Here is an example of a single joint that was brazed using improper purge technique. The outside of the cutout was wiped free of any loose copper oxide material. All copper oxides on the paper are from the inside of the cutout.
This buildup of copper oxide is caused when copper is subject to extreme heat that occurs during the brazing process while there is oxygen present.
Anesthesia personnel usually automatically assume that the medical gases delivered from the wall outlets in the operating room are clean, correct, and safe. While crossed-pipeline accidents always receive significant publicity, an underappreciated patient safety issue concerns possible contamination of these medical gases with substances or materials that could possibly harm anesthesia equipment and, directly or indirectly, the patient breathing these gases.
Medical gas contamination: an unrecognized patient danger.
Moss E.
J Clin Monit. 1995 Jan;11(1):73-6.