Metalpoint Experiments

Koo, I doubt that anyone has done as thorough a testing as you are proposing. Perhaps Natural Pigments? Please let us know the results so that it will be permanently recorded here. The gypsum will probably be too soft but give it a try so that this does not remain an unknown.

You may also want to add to your test barite (baryte), which is natural barium sulfate (blanc fixe), as this was often used in the beginning of the 20th century in grounds for silverpoint, according to a paper conservator at the NGA, where I was visiting last week. Also, instead of using house paint or student-grade acrylic paint where you cannot know the ingredients, why not use an acrylic polymer and add the extender pigments/fillers to it as the basis of your grounds?

Students of mine and I have recently tried artificial aging in an oven to speed up reaction times (and speed up degradation of the cellulose paper substrate 😉 with minimal effect. I also tried simply suspending the silverpoint drawing over precipitated sulfur and this had no effect that we could discern. This was to be expected because at room temp there does not appear to be much volatility of sulfur. I know that some have used thiol containing vegetation (garlic/onions) for this effect, but this seems really un-quantifiable (but probably effective). We will be testing liver of sulfur in the next two weeks or so and will report back our observations.
There is a major qualification here: this is not a technical study but only a fun adjunct to a materials and techniques of Western drawing class  I know this sounds like a copout, given what I just wrote, but unless a student decides to focus on this,  I cannot spend the time to devise a hardcore, repeatable experimental model here. I will leave that to those of you more focused on drawing materials.

As to casein grounds, I would suppose that the method used to hydrolyze the casein could have an effect of the patination of the alloy or metal marks. Residual borax may leave an alkaline surface. An undercured ammonia/casein ground may still be offgassing ammonia which could affect patination.
Having written that, I have only used animal skin glue with pigments and ground silica on paper, thin pigmented chalk-glue grounds on paper and panel, and pigmented true gesso (gypsum-animal glue) ground on panel.

I have had students volunteer to have their silverpoint drawings placed in a drying oven as well so no shame from me.
I seem to remember that gallons of casein paints intended for use in theatrical scene painting also contained an excess of ammonia (well beyond the demands of hydrolysis.) to retard purification,

Let me restate this to be clearer. I am not advocating adding ammonia water to existing artists’ casein paint. I just mentioned my experience.  Ammonia is one of the common alkalis used to put casein into solution.
Honestly, my earlier comment is not from anything authoritative. In the early 90’s I worked with a faux finisher in St Louis. She did all of her undercoats for faux graining using gallons of casein paint made specifically for use for painting theatrical sets. Upon opening the container, you would get a heavy odor of ammonia and the label expressly said to add dilute ammonia to keep the paint from spoiling.
I do not want to take this any further as I do not even remember the brand of casein paint and cannot really comment beyond that.
As to the stability of borax vs ammonia casein.  I am going to do a bit more reading on that and perhaps contact Shiva to see if there is really a definitive answer to that question.