Build A Natural Draft Furnace
Build a natural draft furnace easy. Use physics to produce drafts to force more air into the furnace that provides high heat without the use of a blower. Hot air in the furnace expands and is, therefore, less dense than colder air outside the furnace. This causes the air in the furnace to rise.
Fresh oxygen rich air then enters the furnace to replace the rising hot exhaust gasses. Just like forced air this natural flow causes the fire to burn more intensely. Factors that increase this flow are the fuel being burned, the height of the furnace and the size of the tuyere (entry opening size). Using charcoal as a fuel will yield higher temperatures than wood.
Natural Draft Furnace temperatures of 1200 Celsius or even higher can be achieved!
Step one is to dig a depression in the ground where the furnace will be built. Then, lay in brick or stones to form a floor, the tighter the fit the better. Clay can be used to seal cracks between stones for a really good sealed floor.
Next, you will need to harvest lots of clay to build the walls for the furnace. Add water to the clay if necessary and mix thoroughly. Mix in dried grass to give the clay a stronger structural integrity.
Now, with this clay you will build your furnace walls. To start you will need your walls to be about 6 inches thick. You can allow them to get narrower as you build up if you like. The walls should not be made square. Instead, make the walls round like ring. Each successive addition to the wall will build it higher and slightly narrower.
Leave an opening about 6 inches side at the bottom. This is where the air will enter your furnace. Place sticks or a piece of bark over the opening and the next layer build a complete ring leaving no opening at all. The opening should end up being about 6 inches wide by 8 inches tall.
The clay must be moist to be mold-able. After a foot or two of height is obtained you will need to wait for that portion to dry out. Otherwise, it will not be strong enough to support the weight of the walls built on top of it. When restarting work moisten the top of the clay walls so that the addition of new clay will bind with the old and not simply just sit on top of it.
Over a period of a few days you should be able to extend your furnace upwards. A height of six feet is a good target height.
Now make about a seven or eight inch long piece of pipe about six inches wide out of clay. Make it about nine inches long. This will go through the furnace opening you left at the base and it should be about five inches off of the ground. So place clay under it to support it and then fill in around it to seal all openings other than your pipe opening.
The clay pipe should extend in towards the center of the furnace and be angled slightly downward. Just a slight 10-15 degrees down slope is all that is needed.
Fire up your furnace with small fires several times before going with a large fire that produces lots of heat. This will allow for a more even drying process which will help minimize cracking.
Protect your furnace from the elements when not in use.
Temperatures can be estimated by the color of the glow the fire produces. When a dull red color shows in your furnace opening you are at 1100 F. As it reaches 1300F it will be cherry red. At 1472 F it will be a bright cherry red, sort of a burnt orange. An orange color is 1650F. At 1830 F the color is yellow-orange. At 2000 F the color will be yellow. At 2192 F the color will be a light yellow color. At 2370 F the color will be almost white and the glowing is intense making it difficult to see inside. Pure white is achieved at a temperature of 2550F.
This natural draft furnace is not going to reach white hot temperatures. Luckily you will not need to reach temps that high for most forging endeavors.
Fresh oxygen rich air then enters the furnace to replace the rising hot exhaust gasses. Just like forced air this natural flow causes the fire to burn more intensely. Factors that increase this flow are the fuel being burned, the height of the furnace and the size of the tuyere (entry opening size). Using charcoal as a fuel will yield higher temperatures than wood.
Step one is to dig a depression in the ground where the furnace will be built. Then, lay in brick or stones to form a floor, the tighter the fit the better. Clay can be used to seal cracks between stones for a really good sealed floor.
Next, you will need to harvest lots of clay to build the walls for the furnace. Add water to the clay if necessary and mix thoroughly. Mix in dried grass to give the clay a stronger structural integrity.
Now, with this clay you will build your furnace walls. To start you will need your walls to be about 6 inches thick. You can allow them to get narrower as you build up if you like. The walls should not be made square. Instead, make the walls round like ring. Each successive addition to the wall will build it higher and slightly narrower.
The clay must be moist to be mold-able. After a foot or two of height is obtained you will need to wait for that portion to dry out. Otherwise, it will not be strong enough to support the weight of the walls built on top of it. When restarting work moisten the top of the clay walls so that the addition of new clay will bind with the old and not simply just sit on top of it.
Over a period of a few days you should be able to extend your furnace upwards. A height of six feet is a good target height.
Now make about a seven or eight inch long piece of pipe about six inches wide out of clay. Make it about nine inches long. This will go through the furnace opening you left at the base and it should be about five inches off of the ground. So place clay under it to support it and then fill in around it to seal all openings other than your pipe opening.
The clay pipe should extend in towards the center of the furnace and be angled slightly downward. Just a slight 10-15 degrees down slope is all that is needed.
Fire up your furnace with small fires several times before going with a large fire that produces lots of heat. This will allow for a more even drying process which will help minimize cracking.
Protect your furnace from the elements when not in use.
Temperatures can be estimated by the color of the glow the fire produces. When a dull red color shows in your furnace opening you are at 1100 F. As it reaches 1300F it will be cherry red. At 1472 F it will be a bright cherry red, sort of a burnt orange. An orange color is 1650F. At 1830 F the color is yellow-orange. At 2000 F the color will be yellow. At 2192 F the color will be a light yellow color. At 2370 F the color will be almost white and the glowing is intense making it difficult to see inside. Pure white is achieved at a temperature of 2550F.
This natural draft furnace is not going to reach white hot temperatures. Luckily you will not need to reach temps that high for most forging endeavors.
