German silver parallel rule

Hilary Wright <hilarymwright [at] hotmail__com> writes

>I am about to conserve a parallel rule, a surveying instrument made
>in the UK towards the end of the 19th century and sold in Australia
>where it now is. There appear to be some copper coloured "corrosion
>products" on it at intervals but otherwise the rule is merely dull.
>The curator would like the finished effect to be corrosion free
>though not very shiny and to be lacquered to prevent further
>deterioration. Has anyone any experience in dealing with German
>silver (nickel silver)? Is the copper colour indeed copper due to
>loss of nickel? What does anyone advise on cleaning and protective
>coating?

I would like to support David Harvey's recommendation for
microcrystalline wax as a protective coating: since the shiny
appearance of a lacquer is not required in this case, it is worth
remembering that galvanic corrosion is stimulated by the anodic and
cathodic states caused by  any flaws in a coating, whether these be
due to brushmarks/bubbles during application, or by subsequent
cracking from mechanical or thermal shock. Lacquers tend to be more
susceptible to these problems than waxes. In addition, the aesthetic
quality of the sheen of a hard mc wax is more similar to the
"cared-for" appearance of aged historic pieces than is the gloss of
(even a matted) lacquer.

Lisa Mibach
Heritage Resource Management


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                  Conservation DistList Instance 14:5
                   Distributed: Monday, July 17, 2000
                        Message Id: cdl-14-5-003
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