Tuesday, May 16, 2017

Vitreous Paint Experiments (Malachite)

I bought a small bag of malachite chips to grind into pigment, to go into the William/Isolde scribal gift box. As I was having fun smashing it, it occurred to me that we do have greenish pigments, and I wondered if malachite might not compose one.

So, I smashed it and smashed it until I got cramping fingers and crossed eyes (Actually, not that long or difficult). I got out my handy 325 mesh sifter, as in theory that's the level Reusche grinds to, and sifted the malachite until I had a handy stash in an old spice bottle.

Because I am for scientific experimentation, I wanted to control everything except one variable. I used some Reusche clear glaze base so I could see what the malachite would do on it's own. I realize, I don't know the ratio of glass (or glass components) to oxides in pigments; One period text would say 2 parts glass to one part copper. I made four test chips to compare different ratios, 1:1 through 1:4.

I mixed them on an ad hoc glass palette with a muller. The muller, being glass, seemed the easiest to clean. I noticed how easily the pigment mixed, much like working with commercial (Reusche, Fusemaster, etc) paints and stains. The very fine mesh size seems to promote fluidity. I used a bit more water than I would for "real" but with this small quantity of paint it was rather difficult to get the right water content (I guess I could have tried to drip it off an eyelash or a cat's whisker, but neither myself nor Zod were willing to cooperate with that). The water won't change the performance of the paint, only how it handles on the brush, so having too much water shouldn't impact the results.

I weighed them on my mini digital scale to get the ratios. Although it does .1g increments, the floor seems to be .2 grams. That was what I used as a "unit", so the 1:1 chip is .2 grams of malachite to .2 grams of clear base. The 1:4 is .2 grams of clear base, and .8 grams of malachite. I wrote my name on the chips, for some reason the first thing that entered my head, to test the line work. I then smeared paint on the bottom block of each chip to show various values. I also marked the ratio at the top.

I then fired the chips on my standard vitreous paint schedule, which matches the range for the clear glaze base. The next morning I was fascinated to see significant change in pigment. What I was aiming for was something akin to "Grey green" pigment, a modern sample of which is here placed next to the chips:

The malachite DEFINITELY darkened. To my eye, it also seems more faintly blue than green. Azurite, a deep blue twin of malachite (both are copper (II) carbonate rocks) will turn into malachite when weathered, but nothing in my research shows the reverse. I did find that azurite when heated turns into copper (II) oxide, a black powder. That oxide is then used as a ceramic pigment, making blue as well as green, black, pink, red, and gray colors. I suspect that's what has come into play here, the malachite likely also forms the oxide which would account for the blueish hint and the darker color. I am not a chemist, however, and I couldn't easily find a reference to what happens when you heat malachite. [Edit: A friend reposted my link to get the attention of some chemistry-buff friends, and one Liz pointed me at this link. Malachite does turn into copper (II) oxide.]

I first picked up the 1:4 chip, and immediately noticed the paint flaking off onto my fingers. My fingertips were tinged grey/black/blue. I found, using a wooden skewer, that the 1:3 chip was also very easy to scratch paint from. The 1:2 I could leave some trace, but not much. The 1:1 completely resisted the stick like Reusche paints would. Looking at the reflected light, the 1:1 chip also looked much like a dozen other test chips I've made; the paint is completely glassy and adhered to the test chip. I fire my vitreous paints to the high end for that effect, so this is expected. The other three showed a rough, grainy texture I associate with previous experiments that had too rough an oxide.



Lessons Learned:

  • Yes, a randomly selected mineral MIGHT make a usable vitreous paint!
  • Something near a 1:1 ratio is probably idea to bind the pigment to the glass, though 1:2 was also serviceable. My tests of the period formulas are 1:2, and were very similar.
  • It would be wonderful to find out what actually goes into Clear Glaze. My normal secret trick is to check the EU vendors, who seem to list MSDS's that US vendors do not. Unfortunately Peli doesn't include one for Clear Glaze. Reusche gives them out if you make a special request in writing, whereas Peli just has them on their website. I rather suspect this is because the MSDS sheets rather give away the secrets.