I am a professional artist working in the traditional mediums of egg tempera and metalpoint. In my teaching & writings I work to understand the physical properties of my mediums, and have many chemically related questions. I’m not a chemist – so please pardon the undoubtedly unsophisticated nature of some queries, and let me know if I should be posting in another section of the forum.
In metalpoint drawing a nib of metal is drawn across an abrasive surface, depositing microscopic particles of metal to create a line. There’s no binder – the metal adheres purely through Van der Waals forces and the irregularity of the ground. It’s an ancient technique, peaked in the Renaissance, disappeared for several centuries, is gradually becoming more popular today. The “ground” (drawing surface) is critical - to abrade a metal nib it must have “high PVC” (pigment volume concentrate; a high ratio of pigment to binder) to create a rough, irregular surface (like microscopic sandpaper); and include high Mohs Hardness solids to abrade the metal nibs.
A ground contains a binder (i.e. animal glue, gum Arabic, synthetic polymers, casein, oil, egg oil) + pigments (colored, white and/or transparent) + diluent. There are many different metal nibs (silver, copper, brass, aluminum, lead, tin, pewter, etc). The many possible combinations of these materials, interacting with the environment, produce many different, sometimes inexplicable results. This is what I’d like to understand better.
With that long preamble, here’s my first question. A drawing done in silver (a “silverpoint”) on a lead white oil paint ground quickly tarnished, naturally, to a dark sepia color. This was to be expected. Then, about three weeks later, the drawing entirely disappeared – I would presume because additional tarnishing too degraded the metal deposits. However a few weeks later the drawing resurrected itself, and the silverpoint lines, which normally tarnish to a warm brown, now are a ruddy, brick-wine color (a color I've never seen in a silverpoint drawing). Any ideas what’s going on, how the different elements may be interacting?
Many thanks for your insights,