– What happens when an artist mixes paint for a painting? Are the pigments actually changing physically/chemically? What is actually happening to make the paints change color?

Question

– What happens when an artist mixes paint for a painting? Are the pigments actually changing physically/chemically? What is actually happening to make the paints change color?

In: 4221

Answer 1

Everyone else has touched on the ~~additive~~ E: subtractive color mixing idea, but I want to address this:

> Are the pigments actually changing physically/chemically?

*Generally* not, at least not nowadays, because a lot of research has gone into finding pigments that are good at being pigments – vibrantly colored, as opaque as possible to hide the color of the thing the paint is on top of, etc. – but also *don’t react with each other*, so that they continue contributing the color you expect them to no matter what random combination you mix together.

That hasn’t always been the case. For example, most paints used to include lead(II) carbonate for basically the same reason we put titanium dioxide in paints today – it’s very opaque and a very pure white, which helps the paint to cover up what’s behind it without affecting the color of the paint itself all that much. But lead(II) carbonate reacts with hydrogen sulfide, which is constantly being produced and released into the air in tiny quantities by basically living things due to breakdown of proteins. Lead(II) carbonate + hydrogen sulfide makes lead(II) sulfide, which is this icky dark brown/blackish color instead of white – causing lead(II) carbonate paints to discolor over time.

A certain orange-red lead oxide usually called “red lead” or minium similarly discolors and turns black when it forms lead sulfide, and it can even do this when mixed with other sulfide-containing pigments like vermillion (mercury sulfide) or orpiment (arsenic sulfide). Azurite is an unstable blue form of copper(II) carbonate which degrades over time to a dark brownish-green – a combo of a stabler, green form of copper(II) carbonate + black copper oxide.

There are many more such reactions involving old timey pigments. You may find [this paper](https://heritagesciencejournal.springeropen.com/counter/pdf/10.1186/s40494-017-0125-6.pdf) to be of interest.

It’s partially for this reason – and partially because they’re super expensive, and partially because they’re super poisonous – that many of the pigments discussed in that paper have been phased out of use, and replaced with cheaper, less toxic, more vibrant, and *unreactive* pigments today, like copper phthalocyanine and iron oxide.