LIGHTWEIGHT BUT NOT FLIMSY ★ - Espresso coffee mugs feel lightweight, Compared to normal (soda-lime) glass, our borosilicate espresso shot glassess are much harder and stronger, they are more heat-resistant (enough to safely pour in boiling water!); and they stand up better to acidic drinks. Borosilicate is environmentally friendly, BPA-free and widely used in high-end cookware and drinkware.