Monday, May 01, 2006

Charcoal in smelting - 'Size does too count'

This is modified from a post made this morning to EARLY IRON - related to charcoal in iron smelting:

Its my contribution to take the complex, miss some of the details, then attempt to over simplify it to allow the more knowledgeable something to discuss at length. Bear that in mind:

Daniel wrote:

1. Preferred wood (hardwood, softwood, mesquite, etc) and why

My understanding is that the species of charcoal does not really effect the total energy produced all that much. However the rate of energy delivery and most importantly the amount of waste ash produced will vary by type. Balance this against the fact that from the archaeology of Scandinavia at least (the only set I know about) there is no indication that specific wood types were selected for smelting. The range of types and mixture from the charcoal fragments pretty much matches the range and mix found from pollen remains at the same excavation. They seemed to have used whatever trees were handy when making the charcoal. (In fact its ORE that determines the location of an Early Medieval / Dark Ages smelting operation.)

Most of us look like we are using modern commercial charcoals, which tend to be primarily oak with some hickory. Maybe the odd assortment of other types. My group has been using Royal Oak for the last several years, which we have high praise for. It appears to be scrap from making decorative moldings - and is almost all oak.
Lee and Skip are making their own charcoal in a metal retort. Again they are starting with scrap from a molding plant - and it looks like this is mainly oak as well. (They should really comment on this!)
Mike has made charcoal using both the pit and low stack traditional methods. I believe he was working with dead fall branches from a wood lot. You should nudge him to tell you more.
At Early Iron 2 - commercial charcoal was purchased. This material was mainly oak, and had every indication that it had been made from old rail way ties. We found a LOT of debre and foreign materials - rocks, fused clay - pieces of metal - mixed in.

In any case, all of these charcoals provided good fuels in the end.

2. Has anyone tried using the pressed briquets (Kingsford, etc.)? I don't trust them myself because of the other stuff that's been added to bond them into briquets, but anyone with experience please weigh in.

Because of the way they manufacture briquettes - I can't imagine they would be useful. Briquettes are made of a mixture of clay dust, charcoal dust - and used motor oil. Nasty! The high volume of garbage (the ash and clay) is certain to mess up the interior of your smelter.

3. lump size -- do you break up the charcoal and sieve it for specific sizes?

Ok - now we get to the 'complex made too simple' part:

There is a relationship between the ore type and purity, and the reaction time required for all the various chemical changes to take place inside the smelter.
The reaction time is roughly controlled by first setting the stack height of the smelter. The next variable is the size of the ore particles related to the size of the charcoal particles. As you might guess - these three things work in combination to determine how fast an individual ore particle will heat to the reaction temperature - plus how long it remains in the reactive zone of the smelter.

Air volume works in here too - but I'm leaving that part out for now. (ok?)

Looking at archaeological materials, then MUCH trial and error (on mainly Lee and Skips part) has resulted in the rough dynamics that many of us are working with;

smelter size of roughly 10 - 14 inches (20 - 35 cm)
smelter height above tuyere of at least 16 inches (40 cm) - but ideally closer to 20 inches (50 cm)


roasted ore particle size roughly pea to rice, with the dust included
(although this varies with type of ore)
charcoal particle size of roughly 1 inch (2.5 cm) with the dust screened out.

I have been using mesquite and found it requires *A LOT* of air in order to work well. ...
.. comment on ahistorical blower/bellows that they are using or what they look for in a blower/bellows?

Air is a hot topic!

More exactly, the Sauder and Williams basic process has been proved to work consistently and predictably. High volumes of air (500 - 1000 liters per minute) are required.

One of the basic questions for those of us attempting to work backwards towards Early Medieval methods is how to either :
design and document a large volume air delivery system
modify the basic sequence to allow known lower air volume equipment to actually correctly produce workable iron.

In short - almost everyone who attempts to use low volume air ends up with much slag and little iron. As soon as you bump the air up an order of magnitude - you start getting large masses of workable iron.


No comments:


February 15 - May 15, 2012 : Supported by a Crafts Projects - Creation and Development Grant

COPYRIGHT NOTICE - All posted text and images @ Darrell Markewitz.
No duplication, in whole or in part, is permitted without the author's expressed written permission.
For a detailed copyright statement : go HERE