Point a UV nail-cure lamp at one of these lens elements for three or four days and something close to alchemy happens: the amber clears back to something like glass. Owners of the early collapsible Leica Summicron know the trick well, because plenty of them have done it. Then, over the following months and years, the color creeps back in. Weak tea, then bourbon. So they do it again.
The lens is the 50mm f/2 Summicron that Ernst Leitz Wetzlar built to succeed the Summitar, in the run-up to the M3. The design brief needed glass with an unusually high refractive index and low dispersion, and in the early 1950s there were exactly two ways to get it: dope the glass with thorium oxide or with lanthanum oxide. Leitz's actual plan was lanthanum, in a formulation that would later be sold commercially as LaK9. The problem was that it did not exist yet. As one Leica collector history puts it, that glass "wasn't available until late 1952, and even then only in limited quantities." Thorium was the stopgap that let the lens ship on schedule, used in early collapsible Summicrons dating from late 1951 into 1952, in both the original screw-mount version and, for a stretch, the bayonet M-mount that followed it into production.
None of that is the accident. Choosing thorium was a deliberate engineering decision, made with eyes open about a known, radioactive, but useful material. The accident happened afterward, and it took decades to show up. Thorium-232 has a half-life measured in the billions of years, decaying by alpha emission down a long chain that eventually ends at stable lead. Point a Geiger counter at one of these lenses today, as more than one owner has done for curiosity's sake, and it reads unmistakably hot. What nobody at Wetzlar was solving for in 1951 was what that decay would do to the glass itself over the following fifty years: alpha particles knocking electrons loose inside the lattice, where they get trapped as what physicists call color centers, absorbing blue light and turning the glass progressively yellow, then amber, then brown. It is not grime. It is not a coating problem. It is the glass itself changing composition from the inside, one decay event at a time.
For most of that history, this was treated purely as a defect. Camera technicians developed a fix: bathe the affected elements in ultraviolet light, whether sunlight on a windowsill for weeks or an LED lamp for a few concentrated days, and the color centers empty out, the glass clears, the lens looks the way it did in 1952. Today the same defect is a selling point. Listings specify serial ranges known to fall in the thorium window, sellers confirm radioactivity with a Geiger counter as a feature rather than a warning, and some shooters swear the residual tint adds contrast worth having on black and white film, though how much of that is optics and how much is habit is a real argument among people who own several copies and can't agree.
Here is the part that makes the whole thing feel less like a collectible and more like a small, ongoing chemical process you happen to own. The UV cure does not actually fix anything. It only empties the trap. Because the thorium inside the glass never stopped decaying during the treatment and does not stop afterward, the color centers start refilling the moment the lamp goes off. Clear it, and over the following years — sometimes decades — the stain creeps back, quietly, on its own schedule, for as long as there is thorium left to do the work. Which, practically speaking for anyone who owns one, is forever.