Thanks for the info, looks like Swim is plagued with the same glass etching problem from his H3PO3 rxns. It is starting to make sense, considering that in the end some of it decomposes to phosphoric acid.
Post 523011 (https://www.thevespiary.org/talk/index.php?topic=5210.msg52301100#msg52301100)
(Scottydog: "Cleaning Eudragit Tainted Glassware?", Chemicals & Equipment)
As was previously mentioned, the white salt-like stain is transparent when wet and reappears when dry! Phosphate etching... To think that Swim believed it to bee the result of some kind of eudragit contamination.
No solvent or intense amount of scrubbing will remove this residue.
"Hot phosphoric acid attacks glass."
Over time, will the phosphates stress or weaken the glass? I imagine bees that dream of producing HI from scratch, dedicate one specific flask for that purpose. Just look for the stain and a bee knows which one to use. ::)