In the early modern era, Spanish missionaries, cosmographers, chroniclers, and physicians wrote major studies on botany, ethnography, navigation, Indigenous languages, war, and history, aided by capable, though often reluctant, Indigenous informants. They penned this rich body of scholarship using iron gall ink -- a mixture of tannins, sulfates, and gum -- that facilitated the transmission of ideas in print and manuscript format, as well as the administration of the vast empire. Analysis of ink opens a window onto a laboratory of botanical knowledge that resulted from observation, practice, and artisanal innovation. The "potions lesson" assignment described in this article allows undergraduate and graduate students to make ink using recipes that circulated as marginal notes in notarial records and in grammar and writing manuals. The potions lesson experiment forms part of an assignment in the author's course offering, "Secrets of Nature in the Iberian World," a junior seminar that explores the way people in the early modern period negotiated their relationships with animals, climate, land, disease, plants, and water, and the way they framed their understandings of these categories. Students considered the roles of women and men who molded and shaped the environment to suit individual and collective needs across North, Central, and South America, the Caribbean, and parts of Asia. The course centers on an experiential model with a focus on hands-on learning. In addition to readings and discussions, students engaged with the past through a series of activities in labs, special collections, and museums. This lesson formed part of a unit on plants that considered the relationship between colonialism and botanical knowledge. Because the assignment analyzes the sixteenth century at the height of Iberian exploration and because the composition of the ink circulated across Europe and subsequently the New World, one can easily adapt it to fit a range of classes. Student reflections on the assignment suggest that the experience of spending time outside the classroom greatly refreshed the learning environment. According to the author, the assignment enhances the teaching curriculum in the humanities by asking students to take an active role in the learning process.