cleaning - How can I clean an old metal photographic plate? What are best practices for conservation?

As someone who 'knows about photography' I've been handed some old metal (possibly copper?) plates which look like they were at one time used for in a printing press for some kind of short run, but I'm more than happy to be corrected.

They've been stored in a box in a church for quite a while and have a few mucky bits on but otherwise seem to be in fairly good condition.

I'm looking to identify what they are and recommendation on best practices for cleaning & storage.

Answer

I recognize these. I've made them. It is a copper printing plate. They are screened for use directly onto the paper, probably in a letterpress since they are flat.

They are made by exposing a print (copy) in a large process camera with a vacuum back to hold the film perfectly flat during a long exposure. The resulting very dense high contrast "lithographic" film is then contact printed onto a sensitized copper plate (gum bichromate process). The plate is put into an acid etch bath that eat away the exposed portions and leaves the unexposed portions (a dot screen) in a relief. Ink sits on the high spots that hit the paper when an inked plate (That's what you're looking at) and paper are pressed together in a printing press.

There's more but you'll find everything on the Internet. If not, I can answer any question you may have. I've done it and taught it. I still have a press in my living room that would accept it and make a nice print from it. :)

Edit: I see there're tacks on the edges. They hold the thin chamfered etched copper plate onto a piece of wood - probably plywood - to raise the surface to the international standard type height of 0.918 inches.

Cleaning is easy, wipe it with a slightly oily (keroscene) rag to soften and remove any ink hardened in the surface although it looks well cared-for by the last printer's devil (assistant). The rich patina of the aged copper can be removed to make it sparkle as new; but, that will remove some of the very detailed etched surface of the plate. It is best cleaned not polished to maintain its original integrity. Store flat or on edge. Caution: Copper is a soft, easily-scratched metal. Once scratched, cannot be repaired. For durability, copper plates were chrome-plated.

This would've made a nice cross post with the graphic design group, too.