Tree sap in the cello case?

Rosin has a fascinating history in string playing.

String-instrument musicians have used rosin for centuries. Its use likely dates back to the early development of the modern bow, around the 16th century, when the bow became more curved and capable of sustained tone production. A description of the technique appears in Michael Prætorius’s Syntagma musicum (vol 2, c1617). Rosin use became standard by the Baroque era (beginning in the late 1600s), especially as composers began writing more intricate and sustained lines for bowed instruments.

Rosin is used because bow hair on its own is too smooth to grip the strings effectively. Applying rosin creates the necessary friction between the bow hair and the string, allowing it to catch, pull, and release the string in rapid succession — which produces the vibrating sound we hear. Without rosin, the bow would glide over the string silently or with a very weak tone.

The practice probably developed empirically — musicians experimenting with sticky tree resins to enhance sound — and was later refined into the purified rosin cakes used today. It’s usually made from distilled pine tree resin, and different formulations can suit different climates, instruments, and playing styles.