Tupperware Vent N Serve Mug Black Seal New vented silicone valve that's easier to open and close. Both liquid and airtight, this silicone vent also lets steam escape more rapidly when reheating and can be easily detached for cleaning. And as before, you can take foods directly from the freezer or refrigerator right to the microwave for fast and easy reheating - perfect for leftovers or a meal on-the-go. 2-cup/500 mL capacity. Black Seal.