An artist today takes for granted that the tube of oil paint they purchase will be much like the last one. Plus, they know the art will last for years and years once it’s on canvas. Sometimes, earlier painters didn’t have that luxury. They knew time could be harsh on their materials. At least once, Da Vinci supposedly chose materials of lesser staying power so he could get the tones he wanted.
The heroine, Beatrice, in The Notorious Countess would have either had to mix her own pigments or have someone nearby who would do the chore for her.
Different things were sometimes added to the mix to get the desired effect. Sir Charles Lock Eastlake, in his 1847 book concerning methods of the past, mentions the use of honey to slow the drying of paint. Egg Yolks have helped artists throughout the years. Wine sometimes was used to dilute materials.
Discovering this helped me understand why watercolors were taught to the young ladies in the Regency upper classes instead of oil painting. A student had to be a little more involved if she needed the skills of mixing pigments before picking up a brush.