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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?

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    $\begingroup$ Did you consider the use of an ultrasonic bath, possibly with hot water, but without the sodium hydroxide? $\endgroup$ – Klaus-Dieter Warzecha Feb 7 '17 at 16:58
  • $\begingroup$ How big is the plate? Do you have a scrap plate that can be used for testing? Is the ink itself soluble in water? $\endgroup$ – MaxW Feb 7 '17 at 18:28
  • $\begingroup$ 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. $\endgroup$ – frank n Feb 8 '17 at 18:49
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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.

A common method of putting a good shine on tarnished copper is with weak organic acids like citric acid. Lemon juice and table salt make a favorite home copper cleaning solution. Again, this is only with regards to the cleaning oxides from the copper metal itself, not the ink residues coating it.

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!

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    $\begingroup$ Exposed copper will develop a patina. It is quite possible that the surface crud is a combination of ink and corrosion compounds. $\endgroup$ – MaxW Feb 9 '17 at 4:39
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    $\begingroup$ For what it's worth, I just came across this related question and answer. $\endgroup$ – airhuff Feb 11 '17 at 1:02

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