Cast Iron Dutch Ovens
Lodge cast iron Dutch ovens work really well as kilns for charring. They're black which is efficient at converting light to heat. They're durable and resist warping at high temperatures. The lids fit well to impede oxygen entry, but also lets out the exhaust generated during charring. And they're readily available.
Clicking the image to the left will take you to an Amazon link where you can buy Lodge 1, 2, 5 or 7 quart cast iron Dutch ovens.
Problems to Avoid
Let the kiln cool to ambient temperature before opening. We opened the kiln once right after charring was complete. The charcoal ignited and burned. The fire was easily extinguished by replacing the lid, but it's best to avoid. Let it cool before opening.
Let the kiln cool gradually when finished charring. We rotated the cooker away from the sun once when we were done, then went away while it cooled. Upon our return the rapid cooling had warped the bottom of the kiln, fracturing the metal. Now we let the sun gradually move off of the kiln for a gradual cooling. No more warping thus far.
Over a period of uses the kiln begins to rust on the bottom where the sunlight is focused. This is likely from the high temperatures on the focal spot accelerating oxidation of the iron. Though it still works really well, red isn't as good as black for converting light to heat. Re-seasoning the metal solves this problem.
1 Quart Kiln
We've tested the 1 quart size which works well at charring twigs, grass, and used coffee grounds which covers the gambit from low density (dry grass) to high density (coffee grounds) materials and so should work on pretty much any biomaterials.
Generally good performance capturing about 50% of available carbon, but low capacity.
The 1 quart kiln is a small footprint and requires re-aiming the focal spot every 5-10 minutes or so due to Earth spin.
2 Quart Kiln
We're in the process of testing the 2 quart kiln. Las fall we saw it perform as well as the 1 quart using coffee grounds. More testing to follow this year.
5 Quart Kiln
Will test beginning this season after the spring equinox
7 Quart Kiln
Will test this season if the 5 quart kiln performs well.