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.

I would suggest that you detect if your plates are made from lead or
lead alloy. It is very easy to test on a microscopic slide. Put a
tiny sample of your white powder on the glass with a scalpel and add
a drop of hydrogen nitrate (HNO3, 2M) on it in a fume cabinet. Wait
until it dries. With a light microscope you can see typical
crystallisation if the metal contains lead. They look like
frost-flowers on a window/twigs of fir. If there is lead, you should
be very careful because it is soft metal and hazardous to your
health.

Lead corrodes by organic acids like formic acid and acetic acid.
Many woods emit such fumes. Distilled water is also corrosive to
lead. Your white substance is very likely lead carbonate and you can
clean it with diluted sulphuric acid (H2SO4, 2% v/v) with cotton
swabs and rinse the object with tap water instead of distilled
water. Drying can be done in the air in fume cabinet and coating--if
wanted--with renaissance wax.

Raili Laakso
Object conservation student from Finland


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                  Conservation DistList Instance 17:32
                  Distributed: Monday, October 6, 2003
                       Message Id: cdl-17-32-011
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