Tuesday, May 06, 2014

Some considerations on Iron Ore for 'Turf to Tools'

 Eden  wrote:
Just been up to the nearby iron ore mine and picked up some samples. There appear to be two different types, one a very black and heavy ore which is very rusty, the other the more usual looking ore (From my limited experience).

A couple of things : 
(some of which Eden may be well aware of - this all related to the upcoming 'Turf to Tools' project at the Scottish Sculpture Workshop in mid August)

The direct process bloomery furnaces we will be building really need at least 50 % Fe content to the ore to function. Closer to 60% would be a lot better. This is pretty pure, as modern mines usually run closer to 25 - 30% as a viable ore body. This means it may be hard to find a usable ore at an existing or abandoned mine.

Sauder & Williams are using ore from Colonial Era workings in Virginia. They manage to find good ore because these were deposits deemed too small / too remote for even the technologies of the late 1700's. So looking for long abandoned *small* workings might be a clue.

Your black, heavy ore? Is it magnetic?
This might suggest a deposit containing Fe3O4?

Lee Sauder suggests a very rough 'field test' for Fe content. Take a sample that looks good / typical. Pound or grind it to small fragments (sand sized). Place about a hand full in a glass jar and weigh. Fill the jar with mild acid, something like muriatic / hydrochloric (pool chemicals from the hardware store) or sulphuric (battery acid). Leave this overnight, shake the jar now and then. Pour off the liquid and refill the next day. Repeat until the acid stays clear. Now pour off the last liquid and let the contents dry in the sun a couple of days. Weigh the jar again.
The difference in weights should be the amount of iron removed, this gives you a ball park number for your purity.

It would be great to smelt using a locally available (?) primary bog iron ore.
Problem there is twofold:
1) Even if bog ore is present, actually finding an actual deposit area can be pretty difficult.
2) Even if the geography allows for a deposit, is there enough ore available for an effective smelt. (see below)

For the SSW project, it would be great to attempt a smelt using that distinctive Macaulyite ore. Again, you will have to advise if you can get enough of the material to allow for this. (see below)

As a back up, there are two possibilities I can see:
1) Source a suitable quantity of industrial taconite pellets.
With the long history of industrial iron production in Scotland, I would expect this would not be massively difficult. Perhaps contacting one of the (still existing?) industrial operations?
Usually even an abandoned modern plant has tons of this stuff piled / spilled around. Given that we would be looking for at best a couple of 5 gallon pails of pellets, gathering up enough is more a problem of getting to the source.
This also might suggest attempting to make some kind of sponsorship arrangement with an existing corporation? At the least it would be donation of taconite, but maybe this could be expanded into some project funding? Even covering the cost of the charcoal for the project would be a massive help. (Something like "'Scottish Steel' - From the Past and into the Future") I certainly don't have any problem promoting a local company / business who makes a significant contribution.
2) Making up a suitable ore analog.
This will require sourcing a local / regional pottery supplier. The raw iron oxide, both red (Fe2O3) and black (Fe3O4) should be available as 20 kg amounts. We could easily at least roughly duplicate the components of the Macaulyite by adding other oxides. The primary consideration there would be cost, and to a certain extent the time required for the drying of the mixed paste (?)

Any given smelt attempt will require ideally 20 kg of ore as a working amount.

My experience with this size / type of furnace is that at least 8 kg is needed to basically establish the working environment inside the furnace itself, the slag bowl system. Any amount of ore *over* that 8 kg then goes into the developing bloom.
The experience here is that given a suitable ore, 20 kg input should give something in the order of 20 % + return as bloom.

This yield rises sharply as you increase the amount of ore used. Also because the furnace system is established (heat and slag bowl), there is a big efficiency increase with larger volume ore additions towards the end of the working sequence.
Remember the limits of manpower (and skills). As the blooms get larger and larger, they become significantly more difficult to work. There is a real limit to the ability of human hands to effect larger masses of metal.* (Admittedly, with enough skilled strikers, even the largest blooms possible via these small bloomery furnaces *can* be compacted and forged. You do have to be honest with yourself - how many skilled hands are realistically available?) Despite the improvement in yield / efficiency, this manpower gap is why at Wareham I tend to aim for smaller blooms - in the 3 - 5 kg range.

* Although only a very (!) rough way to think about this is :
Force applied by a hammer is more or less a multiplication of hammer mass times swing velocity.
Control of the hammer declines with the weight of the head, modified of course by the skill and body size of the worker.
All this combines to produce a real limit to heavy a hammer any individual can be effective with - and how much force they can produce.
The bloom mass itself has a resistance to forming. Very (!) roughly this can be thought of as closer to the *cube* of the volume (thus also the weight). So a 5 kg bloom may take something closer to six times the amount of hammer force to compress and shape it over a 3 kg one.
(Note - this is only intended as a rough ball park estimate - I'm really not sure what the exact physics is here!)

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

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