On Thu, Dec 22, 2011 at 9:13 AM, Bruce Walker <[email protected]> wrote: > Permanently? Or does the yellowing eventually return?
Well, the yellowing being caused by the radioactive element (over time) it would be expected to manifest it again, if untreated, until the radioactivity is gone. Not sure what isotope of Thorium was used in the glass production, but the naturally-occurring stuff (232Th) has a half-life of 14.05 billion years so, yes, I would expect it to reoccur in time. (The good news being that it will yellow at only half the speed in a mere 7 billion years). Keep in mind that when you get a yellow lens, you are looking at a lens that was produced in the mid-to-late 60s, so you are viewing the cumulative yellowing of 50 years. I'm guessing a shot of UV once a decade or so should keep it at a manageable level. Darren Addy Kearney, Nebraska -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
