Originally posted by stevebrot Welcome to the Pentax Forums. Thanks for your thoughts. A few points of clarification:
- The thorium is not a coating. It was added to the liquid glass formulation when it was made to change is light refraction characteristics.
- The discoloration is not oxidation. The color is internal to the glass and results from physical, not chemical, changes on the atomic level. Do a Web search for "glass radiation color centers".
- Acid would not be a good solution.
There is a fair amount of strange stuff on the Web regarding the yellowing and how to tell a radioactive lens. One of the most persistent claims is that it is caused by changes to radioactive lens coatings. While many lenses will reflect a yellowish tint, that is not the same as the radiation-induced yellowing seen in a thoriated glass. Efforts to remove yellow-reflecting lens coatings will not remove radiation and will damage the lens.
Steve
Thank you, Steve, I assumed it is a thoriated glass only, without any coating but I was not sure since I never tried to remove the coating from an old takumar lens. This fact should make the removal even easier since there is no need to worry about the coating. Thorium is a not a reactor fuel it is slightly radioactive, probably below average continental runoff consisting of radon etc. there is nothing to worry about, one can test it with Geiger apparatus if concerns are an issue.
if a yellow stain is on the surface of the lens it is likely Thorim oxide Th0, Th02 or any other form of Th2,4+ and therefore removable.
both oxidation stages are yellow so it matches the description.
I doubt he will succeed with UV experiment and permanently remove the issue but I would try different approaches, it can't hurt. If the problem was a fungus of yellow color, UV bleaching on both lens surfaces will probably do the job. I always test for fungus first and dip my lenses in a light solution of hydrogen peroxide, works better than UV
I never managed to damage any lenses, I even ran spectrophotometry test to test for the changed composition of my washes, never found anything worthy of concern. I regularly use moderate solutions of ammonia vinegar ethanol and hydrogen peroxide to restore old lenses and I never managed to damage anything, even when i wanted to