Lens fogging - causes and cures?

In May 2002 a detailed discussion on this topic took place on the photo.net Leica Forum. The following posts were so informative and interesting that they deserve to be quoted in full:

(1) Here is a broad overview of the problem by Niels H. S. Nielsen:

[… Mr Nielsen had bought an old Rigid 50mm Summicron and wanted to know how much it would cost to clean up the glass elements …]
I asked [CRR in the UK] for an estimate on a polish/recoating of the front element plus if necessary, removal of haze & a general CLA of the lens. This is their answer:
To break down the costs involved to restore your lens, is as follows,
The front surface of front optic, re-grinding/polishing and Mgf2 re-coating UK £ 96
The 'hazy' internal coatings may clean with chemicals , but if there is 'etching' of the coating or glass then the only remedy is re-polishing/coating of optics inside the lens.
All older Leitz lenses are 'soft-coated' internally , this means that they deteriorate more readily than their Japanese counterparts which are 'hard-coated'. Consequently , when condensation is deposited on the internal surfaces , after many months/years , the deposit becomes acidic and attacks the soft anti-reflective layer and also the glass itself.
It is highly unlikely that all the optics in the lens have deteriorated too far , but I must stress that chemical cleaning of the inner surfaces will not restore damaged coatings or glass . It will remove the 'cloudy' haze and improve light transmission though .
To service the lens, repolish & re-coat the front element and chemically clean the internal optics would , in total , come to approximately UK £ 175. Subject to examination first.
This may sound expensive, but every surface which requires re-polishing/coating will add an extra UK £ 90 , as the optics has first to be measured t obtain the correct profile and curvature and a moire pattern obtained so that after polishing the lens willl perform correctly.

(2) Andrew Schank followed up:

The part about the soft coatings and years of moisture/condensation attacking the coatings is exactly what Bill Maxwell told me about the older chrome lenses. He should know, as he's taken apart and cleaned over a thousand of them. The coatings on my DR 50 were damaged and not simply cleanable. Bill also told me, (and Sherry Krauter seconded this) that once a lens has had the coatings polished off internal elements, sometimes the thing doesn't perform as well as a factory original lens. Mine lost something after it was worked on-images just didn't have the same look as before the coating went bad and was repaired.
I don't think I would spend a penny on a beat up lens like that one — use it and you'll probably find that except for the widest apertures in backlit situations, it still performs OK. It would be different if it was a smooth, spotless example with some bad glass that was had for a cheap price. Then it might make sense to put some money in it. Just my opinion of course.

(3) Eliot Rosen concluded:

The moisture condensation theory is indeed the most prevalent one today, although the fog/haze in the past has been attributed to other factors (lubricants, whale oil, etc.). You must have had the lens an awfully long time to have had a clear lens originally that developed so much fogging that it etched the glass. My impression was that etching of the glass from the usual fog is not very common, and a cleaning usually imporves performance, unless glass the elements were not re-aligned properly when the lens was re-assembled.

For more information on lens CLA's, see this topic elsewhere in the FAQ.