What is it made of? Egyptian blue is a synthetic compound of copper, calcium, and silica.
Where does it come from? It was first produced in Egypt as early as 3300 BCE but it became well known throughout the Mediterranean world. Outside of Egypt, both Pliny the Elder and archaeological evidence suggest that Egyptian blue was produced in Puteoli (modern Pozzuoli), Italy, and in Spain. In the 1st century BCE, a ship carrying a cargo of amphora foundered off the southern coast of France near Marseille. Found among the wreckage were ingots of Egyptian blue and realgar, both of which were used as pigments. The ship may have originated at Pozzuoli.
How is it made? Egyptian blue is produced by heating a mixture of silica (sand), a copper compound like malachite, and calcium carbonate with natron (a naturally occurring mixture of salt and sodium bicarbonate).
What is it made of? Indigo dyes are derived from the leaves of Indigofera tinctoria plants. These plants contain a colorless chemical that turns blue after being extracted and exposed to oxygen in the air.
Where does it come from? Indigo plants are available worldwide, but Romans imported theirs primarily from India. Woad, a related plant native to Europe, was also used to create a blue dye. However, the blue from woad was not as desirable as that from indigo.
How is it made? To make indigo dye, the plants are first softened in water. This draws out their natural colorant, which is then treated with an alkali such as ammonia so that it chemically bonds with fabric. The blue color appears when the fabric is removed from the dye bath and exposed to the air.
Fun fact! The colorants in indigo blue and Tyrian purple are chemically similar, even though the former is made from a plant and the latter from sea snails.