Tin corrosion

I'm working as metal conservator for the Utrecht University Museum
which houses a large collection of technical and scientific
instruments. I came across two different types of problems connected
with objects that have a surface layer of tin. Together with an
objects conservator I've studied the objects under discussion and we
came up with the following request. Via the distlist we hope to hear
from other conservators or scientists who have done investigations
or have practical experience with this kind of problems.

The first problem concerns tinned iron objects produced namely
between 1750 and 1820. The iron was probably covered with the tin by
immersing the prepared metal  in melted tin. The tinned surface was
then painted with black paint and on top of this a gold decorative
painting was applied. Many of these objects show serious damage of
the tin and paint layer (some with a loss of the layers of up to
80%).  It is not really obvious for me whether the corrosion of the
tin is the reason of the paint loss or whether the paint loss
exposed the tin to the surrounding atmosphere, which resulted in its
corrosion.

The second problem concerns objects with a thin layer of probably
rolled tin. The rolled tin is fixed on glass tubes (Leidse flessen =
Leiden jars) with a glue (probably shellac). The tin is corroding,
expanding and flaking off the surface. In a last stage of the
corrosion the tin becomes powdery. It seems quite likely to us that
the rolled tin is under stress and thus corroded more easily than a
thick piece of tin. The surfaces of the objects should be
consolidated to keep intact as much of the (decorative) layers as
possible. But we're hesitating to take actions as we're not sure
what the deterioration processes are and whether deterioration just
continues after any consolidation treatment.

We would be grateful for any remarks on these problems,

Jan Willem Pette, Margrit Reuss
University Museum Utrecht
The Netherlands
+31 30 2538721
Fax: +31 30 2538700



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                  Conservation DistList Instance 14:48
                 Distributed: Thursday, March 22, 2001
                       Message Id: cdl-14-48-013
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