We always seem to return to the roots eventually, don’t we?
After a year of forging with charcoal made in Charcoal Retort V.2, it was time to address the issues with the system. The main one was that, though much improved from Charcoal Retort V.1, the high operating temperature took its toll on critical components after only a few months. Other important concerns were the lower quality of the “overcooked” charcoal, and the very small batch size.
Version 3 went in a completely different direction, away from the retort style technology to a scaled down model of a traditional Japanese charcoal making kiln. While a retort cooks wood in a completely sealed chamber, the traditional Japanese way uses hot flue gasses from the fuel wood to directly heat the charcoal wood. The charcoal wood is prevented from becoming part of the fuel wood in two ways. One is a firewall that reaches almost to the top of the kiln, and the other is carefully calculated intake and exit openings at each stage to control oxygen flow and consumption. This kiln is based on Iwasaki~san’s design, using his exact opening sizes and chimney lengths, but doubling the length of the charcoal chamber.
Similar to a Japanese wood fired pottery kiln, the design is horizontal and divided into two areas, a combustion chamber, and a charcoal chamber. A half of a 55 gallon steel drum makes up the combustion chamber and it is joined to the bottom of another using a steel clamping ring, but would be easier to weld on. The charcoal chamber is formed from two drums that fit into one another facing top to top, the blue one held pear concentrate and has an unusual tapered lip that slides 4 inches into the top of a standard drum. The seal isn’t perfect but we need to be portable for now so it works as is.
Operating the kiln is much simpler than our previous retort designs, as long as a small fire is maintained until steam stops coming out of the chimney, things should go as planned. Lessons from the first firing that are important to note are the need for all wood to be as dry as possible, and the need to fully insulate the top and sides of the kiln. The flue gasses from the combustion chamber flow up and over the gap in the firewall, through the charcoal wood, filtering from top to bottom as it cools and is pulled out the bottom from under the racks and into the chimney.
Almost as soon as it is lit, the chimney starts to put out very wet and relatively low temperature steam as all the wood inside gives up its non-charcoal contents. Depending on the size and stacking of the wood inside, after between two and six hours, the steam will change to bluish, almost clear, and very hot smoke. This is the time to close up the combustion chamber with steel plate, bricks, and mixed soil and ash and then do the same with the chimney opening. Allowing the fire to go out without sealing out as much oxygen as possible would allow the superheated charcoal to begin burning up, leaving you with nothing but a pile of ashes for your trouble. The next day when the charcoal is absolutely cool, the kiln can be opened and the charcoal sorted, chopped, and screened.
The first run used some deadfall Pine that was kindly provided by some nearby folks. Without insulation the process took 3-4 times longer than it should, and also produced a batch of charcoal with a lot of brown and uncooked wood as well as a large amount of overcooked charcoal. Subsequent batches with a well insulated kiln and drier wood produced very even results and several times the charcoal per batch as the old retort. The relatively low operating temperature and far more efficient use of fuel makes this system a great way to go and I suspect with a proper insulation system like Iwasaki~san’s this design could be lengthened even more without affecting efficiency.
**A note from Iwasaki~san: This kiln must be insulated for operation (exterior photos below were taken before the insulation was installed), the kiln will not operate efficiently or properly cook charcoal without insulation**
Splitting the pine, the punky outer stuff is for fuel wood, the better pieces for charcoal wood:
Stacked with room for circulation, this size and volume of wood will cook in about 3 hours if the kiln is well insulated:
Loaded and closed for the first firing:
The combustion chamber prior to light up, ceremonial Yomiuri Shimbun in place:
The charcoal kiln, note that this should be insulated for proper function:
Drying fuel wood with waste heat from the charcoal kiln:
Maintaining a small fire in the combustion chamber:
Cast iron breakfast with waste heat from the charcoal kiln:
The white steam that indicates wood is still in the cooking phase:
For proper function the kiln should be insulated on the top and sides at very least (not shown here):
The steam is fairly strong, the temperature above the stack relatively low while it cooks:
After 2-4 hours, the smoke becomes clear, blueish, and very hot:
Within five minutes, the combustion chamber is sealed with sheet steel, bricks, and soil:
The chimney opening sealed with sheet steel, bricks, and soil:
Opening the kiln the next morning after cooling:
The first batch had a high rate of uncooked and brown charcoal due to the lack of insulation:
It also had a high proportion of overcooked charcoal for the same reason:
The rack and inside of the kiln after the first firing:
Subsequent batches with the properly insulated kiln produced harder charcoal:
A nice hard, crisp bladesmithing charcoal specimen:
Mixed results, a little overcooked, but plenty fine charcoal for most blacksmithing:
An overloaded combustion chamber glowing at night, this looks beautiful but is way too full for proper operation:
An approximate sketch of the functioning of the kiln:
Read more about the charcoal making process and why we do it: How Charcoal is Made
Read some background information on fuel alternatives: Sustainable ‘Smithing?
A larger traditional charcoal kiln design: Charcoal Kiln V.4
Follow the charcoal making progress: All posts tagged Charcoal
Links to the English Resources on Iwasaki~san’s site: