A thin layer of PTFE material or grease is usually applied to the ground-glass surfaces to be connected, and the inner joint is inserted into the outer joint such that the ground-glass surfaces of each are next to each other to make the connection.
Both PTFE tape, bands, and fluoroether-based grease or oils, but not silicone-based, all emit perfluoroisobutylene fumes, as well as multiple other breakdown products, as they approach and exceed their working temperature limits (ca.