On this run, when I took the lid off, the bag that I used to start the fire had turned to carbonized paper. If you look close, you can sort of see that the part that had been burning before putting on the lid has turned to ash and is gone, but the rest of the bag is still there and you can even read what was printed on the bag, but soon after I grabbed it with my meaty fist (my hand doesn't seem to take good pictures) it crumbled into small pieces of bio char.
A chunk of wood was almost the same way, before I picked it up, it was an intact piece of charred wood, but it easily crumbles and has no ash residue.
This time I ended up with about a third of a barrel of bio char from a loosely filled barrel of maple limbs that were about 12-15" long. When I've used short (4-6") pieces of cedar, I've been able to pack the material into the barrel a little tighter (I still don't worry excessively about getting it 'just right') and I'll end up with more bio char when I'm done.
After you've made your bio char, it's time to turn it into the makings of terra preta. I usually do that by cleaning out the stock trailer after hauling some cattle (if you don't happen to have a stock trailer that needs cleaning out, you'll have to figure out something else, like using compost or cleaning out your chicken house).
I usually start by throwing a bucket of stock trailer cleanings into a wheelbarrow, then throw in a bucket of bio char, chop it up and mix it together a little with the shovel, then another bucket of manure and a bucket of bio char, etc. until the wheelbarrow is full.
Spray a little water once in a while to help "charge" the bio char with the beneficial bacteria and microbes in the manure. If you are really daring and also of the scientific mind, try leaning down close to the wheelbarrow and taking a good whiff, and you should find out that the bio char has already started to absorb all the aroma from the manure, compost, etc. That aroma absorption effect is a simplified explanation for how the bio char works to create your terra preta. All the little microscopic pores in the bio char absorb and release micro and macro nutrients, microbes, etc. and release them to the plants.
When I spread my bio char mixture in the garden, I'll cover about 100 square feet with an overflowing wheelbarrow. When I first spread it , it almost looks like I have turned the the entire area black. But as soon as I run the tiller over it, the "OMG, I've ruined this garden forever" black color is toned down, and I end up with a nice dark looking soil with scattered chunks of bio char on the surface. Those chunks of bio char will start to crumble and be incorporated with moisture, plant growth, and tilling (I tend to think that those chunks laying on the surface might be absorbing Nitrogen and Phosphorus from the air).
I tilled this before replacing the worn out tines on the tiller, so if your tiller isn't as worn out as mine, you'll get an even better result.
Next time, I'll try to gather up some of the links I've found that have helped me figure out some of this stuff that I think I know about Terra Preta and TLUDs.