Primitive Technology: Natural Draft Furnace
Published on Oct 27, 2017
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 the size of the fuel bed as these effects 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 was 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 “The mastery and uses of fire in antiquity” by J.E. Rehder. It was designed to have a space velocity (airspeed 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 inside 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 labor 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 labor they provide and simplified infrastructure (fewer workers and no bellows required during operation).