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?

While it is always difficult to offer advice on a specific object
with out examining it, I can offer a few general responses to this
query. German Silver (the 19th century name) is an alloy of
copper-zinc-nickel that has a white, silver-like appearance and is
corrosion resistant. The green patches on your rule are probably
some copper corrosion products and may have developed from the use
of materials used to maintain the rule, such as organic oils or
waxes. If the green patches have the morphology of a waxy film
rather than localized cells, then you probably have this type of
organometallic corrosion. In my experience this would be a fairly
easy cleaning problem, as the local application of white spirits can
remove much of the waxy film. If you have localized corrosion cells
or raised "green" nodular corrosion then you might consider options
such as the use of dilute formic acid, localized electrolytic
reduction, or mechanical debridement of the corrosion products.

Another thing to investigate before undertaking treatment  is
whether you may have an aged varnish on the metal components of the
rule.Often mechanical and scientific instruments were coated with
organic varnishes and this should be evident under UV light.This
will help you rule-out or rule-in some of the treatment options (all
puns intended!;-).

As far as using a protective coating I would avoid one unless the
object will be subjected to elevated humidity levels, salt-air,
handling with bare hands, or pollution. I would recommend a
microcrystalline wax first and a lacquer last, especially if your
intent is not to polish and expose fresh metal to the environment.

I hope that my response has been of some help to you,

David Harvey
Conservator of Objects
The Rocky Mountain Conservation Center
The University of Denver
2420 S. University Blvd.
Denver, CO  80208
303-733-2712
Fax: 303-733-2508


                                  ***
                  Conservation DistList Instance 14:3
                  Distributed: Thursday, July 6, 2000
                        Message Id: cdl-14-3-001
                                  ***