Making charcoal

One metal biscuit tin inside another metal biscuit tin. We started out this way for fear a single skin might melt in the fire but so far, that has not happened. Each tin is filled with small pieces of timber and the lids fitted tightly back on. The whole lot is put inside the chiminea in the back garden on a day when nobody has their laundry out on the line and a couple of hours later, voila. Gives the tins a pretty finish too though I wouldn't choose to keep biscuits in them any more.
I hoped to be able to recognise the pieces of timber but they've moved around too much inside the tin getting it in and out of the chiminea for their relative positions to have remained constant.