Saturday, January 26, 2013

Bloomery Smelting - Variations on ... Everything!

Jur de Stoute said :

When you are working with the hand bellows there is another variable coming into play and that is weight of ore charge. About 250-500 grams/charge works about best. If you charge more/charge pumpinng will be ALOT harder, because it chockes the column more. Just drop on the weight per charge and after a while things start to work just fine again.

My usual is to distribute the ore as evenly as possible through each addition of a 'standard bucket' of charcoal.

DD 2 ore analog with the 'standard scoop' - Vinland 1, May 2009
The tool we use for the ore additions is shown in the image above. It holds a bit more material than you could hold in a single cupped hand.

One of the problems we all face in these kind of conversations is that I'm sure our experiences all roughly match - but we are all using slightly different measurements. Both in terms of tools and how we keep track / records.  This goes back to the thing I mentioned about 'kg charcoal per hour' being one thing most of us can record (or calculate from our methods).

On charcoal, we have a 'standard galvenized bucket' sold through hardware stores all over N. America.
Dave Cox adding charcoal in our 'standard bucket' - June 2006
Now, the charcoal does vary in density based on wood species. The most commonly used types here in N A are oak, hickory or maple (or some combination). There is some research to be considered here (??)
The charcoal does also vary in weight per volume due to how dry it is. When my team noticed this, we started trying to measure the weight of our 'standard bucket' at each smelt. The variation is easily 10 - 15%.
Of course another wrinkle there is that we certainly do not weigh each individual bucket, and there is certain to be variations in how full any given bucket might be. On a guess - another 5 - 10 % at least.

On the ore, we originally just counted 'scoops'. We would measure and weight three, take that average, then multiply number of scoops against that calculated weight to get our ore total.
We again realized there was considerable variation there. Now we measure out the ore by full kg (or 500 gm multiples) into cans, then use the cans into the scoop to make it easier to add to the top of the furnace.

The density of the specific ore types we use can vary considerably. The same 'scoop ful' can vary from the DD analogs in the range of 275 - 300 gm - to as much as 600 + gm when the ore type is taconite or hematite grit.

The stack height of the furnace will come into play here too. As the charcoal burns, individual pieces will turn, tumble, and change size. This all will result in the ore particles all falling at slightly different rates. In effect they should start to distribute themselves fairly evenly though the entire volume.
The size of the particles, and the relationship between ore size and charcoal size, will certainly effect this as well.

That being said - I think you may be on to something Jur!
We are always careful to spread the ore in a given charge as equally as possible through an individual charcoal addition. So we never lay all the ore as a single 'slug'. (And yes, many earlier published illustrations suggest this as the method, which I do agree is NOT the correct way to proceed!)
We very rarely charge more than 3 kg of ore into a single charcoal measure (for us the average 'bucket' is about 1800 gm). In method, we would be splitting that ore amount into four roughly equal applications, covering each with a 1/4 bucket of the charcoal.  More typical is charging at 2 kg to one bucket (so roughly 1 to 1)

One consideration on that advise may be the surface area of the furnace as well. I think we all can see that 500 gms ore over a 20 cm diameter results in more of a 'plug' than with a 30 cm furnace. (Our standard here is between 25 - 30 cm interior diameter at the top.)

I think this all just re-enforces what Mark said : Bloomery Smelting is an ART.

Something for those not familiar with historic artifacts:
It is common to find a number of slightly different furnace layouts at a larger 'industrial' ancient iron smelting sites. Usually showing signs of a single firing only. Then there will be one version built over and over again. Obviously the ancient masters would take their generalized knowledge of the iron smelting process, then tweak the furnace design to account for the local clay, ore type, maybe even the charcoal available. Once an 'ideal' combination of design and method was determined - they just repeated that combination over and over.

We do all need to remember that this same problem of matching our furnace and method to our individual local ore does confront us all!

(again taken from a post to Don Fogg's Bladesmith Forum )

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

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