Durable silicone boots designed specifically for borosilicate glass tubing! Engineered to fit the consistent OD of glass tubing these boots are easy and quick to install on the ends of your tube and provide a reliable sealing grip on the glass. Each boot has a hole in the center for your blow hose swivel to accept a 1/4" swivel.. Each boot is flexible and can stretch to fit larger tube than it's nominal size. The silicone is rated for 450 degrees F (230C).