hey all - finally an issue i can chime in on! lol
my wife has always cooked on electric ranges, and INSISTED we buy clover-leaf looking copper wire units to put below all glassware on stove - corning & otherwise. check the scan - hope it works. they run about a buck for 2 at dollar stores.
with electric elements, the elements heat the glass directly, in the area it is in contact with with, so adjoining molecules, within the design of the element can be at extremes temp wise, a thermal shock thingy. glass is thermally transparent, which is why you don't have this problem with radiant cooktops (at least thats what we were taught to say when i sold them at sears!) since the heating element is BELOW the glass, not in contact with it.
on gas ranges, (or hot plates) the prob isn't the flames, its the metal burner pot holder thingies (sorry to keep using these trade-specific scientific expressions, makes me sound like a SNOB, i know!)
that CAN heat up and create hotspots. usually best to start temp low then gradually build up if need be to temper the project.
remember, the glass lets heat pass through, more or less, so if sludge is a large WET mass, it will remain at 212 degrees until water is evaporated, then jump QUICKLY once its gone, up to its melting point.
since corning ware, the good stuff is getting harder to find at garage sales. a buck is cheap insurance.
i think.
hope this helps.