Wet cast iron

Neil Mahrer <neil.mahrer [at] jerseyheritage__org> writes

>A 19th century steam and water mill is being restored here in
>Jersey.  One feature that has been exposed in the renovation is the
>housing for a large flywheel that was set with its axle at floor
>level in one of the rooms.  The depression into which the 4 meter
>flywheel originally sat was lined with rivetted cast iron sheets to
>keep the ground water out.  This iron structure has been completely
>submerged in fresh water for about a century and has now been pumped
>out and cleaned (it still however stands in waterlogged ground). The
>iron is in very good condition but damp patches are appearing at
>some of its rivetted joins.  I am hoping for some advice about what
>should now be done to protect the structure which will now stay in
>situ, dry and on display.  I was unsure whether painting the iron on
>the exposed dry side might do more harm than good.  The structure
>cannot be extracted to allow a coating to be applied to the wet
>side.  Any help would be gratefully received.

Unfortunately, painting one side of corroded iron is like painting
one side of a sponge; if water continues to penetrate it is likely
to pop off the painted surface, just as rising damp pushes off paint
on masonry.

You do not describe how the structure was "cleaned", but even though
the water in which it sat was "fresh", I would suggest an analysis
of the "waterlogged" water to determine its potential future impact.
Is this an acidic bog, or constantly moving fresh water?

In theory, the pieces should be cleaned of corrosive salts (if
present) by chemical soaking and/or electrolysis, dewatered, and
treated with a sequestering agent to reduce ongoing corrosion. The
application of a coating would depend on the installation
environment, as it could either help or harm.

I would suggest that you consult with a conservator of
archaeological metals, and possibly a heritage engineer regarding
treatments and the pros and cons of drainage and galvanic protection
systems.

Lisa Mibach
Heritage Resource Management
Canada


                                  ***
                  Conservation DistList Instance 22:6
                  Distributed: Saturday, July 19, 2008
                        Message Id: cdl-22-6-002
                                  ***