Reader question: “I’m wondering about cobalt and the story about blue colours being so expensive in the past – is that true and does it have any importance for the evolution of art?”
It is true! But it’s actually not cobalt blue that you’re thinking of, it’s ultramarine.
The history of colours in art is really weird and interesting. It’s true that the availability of various colours has often determined which ones are used and what importance they have. This is especially true the further back we go in history, when all colours were not readily available in the nearest art shop.
A surprising variety of methods were used to make different colours. To make bright red, for example, an early 8th century process was described by Persian alchemist Jabir ibn Hayyan: you had to heat mercury and sulfur in a flask, vaporize and recondense it, and then grind it to create a red colour.