Archived posting to the Leica Users Group, 2007/11/07
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Thorium oxidizes to a brown/black color and this could account for the coloration of the lenses. I'd say this would be a more logical reason behind the yellowing of the glass with the radioactivity being coincidental. I'd like to hear about the yellowing effect in these rare-earth lenses from someone who worked with their manufacture as well. My SMC Takumar 50mm 1.4 is one of my favorite lenses and probably better than any of the Leica glass I own or dare say, better than any of the Leica glass produced up to that point in time. The half-life of Thorium-232 is several billion years however there are trace isotopes that have been used industrially (Th-234 and Th-231) with half-life durations measuring about one day to one month. Not a chemist or physicist, but google works wonders. PhilFo On Wednesday 07 November 2007 17:54, Marc James Small wrote: > At 05:10 PM 11/7/2007, Leonard Taupier wrote: > >I have an old Pentax 50/1.4 Super Takumar that has, over time, > >yellowed due to one of the elements containing Thorium, a mildly > >radioactive element. The addition of thorium makes a very high > >refractive index glass, and a very good lens. This lens is probably > >one of the most famous lenses with this element but I'm sure there > >were more. > > Len > > I love you like a brother, but be conscious > here. The yellow cast may well result from > decaying coatings. Disassemble the lens, remove > the lens coatings, and then contemplate the > result. If an element is yellow, then, yes, your > original thesis may be correct. > > It takes thousands of years for radioactive > elements to decay. I am baffled at the thought > that the passage of only a few decades can lead > to some change in the color of glass > elements. Maybe I am a babe in the woods -- you > guys have locked my heels over the years on > things ranging from the utility of Tilley hats to > the vibrant necessity of US television -- but I > just do not see this as change as a reality. > > Do we have any nuclear chemists on the List who > can add anything to the discussion? Jerry > Lehrer, I realize, will tell us of his experience > at the Brooklyn Home For Broken Boys or whatever > it is called, back in 1921 <he grins>, but I > really would like to know the straight skinny > from guys who regularly have worked with thorium or with thorium glasses. > > And, Jerry, you STILL owe me those promised > serial numbers from your studio Rollei. <he grins, yet again> > > In short: can Thorium glass really change color > in a few decades? We hear this all the time. But is it real? > > Marc > > > msmall@aya.yale.edu > Cha robh b?s fir gun ghr?s fir! > > > > _______________________________________________ > Leica Users Group. > See http://leica-users.org/mailman/listinfo/lug for more information