How to Fake a Pearl
The term "alchemy" commonly refers to the attempt to transmute baser metals into gold, but its original definition is simply the chemistry of the Middle Ages and the 16th century. Among the chemical reactions claimed by some alchemists is the ability to turn black pearls into lustrous white pearls. As early as the fourteenth century transforming dark pearls into "pearls most beautiful, to every test" was touted in Italy. As the gateway to the East, Venetians were in a prime position to fake pearls for the wider European market. By the sixteenth century Venetian pearl merchants imposed strict corporal punishments on those caught forging pearls.
By the sixteenth century, therefore, the alchemical "recipes" for transforming pearls were not necessarily being published, but some of the techniques were still being written down by hand and circulated or kept for reference. One surviving manuscript was recently acquired by the Rare Book & Manuscript Library. It is a late 16th- or early 17th-century guide to forging pearls. Many of the recipes describe creating white pearls literally in vitro from a paste based on talc or alum. Even Leonardo da Vinci gives a recipe very similar to one found in Penn's manuscript; it involves softening seed pearls in a flask using lemon juice.
To see the complete manuscript, request Misc Mss (Large) 3 in the reading room of the Rare Book & Manuscript Library.
"To dress up black or brown pearls, in order to render them beautiful and white,
to enlarge them as you please while maintaining their luster, hiding their true blackness and nature"
"Method to obtain the paste"