Using an old paint can as a retort is an easy way to make small quantities of Bio-char for use around the homestead. I use char mainly to add to the cover material mixture that I make for composting because it's great at absorbing odors. But it has many other uses as well.
The messy part is cleaning out the old, dry paint. But if you find one that has been mostly emptied by the paint brush, it's usually pretty clean and the first fire takes care of the residue.
Here I made some Bio-Char mostly from the spiky little balls from the neighbor's Sweetgum Tree, some twigs and part of an avocado seed.
There is zero ash and the best part is that you can burn stuff that is usually a pain to burn in a regular fire or a TLUD.
To make sure the can doesn't blow up, it's advised to punch a tiny little hole in it. But frankly, mine have always vented around the lid without a problem. I simply put the can into the center of the fire area making sure that the escaping gases will be directed downward and inward toward the fire. The gases, (smoke) that is forced from the can help feed the fire.