GREAT DISCOVERY! (For me anyway). In my mobile fluorescence museum I have 1504 square inches of Hoya U325C filter glass. It’s been there for 3 or 4 years and all of it, to a greater or lesser extent, has developed an ugly white encrustation. Why? I don’t know. But I’ve tried hours and hours of scraping, steelwooling, household cleaners, various solvents and more to get rid of it. I’ve never been more than 50 or 60% happy with the results. Today in desperation I tried hydrochloric acid – useless. Then I tried phosphoric acid. A miracle! The white stuff wipes off as easily as wiping down the kitchen bench, leaving the shiny glassy surface as it was when new. How about that!

Great to know it! I am waiting from ebay a fotodyne transilluminator 3-3000. I guess it has the same Hoya filter. I’ll check if it has the same problems of yours. In case I will try with phosponic acid too (sold as normal household cleaner, not necessary to go to specialized stores). Thanks for sharing this great discovery with us.

The white scum is usually water damage. Hoya filters are susceptible to moisture. All my Hoya glass is in 8″ x 4″ ’tiles’, a few cut to cut to 4″x4″ or 8″x2″ where necessary, butted up to each other either sideways or lengthwise (eg. 2 8x4s and a 4×4 needed for twin 95Ws). They are all installed in display cases in the walk-in trailer. The only water they could encounter is humidity. The very strange thing is that there is no pattern of infection. Some tiles were completely covered with a uniform coating of the stuff, others the stuff was creeping in from the edges, others had little or large round blobules randomly spread, etc. Contiguous tiles could be affected quite differently and very occasionally not at all. I wonder if the recipe Hoya uses could change a bit from batch to batch?