In the seventeenth century, the Swiss scholar Théodore de Mayerne wrote a recipe book for Baroque painting. In the middle of this manuscript, he drew colorful circles to test various pigments for their colorfastness and cracking during drying. Mayerne acquired his knowledge from books and conversations with numerous pharmacists, craftsmen, and artists, including famous names like Rubens, Antoon van Dyck, and Artemisia Gentileschi. With Rubens, he discussed the use of turpentine oil, blue pigments, and varnish, among other things. It is highly likely that these conversations took place while Rubens was painting a portrait of Mayerne in London.