Why this sentence is no error:
Egg tempera, a painting medium (in which) pigments (are mixed with) egg yolk, (was favored by) several prominent twentieth-century United Sates artists, (including) Social Realist painter Isabel Bishop.
I think that tenses have to be parallel so I pick “are mixed with”???
Because the painting mentioned is a fact and it does not change through time, we can use the present tense.
thank you @collage so much