Obviously oxygen has to be eliminated from the burn, that is why the fire is built layer by layer, as soon as ash appears on the surface, you add more fuel; this creates a lack of oxygen in the material below as all oxygen is being consumed at the surface. If you add more material before ash appears, then you will not eliminate the oxygen from the lower layer, if you let too much ash appear then you are losing char quantity. Once you have filled the pit or exhausted your material supply, you can extinguish the fire thoroughly with the aid of a hose. Some people bury the pile (like the indigenous method), but then you have to wait a long time for it to cool completely and when using water, the char structure can be helped supposedly. The way to ascertain if your char is good (without equipment) is firstly if it makes a satisfactory tinkly noise when you drop it, and secondly if when you rub it on your hands, that any residue left behind is minimal and rinses off with just water.