What do I need to take into consideration to take video of molten glass which I will later postprocess?
What this comes down to is what's referred to as dynamic range. The wider the dynamic range the more detail you will see between the darkest darks and brightest lights. The human eye has a very large dynamic range, much larger than a camera. Cell phones are alright but dont have a very large dynamic range compared to a dslr or camcorder.  (This is immediately evident if youve ever tried to take a photo with your phone of a beautiful sunset only to be underwhelmed by the resulting photo compared to the reality.)

It sounds like all you need to do is manually adjust the exposure down a few stops until just the glass is exposed. If you have a dslr try snagging a set of ND filters for the lens you'll shoot with. The ND filter (neutral density filter) works to allow you to adjust the exposure of the image if you are dealing with extreme brightness either from the environment or due to your aperture/shutter/ISO settings. 

Hope this helps! Feel free to message me if you need any help!