Lens Glue

    Have you ever been disappointed to find that one of the cemented lenses in a
nice piece of surplus optical equipment you have acquired has separated? Or perhaps
you have purchased an old pair of binoculars for a song at a garage sale, only to find
after a closer inspection that one of the objective lenses has suffered glue failure and
is unusable due to the spider-web cracks, voids, and cloudy gunk between the
elements.

    If and when this happens to you, don't despair. In most cases this can be easily
remedied in a few minutes in your kitchen at home.
Most older cemented lenses are held together with Canada balsam, which is the
same stuff we used in junior high school to mount the cover glasses to microscope
slides. More modern lenses are held together with some more serious stuff, and it takes
a bit more effort to get the elements apart.

    To separate the older Canada balsam-glued lenses, simply place the lens in a
saucepan (or coffee can), cover it with water, and put it on the stove and slowly bring
it to a boil. In just a few minutes you will find that the glue has softened and the lenses
have separated. For smaller lenses you will find that the action of the boiling water has
enough force to shake the lenses loose. Larger lenses may have to be twisted or slid
apart by hand (wear gloves!). Keep the lenses submerged, remove from heat, and let
the glass in the water cool slowly. Do NOT get impatient and add cold water to the
pan! The thermal shock caused by this rapid temperature change is enough to crack the
glass. I learned this the hard way with a nice 50mm objective lens many years ago. It
split the flint element neatly in half, right down the middle! After cooling, the residue of the glue is easily and thoroughly cleaned from the lenses with acetone, which leaves them ready for re-cementing.
 
    If the above method doesn't work to separate your lenses, then they have probably been joined with a
tougher, more modern adhesive. One of these is dealt with below.  But to continue: Since in recent years Canada balsam has apparently disappeared from the scene, a common modern alternative is an ultraviolet-cured optical glue made by Norland Products, Inc., 695 Joyce Kilmer Ave., New Brunswick, N.J. 08902. It is also available from Edmund Scientific and other optics suppliers for about $20 for a
1-ounce bottle. They produce a series of these cements, but my preference is number
61 which has a similar refractive index (1.56) to BK7 glass, and a tensile strength of
3,000 psi. The spectral transmission of Norland 61 falls off rapidly below 0.4 microns
wavelength, is flat at about 98% transmission from 0.4 to about 2 microns (covering
the visible range and well into the infra-red), then falls off in the infra-red to about
30% at about 3.3 microns, and back up to 90% from about 3.5 to 5 microns.

    This adhesive has a relatively short shelf life though. The literature claims at
least 4 months if stored in a cool dark place. A bottle I bought in 1992 or '93 had an
expiration date in 1994. The stuff was still usable in '95 or '96, but in the spring of
2001 it was harder than a block of wood. A bottle I bought in April of 2001 was still
fine in April of 2002. (I store the stuff in a zip-lock plastic bag, which is inside a
brown paper bag, which is inside a cardboard box, which is on a shelf in a cool
basement that rarely sees temperatures as high as 65 degrees Fahrenheit.)
To use this glue, just place a drop or two between the lenses (it has a viscosity
just a little thicker than vegetable oil), squeeze any air bubbles out, and center them
well with a V-block or a piece of angle iron. The Norland literature says that it can be
stiffened up a bit, or partially cured for final lens alignment by exposing the joint to a
100 watt mercury lamp at 6 inches for 10 seconds. The final cure is accomplished by
another 5 or 10 minutes of the same exposure.
Since I do not have a mercury lamp, I have opted to use sunlight which is rich
in ultra-violet rays. I have found that partial curing can be accomplished with a 10-second exposure to the afternoon sun through one layer of window glass. This stuff
sets up very rapidly though. A 20-second exposure to sunlight will seize the joint fast,
so be careful. After this I generally let the joint sit in the sunshine for a half hour or so
just to be sure the sun has finished doing all it can do to the glue.
The literature also mentions that optimum adhesion to glass comes "with aging
over a period of about 1 week in which a chemical bond will form between the glass
and adhesive. This optimum adhesion can also be obtained by aging at 50 degrees C
for 12 hours."

    If and when you may need to un-cement glass that has been bonded with this
glue, methylene chloride is the solvent to use when the glue is fully cured, and acetone
works when the cement is still not fully cured.
I recently had some one-inch prisms I needed to unglue, but unfortunately
learned that methylene chloride is getting hard to find in my area's hardware stores.
Several years ago I could buy small amounts of 90% concentration in the hardware
store which sold as some brand of Plexiglass (acrylic) glue, but nowadays the
"Plexiglass glue" has become one of a number of other solvents.
As a substitute, I tried a brand of paint stripper that mentioned methylene
chloride as the first ingredient, with acetone, methyl alcohol, and Stoddard solvent
making up the rest. I placed the prisms in a glass container and filled it up with the
stripper and waited. And waited. And waited - in fact, several weeks of waiting to no
avail. Don't bother with trying paint stripper.

    Finally, I made a trip to the local plastics supplier in hopes that he would sell the
stuff, but he informed me that he doesn't sell methylene chloride anymore because of
some packaging regulations or some such nonsense. However, I did find that he sells
a "Clear, Water Thin, Very Fast Curing Solvent Cement For Joining ACRYLIC". The
can listed the ingredients as methylene chloride, trichloroethylene, and methyl
methacrylate monomer. I paid the six dollars for a pint of this stuff and poured it into
the prism container. Over night the first pair of prisms had cleanly separated. After a
couple of days however, the second pair had still not separated, though progress could
be seen around the edges of the joint. I realized that the prism pair that had separated
first was on the bottom of the container with the second pair resting its weight on top
of it, adding a bit of shear force along the direction of the joint. Within 12 hours of
adding a one inch length of one inch diameter stainless steel rod as a weight to the top
of the remaining prisms, they had cleanly separated. The successful can of chemical
is made by IPS Corporation, and is called "WELD-ON 3".

    I thought I might pass this experience on to others that are walking a similar
path. Enjoy!
 

- Jim Sapp
April, 2002