Menzel round cover slips are made of D 263 M by Schott AG, which is a colourless borosilicate glass. Due to its composition it shows an excellent resistance to chemical attack. - Hydrolytic No. 1, accurate cutting, free of flaws and blisters. - Thickness No. 1 (0.13 to 0.16 mm). Menzel round cover slips are made of D 263 M by Schott AG, which is a colourless borosilicate glass. Due to its composition it shows an excellent resistance to chemical attack. - Hydrolytic No. 1, accurate cutting, free of flaws and blisters. - Thickness No. 1 (0.13 to 0.16 mm).