Our 2000ml Erlenmeyer Flasks are made of Borosilicate Glass (aka Pyrex) which allows for direct boiling and cooling of your starter wort*. They take a size #8.5 or #9 stopper if you wish to use one but we prefer covering with aluminum foil loosely to allow oxygen to enter while stirring.
*Of course, use of some common sense is appropriate. Before moving the flask from direct flame to cooling bath, consider waiting 5 minutes and perhaps use a room temp water bath for a minute or two before adding ice. Borosilicate can handle wide temp swings relatively quickly, but boiling to ice without any pause is a bit much to ask.
