Mercury dyes

Cinnabar (mercury sulfide)

Deposits of natural cinnabar (ore) are found in all parts of the world.
Cinnabar was already known in ancient Egypt, Greece and Rome.
Natural cinnabar is called "mountain cinnabar." It ranges in hue from bright yellow to dark red. It is only subject to cleaning and grinding before being used for painting. The better ground, the nicer the color.
Mercury sulfide comes in two varieties:

1.black color (amorphous),
2.red color (crystalline).

Cinnabar is a crystalline sulfide of mercury. Impermanent, it blackens in the light.
In old Dutch paintings it behaved well, as it was insulated from light with either a varnish (varnish) or other laser red.
Cinnabar behaves better on black underpaintings.
Nowadays, due to the depletion of deposits of mercury sulfide, this product is obtained artificially.
It is impermanent in lead and copper combinations.
The blackening of cinnabar in the light is explained by the transition from crystalline to amorphous (amorphous) structure. The durability of cinnabar on light depends on the binder on which it is rubbed. It darkens most strongly from varnish, then from oil, tempera, and least from glue.
It depends on the refractive index.
On resins it is more transparent, transmits more light and thus darkens.
If you want to cover cinnabar with varnish, you need to insulate it with a layer of glue (gelatin).
On oil gesso, cinnabar behaves less well than on glue gesso. It darkens the most from Kapay balsam (it has a higher degree of light refraction than other resins). Used in all techniques. oil takes 15 to 20%, but dries slowly.
Recognition:
When heated, it evaporates without residue. Acids and alkalis have no effect on it. Heated to 100 deg.C. it blackens, after cooling it returns to its color. Heated to 400 deg C it blackens irreversibly.

Mineral yellow (sulfur-mercury salt compounds with mercuric oxide)

Bright yellow (lemon). In the light it blackens. In combinations with organic compounds it decomposes.