Hello All, On the topic of Borosilicate glass, I just want to relate my experience. I made a glitter lamp out of a 2000ml Borosilicate glass graduated lab cylinder. I heated it with a clear reflector 100 watt bulb from a Grande lamp. the bulb had a small space between it and the bottom of the cylinder. I wasn't worried about heat because my thinking was that - Borosilicate glass is used over open gas flames in the laboratory. I figured that the gas flame must be hotter than any light bulb could be so I wasn't worried. To my surprise, after a couple of days, the Borosilicate glass cracked on the bottom and burst. It did flow beautifully when it worked though. Has anyone else had Borosilicate glass crack from heat? I have another Borosilicate glass vessel I'd like to use for a lava lamp, But I'm very wary. Is this common? If you have ever worked in a lab and had Borosilicate glass crack from heat, I'd appreciate hearing about your experience. My thinking is that the clear reflector bulb with the mirror finish inside must focus intense heat on one small spot and that spot gets much hotter than the surrounding glass.
a short Answer: Borosilicate Glass is not all the same.
This kind of glass is a very clean and high quality glass, but it does not mean, that something is definetely heat resistant, only because it is made with this Material. The globe needs to be tempered like 24 hours, which makes them really expensive. Some glassware which is boro, is a a clean and more heat resistant Material, but it is not the same as something which is really made for this use case.
Our big globes for example are specially made for the use in our Lamps and are tempered.
Anyway, you should not place a bulb directly on the glass, a distance of 1-2" does not really change the flow but keeps you always on the safer side.