Originally posted by Elroy Jetson Canada balsam fir gum which, when dry, apparently has properties nearly identical to glass
Except for one important fundamental characteristic: the balsam is a primitive polymer, glass has a largely crystalline structure. Both materials behave differently outside of common operating ranges.
Originally posted by Elroy Jetson I didn't see any information on how much heat would be required to soften it 40 years after assembly, but it would be useful to know.
You can separate the elements cemented with Canada balsam by softening the adhesive by heating them in an oven at 150°C for about an hour*, and after separating the elements cool them down (
carefully) and soak the separated lens elements in acetone. This should remove the balm from the lens surfaces and then you can attempt re-cementing them with a modern UV curing optical cement.
Warning:
I do not recommend anyone attempt the following high risk technique: It is also possible to use cryogenic cooling to separate lens elements with liquid nitrogen, which causes the Canada balsam to become extremely brittle** - brittle enough that it is possible to shatter the cement between the elements with a quick tap, right at the point of interface with a finely honed blade, after separation the balm simply flakes off.
* the diameter and thickness of the elements have a profound effect on the amount of time needed, this is just a standard recommendation.
**the glass is still significantly stronger then the frozen basalm, but extreme care should be taken when handling glass at these temperatures.
Last edited by Digitalis; 12-19-2015 at 10:44 PM.