OK, I have attached two nice closeups below. I'm sticking to my earlier assertion that the amber crystals are some chemical that is precipitating/outgasing/growing out of the aging rubber, for the following reasons:
1) They are found all along the rubber seal material, even on the upside down and vertical parts. I have a hard time believing that dessicant sprinkeled into the airgap between the windows 40 years ago would not have shaken down to the bottom.
2) One of my wing windows is original and is sealed with the black stuff (that has crystals growing out of it). The other has been repaired at some point and was done with some kind of grey glazing putty. They both have some moisture (fog) between the panes, but no crystals in the grey glazing putty.
3) If I look really closely, I can see tiny crystals that are just emerging from the rubber.
Some time in the near future, I will pull the window frames apart and rebuild/reseal the windows. I'd be glad to save a sample of the crystals if someone has access to a mass spectrometer and would like to do a detailed analysis. Any grad students out there looking for a thesis?
Since there seems to be some fans of Wikipedia on this thread, I went ahead and did a little search for "crystalized rubber," and low and behold, got a wiki hit:
Strain crystallization - Wikipedia, the free encyclopedia
Not a very enlightening article, but the gyst is right here:
is a phenomenon in which an initially amorphous solid material undergoes a phase transformation
due to the application of strain. Strain crystallization occurs in natural rubber
, and some other elastomers
. The phenomenon has important effects on strength and fatigue properties. There are various techniques for measuring crystallization in rubber, including: x-ray diffraction, specific heat changes, and density changes.
Because I tend to over-think these things, I did some more looking, and I have bad news for my budding grad student that might have gotten excited at the above. The study has already been done, you can find the dissertation (bed-time reading) at the link below (there are some sections in German, but don't them them slow you down). It gets really interesting around page 92 "Deformation-induced crystallization in polyethylene-polybutene copolymers depends on temperature and strain rate in a similar way as does polyisoprene rubber, although the molecular rearrangements taking place are dfferent. In the early stages of drawing an unoriented crystalline polymer, spherulites become..."
Ah...so if I take rubber and squeeze it between two sheets of glass for 40 years, it might undergo a phase transformation, producing crystals. QED. Guess I still have to take the window apart or live with them.