[KooSchadler]

I'm an artist who draws with metalpoints, a medium more commonly known as "silverpoint" (famous examples are some of the drawings by Leonardo da Vinci and Albrecht Durer ).  A metal nib is drawn across an abrasive ground, leaving deposits of metal to create a line (i.e. same as a graphite drawing, except you use metal instead of graphite, and the drawing surface must be more abrasive than paper). 

Generally it's difficult (if not impossible) to get a true, black line in metalpoint; only a very dark grey line from a silver nib is possible.  However there is an artist who claims to get true blacks from a pure silver nib.  His description of how he does so is as follows:

"Using a pure silver nib on a homemade ground [made from animal glue, marble dust, zinc oxide and bone ash]..I apply a silver [nib] to the drawing surface by striking it first on a piece of 800-grit W&D sandpaper, then applying it quickly and directly to the drawing surface. This removes the oxidised layer so the silver reacts with the ZnO in the mix to produce very dark Silver Oxide. It gets you to 9 on a 11 step value scale [11 being a true black].  If you delicately wet a small area, THEN apply the silver nib, you'll get to a true black.  PLEASE NOTE: this takes a bit of practice to do. The ground is water-soluble: too wet and you'll go through it; not wet enough and you won't get a true black. You can't go over it, you get ONE shot. Strike the silver as described above, then very gently rub the stylus into the dampened area and gradually increase pressure over 20-30 seconds. The harder you press, the darker the tone".

I think the artist is suggesting that the silver nib interacts with the zinc oxide in his moistened ground to instantaneously produce black silver oxide.  In videos of his method he achieves the black immediately - the line being applied is a rich, charcoal black. My question for you chemists is: 

Can a pure silver nib, upon contact with a moistened ground containing a percentage of zinc oxide, immediately tarnish to a pure black, silver oxide?  If so, would it appear as a consistent black (as it seems to be in this artist's backgrounds), or would it be a varied, irregular black (dependent on the degree of oxidation of the silver). 

I think the artist may actually be incorporating other mediums into the drawing (i.e. working with graphite atop silverpoint); he's not achieving his true blacks via silver alone - but I don't chemically understand what pure silver is capable of, and don't want to mistakenly question his claim until I know more.  Thanks for you insights.

[Corribus]

A few things from my perspective:

Generally solids like these do not directly react without a solvent (e.g., water) containing an electrolyte to mediate the flow of charges. Not that something like this can't happen, but if you're using distilled/deonized water, any kind of single exchange process like the one you are describing would be pretty inefficient.

More to the point, silver is below zinc in the reactivity series. This basically means that zinc oxidizes much more easily than silver. Which means that going from zinc oxide plus silver to silver oxide plus zinc is not thermodynamically favorable. In point of fact, the reverse process, which is spontaneous under certain conditions, can be harnessed as the basis of a battery. You'll notice that the chemical reaction to describe the battery's action is:

Ag₂O + Zn --> ZnO + 2Ag, E° = 1.56 V.  The positive standard cell potential indicates the forward reaction is spontaneous.

After the battery is discharged, the cathode would be coated in silver metal and the anode in a layer of zinc oxide. The regenerate the battery, you'd have to apply an electric force to regenerate the silver oxide and zinc surfaces. That the battery would require energy to recharge indicates that the process mentioned in your post is not spontaneous.

So, my intuition tells it's unlikely that freshly ground Ag will react spontaneously (to say nothing about quickly) with zinc oxide to form silver oxide. Unless there is something we're not being told.

A final note is that pure silver does not react easily with oxygen but it does react fairly efficiently with sulfides. We would still call this "oxidation", which does lead to some confusion. But silver tarnish is mostly due to surface coatings of silver sulfide, not silver oxide. Pure silver sulfide is also black. However less pure forms of silver (e.g., sterling) have substantial amounts of other metals like copper that may be much higher on the reactivity series and thus more likely to react with oxygen, particularly if the medium is acidic. So, you may have to clarify what you mean by "silver nib". What formulation/purity of the silver is being used?

[KooSchadler]

Oh, that's very helpful, thanks so much.  He is using a pure silver nib, not sterling.  I'm skeptical he's achieving true blacks with a pure silver nib alone, but I wanted to give him the benefit of the doubt.  I'm not sure but I think he creates a very abrasive ground, uses a pure silver nib to apply a layer of silver, next works with a very soft graphite pencil on top of that, then finishes with a silver or gold nib to give the graphite layer a metallic shine.  His work is beautiful and very accomplished, but I think it's a bit misleading to other metalpoint artists in terms of how he says he does it.  I appreciate your insights. 

[Corribus]

Assuming the result is authentic, I suspect something else is going on. I could throw out some wacky theories.

For instance: Graphite is itself electro-active. Graphite is a layered material consisting of stacked flat sheets of coordinated carbon atoms. When the sheets are separated (exfoliated) you get graphene, which has even more interesting properties. Certain metals can intercalate into graphite - i.e., individual metal atoms can insert themselves in between the graphite layers, separating them. Since graphene is very electro-active (it's electrons are, say, highly mobile), the graphene can change the electrical properties of the metal atoms and vice-versa, enabling some unusual chemistry and physics. Moreover, my experience is that good quality exfoliated graphene is much blacker (truly, it's about as black as you can get) in appearance than graphite, which has a dark grayish cast.

If the artist is layering silver and graphite, it is possible that silver atoms diffuse into the graphite, which leads to some exfoliation of the graphite layers, giving them a more pure black-metallic appearance. I did a quick search and there is some work (not much, but some) to establish that Ag can intercalate into graphene, so this isn't a ridiculously far-fetched idea.

Well, that's just wild conjecture, might be totally wrong. But what is certain is that graphene-metal interactions are quite complicated, lead to some interesting properties, and are still a very active area of research. So I guess anything is plausible here.

[KooSchadler]

Very interesting wild conjecture... It could be the artist is getting a reaction he doesn't understand or hasn't correctly identified.  If it is the reaction you describe, it still seems odd that he achieves such an instantaneous and consistent application of pure black.  Attached is a close up shot from one of his videos of a drawing in progress - it's too low resolution to clearly see what he's doing, but the image at least clarifies how deep and truly black the marks from his "silverpoint" tool are.

[Borek]

I wouldn't trust video/pictures too much, you can do plenty of histogram tricks in postprocessing, making the black look deep or bland. And these days many of these adjustments are done by cameras automatically, just to make pictures pleasant.

[Corribus]

Well yes there is always that. Which is why in science a photograph is basically the lowest form of evidence. Useful to get a point across but that's about it.