Does your Hoya Shortwave Filter have White Scum Disease?

Graham Fraser on April 3

Graham Fraser posted a significant discovery on Facebook which is worthy of documenting here in the blog. The post, and comments, are quoted below: ===================

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.

You may have better results with a can of Coca-Cola (flavored with phosphoric acid) than with anything containing phosphonic acid. I did industrial water treatment for 12 years; I tried using phosphoric acid to remove scale deposits but found it far too weak for that purpose. Phosphonic acid and its variants are used as "crystal modifiers", which tend to distort the crystal formation of scale minerals such as calcium and magnesium carbonate, keeping them from forming the usually dense and tight layers of scale on heat transfer and water evaporating surfaces, making this chemical more of a preventative rather than corrective method. Coke is way cheaper, too. LOL

Phosphoric acid

Thanks Steve. I have some orthophosphoric acid at home, so I prefer to drink coke... BTW I think that the amount of phosphoric acid in coke is very little and, above all, mixed to many other colorants and a lot of sugar. Probably it is better to use pure or diluted phosphoric acid.

Great discovery Graham. FWIW - the white scum is usually water damage. Hoya filters are susceptible to moisture. I don't believe it has anything to do with solarization (but cannot absolutely confirm that).

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 4x4 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? Anybody know what the stuff actually IS? Might help to understand why phosphoric acid dissolves and removes it instantly. Sorry, can't send photos ... the stuff ain't there any more!!!


I wonder if there is enough phosphoric acid in a can of Coca-Cola (used to provide tartness, somewhere between a teaspoon and tablespoon's worth. True story) to remove that encrustation. Coke's not just good for removing crusties off of car battery terminals, you know. ;)

Good to know, thanks! I was advised to use ceramic stove-top cleaner. I think humidity may cause the problem?

That's one of the things I had tried without success. Looks like humidity could be the culprit, but doesn't explain why some panes are badly affected while others, even adjacent ones, are affected differently or sometimes hardly at all.

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