Here's a video of how I make bio-char in my masonry heater, using two tin cans that fit together nicely to seal out oxygen.

Charcoal is a major ingredient in the super-fertile, man-made, Amazonian soil called Terra Preta. Terra Preta is being intensively studied as a hopeful solution for the impending agricultural and climate crisis. People have now been experimenting with a lot of complicated methods for making charcoal for Terra Preta, in as clean and efficient a manner as possible, involving double chambered barrels, and tubes and chimneys, and such. The quest is to make charcoal with as few greenhouse emissions as possible, while utilizing the heat generated from the fire for something useful. I've become involved with experimenting with some of these methods on a home-scale, and I will let you know how it goes in future articles. In the meantime, I came across a very simple way of making some charcoal, in a substantial enough volume for your garden beds, in a video on YouTube by Greenpower,.

Terra Preta, or "Dark Earth" is the name for the super-fertile, man-made soil, covering a combined area, the size of France, that has been discovered in the Amazon. The above video and it's accompanying article at 
http://news.nationalgeographic.com/news/2008/11/081119-lost-cities-amazon.html  gives an excellent synopsis of what it is, and why it's such an incredible and significant discovery.

Terra Preta is now being intensively studied as a hopeful solution to the impending climate and agricultural crises.
 Terra Preta seems to have many ingredients that seem to act synergisticly, but the key ingredient is charcoal. Below is an excellent video on how to make your own charcoal, in a non-polluting way, on a family garden-sized scale. The finished charcoal is ground up, and moistened, and mixed 50/50 with compost (and other needed soil amendments), and then it is tilled into the garden soil.