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First off, I checked and there are indeed a number of questions on copper patinas, but none which deal with resilience.

Context

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.

enter image description here

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.

enter image description here

Figure 3, the glorious coating-like verdigris patina I'd like to get.

enter image description hereenter image description hereenter image description here

Figure 4, this shows the same coating-like verdigris, but on bronze rather than copper.

enter image description here

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.

enter image description here

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.

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    $\begingroup$ Many of the cast objects that have attractive patinas are not copper but bronze (a copper alloy with tin and other components which is much stronger than copper): these won't have exactly the same patina as copper. // Also exposure to "sulfur compounds" will usually create sulfides not sulphates so your doubt about the wikipedia claim is wrong. $\endgroup$
    – matt_black
    Feb 5, 2023 at 0:11
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    $\begingroup$ @matt_black indeed “The Spirit of Detroit” is bronze — corrected that. But the patina seems indistinguishable from what is seen on copper statues, including the Vercingétorix Monument or the statue of liberty, neither of which look crusty or corroded. Also, copper shingles are made out of copper, bronze shingles seem much rarer. $\endgroup$
    – TheChymera
    Feb 5, 2023 at 1:01
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    $\begingroup$ When the US Statue of Liberty was repaired in the 1980's, there was considerable discussion of how to restore the green patina. Sodium carbonate was used to "sandblast" the statue, followed by ammonium compounds such as NH4Cl to restore the verdigris. See quora.com/… and en.wikipedia.org/wiki/…, for some info. Durability on your copper or bronze material is another question. $\endgroup$ Feb 5, 2023 at 2:01
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    $\begingroup$ I it contains copper , it has some level of toxicity. $\endgroup$ Feb 5, 2023 at 16:24
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    $\begingroup$ I assume artists involved in art restoration or aging may have much more practical, empirical experience with this topic. Most chemist may understand better the underlying chemical principles and processes. But they cannot easily guess the result from basic principles, unless they have by chance experience in this area too. $\endgroup$
    – Poutnik
    Feb 8, 2023 at 10:15

1 Answer 1

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I'm not sure this would be helpful or not, but just for curiosity, when I check for Verdigris patina, this page came up. Accordingly:

  1. Subtle Copper Patina Recipe: Make a solution with 3 tablespoons of 5% White Vinegar ($\pu{45 mL}$) and 1 teaspoon of table salt ($\pu{5 mL}$) Make sure to stir the solution very well until the salt dissolved completely. Let the copper soak for $\pu{30 min}$. Removed the copper and let it dry.
  2. Green Copper Patina Recipe: Make a solution of $\pu{45 mL}$ of water and $\pu{15 mL}$ of Miracle Grow plant food in exact ratio (3:1). Make sure to stir the solution very well until all Miracle Grow dissolved completely. Let the copper soak for $\pu{30 min}$. Removed the copper and let it dry. The colour will develop gradually while it dries.
  3. Blue Copper Patina Recipe: Make a solution of $\pu{45 mL}$ of Red Wine Vinegar and $\pu{15 mL}$ of Miracle Grow plant food in exact ratio (3:1). Make sure to stir the solution very well until all Miracle Grow dissolved completely. Let the copper soak for $\pu{30 min}$. Removed the copper and let it dry. The colour will develop gradually while it dries.

Copper Patina

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    $\begingroup$ (+1) Nice! Now I have to go out to my shed and look at the ingredients on my box of Miracle Grow, which, sadly, never seems to save any of my plants. I think this is actually my fault: I have the opposite of a green thumb. ;-) $\endgroup$
    – Ed V
    Feb 12, 2023 at 17:45
  • $\begingroup$ Tried something similar (updated the initial question) sadly it's just like a brittle sheen that forms, even after a month. Breaks apart from just touch, water dissolves most of it :/ $\endgroup$
    – TheChymera
    Mar 31, 2023 at 7:52
  • $\begingroup$ I am not surprised. It was from an internet post, most of the time just fabrications. :-) $\endgroup$ Mar 31, 2023 at 19:25

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