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Moderator Answer
(koo schadler)
Hi Ellen,
Mirjam has addressed the question well. In my usual, long-winded way, here are a few more comments.
As you of course know (but for the sake of readers who don't): a metalpoint line is created by deposits of metal abraded from a metal nib by an abrasive drawing surface. Abrasion is created by (1) TOOTH in the surface (the result of high PVC - lots of solids in the ground; more irregularly shaped solids, like green earth, help abrade too); and (2) HARDNESS in the surface (given by the hardness of the pigments/solids in the ground).
A logical assumption is that very large sized pigments in a ground create an especially toothy surface (akin to rough sandpaper), and this creates an ideal level of abrasion. This isn't quite true. Metalpoint has no binder. Metal marks adhere via a mix of particles getting “trapped" within the “nooks and crannies" of a support or ground (mechanical adhesion) + electrostatic attraction between particles (Van der Waals forces). A ground with especially large tooth has larger and fewer interstices into which metal deposits can lodge. At the same time, a large toothed ground abrades larger particles of metals, and large particles are less likely to attach to the surface as well as to each other. So, at some point, a too rough, sandpaper-like ground actually makes for less well-attached, stable marks.
In my limited experience with Golden's pastel ground, I found it relatively rough compared – as it's designed to be, to abrade pastels. Metal particles don't adhere quite as well on this surface as on others, in my experience. This isn't to speak against it as a ground for metalpoint, just to clarify its qualities.
FYI, you didn't ask about this, and I'd never want to talk an artist out of using a material or method they like…but, my two-cents worth is that Artboard Gesso and Natural Pigments Tempera Ground abrade the best of the synthetic polymer grounds I've tried; and their microscopic irregularity is small enough that particles adhere well - albeit not permanently, since that's not possible, as noted next….
As you say, the idea that metalpoint isn't erasable is not true. The mechanical adhesion and electrostatic attraction that attach particles to a surface can be affected in various ways – erasing (tho' one has to be careful not to mar the ground; a kneaded eraser works best); applying solvent (water or alcohol – but again, this can damage the ground so must be done carefully); degradation of the metal particles (which are prone to corrosion). All the above are abetted if adhesion is less strong from the start - for example, if large metal particles are abraded atop a rough drawing surface (i.e. the “nooks and crannies" are so large that particles don't embed as securely). In short, the deductions in your last reply all seem correct to me.
Regarding the solubility of the ground: most synthetic polymers are touch dry within 20-30 minutes, but the actual cure time – the point at which the polymer becomes insoluble – varies depending on brand, dry time, application method, paint thickness, number of layers, underlying support, temperature/humidity, etc. So whether water is actually dissolving the ground, or merely helping to dislodge/lift particles, it's hard to say. But yes, water can encourage the lifting of metal particles and is one way to “erase" (although water can also affect the appearance of the ground, so this must be done very carefully).
The ultraviolet light stabilizers (UVLS) in a varnish aren't there to protect the underlying artwork from UV radiation; they protect the varnish itself from degrading as quickly from UV light (at least, that is my understanding). Also, while a support or ground (specifically the binder) may be degraded by light, most metals are not affected by UV light. Metals degrade via corrosion, which is caused by lots of things: moisture, oxygen, atmospheric elements (sulphur), etc. So the main reason to apply a varnish to a metalpoint drawing is to protect the metal deposits from detaching and from corrosion. However, there are complications to varnishing a metalpoint drawing (the varnish embeds into the irregular surface, and is hard/impossible to remove), and not everyone agrees that the benefits outweigh the costs – it's a personal decision, with pros and cons either way.
There are so many types of synthetic polymers – some water-based, others solvent-based – that it's hard to comment on the pros/cons of layering many different sorts. As Mirjam notes, testing is key – even then, surprises can occur at some point down the road. Talk to the manufacturer (Golden has a fantastic tech team – call or send an email). Simple systems are easiest to work with, maintain and diagnose when something goes wrong. But I know the muses can lead us towards complicated systems, and I'd never talk an artist out of a conversation with their muse (your planned layering sounds very enticing to me; I love to scumble over metalpoint, it's magical). Labeling the back with your materials and methods will help if future conservation is needed.
Koo
