Osmund iron

Eero Ehanti <eero.ehanti [at] helsinki__fi> writes

>Does anybody have experience or knowledge about Osmund iron? The
>term refers to small wrought iron bars, which were, according to
>written sources, typical merchandise in medieval times, but it seems
>that very few examples have been found and conserved. ...

You bring up a subject for which I have great fondness. Many years
ago, before I started my career in conservation I spent several
years researching and reconstructing an American Bloomery furnace in
which we made wrought iron directly from ore and then had extensive
metallography conducted on all of the stages of the process by The
Museum Applied Science Center for Archaeology at the University
Museum, The University of Pennsylvania, and also by Dr. Robert B.
Gordon at the Department of Geology at Yale University.

It has been some years since I have read deeply in archaeometallury
but I will share a few observations with you.

>From what I understand there are claims that "Osmund" refers to a
process, later called the Walloon process, in which cast iron was
subjected to an intense oxidizing fire. As it started to melt the
forming bloom was held on a long metal rod and was rotated in the
fire to exposure more surface area to the oxygen. As this happened
the iron was decarburized and with the resulting higher melting
point it was eventually formed into a raw bloom or more or less low
carbon or wrought iron to be then repeatedly hammered and headed
into bars. I ran across other references that "Osmund" iron was not
folded and welded (sort of an ironmaker's kneading process to make
the product more homogenous). I have also seen references to
"Osmund" being a specific size of the wrought iron bars.

I would think that beyond recording the size and weights of the bars
and doing x-radiography to see if there are any stamped markings on
them (and assessing the corrosion crust), that the microstructural
analysis would prove very interesting to see if a determination can
be made as to the process of manufacture. If it were indeed "fined"
from cast iron I would guess that you still may find remains of
graphitic iron in the microstructure of the iron that may have
escaped the oxidizing fire. This takes a careful and experienced
historical metallurgist to conduct this sort of examination and
interpretation as graphitic iron can certainly be accidentally
produced in the bloomery or direct process as well.

It would also be interesting to see how heterogeneous the
microstructure is when compared to irons from the same time period
from other sources. There are very few instances where you can
attribute a specific iron artifact to a specific ore, but getting a
qualitative composition of the iron is always helpful in
ascertaining what may have been in play and how in the quality of
the iron that resulted (for instance phosphorous and sulphur iron is
generally not the best wrought irons). Also the size and
distribution of slag within the iron is always a general indicator
of the quality of the wrought iron bars.

The Journal of the Historical Metallurgy Society is a great source
of information and I believe that they are on the internet now.

As far as conservation goes I would recommend the obvious--such as
not using any conservation process that would affect the
microstructure, and creating a very stable storage for the iron. If
they are stored wet, keep them wet and start desalination--if only
to monitor for chlorides and change the water regularly--keep the
temperature low to avoid biofilms from forming. If they have been
removed from water then keep them as dry as possible and use oxygen
scavengers in enclosed microenvironments until they can receive full
conservation.

I did do Google search for Osmund Iron and found a slew of
references--one that seemed as if it might be helpful was this one:

  <URL:http://forskningsbasen.deff.dk/ddf/rec.external?id=dtu165515>

David Harvey
Conservator
Los Angeles, California


                                  ***
                  Conservation DistList Instance 20:43
                  Distributed: Tuesday, March 6, 2007
                       Message Id: cdl-20-43-001
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