Steve Boxley inquires concerning the "aggressive conservation technique of
annealing" as a conservation measure.
 
These heat treatment methods essentially attempt to remove chlorides from the
artifact by volatilization. In addition, there is thought to be the additional
benefit of converting at least some of the corrosion products to metallic
iron.
 
The approach uses a hydrogen reduction furnace -- an electrical furnace that
can heat objects in a stream of hydrogen and or nitrogen. There are two
techniques still in use. One uses temperatures of upwards of 900 to 1000
degrees centigrade. The problem here is that the temperature is so high that
the metallurgical changes occur. The other approach uses temperatures of
around 400 degrees centigrade, followed by immersion in sodium hydroxide
followed by washing. In this second case, although hydrogren is still used,
the goal is to make the iron more porous, allowing the water rinses to more
effectively remove the chlorides. While not as likely to cause metallurgical
changes (but be aware that some metals can be changed under very low
temperature conditions), it also takes up to 40 weeks to remove the chlorides.
 
Both techniques use hydrogen, which of course in combination with oxygen
(i.e., air) forms a highly explosive mixture. Add to this an electric furnace
and, well, you probably get the picture -- not something you want to do in
your basement.
 
There are a lot of other metal conservation techniques and I would be very
reluctant to embark on this approach unless _all_ other approaches have been
thoroughly explored and ruled out.
 
Best,
 
Michael Trinkley, Ph.D.
Director
Chicora Foundation, Inc.
PO Box 8664
Columbia, SC  29202-8664
803/787-6910
Email: [log in to unmask]