Molded from premium polystyrene, this test tube rack feature round holes and holds 17mm test tubes in width. Disassemble for easy storage. Plastic clear test tube caps or stoppers ensure a leak proof seal. The plastic cylindrical test tubes are manufactured from high quality polystyrene. PS Tubes are not autoclavable or dishwasher safe but are chemically resistant and clear like glass. Polystyrene has the properties of glass but at a much lower price point. Since polystyrene is not as hard as glass it will have minor scratches.