Viable methods of repair depend on the extent of the damage and type of cement involved.
Repairing minor separation (including small discrete bubbles and crazing)
Balsam is used in the preparation of permanent histology slides, which may develop separation and bubbles. A trick I learned from a lab tech is to stick the slides in an oven. The heat melts the balsam, allowing it to self-heal as the bubbles migrate out from under the cover slip.
The same approach can be applied to lenses. Set the oven to just over 300F, and let the lens sit for 15-30 minutes. Make sure the concave surface of the lower element is facing upward so that bubbles will migrate upward, toward the edge of the seam. If bubbles migrate toward the center, just turn the elements over and repeat until all the bubbles have migrated out of the lens.
Repairing major separation
If baking the element for up to an hour doesn't resolve the problem, the elements need to be completely separated and recemented.
Separate the elements. Balsam melts at about 300F (149C). So methods discussed on forums, such as boiling in water (212F/100C), will not work. Cooking elements in oil will work, but is unnecessarily messy. Put the element in an oven set to just above 300F (149C) for about 15 minutes. Then carefully slide the elements apart while they're still hot.
Another option is to use a hot plate (or equivalent). Watch for the balsam to melt.
Centering the elements. Most lens elements are designed so that the mechanical center is the optical center. Most lens housings are designed to hold elements so that their optical centers align. So setting the cemented elements in their final resting place will appropriately align them. It's also possible to build a simple jig to hold the elements while they set.
Recementing the elements. The difficult part is obtaining a suitable optical cement. Canada balsam does not appear to be available at everyone's favorite auction site, except as essential oils or food supplements.
I have tried a few different materials, but they are unsuitable for use in photographic lenses. Further attempts are pending my ability to source real balsam or another suitable material.
- Consumer ovens may not be accurate, so you may have to increase the temperature.
- Wear eye protection and heat-resistant protective gear.