Help needed: cleaning lens from firecracker particles

[resummerfield]

Did it make a difference? I would think that just leaving the sleeping dog lie would be the more prudent thing to do.

I was able to scrape off the particles, but the damage to the glass surface remained. In my case, the bubbles were light colored, and in bright light they would sparkle, and removing them helped slightly. If the particles are dark, I think I would agree with Brian and leave them alone.

[cyberjunkie]

I left the lens in the hands of friend, who's a retired repairman. i try to refrain from asking his help, cause he does not want to be payed, and he saved my bottom way too many times...
Whatever... the particles went away (AFAIK he uses isopropilic alcohol and some product for dishwashing machines), but the impact was strong enough to produce some small cavities on the surface of the front glass.
The damage is all over the map: some particles cleaned, others left damaged coating, 3 of them left a visible indent on the surface of the glass.
My best guess? I think that the cleaned lens will perform much better than before, and that the reduction of contrast and the increased flare will be barely noticeable.
If i'm right, i will not do the china ink stuff. Being also a collector, i'm probably too sensible to the "beauty" of my preferred/beloved optics. Therefore i think i'l do my best to find a replacement glass in due time. Placing spots of black ink on the front of the objective would be like defacing it. Can't help but feeling that way...

Thanks to all those who offered their advice.

cheers

Paolo

[Donald Qualls]

The particles are most likely aluminum oxide: the stuff inside a firecracker that make the "bang!" is flash powder, usually aluminum powder with a strong oxidizer such as potassium perchlorate. If the cracker was close enough to the lens, particles of aluminum oxide formed in the flash might have arrived still hot enough to melt to coating, and thus effectively welded themselves to the lens. Removing them will be virtually impossible, and completely impossible without leaving craters in the coating. The best results you can get would be to individually paint each particle black (extremely tedious, but possible with something like India ink and a "brush" made of a splinter of wood) or to chip each one off and fill the remaining crater with something black (India ink is a good choice here, too). Given the extremely high risk of adding scratches to the craters if you try to chip the particles off, I'm going to suggest either leaving the particles alone, or blackening them in place.

If the lens is valuable enough, it might be worth having it disassembled and the coating stripped from the front element and reapplied (this might or might not require separating it from the next element if cemented); there used to be a service (in Russia, as I recall) that would do that kind of work for cost low enough to be cheaper than replacing a top-end lens, but who knows if they're still operating, or what if any loss of performance there would be due to their coating not being identical to the original.

Edit: Woops, just reread your last post, Paolo, and realized the "cleaning" is already done, and the craters present. You'll get some flare (that will fill in shadows a bit), but probably little or no loss of resolution; the flare is the only thing likely to be noticeably changed by blacking in the craters. Replacing the front group might or might not give a perfect match with the rear group (in other words, it might be worse than leavin the lens as it is now). Recoating the front element is the course most likely to fully restore the lens performance, but still without any guarantee, because the new coatings won't be the same as the original.

[Harold_4074]

New coatings will also not correct the refraction occurring due to the shape of the craters. But India ink is water-based, so spotting out the worst damage and removing it if the lens ever goes on sale would be an option.

[Donald Qualls]

New coatings will also not correct the refraction occurring due to the shape of the craters. But India ink is water-based, so spotting out the worst damage and removing it if the lens ever goes on sale would be an option.

IF the damage is only to the coating, stripping the old coating will remove it, and the new coating will have (approximately) the same optical surface as the original. If the damage goes through into the glass underneath, new coating will only reduce the visibility of the damage, while doing nothing optically to improve it.