More about colours - Ultramarine
Until 1836 the colour ultramarine used by artists was obtained by grinding the stone lapis lazuli. Ultramarine means over the sea and was so named because this rock, used by the ancient Egyptians to cure melancholy, enhance intelligence and confer the wisdom to use it, was first found in one place, above 5000 metres, in the mountains of Afghanistan. To protect the mines and deter thieves, any who were caught were put to death. Even the miners worked chained to the walls of the shaft. It is the tiny crystals of pyrite, that look like stars in a midnight sky, that prove the stone is true lapis. One company still makes the pigment using the ancient method. First, small pieces are heated then quenched. Now brittle, the material is ground and washed. Next, the washed pigment is mixed with dough formed from a little wax, rosin and linseed oil. The dough is kneaded in a weak solution of soda lye. The alkaline water extracts the finest particles of colour which settle out while the dough retains the impurites. This extraction can be repeated several times, but the first produces the purest and deepest colour.
Orpiment: Pigment of Gold. Used from the earliest civilsations, it was claimed that this could only be lightened by using white made from burned hartshorn, (deer antler) also used in Regency times added to water to revive ladies suffering from fits of the vapours. Orpiment is also known as arsenic yellow, which is a warning in itself. Nor should it be touched with an iron knife. (I haven't yet managed to find out what would happen if you did.) One form is known as King's Yellow. The other is red orpiment, sometimes called Realgar. Both are poisonous, not reliably permanent, and have only limited compatibility with other pigments. Painting has probably become considerably safer since they were replaced by synthetic colours.