How Bad Is It?
Is it bad to pour hot liquid into a glass? According to John C. Mauro, Ph.D., a professor of materials science and engineering at The Pennsylvania State University, unless a glass is specifically made to withstand hot beverages, an effect known as thermal shock could cause it to break. While the inside of the glass expands due to the higher temperature, the exterior stays at room temperature, creating fracture-inducing stress.
So if you’re preparing iced coffee or tea, know that starting with a cold glass full of ice cubes can create thermal shock. Dr. Mauro says that while the trick of putting a metal spoon into the glass will conduct some of the heat away from the liquid, it won’t necessarily prevent shattering. Double-walled containers and borosilicate glass are typically safe for hot liquids, but always check the packaging to be sure.