Thursday, January 22, 2009

Charcoal to Bar in Colonial Furnaces

This is a commentary edited from a posting to the 'Early Iron' discussion:

" ... Can anyone come up with a ballpark figure (SWAG) for how much charcoal it takes to make a ton of bar iron? There really aren't any good figures for the 17-18C, because they frequently measured by the basket-load, and very few of them wrote down consumption figures anyway..."

James is referring to Colonial Era (American) bloomery furnaces. These operate more or less the same way the Viking Age prototypes I have been working with. Most significantly, Colonial bloomeries are easily an order of magnitude larger, with bloom sizes into the multiple hundreds of pounds per smelt. Mike McCarthy has been especially interested in this type

This is my usual 'talk around the numbers' :

The raw machinery of the smelter takes a certain amount to operate in terms of heat budget. There is going to be a more or less fixed amount to get the furnace from cold to operating temperature. One of the biggest factors here is going to be raw volume and the furnace design. Numbers from small furnaces like we are using may not be the easiest to transpose, as there is a square to cube relationship between surface area (loss) and volume ( production space). My own furnaces tend to be pretty consistent, I think my smallest was 20 cm diameter and the largest about 35 cm. I'd bet most of those on Early Iron keep to the historic pattern of 'build one smelter, get it working, duplicate as required'.

With a small ore addition smelt, the ratio of fuel to iron is going to be the highest. I personally have found that (again in that standard 25 - 30 cm diameter by 65 cm tall furnace) that I need at least 8 kg ore (would vary by purity) to kick start the reaction. Looking back over my own notes, it looks like I'm using roughly 30 kg of charcoal to get the first 1 - 2 kg bloom. If the smelt keeps going, then at 60 kg the blooms produced are climbing up to the 10 kg range. At some point, the 'kick start' amounts would statistically disappear into the large bloom production numbers. (You want to find out from Lee and Mike what kind of raw material to iron production numbers they got from the huge bloom they worked on last Smeltfest.)

The size of the reaction volume in the furnace is dependent on air delivery. We have built furnaces up over 100 cm. Our usual air volumes are set by * diameter * (in the order 800 l/m +/-). What happens is that the reaction column (in our standard furnace) remains pretty consistent at about 40 cm or so above the tuyere. With a taller column, what seems to happen is that the exhaust gases in the upper area serve to preheat charcoal and ore. There can be no chemistry, as there is no free oxygen / carbon monoxide to drive it. What does seem to happen is that the burning efficiency improves, the furnace running hotter, so less fuel required to process the identical quantities of ore.

The dryness of the charcoal has shown to be a huge factor. Charcoal will absorb water from the air, and its amazing what even a little bit of dampness in the fuel can do to change consumption. We have been able to purchase fuel from a local fellow in Ontario who makes charcoal the same way we are making iron. This Black Diamond charcoal comes so fresh its almost still warm - and bone dry. It makes a big difference.

A couple of the others (on Early Iron) had mentioned a rough 1:1 ore to charcoal ratio.
In my own notes, I have recorded all the charcoal used, from first
addition (after wood pre-heat) through to initial compaction in the furnace. More typical is something closer to 2 : 1 (charcoal to ore) over the entire furnace cycle. Most of us over here are getting yields in the range of 25 % (ore to bloom) . For myself I have seen in our larger ore smelts (45 kg ore) giving returns closer to 40 percent. So taking the lower yield, that means 50 kg charcoal is being consumed for a 12.5 kg bloom (more or less). Of course the 'Colonial' era furnaces that you were referring too might be basically be 'kept hot' through a continuing working series, which would significantly reduce overall consumption.

The original question also specifically mentioned 'bar iron'. All of the above just applies to the ore to bloom phase. I think Lee Sauder has kept some notes on this second stage of the process, bloom to billet. Peter Crew had reported ore to working bar conversion at roughly 10 %. (And I've got to admit, that seems pretty low to a lot of us.) I personally have not worked enough raw blooms into bars to say much useful (those blooms are like our children!) I think Lee had figured that his own ore to billet conversion was on the order of 25 %. What about billet (block) to actual long rods? Most of us are undertaking the bloom to billet phase in coal (or in some cases propane) forges. I personally don't have enough time with charcoal forges to estimate the fuel requirements. (And my charcoal work has been with a much smaller Viking Age forge at that.) On a real, real rough guess, I would not be surprised to find that you might consume as much charcoal taking a bloom to rods as you did taking ore to bloom.

I would expect the overall requirement for charcoal - from lighting the smelter through to pulling off those final working rods, to be quite high.

Lee Sauder later made the following comment:
" For a long time we seemed to burn 8 to 10 lbs of charcoal to produce 1 lb of bloom. It's only over the last few years that we've gotten that down to 5:1, ... The best I found was 3.2 lbs charcoal per lb of bloom, for the second smelt in a furnace that was already hot.

Good blooms give us a bar that is 60% to 75% of the bloom weight."

PS - For those truly interested:
The Early Iron Experimental Group was formed after Mike organized the first Early Iron Symposium in 2004. This Yahoo based discussion is confined to "individuals who have a keen interest in the smelting of iron from ore". It is an invitational group, and I am the moderator. Any of my readers with a serious interest in experimental iron smelting who might wish to join and participate should contact me directly.

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