When heating Pyrex on a metal heating element directly, the different rates of expansion of the layers of tempered heat resistant glass WILL cause the glass to explode, sending shards of hot, sharp glass in all directions! I had a glass pie plate I mistakenly heated when I turned on the wrong burner. Fortunately, I was wearing a long sleeve shirt and heavy jeans. Shards of glass flew into every corner of a 10 by 12 foot kitchen, behind appliances, even melting the flooring where it fell.
Got lucky that time, but when I moved from that house I was scraping the glass out of the melted flooring before I left.
If you use an direct electric element, get a diffuser from a lab supply company. Not expensive, and worth every penny. My range is now a glass top electric and the top itself works as a diffuser and I have never had a problem with starters, except for boil over. What a sticky burnt mess! Fermcap-S is my best friend there!