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My son asked me about the discoloration on our outside brass doorknob, and I told him that it was verdigris, explaining that it was basically rust that formed on brass, bronze and copper.

I know what it is and its origin (Old French "vere grez" "green of Greece") but I had no answer when he asked why we don't just call it rust.

Any ideas? One of the definitions of rust is:

any film or coating on metal caused by oxidation

so it would seem to be a correct term to use, but I have no idea why it is not used(at least in my experience) when referring to oxidation of copper.

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    Verdigris forms on brass, bronze or copper, rust is found on iron or steel. – user067531 Jul 13 '18 at 20:49
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    Verdigris isn't copper oxide (which is black), it is copper carbonate. The most likely place to see patches of copper oxide is on copper pipes used in plumbing, after soldering joints with a gas torch. – alephzero Jul 13 '18 at 21:50
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    Why is a male deer a stag but a male elephant a bull? One of the definitions of bull is an adult male of certain large mammals, and deer are relatively large mammals, it would seem to be a correct term to use. – choster Jul 13 '18 at 23:04
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    Verdigris forms on bronze statues. – Lambie Jul 13 '18 at 23:16
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    @HotLicks Bet you Lady Liberty's got some...under her robes, of course. On the other hand, as Venkman said in the second film, she is French, so one never knows (about the bra, that is). – Deepak Jul 14 '18 at 6:55
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Verdigris and Rust are both oxidation, but it depends on the metal which is oxidizing. (as user110518 stated)

verde = (latin) green

gris = (latin) grey (also at black) [ty @all]

rust = (old english) red

https://en.wiktionary.org/wiki/griseus

https://en.oxforddictionaries.com/definition/verdigris

https://www.merriam-webster.com/dictionary/rust

  • from the link you provided: " a comparable coating produced on a metal other than iron by corrosion". This suggests that copper oxide can be considered "rust" unless "comparable" necessarily includes color – Michael J. Jul 13 '18 at 21:25
  • yea i think verdigris rust is like square rectangle... – tidbertum Jul 13 '18 at 21:34
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    Copper oxide is black, but that fact is is irrelevant, because the green color on copper exposed to moist air is mostly copper carbonate. – alephzero Jul 13 '18 at 21:45
  • im no chemist, but looks pretty green to me @alephzero : google.com/… – tidbertum Jul 13 '18 at 23:28
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    google images (and/or the people who label / tag images of copper exposed to air) are not chemists either. en.wikipedia.org/wiki/Copper(II)_oxide has a picture: it's a black solid or powder. I had always assumed that the green patina that forms on copper was oxide, and would have made the same mistake until a minute ago when I read @alephzero's comment! (Wikipedia says it's used as a pigment in ceramics "to produce blue, red, and green, and sometimes gray, pink, or black glazes." IDK how that works, though. (Also, Copper(I) oxide is red.) – Peter Cordes Jul 13 '18 at 23:31
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The definitions linked from the question highlight three main differences between rust and vertigris:

  • Sorry about the cross-post. – Phil Sweet Jul 13 '18 at 22:44
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    @PhilSweet by 'cross-post' do you mean posting the same answer at the same time? No problem, but I wouldn't consider your answer a cross-post anyway because it had different info than mine. – tookie_styles Jul 13 '18 at 23:08
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Others have already mentioned that verdigris is not caused by an oxidative process. So that is answer enough. But to go further, even if verdigris were caused by an oxidative process:

  • According to my Merriam-Webster dictionary (10th ed) the word verdigris has its origins in the 14th century, well before chemical principles of oxidation were understood. So why wasn't verdigris called rust? Because the deposits look entirely different and no one considered them as chemically similar processes.

  • Today, it would be misleading (at best) to call verdigris "rust" because rust also carries the connotation of a rust (brownish-red) color.

  • Rust is a sub-type of oxidation. Not all oxidation is rust. The term rust is reserved for oxidation of iron-based alloys. Even if the uncited definition in the question comes from a reliable source, and might be technically correct (or jargony) in certain contexts, I doubt that context would extend to everyday descriptions.

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Vert-de-gris would have had a special name because it was of commercial importance. Rust, less so.

In modern technical usage, the term refers to Copper(II) acetate monohydrate ("neutral verdigris") or Basic copper (II) acetate ("blue verdigris" or "green verdigris").

A 19th century (the 3rd French edition published in 1892) book (Traité de chimie industrielle by Johann Rudolf von Wagner) gives some uses that were in use at the time: pigment for oil paints and water color; to make Paris green, for dyeing cloth, and in gelding bronze (specifically, for giving a reddish finish to fire-gelded bronze).

Even at the time of writing, the author notes "verdigris is used much less often than previously, as it has been replaced by copper sulfate in many of its applications."

Going further back, the same substance is listed as a drug in Dioscorides' Materia Medica (1st century AD). The author notes that it's produced in Cyprus, which had mining industry in antiquity. The word "copper" itself comes from the place.

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Verdigris isn't rust because it isn't a product of oxidation. It is a different chemical reaction. The natural stuff is either copper carbonate hydroxide, or dicopper cloride trihydroxide. The artist's pigment is copper acetate.

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    While this is true, I don't think it's any explanation for having two different terms, considering they long predate modern chemistry. – choster Jul 13 '18 at 23:06
  • @choster We have a habit of retrofitting old terms to our brave new world's understanding. Your car's dashboard no longer stops you from getting splashed by mud from horses' hooves. – Phil Sweet Jul 13 '18 at 23:37
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    @PhilSweet and my glove box doesn't contain any gloves – mgb Jul 14 '18 at 4:20
  • @PhilSweet I don't think that was his point. – Sneftel Jul 14 '18 at 18:45
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Whether or not rust is also applicable to verdigris... why shouldn't it have another name? It's like asking why we call crabapples crabapples and not apples... they might be apples but why can't they have its own specific name?

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