Friday, March 25, 2011

Heat under AFRICA

This is the third of my short overviews of the activites at Smeltfest with other members of the North American Iron Undergound. Note to readers : I’m reporting on events several days in the past at this point.

Work on Tuesday started with preparing the large African prototype furnace for firing.

This is the interior of the furnace, showing the set of the individual tuyeres, with four set in each of the four arch quadrants. The slag pit below the furnace has been filled with short lengths of green branches, each about 15 inched long.
This is the same shot, with the bed of the furnace established. A thin mat of broken straw covers over the stick ends, topped with a thin layer (about 1 cm) of charcoal fines. Its fully expected that the ignited charcoal will establish its own working level. The layers seen function to keep smaller pieces of burning charcoal from falling straight down between the sticks into the pit.*At this point the first layer of clay tuyeres have been installed into the open arch. The initial spit wood fire has been started. Next the remaining gap will be filled with a loose high sand and rough clay mix to seal the base of the furnace completely.
At that point, the furnace was charged with small pieces of dry wood. About enoung to fill a standard garbage pail had been pre-split into roughly 1 x 1 x 10 - 15 cm long splits. On top of that material was added larger flat pieces mostly the same length.perhaps twice again as much material.

The heat created operated the furnace like a big charcoal retort at this phase, resulting in considerable smoke from the stack. This certainly indicated that the tuyeres were in fact drawing enough air to produce a significant natural draft. Jeff figured it was more than time enough to ignited the exhaust gasses, touching these off with burning paper at the lower charge door. The effect was quite dramatic...(The handle for this can be seen in the bottom of the pipe in the image above.)

Once that wood fuel was obviously fully ignited, rough charcoal was added to fill the lower chamber. Of course some of the wood had burned away by that point. It took about two 40 lb sacks of charcoal to fill to the ‘neck’ where the shaft starts. Jake Keen, the smelt master for this experiment, is seen adding charcoal via the charge door in the image above
The charging cycle of 5 lbs of charcoal and 5 lbs of roasted and crushed Lexington rock ore was started almost imediately after.
The furnace would settle down to a fairly uniform consumption rate of 5 ore plus 5 charcoal roughly every 10 - 15 minutes. Of course there would be variations on this! Twice the furnace almost stalled out, with internal temperatures obviously dropping and consumption greatly increasing. With a total of 200 lbs of prepared ore on hand, many long hours of furnace tending were expected. The initial fire had started about 2 pm on Tuesday, it was fully expected to take 24 - 36 hours to run the complete smelting cycle.
One effect that came to be relied on is seen above. As each individual charge was fully ignited inside the furnace, the stainless steel stack pipe would start to glow a dull orange to red along its whole length. (The image above is hand held at about a half second exposure.)

I personally ran out of juice by about midnight Tuesday. I set my alarm for 4 AM and grabbed some sleep. Various others retired at other points. When I got back to the site. Jesus Hernandez, Dick Sargent, Lee Sauder and of course Jake Keen where the only ones still functioning.
The last charge was made about 5:30 AM, so roughly 14 1/2 hours after starting. It was agreed that many hours would be still be required until the ore already involved would burn down to tuyere level. We all went back (or to!) bed....

(next entry : opening the furnace)

1 comment:

Polymarkos said...

Do you have a vid or pic of lighting the exhaust gasses? Did that increase the draft?


February 15 - May 15, 2012 : Supported by a Crafts Projects - Creation and Development Grant

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