Solid Cone Stopper Penny Head Size 24/29 joint Made of high quality borosilicate glass Solid cone stopper with penny head. Joint size 24/29. Made of high quality borosilicate glass. 2.6" tall. Eisco Labs Borosilicate glass has the following heat tolerances: Strain Point -515°C | Annealing Point 565°C | Softening Point 820°C.