Choose Dunlop Pyrex Glass Slides for a warmer, thicker tone accentuating the middle harmonics of your sound. Today's most popular slide. Processed from high quality Boron Silicate. Heat treated and annealed to produce a flawless tube. The 204 is a medium knuckle, medium wall thickness slide that fits the ring size of 10 with the dimensions of 20 x 25 x 28.