5.2 NSS test5.2.1 Reference specimensTo verify the apparatus, use four or six reference specimens of 1 mm ± 0,2 mm thickness and 150 mm × 70 mm, of CR4-grade steel in accordance with ISO 3574 with an essentially faultless surface2) and a matt finish (arithmetical mean deviation of the profile Ra = 0,8 µm ± 0,3 µm). Cut these reference specimens from cold‑rolled plates or strips.Clean the reference specimens carefully, immediately prior to testing. Besides the specifications given in 6.2 and 6.3, cleaning shall eliminate all those traces (dirt, oil or other foreign matter) that could influence the test results.Thoroughly clean the reference specimens with an appropriate organic solvent (such as a hydrocarbon with a boiling point between 60 °C and 120 °C) using a clean soft brush or an ultrasonic cleaning device. Carry out the cleaning in a vessel full of solvent. After cleaning, rinse the reference specimens with fresh solvent and then dry them.Determine the mass of the reference specimens to ±1 mg. Protect one face of the reference specimens with a removable coating, for example an adhesive plastic film. The edges of the reference test specimens may be protected by the adhesive tape as well