First off, I checked and there are indeed a number of questions on copper patinas, but none which deal with resilience.
I own a number of copper writing utensils, which I'd like to get a nice patina on, preferably verdigris.
What I came up with thus far
Looking up the topic here and on wikipedia it seems that copper can patina either in brown or in blue. According to an unsourced claim from wikipedia, exposure to chlorides leads to blue-green coloring, while exposure to “sulfur compounds” [sic] leads to black-brown coloring. I find this highly doubtful, since copper sulfate forms bright blue crystals... so it might be more complex.
What I can conclude from natural experiments (e.g. old copper pennies), is that “normally” copper objects will patina black-brown (Fig. 1), and sometimes will develop some blue-green spots on them, which however bloom out of the penny, appear corrosive rather passivating (i.e. more like iron oxide than aluminium oxide), and are rough and brittle (Fig. 2). This stands in stark contrast with the patina I see on statues and on copper roofs (Fig. 3), which is indeed somewhat rough, but is never brittle, and looks more like proper passivation than corrosion.
I also consulted youtube for verdigris how-to's, and they all end up producing very brittle looking crystals, which are only preserved by adding some sealant to the metal after treatment — for what it's worth the color also doesn't really match what I see from statues/buildings (the architectural patina is more green, less blue, and less vibrant).
Someone brought up in the comments, that the passivating light turquoise patina is specific to bronze and not copper. Comparing Fig. 3 to Fig. 4, it seems it can be obtained on either, but I'm not asking about bronze in this question.
Figure 1, brown patina, as naturally develops with all handled copper objects.
Figure 2, brittle blue patina, as I've seen in isolated spots on pennies and analogous to what a lot of youtube videos show how to create — this is specifically what I don't want.
Figure 3, the glorious coating-like verdigris patina I'd like to get.
Figure 4, this shows the same coating-like verdigris, but on bronze rather than copper.
So I guess my questions are:
- Is it possible to obtain a patina on objects which I handle which looks more like turquoise copper roofing and less like old pennies?
- If so, how?
- Should I be concerned about it being (significantly) toxic?
Addendum (not a self-answer since I still wasn't able to figure this out)
I tried a few experiments, basically taking a copper plate, cutting it to pieces which fit into a falcon tube, and sealing them in with tissue paper soaked in either vinegar (left) or ammonia (right). I left them there for about 1 month at ~10°C.
The first row is what they look like straight out of the tube, the second row is after handling them with dry gloves, the third is after handling them under water for a while. The Vinegar-treated plate basically had all the colour wash off immediately, not sure if the ammonia treated plate has truly resilient colouring either. This can't be what's on statues... Some of the colour indeed bleeds, but there's some layer that forms that's not water-soluble.