Friday, October 26, 2007

Historic Blooms?

Some thoughts on Blooms

I have been spending more time than normal in discussions related to the current series of experimental iron smelts. These include members of our working team, iron masters Lee Sauder & Skip Williams, and researchers Kevin Smith, Arne Espelund and Birgitta Wallace. (In various combinations, with separate topic threads intertwining).

I wanted to pull together a couple of things for the interest of my readers (and fellow pyromaniacs). Please remember that the following represents ideas from any (sometimes all) of the people mentioned above, who may not be clearly represented. With that large grain of salt taken - read on...

A reasonable question was raised about how I keep comparing the blooms we have been making compare to actual Viking Age artifact blooms.

First a note on our sizes:

I had decided to keep the sizes of our blooms into the small end of those found from the Viking Age. As was pointed out to me by a couple of people, historic blooms tend to range closer to 8 kg on average, with a few samples as much as double that weight. From what we have learned from Lee and Skip, and our own direct experience, once you get the iron bloom ball rolling, its actually pretty easy to just keep packing on the size. Inside the reaction, the furnace reaches a point where to maintain a consistent burn rate (at roughly 6 - 10 minutes for 10 litres) you effectivley dampen down the heat by adding ever larger charges of ore. In some smelts we have seen ore additions raised to as much as one and half times (by weight) of the charcoal amounts, inside a consistent consumption. When that happens the end product are truly monster blooms - in the range of 20 kg (Lee and Skip have gotten even larger ones).
In truth, there is a basic amount of fuel expended to get the interior of the smelter at the conditions for the creation of any bloom in the first place. This will vary by the construction and size of the smelter, does tend to represent 50% or better of the total fuel consumption. It makes practical sense to just keep piling on the iron once you have gone to all the work to get things happening in the first place.

The counter to this is : How do you work that huge lump of metal afterwards? For those other modern blacksmiths reading, imagine hand forging a piece of iron which is an irregular half ball shape, roughly 10 cm thick by 20 wide. The obvious solution is - POWER HAMMER. But what if you did not have any? Even trying to re-heat such a large mass after it has cooled is a daunting task...

We also have been influenced by our initial starting point in all of this - the Viking Age iron smelt at Vinland by members of Leif Ericsson's crew circa 1000 AD. The written reports suggest roughly 3 kg of iron were produced at that first smelt in North America. (Although its important to note that I'm not sure if that number may refer to the estimate of the workable iron produced, not necessarily a measure of the weight of the bloom out of the furnace.)

Taken altogether, the DARC series of smelts have kept the size of the blooms produced in the range of 3 - 5 kg. I certainly feel that if we can make 3 kg of good workable iron, we could have easily produced 10 kg with just a bit more smelt sequence. It also leaves us with a mass of material which is much easier to manipulate into the consolidation phase of the process.

Second - physical appearance:

Kevin Smith commented "...those who wrote about these Norse bloomery furnace blooms were convinced that the archaeological examples in question were blooms that had been consolidated to that shape and density through initial forging/welding, perhaps in several steps, after removal from the furnace."

Kevin is exactly correct that many of the artifact blooms show a distinctive 'hockey puck' shape, a flat sided disk, often sliced from one edge into a 'pac man' profile. As he states, this specific shape is clearly the result of heavy compaction hammering of the bloom. There are also however a number of samples which have a clear convex / concave bottom and top shape to them. These are almost identical to what we are pulling out of the furnace from our own smelts.

Iron Bloom - Oyane, Telemark, Norway (19.5 cm dia.)

'Resurrection' Bloom - Wareham, 10/2006 (18 cm dia.)
bottom uppermost

On extraction, the bloom mass will have a dished bottom surface, with the upper side either flat or slightly dished in. The top surface is normally well compacted, the lower side somewhat less so, with the lightest structure to the circular edges (especially the side furthest from the tuyere).
We often end up doing not much more than a very quick surface compaction on the still hot bloom after it is extracted. With a good quality dense bloom, the core is noticeably very hard, with lacy material attached to the edges. The first working over with hand sledges will either compress in, or often just knock away this attached material.

(more to come)

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

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