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.