Tinware

This is in response to Melissa Heaver's query concerning the
corrosion of tinwares.

Tinplate is a material that was formed by first pickling wrought
iron sheets to remove fire scale, dipping the sheets in tallow or
palm oil, and then finally heating the prepared sheet iron to a
cherry red color and immersing them in several baths of molten tin.

Tinplate corrodes because micropores are present in the tin coating.
Tin is a more noble metal than iron and thus the iron corrodes at
the base of the pores. The iron salts are carried by an electrolyte
(moisture in the form of RH) and forms a hard expanded corrosion
crust along with ferrous staining of the surface.  Often the tinned
surface lies intact underneath the rust.

The reason why so many kitchen items are tinned, both ferrous and
copper alloys, is that tin salts are non-toxic.  The reason why tin
cans don't rust on their interiors is because the complexing nature
of the tin ions by food products greatly reduces ionic activity and
there is a reverse in polarity of the tin in an active direction.
Thus the interior of the tin can becomes cathodic and the exterior
becomes anodic, leaving the exterior tinning to become the
sacrificial component of the electrochemical system.

The solder joins, usually a lead based solder, are usually stable
because of the less noble nature of ferrous metal.

So, my best advise would be to reduce the exposure of these objects
to relative humidity which drives electrochemical corrosion.  Get a
conservator out to survey the objects and to make specific
recommendations.  Do not try to clean these objects yourselves
(especially if any of these have further surface treatments such a
paint).

I treated about one hundred objects for a kitchen exhibit several
years ago, the majority of them tin plate, and they were among some
of the toughest, nastiest, treatments that I've encountered.  (ie.,
a raised, hard, steely ferrous corrosion crust with a soft tin
surface underneath, mingled with layers of historic cooking grease)
Yuck, these artifacts did stink!

David Harvey
Associate Conservator,
Metals & Arms
The Colonial Williamsburg Foundation
P.O. Box 1776
Williamsburg VA 23187-1776  USA
757-220-8039

                                  ***
                  Conservation DistList Instance 12:7
                  Distributed: Thursday, July 2, 1998
                        Message Id: cdl-12-7-003
                                  ***