S'well Tumbler with lid onyx 18oz S'well Tumblers feature Therma-S'well technology - a triple-layered, vacuum-insulated construction made of high quality, 18/8, food-grade stainless steel walls plus a copper wall layer for superior temperature retention. It means that beverages will stay cold for up to 17 hours and hot for up to 4, while maintaining a condensation-free exterior. The 530 ml (18oz) tumblers are perfect for on-the-go drinking, featuring a slide-open lid meant to create a splash-proof experience.