| Repair of Cemented Lenses |
| Information provided by John Toeppen |
Old lenses sometimes develop a cloudy or cracked yellow layer between two glass elements. This is because the lenses were typically cemented using a transparent tree sap called canada balsam. It usually has bits of wood floating in it and it must be warmed to allow it to flow like honey. Since it warms the optic unevenly it can stress and warp otherwise good optics. If you ever buy and use this material you quickly learn to appreciate UV cured optical cements. Sunlight and blacklights will cure UV materials but tests need to be done on microscope slides first. Canada balsam is easily remelted with a hair dryer and cleaned with many light solvents.
If you do not have a good spanner wrench to remove lenses from cells, it
is common to grind a steel edge to key to span the ring and get both
slots of the threaded ring at once. Removing a lens from a threaded
mount usually requires a suggestion of acetone on the thread of the
rings holding the element into the cell. It was common practice to mix
Duco Cement with a little acetone and put a drop on the threaded ring to
secure the ring. Excessive solvent could separate doublets that are
still good and dissolve other plastic parts.
Examine the edge of the doublet and determine if the edges of the lenses
were aligned to each other when cemented (typically an autocollimator is
not used to align doublets). Before you separate paired lenses it is
common practice to use a pencil to mark the clocking (relative
rotational positions of the elements) as it is usually important with
custom lenses. Use Norland or Sumner optical adhesives to recement lenses: https://www.norlandprod.com/adhesives/adhindex.tpl
If you have lenses that have become separated they will be of little
value for imaging. The camera may be collectable or not. If may be
worthless if you try to fix it, or it may be worthless if you leave it
alone. It is therefore suggested that the first optics that you practice on are
not of high value.
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