Strange pattern in Angulon 90 6.8 front element - balsam separation?

[Steven Tribe]

Actually, synthetic adhesives are superior to Canada balsam in almost every way...
.

Except -well, for earlier synthetic glues, at least - preventing sudden loss of adhesion, through thermal stress.

[Nodda Duma]

Not even then.

Norland 61 was developed in the 1960s specifically for NASA because Canada balsam could not survive the rigors of space flight (think temperature cycling). Aside from a wider range of operational and storage temperatures -- and UV curing -- it also remains pliable long after Canada balsam hardens (lower outgassing). NOA 61 is still in wide use today... it is considered a "good run of the mill" adhesive for doublet bonding, and chances are most of the lens doublets in any of your modern camera lenses are bonded with NOA 61 or something very similar.

Failure correlation isn't to adhesives. Failure correlation is to large CTE mismatch of the elements and exposure temperatures beyond design intent. By that I mean they would fail regardless of the adhesive. It wasn't as easy to match CTE's back then due to more limited selection of glass types -- especially in the post-war period with introduction of higher index / lower dispersion glasses that designers wanted to use. If the designer wasn't careful with controlling the bonding surface radius of curvature, then failures were more likely as the years pass.

Interestingly enough, I've participated in analysis and testing of doublet debonding at temperature, and we found that the bond joint most often fails at cold rather than hot temperatures. The reason is that optical adhesives soften (become more pliable) at hotter temperatures, and become harder (more brittle) at cold temps. The adhesive can thus survive stresses at hot extremes that it can't survive at cold extremes. Thermal shock (significant, rapid change in temperature) is also a cause.

[Steven Tribe]

Interesting.
The cooling problem could explain why I have seen a number of failures in connection with air transport and associated winter storage in Warehouses etc.

[Lachlan 717]

I've found several of the (inner-ring marked) Fujinon lenses get this effect, particularly on edges.

FWIW, I haven't noticed any image degradation from it, though.

[kaif]

Thank you everyone for their detailed advise and information!

My initial instinct would be to just return it, since at the price these go for, it hardly seems worth trying to repair it. Part of me would be curious just to see how much impact this has on the image quality, and with a very rough test putting it in front of the sensor of a Sony A7s I didn't notice any obvious degradation even with some shifting towards the edges of the image cone - but then, I don't have an undamaged lens of the same type here, to compare it to.