Permanent and non-permanent dye combinations

The durability of dyes used in past centuries can be observed on the works of many painters.
Nowadays, a whole range of newer and newer dyes are being developed, and the persistence of these dyes to combinations with others must be subject to prolonged observation.
The phenomenon of the behavior of dyes in different combinations is of momentous importance in painting, and that is why both scientists and painters deal with these issues, using their own experience.
Scientists use various accelerated methods to observe the behavior of dyes in combinations, as well as the effect of light on them.
One such method is to grind the dyes with distilled water and expose them in test tubes to sunlight (appropriate dye combinations are made beforehand).
In some cases, changes occur after just a few hours.
The durability of the combination of dyes depends not only on the type of color substances being combined, but also on the purity of the product itself. For example, cinnabar (especially lighter shades) contains sulfur in the free state or its compounds formed during its production.
Sometimes, too, lead white is poorly purified and contains a certain amount of so-called "lead sugar" .
Cadmiums can contain sulfur in the free state.
All such impurities have a bad effect on the combination of a given dye with others. For example, poorly purified cadmium and cinnabar with untreated lead white in combination will give lead sulfate.In contrast, pure cadmium and pure cinnabar will not darken with lead white slightly contaminated.
Pure ultramarine blackens with untreated lead white. With pure white it does not blacken.

Dyes most durable in combinations

1. earthy minerals
Chemically inactive. Yellow and red ochre, bolus, natural ultramarine, earth green, umber.
2. mineral artificial
Iron reds, marls, cinnabar, blue and purple cobalt, ceruleum, green chromes, barite white, organic blacks.

Medium-term

1. mineral artificial
More chemically active. Lead and zinc white, Neapolitan yellow, dark cadmiums, cobalt green, ultramarine of various shades.
2. organic
Alizarin crackers.

Small and impermanent

Lithopone white, light cadmiums, aurypigment, minia, cinnabar, titanium white, light chromiums, aurealine, copper.

1. organic
India yellow, light craplacs, carmine, gumiguta.
Note that some permanent dyes are sometimes sensitive to exposure to light and lose color in combinations even with permanent dyes.
An example of this is cinnabar, which loses its color in combinations by changing its molecular structure (from crystalline to amorphous), and not due to combination with another dye.
Mixing permanent dyes with non-permanent dyes yields non-permanent combinations.
Of great importance for the durability of a dye is the binder.
There is a small group of dyes that gain durability in combinations:

• Lead white combined with pure cadmium darkens more slowly from hydrogen sulfide than on its own.
• Gumiguta is more durable with yellow cadmium and yellow baryta.
• Kraplak behaves better with whites than alone.