a rental toilet has a dark clay looking stain in the bottom of a toilet that the renter was unable to remove. At first I thought it was very hard water, however, there was no evidence of stains in a second bathroom or any other fixture. There is evidence of cat litter in the area and I was wondering if cat litter could form a tough clay layer in the bowl? Also, if cat litter is in the toilet, could it be in the drain piping and be a problem waiting in thwings. Can anyone suggest a way of removing from the bowl?
Cat litter is typically bentonite clay, so even if it has settled to the bottom & stuck to the surface it should be possible to remove it as it won't harden & set while under water (unlike cement).
I'd try using some sort of hard plastic spatula-like impliment to scrape the bulk off, followed by a vigorous scrub with a stiff-bristled toilet brush.
As for the rest of the drain plumbing ... if it is clay cat litter and it has settled somewhere inaccessible - you may well have a potential problem brewing.
You probably won't know until something gets stuck somewhere (or you go to the trouble of pushing an inspection camera down the pipes).