Metal printing plates

Eric Alstrom <eric.c.alstrom [at] dartmouth__edu> writes

>The plates, which could be zinc (but I'm not positive) are covered
>by a white substance, which originally was thought to be mold.  I
>don't think it is mold, but rather some oxidation on the surface.  I
>am not a metals conservator, but was asked to experiment with one of
>the less interesting plates.  I tried washing in DI water and
>lightly scrubbing the surface.  The white substance seemed to
>disappear while wet but reappeared just as much as before when it
>dried.

Corrosion products on zinc are white and tend to form if the zinc is
left wet with reasonably pure water, eg the underside of a
galvanised steel roof where condensation remains.  In corrosion rate
monitoring work, I have removed these products with a 10% solution
of ammonium acetate.  Other solutions are described in ASTM standard
G1.

Magnesium is one of the more reactive metals and it is not
surprising that unprotected magnesium alloy plates have corroded.
There are electrochemical treatments that protect magnesium alloys
in the same way that routine anodising processes are used for
protection of aluminium alloys used in doors and windows.

And finally, the initial banning of zinc plates seems an over
reaction to a problem that could probably have been corrected by
reducing the impurity content of the alloy.

Dr Graham Sussex
Sussex Materials Solutions Pty Ltd
100 Power St
Hawthorn  VIC  3122
+61 3 9819 1998
Fax: +61 3 9815 1260


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                  Conservation DistList Instance 17:34
                Distributed: Thursday, October 16, 2003
                       Message Id: cdl-17-34-008
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