RockSlide Guitar glass Slide medium size: 19.5mm x 60mm. Moulded Glass Rock Slides are individually hand moulded in the USA by master lampworkers. These have all the same features as brass Rock Slides with the Knuckle Cut, Finger Rest and Tapered Interior. These fit almost exactly like their brass counterpart but are slightly longer. The main difference being that they are much lighter & faster so you can really cook with these on an electric guitar. Because of unique tapered interior design they do have enough mass to light up your acoustic. Medium Rock Slides have perfect weight for most styles, heavy enough for long sustain on acoustic guitars, but light enough to easily control for fast electric playing.