[KooSchadler]

Hi,

I am a metalpoint artist (a Renaissance technique in which one draws with a metal nib, most often silver), not by any stretch a chemist, who has posted here before in an attempt to understand my medium better.

Copper is among the many metals (any metal MHS 3 or less) that one can draw with on a specially prepared surface.  It leaves a warm grey mark.  The problem with copper is that it can tarnish in unpredictable ways to so many different colors (red, brown, black, yellow green, blue).  Sometimes this enhances a drawing, sometimes disrupts it. Just recently I've heard from a couple of metalpoint artist that they have also had a copper point drawing completely disappear.  I've seen this with nickel, lead and zinc drawings, that oxidize to white (so the lines don't show up on white paper) - but never copper.

My questions are…
1. Can copper oxidize to white? 
2.  I've read of copper chloride possibly turning to white - is this accurate?  If so, what in the atmosphere might cause a copper point drawing to oxidize to copper chloride?
3.  Any other ideas about what might cause a copper point drawing to "disappear".

Thanks, Koo Schadler

[Borek]

Can copper oxidize to white?

Quite unlikely.

In most compounds copper is present as a hydrated cation, and this cation is always in some shade of blue/green. Some salts are white if prepared as anhydrous, but keeping them white requires extremally dry conditions. Air always contains enough moisture to hydrate these compounds.

Some compounds contain copper that is not hydrated, but then it is in the red/brown/black area.

These are not hard rules, there is always a chance in some exotic cases it will look differently - but I am not aware of such compounds.

I've read of copper chloride possibly turning to white - is this accurate?  If so, what in the atmosphere might cause a copper point drawing to oxidize to copper chloride?

Never heard about it. Anhydrous copper chloride is the brown color branch and gets blue/green when absorbs water from the atmosphere.

Oxidizing copper to chloride will be possible if there are either chlorine (Cl₂) or hydrochloride (HCl) present in the air. Both are gaseous in normal conditions, but - again - even if they are present (for the latter it is enough to have a not so tightly closed bottle of muriatic acid present somewhere nearby) chloride that will be produced will be bluish. To make things more complicated I would expect Cl₂ to produce some basic chloride, in which part of the counterions are OH^- (it will change the hue, but we are still talking about something in the blue-green range).

Any other ideas about what might cause a copper point drawing to "disappear".

Is the line just a cut/scratch? If so, any corrosion that will destroy surface deep enough will make it disappear. Other than that no idea.

[Hunter2]

The question is, which copper chloride we talking about .
Copper(I)chloride CuCl or copper(II)chloride CuCl2 .
The first one is white,  the second  one blue green.
https://en.m.wikipedia.org/wiki/Copper(I)_chloride#:~:text=Copper(I)%20chloride%2C%20commonly,)%20chloride%20(CuCl2)

https://en.m.wikipedia.org/wiki/Copper(II)_chloride#:~:text=Copper(II)%20chloride%2C%20also,chemical%20formula%20CuCl%202.

[Borek]

Yes, but CuCl on the natural surface of copper in the presence of air is highly unlikely, it gets oxidized to Cu(II).

[KooSchadler]

Is the line just a cut/scratch? If so, any corrosion that will destroy surface deep enough will make it disappear. Other than that no idea.
[/quote]

Metalpoint drawings are done on top of "grounds" or coatings that have an inherently abrasive quality (a high ratio of solid particles in the ground creates a toothy surface; and the solid particles are harder than the metal nib).   So the ground abrades the metalpoint drawing tool, which depositing small particles of metal, creating a line.  It's actually not much different from a graphite pencil, except the drawing material is some sort of relatively soft metal instead of graphite.  So it's more than a scratch, it's actually particles of metal that are either converting to white or transparencies, or somehow detaching from the toothy ground.  Can't figure out how either is happening….

[KooSchadler]

Another chemist I spoke with told me this:

"Copper salts are green or blue and often transparent but the thinner they are the less obvious the colour is. Think of laying one sheet of coloured glass over another; the double thickness is a lot denser in colour. I dont know how thick the copper salts need to be to create a visible colour but there will be a point where if there is not sufficient thickness it will not be detectable. I have no idea whether this might occur with oxidised copper point."

Any thoughts on this being a possible explanation?  If a copper nib is abraded by an abrasive ground (think microscopic sandpaper), would the particles be small enough to appear transparent?

[Borek]

Any thoughts on this being a possible explanation?  If a copper nib is abraded by an abrasive ground (think microscopic sandpaper), would the particles be small enough to appear transparent?

No idea if that's a correct explanation, but they definitely can be up to something.