Thursday, June 01, 2017

Mixing Bog Ore Analog ('DARC Dirt')

(Back to our regularly themed topics?)

I am currently involved in the planning phases for two major museum based projects.
July - To L'Anse aux Meadows NHSC with a team from DARC.
August - To the Scottish Crannog Centre (Aberfeldy) - 25th Anniversary event

Both of these involve iron smelt demonstrations.
In both cases there is discussion on the actual ore type to use. I have suggested that the bog ore analog, initially developed by DARC for our 'Vinland' research series, should be used.

As a fast review, 'DARC Dirt' was initially developed to provide the team, working here in Central Ontario, with a consistent and dependable iron ore type. This was important because originally, there were too many other unknown variables effecting the working design of the furnace and operating method. Experience has since clearly proven that ore has the biggest single impact on results : You can have a good furnace and proven experience, but if the ore is less than ideal - you are just not going to get good results (yield and density)

'DARC Dirt' bog ore analog - added to a working iron smelting furnace (Vinland 1?)

If you are interested in the long development sequence of our bog ore analog, the easiest way to read the many earlier commentaries is a simple search :'bog+ore+analog'


There are two types of iron oxide easily available via pottery supply.
Black = Fe3O4
Red  = Fe2O3
Although the black does create a higher overall iron content per weight, in the past it was significantly more expensive (about double).
Checking today's prices at the Pottery Supply House (Brampton, Ontario), Both the black iron oxide and the 'Spanish Red' are priced at $55 CDN for a 50 lb (22.6 kg) bag.

The problem with this material is that it is extremely fine - even more than baking flour. If you attempt to add it straight from the bag, most of it is just going to blow straight back out the top of the furnace.
Originally we had been attempting to match the chemistry and texture of natural primary bog iron ore as uncovered in the archaeology at L'Anse aux Meadows (Vinland). The reports indicated '10 % organic matter'. To simulate this, our analog uses the same amount of whole wheat flour.

What happens in production is that the flour acts as a kind of binder to the iron oxide powder.

The method is to add one standard bag at 2.5 kg of whole wheat flour, purchased at the local grocery store, to each full bag of the oxide.
- The best way to do this is to dry mix these ingredients together first, ensuring the flour is spread evenly through the mix.
- The low tech version (1) is to make up two batches, half of each of the main elements, into standard 5 gallon / 20 litre plastic pails. Blend by hand.
- The ideal way to add the required water to the mixture is to use a third pail. Fill about 1/4 of the bucket with water, then add the dry mixture on top. (If you add water on top of the powder, it never penetrates down fully to the bottom layer and corners to correctly mix.)
- Again the simplest (and most effective) way to mix up the water and powder is with your hands. Yes, you will end up covered from elbows down. Yes, the fine powder gets about everywhere (wear old clothes!). Yes, the fine powder seems to get lodged in every small crack and wrinkle of your skin, and seems to take days to completely wash out (!).
- The ideal consistency is roughly between that of peanut butter and mayonnaise. Generally this is about 1/3 water to powder by volume. If you make it too thin, it is easier to mix, but will take much longer to dry.

The next step is setting the paste out to dry in the sun. The ideal thickness is about 1/2 - 3/4 inch / 1 - 2 cm.
If you have the space, the simplest is just spread out the paste on to plastic sheeting, thin painter's drop sheet tarps works very well (and easy to cover over with the same at night. I have a set of cheap plastic trays, about the size of cookie baking trays, which makes it easy to move the drying analog under cover in case of rain.
Obviously your local weather is going to effect drying time. Here in Central Ontario, I need roughly 4 - 6 days (depending on time of year). In real wet weather, I have spread out the analog inside the workshop with a large room fan blowing over the surfaces. This typically doubles the drying time, but does work reasonably well.

Ideally the paste dries out to about the consistency of an oatmeal cookie. It should break under your fingers, not flex and mush (still too wet in that second case).

The dried analog needs to be broken up. Ideally none of the pieces should be much larger than 1 1/2 inches / 3 cm = 'half walnut' sized.

I normally save a 100 gm sample of each analog batch. This is placed in a metal pan, then baked (to a dull orange) in the gas forge and re-weighed. This to primarily determine the proportion of water still remaining after the drying sequence. (Although this method likely burns out some of the organic contribution as well.) Results vary by individual batch, but typically the water loss thus calculated is roughly 15 % on average. (2)

We often slightly 'enrich' the analog mix - by adding 10% / 2.5 kg of gathered hammer scale from my forging operations. I sweep this up daily using a large magnet. There is certainly some additional silica content here (from dirt clinging to the hammer scale on the forge floor. The effect on overall yield is slight (if anything), but this additional iron oxide (as Fe3O4) is certainly available.

There is always some loss in the mixing process. This is balanced by the additional weight in added water. So typically one batch of the straight red oxide to bog ore analog produces about 26 - 28 kg of ore for the furnace.

(1) Given how messy this process is, and how often I do this, I purchased a small size electrically driven home concrete mixer. This unit is designed to mix one standard 50 lb bag of concrete mix - the same weight and volume as the analog.

2) I will often take this into account when recording the final yield number on any smelt using analog. The other natural ores being used by other experimenters rarely have any significant weight as water in their material and measurements.

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

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