Draft Furnace

Primitive Technology: Natural Draft Furnace

I built a natural draft furnace to test ideas about how hot a furnace could get without the use of bellows. Natural draft is the flow of air through a furnace due to rising hot air. The hot gasses in the fuel bed are more buoyant than the cold air outside the furnace causing them to rise. Fresh combustion air then enters the base of the furnace to replace the rising combustion gasses, keeping the fuel bed burning. This effect increases with: 1. the average temperature of the fuel bed relative to the outside air and 2. The height of the furnace. Two other important factors are the size of the tuyere (air entry pipe) and lump size of the fuel bed as these effect the resistance to airflow through the furnace. The furnace was tested with wood fuel and some ore was melted but produced no iron. High temperature were indeed produced (probably about 1200 c). These types of furnaces were once used for smelting copper and iron ores in around the world in ancient times, usually using charcoal as a fuel and in some cases wood too.
I designed the furnace using a formula from the book β€œThe mastery and uses of fire in antiquity” by J.E. Rehder. It was designed to have a space velocity (air speed within the furnace) of 6 m per minute which is recommended for iron smelting. The furnace was 175 cm in total height but with a height of only 150 cm above the tuyere. The height between the air entry and the top of the furnace is what determines the strength of the draft, the space beneath the air entry is not included in the formula. The internal furnace diameter was 25 cm. The walls were about 12.5 cm thick at the base but got thinner with height. The tuyere (air entry pipe) was 7.5 cm internal diameter and about 20 cm long. The tuyere was placed into an opening in the base of the furnace and sealed with mud. The whole thing took about a week to make due to the slow drying time that was assisted by keeping a fire burning in side it. The furnace was designed to use charcoal (which in this case should be 2.5 cm diameter lumps) but I used wood to test it instead as it was easier to acquire. To test its melting ability, bog ore was found further down the creek and roasted. The roasted ore was then crushed and stored in a pot.
The furnace was filled with wood and lit from the top. The fire burnt down the furnace producing charcoal. On reaching the tuyere the fire then started burning the charcoal. Wood was also continually added from the top along with a few small handfuls of the roasted bog ore (not shown in the video). The temperature of hot objects can be visually estimated from their incandescence. After about an hour, the light coming out of the tuyere was high yellow to white hot indicating a temperature of at about 1200 c. Colour temperature charts vary but white hot is usually given to be at least 1200 c, examples of these charts can be found on the internet for reference. It was uncomfortable to stare into the tuyere and doing so left an after image when looking away, indicating the strength of its brightness. After about an hour and a half the furnace was left to burn out. When opened the next day the tuyere was covered in slag with bits of slag found on the furnace floor also.
This experiment shows that high temperatures can be achieved without the use of bellows or charcoal, which might significantly reduce labour in the production of iron. The furnace was technically easy to build as it was a simple vertical cylinder. When running, the wood added to the top of the furnace converts to charcoal in the upper part of the stack and is consumed in the lower part. The ore I used was new to me, normally I use iron bacteria as an ore. This new ore produced no metallic iron so I’m inclined to use iron bacteria in future. Natural draft furnaces were once used to smelt copper and iron ores in the past, usually with charcoal fuel and less frequently with wood. The main benefit of these furnaces seems to have been the reduction in labour they provide and simplified infrastructure (fewer workers and no bellows required during operation).

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25 Responses

  1. Primitive Technology says:

    I built this natural draft furnace to see how hot it could get. Because of the high stack the draft is so strong (6 m per minute) that bellows aren’t required. The purpose (eventually) is to smelt iron metal from ore. The ore in this video was bog ore and not iron bacteria like I usually use. It produced no metallic iron whereas the iron bacteria usually makes even a small amount by comparison. It was magnetic however indication the presence of iron oxide. I estimate it reached temperatures of about 1200 c.

  2. peter penner says:

    do you think it is possible for you to produce enough food and clean water to live exclusively from nature? because only with enough food and clean water you can survive.

  3. Jonathan Benitez says:

    In five minutes and forty six seconds, he’s made me question living in the twenty first century. And know I wanna live off the grid.

  4. Mr Spelly says:

    Wacht him make a plane in 1 year or 10000 years

  5. Seohyun Yoon says:

    Wow…oh..l’m korean gilr um…
    It’s very …..ha…l don’t khow

  6. Ernest Yeap says:

    Frankly i too dont mind building my own home n living by nature..

  7. 240pixel says:

    Cant wait for you to start making metal tools

  8. jawadad802 says:

    so how much more efficient would it have been if you had used charcoal? would the temperature get higher then 1200Β°c or would you just reach the 1200Β°c faster? and would adding an adjustable cover to the inlet allow you to control the airinlet and optimise temperature? I really wanna see you reach that first handmade iron tool…its like watching a game of civilization on really really really slow turnbased settings…but with a much more enjoyable soundtrack…watching you mould that dirt into something usefull is like mental yoga….great channel man!

  9. Robert King says:

    Use it as a bong

  10. Jose Lopez says:

    I’m beyond impressed that you learned to build this by hand! Unbelievable man!!!

  11. Jason Crooked says:

    Do you have a Facebook page where I can send people? I don’t want to share your videos on there unless it’s linked to you.

  12. Rebecca Thraves says:

    The thing is , how does he make the fire without using matches or something

  13. tear728 says:

    Anyone else get the feeling this guy like to play with his own poop?

  14. Redstar Kachina says:

    This guy needs his own show on Discovery/National Geo.

  15. Joe Williams says:

    Man has come a long long long way .

  16. Proboscis5000 says:

    Next year he will be manufacturing gameboys.

  17. Danny Martin says:

    Here is an idea – making a blowpipe

  18. Ruben de Jong says:

    I got a new headset and it’s very good but now i can hear that you aren’t that far from “civilization”. *sad face*

  19. Ilhamrio Annannda says:

    sir make a somethinng from metal, and show me how make a smoke beef

  20. mrspeigle1 says:

    It may be an issue of burn time, some of the bloom furnaces ive seen the project seemed to last all day even with a bellows.

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