I have not been following this particular discussion but we were once told to minimize handling as even oils from fingerprints can cause discoloration over the [very] long term.

I will add, however, that a very powerful way to illustrate “five centuries of map making” is a side by side display of a woodblock and a woodblock prints; a copper plate with a  pull from copper plate; and a lithographic stone with a lithographic print. 

Ed Redmond

From: Maps-L: Discussion Forum for Maps, Air Photo, Map Librarianship, GIS, etc. [mailto:[log in to unmask]] On Behalf Of Joel Kovarsky
Sent: Sunday, November 16, 2014 3:19 PM
To: [log in to unmask]
Subject: Re: 'Enhancing' copper plates; I suggest NOT (at least per se)!

On 11/16/14 11:48 AM, Virginia R Hetrick PhD wrote:

Hi, Susan -
 
I'd suggest NOT doing any enhancing of any kinds.  

This might be accurate, in terms of "enhancing" (a bit unclear as to how that term is being used in the context of these copper plates): http://www.sha.org/index.php/view/page/process#A

Their comments:

Metals

Dry-brush with soft paintbrush. Dislodge soil with soft tools. Best to not clean

Organic remains attached, or incorporated into corrosion. Loose corrosion layers. Multiple components. Corrosion in the presence of water.

I would presume similar concerns would apply here, but it is always good to talk with the experts as several have suggested.

            Joel Kovarsky