I clicked the "send" button prematurely. Here is the whole story:

The kitchen drain has not been properly vented for over a hundred years, if only because the obsolete, now code-violating drum trap is more than 6 feet from the vent. I added a further code violation to please a possible inspector when I installed a second, currently approved P-trap underneath the kitchen sink in series with the old drum trap. This usually works satisfactorily, but every 6 or 12 months the system needs a little encouragement with sodium hydroxide flushed after one to twelve hours with a kettle of boiling water. Optimally I would open the drum trap to clean it, but if I remember correctly my attempt to do do many years ago was unsuccessful. A hired plumber would be required by code to tear it all out and replace everything and cause a lot of trouble and expense. Been there and done that.  Now, of course, my hips would veto all my own efforts to try to clean out the trap. Wer spricht von siegen? Ueberstehn ist alles.