Borosilicate glass flasks are designed to be put on an open flame and moved from heat to ice without cracking. Perfect for yeast starters and yeast culturing.
Safety Note: While flasks can be put on direct flame, even heat distribution is important and electric heat is not recommended. The safer, preferred method for heating would be a double boiler set-up. Use a 3 Gallon pot (or similar) large enough for the flask to fit down inside, fill the pot with water about halfway up the side of the flask. Heating the water to boiling will, in turn heat the contents of the flask.