'When would you choose casein over gouache?'

A question from a reader leads to buttery fun...and strange smells.

Sleepy Knitter asks: "When would you choose casein over gouache OR gouache over casein?"

Sleepy continues: "If I were you and preparing my plein-air kit for the day, what would make me choose the casein kit over the gouache kit, or vice versa? I’m debating whether to upgrade my student gouache kit to a professional one or instead to buy a professional ca…