Manufactured from ASTM 438 type 1 Borosilicate 3.3 Glass. Precisely balanced at 20° Centigrade , while the joints are 10/18 size. The capillary stoppers are not interchangeable since the final adjustment for volume is made on the stopper. Adjusted bottles are calibrated to ASTM Test Reference D-369. Just fill with your sample and weigh. Each bottle and stopper convey the same recognizing number. Comes with Individual Calibration Certificate.