In scientific glassblowing (and, I'm sure other aspects of glassworking as well) the following is observed in the process of 'flame cutting' whereby glass is severed by gradual thinning and then 'ripping' by application of a torch: If the flame is kept too sharp at the moment when the glass is paper thin, there appears to be a physical transformation of a fragment (usually the size of a small fingernail paring) of the glass to a 'harder' or more quartz-like material. This forms what I refer to as a 'quartzlet', for lack of a better term. My questions are, what's happening, and what are these 'things' actually called?
My guess is that the glass is being flame-depleted of flux and forming a more purely SiO2 material. When working with borosilicate, the boro-glass appears red-orange in the flame (the 'flare', due respectively to the Li and Na fluxes), but the quartzlet appears water-clear with perhaps a faint hint of green. It appears to have the transparency of glass as opposed to any opacity or grainyness expected of a ceramic, which some glassblowers have told me it 'definitely' is. In a polariscope the quartzlet induces substantial strain into the workpiece and thus must be cut or 'picked' out. Quartzlets are very frequently the source of micro-leaks in high vacuum systems. In the process of removing (picking-out) the quartzlet, it doesn't appear to soften appreciably at temperatures where the boro-glass is quite plastic.
I'm sure this phenomenon is common in other glass disciplines, and it's something I've been curious about for a number of years now.
Last edited 02-18-03