Made of top quality 3.3 borosilicate glass. Designed according to ISO 1042, DIN 12664 and calibrated to contain (TC, In). The conformity with “Eichordnung” (the German Federal Weights and Measures Regulations) is certified through the official “DE-M (year)” sign on each flask. Each flask has a batch number printed on the flask. Batch certificate included (to be downloaded free of charge here)