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.

The description of both the plates and the white substances is
consistent with zinc. The white substance is probably a combination
of zinc carbonates, zinc oxides and possibly zinc acetate, with zinc
carbonate being the predominate species.  Zinc carbonate is formed
when zinc metal is exposed to moisture and CO2 in the atmosphere.
Organic acids from the timber packing crate would contribute to the
oxidation process. Zinc carbonate is sparingly soluble in water and
would seem to disappear when wet, only to return upon drying.

To remove the oxidation I suggest you experiment with washing the
plate in dilute sulphuric acid (0.05M). The carbonates should
dissolve in this solution and you should get minimal etching of the
zinc metal. After the acid treatment I would rinse the plate
thoroughly with clean water, followed by dewatering with an alcohol.

George Bailey
A/g Manager
Conservation of Large Technology
Australian War Memorial


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                  Conservation DistList Instance 17:32
                  Distributed: Monday, October 6, 2003
                       Message Id: cdl-17-32-009
                                  ***