Borosilicate glass tubes, great resistance, ideal for laboratory ware. Complies with ASTM Specification E438, Type I, Class B glass requirements. These are premium quality tubes with sturdy, uniform bottoms and consistent lengths. The 6 x 50 mm size is often referred to as a 'Durham tube'. Trays are shrink wrapped in plastic film with tubes remaining in constant alignment for protection against in-transit breakage and contamination. Designed from ASTM Specification E890.