Much of the time is related to scale - a one gallon paint can in the
barbeque might well get done in under an hour, and a gigantic killer
pile might take 3 days or more.
Your basic small-scale method is to put the wood to be charred in an
almost-sealed can and cook it. Your smarter or more efficient methods
arrange the non-sealed portion to vent the steam, and eventually gas, in
such a manner that once the gas (from the cooking wood) gets going you
don't need any more external fuel to cook it - the gas coming off the
wood is used to cook it.
Here is a nice explanation on a slightly larger scale - scale down to
fit your needs...
I make my charcoal in 45 gallon barrel. The time it takes to make it ranges
from 3 to 5 hours from start to finish depending on how green or wet the
wood is. I average between 2 1/2 and 3 buckets full of good charcoal per
I'll second the method Ecnerwal posted.
I've used it, although not to that scale.
If your wanting to make charcoal, make as big a batch as you can stand
each time. I'm guessing your wanting to burn it in your forge? If so
you can go through it quite quickly.
For small batches I've used an old cookie tin with afew holes punched
in the bottom. just fill it pretty tight with some small pieces. I
stacked mine vertically. set it in/over a good fire for afew hours.
I've used the smaller batches for charcoal "fuel" for small fireworks.
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