Removing PTFE marks on glass bottles after using magnetic stir bars
Stir bar has a very rough grinding action. It is quite well known among silica manufacturers that if you stir micron sized silica with a stir bar for a long time in a reaction mixture, the spherical particles are damaged to some extent. In such cases, overhead stirring is preferred. This is not applicable to your case , I guess. If you had soil as well, the scenario is no different. It is a grinding action of the stir bar on glass. The "frosting" of glass is like rubbing a sand paper on glass for a long time. Nothing can heal this, except a glass-blower's flame treatment. Of course this is not worth it, as it is not going to affect the analysis of ammonia. I don't think that this is PTFE on glass. It will not stick to glass and secondly it is so hydrophobic that it would be floating on your solution rather than firmly sticking to glass.