Sunday, February 14, 2010

Considering the Evenstad Furnace

The following is adapted from an ongoing e-mail conversation I am currently having with professional metallurgist turned archaeological researcher Arne Esplund. He is referring to a Norwegian patterned furnace, described in some detail by writer Ole Evenstad in a manuscript from 1782. Readers interested are directed to "Iron Production in Norway During Two Millenia" by Arne Espelund, 1995 (ISBN 82-992430-3-3)
The Evenstad furnace was intended for small scale commercial production in a rural setting, using a direct process bloomery furnace.
" Another point: why was the Evenstad method established if people could use a simpler method, a sort of prolongation of the medieval method, as it was perceived? There are some 500 slag heaps from Evenstad smelting in Norway alone, in Sweden probably more. "

Arne Espenlund
(from a private e-mail - 2/13/10)

If I could hazard a guess on the Evenstad method?
A couple of things come to mind, looking over the system in the manuscript:

1) Use of straight wood.
This saves a lot of physical labour. Making charcoal correctly (!) is at least half as much labour as mounting the smelt itself. The Evenstad process lets you skip that. (see a recent post : 'On the Economics of Making Charcoal')

2) More durable structure.
Although its going to be more work to build the brick structure for the Evenstad furnace, that expenditure is going to be reflected in just what kind of operation using this smelter would be. This aspect is rarely commented on by various researchers.

In the Viking Age, one system is that of seasonal iron production on family farmsteads - for household use primarily. This might be no more than one smelt a year. Even a simple furnace will endure a single smelt. My own experience here, where we smelt only 3 - 4 times a year, is of use. The damage due to weather most often exceeds the damage due to the smelt itself. So if you were a householder, it might prove just as easy to build a brand new furnace every year - if one or two smelts was all you needed to undertake. Not too surprisingly, such locations are not going to be marked by much by way of archaeological remains. We have one solid clay construction furnace here that has been out in the weather since 2003 at this point. It is little more than a ring of slumped clay with fragments of sintered ceramic and slag pieces inside.

Furnace Remains - June 2003 solid clay construction, as it appeared in early Spring, 2009

Most ancient sites excavated are in fact 'industrial' level production. Large numbers of furnaces built by groups that make their living from smelting over and over as the primary activity. Here you would want physically robust furnaces, able to withstand a dozen or more firings. It would be worth building massively to ensure this. Individual furnaces would show repairs between uses. (We have run a single half banked clay cobb furnace FIVE times before it failed).

So I see the Evenstad furnace as a 'semi professional' equipment. The Evenstad system was intended for use on household sized farmsteads, but as at a significant production level, those blooms to be sold for straight cash (ie, not for direct use on the farm itself). So this places it in between the older 'once a season' and the full scale (year round?) industrial level. Again, the expected use cycle requires more durability, and the expected reward makes the more elaborate construction a good investment.

Coupled directly with this is the suggested use of double, perhaps even triple (?) end to end smelt cycles in Evenstad. There certainly is a significant gain in charcoal expenditure and time against production when you take an already full temperature furnace and after snatching out a first bloom, immediately re-fill and run a second smelt. I personally have only done this twice, the first time more or less by accident, the second time quite deliberately. That would the the 'hot swap' , June 2007 - These experiments do not illustrate the best numbers, but in terms of charcoal consumed against weight of bloom resulting, the second smelt was significantly more efficient.

3) Ability to utilize one structure for both smelting and as a re-heating hearth for bloom consolidation steps.
Although it is possible to use a smaller cylindrical clay furnace to re-heat the bloom after its initial extraction temperature has faded, this has proved quite difficult.
The funnel shape of the Evenstad furnace would certainly be much easier to operate inside of. Controlling the exact location of the bloom mass within that furnace, as if it were a giant forge, would be much more simple. As the bloom is compacted into the final puck shape, it also expands in diameter as it collapses downwards. Inside a cylindrical furnace, it easily can become too wide to even fit. The clay furnace walls are also not that durable or stable. As the bloom is reheated, it must be constantly adjusted for position to achieve an even heat. It is very easy to critically damage those thinner clay walls.

All these practical reasons, (things we have directly experienced in our own smelts), certainly suggests to me that the Evenstad furnace structure would provide most definite advantages if your goal was a yearly round of repeated smelts aimed at even limited commercial bloom production. Fitted with a simple wooden lid, this type of furnace would certainly over winter well, and would likely require only minimal repair for the next season's use.
I don't see anything here that reduces the wisdom of the Evenstad system - considered against the application it was intended for.

1 comment:

Polymarkos said...

With as much smelting as you do, why haven't you built one of these in your yard?


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