Some recent questions had surfaced on Early Iron about charcoal. I warn you that I did not research this myself, someone around the edges of my team did (Thanks to Russ, who was actually looking into wood fired pottery kilns). But the topic did come up, and this is what I remember.
- If the source wood is completely converted to carbon, the thermal energy available per weight unit is pretty much identical - regardless of species. So if you are measuring by weight, it will not change things that much.
- The different species do have different densities. This could prove important if you are measuring by volume (and most of us are using a standard 'pail'). The softwoods will have a lot more volume against weight. Certainly here in North America, most of us are using 'BBQ' charcoals, which generally are oak or hickory. If you suddenly changed to pine fuel, you might have to adjust your 'standard pail' sizes.
- The different species will also have different structural strength - more or less how fast a given piece will crumble as it burns. This is true, but given the small pieces we cut our charcoal to, this is not likely to be significant.
- The single largest factor in the performance of a given charcoal is going to be its moisture content. The drier the fuel, the hotter it will burn. Charcoal will absorb water out of the air, so a rough rule of thumb is the 'fresher' that charcoal is, the more efficient it will be in your furnace. Note that water quickly can modify the weight by volume, and throw off your calculations. (If you were always using a fixed weight, a damp charcoal would have less effective carbon for the same weight, at the same time it also effectively lowered the combustion temperatures!) There is no doubt that a bone dry charcoal gives the best furnace performance.
- Various commercial charcoals can vary considerably in how effectively they have been carbonized. Also how much 'garbage' they have included (which of course gets picked out during your grading for size process). The worst I have worked with is the American 'Cowboy' brand - double fist sized rocks, rail spikes, half head sized chunks of unburned wood. They had been taking old railroad ties and converting these for the source material.
- The different species appear to leave different amounts of ash behind. This looks to be related to the density mentioned above. Pine certainly leaves the least amount of a fine ash (which pretty much all blows out the top of the furnace). I mention this as we had one lot of charcoal which was from tropical hardwoods from South America (Early Iron 3 if I remember correct). This extra ash increased the slag generated noticeably. (Sometimes hard to tell with so many other variables!)
We stumbled into this through looking back at our measurements of a number of experiments. Although we were using the same pail measure - the weights of the charcoal charges sometimes varied 10% between individual charcoals used. I have kept all my records listed as 'by the pail', which in our case is 9 litres.
Generally though, I would suggest that making sure your fuel is dry is the most important concern. Second would be to use the same source of charcoal (both species and manufacture) as much as possible. You will vary your specific method to suit your furnace layout, ore type and available charcoal. My read of ancient European smelting sites is that they used the trees closest to hand for smelting. Ore appears to be the determining factor in the location of a smelting operation, with other details modified to get the best yields from that ore.
(PS - a commercial plug for Royal Oak, who have donated several thousands of kilos of charcoal to my effort to date. I have consistently found the Royal Oak to be the best commercially available charcoal of half dozen or so I have worked with.)
(PPS - Bruce Cowan's Black Diamond charcoal, has ran afoul of the Ministry of the Environment, and forced to shut down. Black Diamond was an exceptional fuel for our smelting, coming perfectly sized and always bone dry. A real loss!)
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