These clean-lined glass mugs are made of German borosilicate glass, with a durability that belies their delicate appearance. Originally developed for scientific and industrial use, borosilicate glass is made by adding the natural element boron to the traditional glassmaking components of silicate sand, soda, and ground lime. The resulting glass is highly heat, chemical, and thermal shock resistant. The optional lid helps keep beverages warm and makes a handy place to set your tea bag. Loose tea or ground coffee can be brewed directly in the mug using the optional stainless steel micro mesh strainer. Dishwasher and microwave safe (metal strainer - not microwave safe). Lid and strainer fit both sizes of mugs.