Dear Matthew

> Does anyone know (or have a guess at) the approximate ratio of green wood to
> finished charcoal (i.e. in terms of mass - how many kilograms of wood to
> produce one kilogram of charcoal)?  What about how much charcoal was
> required to keep a small smithy in business (i.e. what could be produced
> with a tonne of charcoal - in terms of horseshoes, blades, whatever)?  
> 

We are currently looking at exactly these issues with respect to 
iron-making. To accompany our experimental iron smelting we have 
started a programme of charocoal burning, and over the summer 
we will be experimenting with a series of single-species earth 
clamps - with the product being used for smelting and smithing. I'll 
let you know when we get some real data, but our first clamp had 
approximately 6 cubic metres of stacked oak and generated 
around 180kg of charcoal. I'm not yet certain what the density of 
our stacking is (though others have suggested that solid 
volume/stacked volume of around 0.6, and stacked density of 
0.7t/cubic metre applies to stacked cords). We estimate that 180 
kg should give us two smelts capable of generating 5kg blooms - 
but we are not quite that efficient yet! This figure excludes the 
onward working of the bloom - which would require an additional 
large supply of charcoal, which we haven't measured yet.

If the estimate of stacking density below is true, we got 180 kg 
from around 4 tonnes of wood - a rather poor yield (but it was our 
first attempt)! However, a similarly-sized clamp constructed in 1985 
was estimated to contain only 2.5 tonnes - if this applies to ours 
too, then the yield was more respectable (14:1). We need to check 
the density out this week.

We are told by our charcoal "tutor" to expect a ratio of 8:1 (wood 
wt :charcoal wt) for dry wood and 12:1 for less well seasoned 
material. We have also made some small charcoal samples in the 
lab furnace - but don't have the yield values to hand just now.

The website www.connerprairie.org/cp/fuel.html gives accounts of 
charcoal making in earth clamps gives yields of 35-45 bushels / 
cord. Unfortunately this delightful (and useful) measure wasn't 
ratified as an SI unit!

In the UK the statute cord (but there was much regional variation) 
was 128 stacked cu ft (3.6 cubic metres) and weighed 2.5 tons; 
the long cord was 175 cu ft (3.4 tons). Bill Linnard gave figures of 
coppicing from well-managed woodlands yielding 12 long cords / 
acre. i.e. 1200 solid cubic feet weighing 30 tons green or 20 tons 
seasoned. So you should get 2.5 tons of charcoal/acre. Coppicing 
rotations vary from 16-20 years - giving 0.12-0.16 tons/acre/year 
over the full cycle (0.35 tonnes/hectare per year).

In other words 1 hectare of well-managed woodland would keep our 
bloomery supplied for 4 smelts/year, producing around 20kg 
iron/year, excluding the bloomsmithing operations. (corresponds to 
annual production of 1 tonne from 50 hectares i.e. 1 tonne from 
17.5 tonnes charcoal)

This compares with Hanbury's estimate (1704) for making bar iron 
at Pontypool (by blast furance and forge) of 4-4.5 long cords/ton. 
i.e. 15 tons charcoal per ton of bar iron.

There must be enormous regional variation in all the above figures - 
hence the need for further experiment, but they at least give a clue 
to the magnitudes involved. (DISCLAIMER: at least I hope they do - 
I've done some of the calculations "on the fly" and my head is still 
muzzy from a long day's smelting yesterday!).

Tim


Dr Tim Young
Email: [log in to unmask]
Phone: 01222 747480
Fax: 0870 0547366
Mobile: 0802 413704


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