Effect of sodium hydroxide solution on copper plate

I have a copper etching plate that has over 100 years of dried ink in the lines of the plate. Someone suggested flake sodium hydroxide as a possible solvent for the ink. My concern is preventing damage to the copper itself.

Suggestions? For example, what should the concentration of the solution (in water) be?

• Did you consider the use of an ultrasonic bath, possibly with hot water, but without the sodium hydroxide? – Klaus-Dieter Warzecha Feb 7 '17 at 16:58
• How big is the plate? Do you have a scrap plate that can be used for testing? Is the ink itself soluble in water? – MaxW Feb 7 '17 at 18:28
• The plate is 19" x 29" and due to the age of the plate, I am unsure of the makeup of the ink, but probably oil-based, but certainly not water based. I have tried other solvents, i.e. denatured alcohol, mineral spirits, water, benzene, kerosene. I get some small toned liquid release, but I think this is more removal of dirt and certainly not the ink. Ultrasonic bath is out of the question due to size. Talked to Graphic Ink & Chemical Co. in Chicago, and they suggested sodium hydroxide, but I hesitate to use it until I know the plate will be undamaged. Will try on the back of the plate to test. – frank n Feb 8 '17 at 18:49

Regarding the copper metal itself, the only down side I see of using a $\ce{NaOH}$ is that it is unlikely to remove some of the oxides that would be covering any exposed copper. The initial etching process would have mainly produced water-soluble $\ce{CuCl2}$, likely with a bit of insoluble $\ce{CuO}$. $\ce{CuO}$, $\ce{CuCO3}$ and some $\ce{Cu(OH)2}$ would have likely formed on any unprotected copper surfaces over time. $\ce{NaOH}$ could remove $\ce{Cu(OH)2}$ and $\ce{CuO3}$, but not $\ce{CuO}$. It also should be unreactive toward the copper plate itself.
My bottom line advise is to do what I’ve seen in your last comment that you plan to do; test a portion of the bottom side of the plate with the $\ce{NaOH}$ solution to verify that this solution cleans the ink residues without harming the copper. I think you can do this with confidence that the copper metal will be unharmed. If the ink residues are removed and you want to remove any remaining oxidation to create a fresh, shiney copper plate, you could dissolve just a bit of salt into lemmon juice and wipe the plate with this solution until shiny!