8. Method A
8.1 Use the oil to be tested as received. Do not filter the oil through paper.
8.2 Promptly place the prepared copper strip in a clean 250-mL flask to which has been added 250 mL of the oil to be tested. Place the bent copper strip standing on its long edge so that no flat surface lies along the glass bottom of the vessel. Lubricate the ground-glass stopper with a small amount of the test specimen. Bubble nitrogen through the oil in the flask by means of a glass tube connected to the reduction or needle valve of the cylinder (rubber connections must be sulfur-free) for 1 min, and quickly put the stopper loosely in place.
8.3 Place the stoppered flask (immersed to the neck in the event an oil bath is employed) in the oven at 140°C. When the oil in the flask has reached approximately 140°C, tighten the stopper more firmly (Note 4). Remove the flask after heating for 19 h more or less 10 min at 140 more or less 2°C. Carefully take the copper strip from the flask and wash with acetone or other suitable solvent to remove all of the oil and let air dry. Do not use pressurized air to dry copper strip.
NOTE 4 - The corrosive effects of unstable sulfur compounds present in the oil are reduced when aged under strong oxidizing conditions such as when oxygen is present. It is imperative to keep the flask well sealed after nitrogen bubbling. If the oil visibly darkens during testing then most likely oxygen has entered the vessel and oxidized the oil. In these cases, the test procedure has become compromised and must be repeated.
8.4 To inspect, hold the test strip in such a manner that light reflected from it at an angle of approximately 45° will be observed.
