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Moderator Answer
(koo schadler)
Tom Mazzullo knows as much about metalpoint as anyone, and covers the traditional gesso option well. But I love this topic and can't resist adding several more thoughts – I hope some prove helpful to your project.
The abrasion in a metalpoint ground is primarily created by…
1. High Pigment Volume Concentrate (PVC). A ground/paint that has a high solid/pigment content relative to the binder, so that the solids/pigments protrude above the surface of the binder. This creates an irregular, rough surface (on a microscopic level, generally) or "toothiness" that abrades the metal nib.
2. Hardness. Solids/pigments within a ground are very hard; i.e. have a Moh's Hardness Scale (MHS) rating higher than the metal nib being abraded.
Of the two I think hardness is as important as toothiness – but first I want to look at a rough surface.
HIGH PVC/TOOTHINESS. Metalpoint has no binder. Metal marks hold on via a mix of particles getting trapped within the "nooks and crannies" of a support/ground, along with electrostatic attraction between particles.
If a ground is too smooth – i.e. there are not enough irregularities in the surface - metal marks may, over time, lose their "grip" (so to speak) and a drawing can appear to "fade".
However if a ground is very rough (akin to sandpaper) it has larger and fewer interstices into which metal deposits can lodge. A large tooth ground also abrades larger metal particles that are less likely to attach to the surface and each other. So, at some point, too large-toothed a ground makes for less stable metalpoint marks.
For the above reason I don't generally recommend very rough pastel grounds (I've tried several) for metalpoint. I realize you like the Golden Pastel and needn't listen to me…. I just want to clarify what I see as potential shortcomings of a very rough ground.
HARDNESS. To the extent it can be determined, the hardness of the solids/pigments within a ground is one of the most important considerations in creating an abrasive surface. Simply put, if ground solids are harder than a metal nib being drawn across that surface, the ground abrades the nib.
You can add extra solids/pigments to most grounds to increase both PVC and hardness. However, too much added solids can lead to chipping/delamination of ground from support, and/or a brittle, friable ground that crumbles under a metal nib. So, a conservative starting point is adding 5% of the total volume of ground. If there are no problems, gradually increase the amount. Material experts generally recommend no more than 10% added solids to an existing ground. In practice I've added more with success, depending on the binder. Experiment and test (see below).
TESTING SOLID/PIGMENT CONTENT IN A GROUND
1. Flexibility. Apply one or two layers of the amended ground to a flexible support (i.e. 90# watercolor paper). Gently bend in various directions and see how it reacts. If the ground cracks or shatters into pieces = too much pigment. (However, for the actual drawing, always apply multiple layers of a high PVC ground to a rigid support, as you're planning to do via HDF).
2. Adhesion. Do a cross hatch adhesion test to see if there's enough binder for the ground to properly adhere.
3. Diluent Sensitivity. For water-soluble grounds, take a wetted Q-tip and rub the surface to see if ground lifts or dissolves (but don't rub too aggressively or you'll merely remove ground through abrasion). The ground should stay put. If you can't see a white ground on a cotton Q-tip, warp the end with a small piece of wet, blue shop rag.
EXTENDER SOLIDS/PIGMENTS. Extenders must be inert. Particle size is best within the .1 to 2 micron size. A pigment's color and opacity/transparency are also factors. Additionally, some extenders give unexpected results, i.e. I've found that pumice brings out more of a metal's color. There are many options, but here are a few of my favorites for increasing abrasion, followed by the MHS rating. (FYI, a pure silver nib is 2.5 MHS; sterling silver is about 3 MHS).
Bone Ash 3
Zinc White 4
Pumice 6
Pyrite 6.5
Titanium White 6.5
Silica 7
Of course, always wear a dust mask when working with powders or sanding a high PVC ground - especially silica!
TINTING A GROUND. Tint with either powdered pigments, dispersions or compatible paints. I tint with powders only if they're large micron size, low tinting earth pigments (because they disperse readily). For all other pigments I use a commercially produced dispersion to tint.
HEIGHTENING WITH WHITE. I've used all of the following, at various times, for white highlights on metalpoint, and all have worked well for me, no sinking in. They have different pros and cons (solubility, density, fineness of line, etc.): White Chalk, Colored Pencil, Pastel Stick, Chinese White Watercolor, Gouache, Casein, Egg Tempera, Acrylic Paint.
COMMERCIAL GROUNDS. Finally, If you're content with Golden's regular Acrylic Ground, that's great. But I've had a lot more luck with metalpoint on Golden's Sandable Gesso (excellent), Absorbent Ground (good) and their High Load Acrylic Paints (good).
Two other synthetic polymer grounds that work really well for metalpoint are Art Boards Gesso (one of my favorites) and Natural Pigments Tempera Ground. Both sand well and are nicely abrasive.
Extender solids/pigments may be added to any of these commercial options to increase abrasion or to tint.
My all-time favorite metalpoint ground is Natural Pigments, Rublev Traditional Silverpoint Ground. It comes in powder form and needs to be hydrated (two hours), then warmed in a double boiler. It can be applied right away but, like traditional gesso, becomes smoother if put in the refrigerator overnight, then rewarmed and applied the next day. Of course, you can make a traditional ground from similar ingredients yourself, but the above is convenient.
I prefer all of the above over traditional gesso for metalpoint – but every metalpoint artist has their favorites, and time will tell what works best for you. Good luck with your project!