TheresaBee:
I make 320 ppm frequently.
Recently I noted one batch during which the glass became very 'silvered.'
The only thing I had done differently was change my heat to <150 degrees F.
I don't know 'scientifically' if the heat change was the cause ... but since then I have had my heat such that the water was near a boil, but not boiling and I haven't had the silvering issue since.