The glass becomes molten at temperatures ranging from 900°F to 1600°F, depending on the type of glass being manufactured. Standard microwaves cannot produce that much heat in any amount of time. Because the microscopic air bubbles trapped in a glass container may shatter, crack, or explode. On the other hand, a microwave will not be able to melt glass. As a result, you can microwave cold glass containers (microwave-safe) for as long as you need without worrying about melted glass.