There is nothing more irritating that a rod of expensive glass that self-destructs when it gets anywhere near a source of heat. No matter how careful you are, it just flies off in little, unusable bits.
I have been hoarding some rods of River Rock (Vetrofond 931, once created, never reproduced) until I have been worthy of it. I have fooled around with it a little bit, but it shocks easily and I only have lots of bits and pieces of expensive, rare glass to show for it.
Today I tried an experiment. I tipped out all my little bits and pieces of River Rock into a small aluminium tray. I then melted a small blob of clear glass on the end of a stainless steel chopstick.
Next, I picked up the smallest crumb of RR, and melted it in. I then picked up the next crumb, melted it in, etc.
Once I had a big enough gather of molten RR I picked up a bigger chunk. The trick is to ensure that the molten glass can be pushed around over the top of the cold chunk. This seems to minimise the toxic shock that even small chunks of very shocky glass can experience.
Keep heating up your gather, pushing it around another chunk, melting it in, etc until the gather is about the size of a small grape. Use tweezers, an old mandrel, or needle-nosed pliers to pull a very thick stringer.
At the end of this exercise I had enough thick stringers of RR to reverse-encase two focal beads. The stringers did not explode. Not bad…