Solid Cone Stopper Flat Head Size 19/26 joint Made of high quality borosilicate glass Solid cone stopper with flat head. Joint size 19/26. Made of high quality borosilicate glass. 1.6" tall. Eisco Labs Borosilicate glass has the following heat tolerances: Strain Point -515°C | Annealing Point 565°C | Softening Point 820°C.