RE: [Rollei] "Balsam" issues with some Zeiss lenses?

[Richard Knoppow <dickburk >]

At 01:30 PM 11/06/2001 -0000, you wrote:
>Joe B. wrote:
>
>"A repairman I spoke to tonight said he's seeing a lot of Zeiss lenses from
>the
>60's with what people call balsam problems- he says it is actually optical
>cement and not balsam that is giving this problem with these lenses."
>
>I've heard this too, from a specialist Rollei repairer in London. The lens
>we were discussing was the 135mm f4 Sonnar in the Tele-Rollei, and he warned
>me to check *very* carefully for this effect. His optical specialist has had
>the cemented elements of two Tele taking lenses sitting in a bath of
>whatever-it-is-they-use-to-take-them-apart for over *twelve months* and they
>won't separate (so they can't recement them).
>
>www.ffordes.co.uk has three Tele Rolleis for sale on its site, and when I
>enquired about the lens condition by mail a few weeks ago I was told that
>two of them had separation problems in the taking lens.
>
>-- 
>David Morton
>dmorton  uk
>
  FWIW, A company called Summers Optical makes optical cements and
solvents. Their web address is:
http://www.emsdiasum.com/Summers/optical/cements/default.html
Even if you
are not interested in taking on recementing yourself the primer here makes
interesting reading. 
  Synthetic cements have been used for nearly all lenses from the late
1940's. A few manufacturers began using them even earlier especially for
aerial lenses for use at high altitude. These lenses are subjected to
temperatures which will almost instantly crystalize Canada Balsam, making
the layer cloudy and the lens useless.
  Many kinds of cements have been used. The early ones were mostly
thermosetting. While synthetic cements should have a much longer lifetime
than Canada Balsam there are subject to some problems in assembly and
curing. I've seen some lenses, including Zeiss lenses for the Contarex,
which had what looked like large bubbles in them. This is the cement
separating. I have also seen a few Kodak lenses where the cement layer has
become turbid, looking like wax paper. 
  Many lenses can be recemented. If the elements are not completely
separated the technique is to bathe the lens in a hot solvent solution. The
solvent Summers sells operates at around 340F. The problem is that
sometimes the thermal shock can cause the elements to fracture. The
Summer's solvent is started cold to avoid this problem. Once separated the
lenses can be cleaned with Acetone and pure Ethyl alcohol and recemented.
Summers sells both binary type and UV setting cements. I've used the
conventional binary type. This requires curing at 130F for an hour. There
is also a room temperature curing cement but I prefer to have the longer
working life of the mixed cement. The temperature is not critical and the
recementing procedure is not too hard to do. 
  Most cemented elements have edges which are carefully centered. When
these are clamped together the entire assembly will be centered correctly.
The difficulty comes with lenses with different diameter elements, such as
the Schneider Angulon. I've not recemented a finder prism but would guess
that its practical to do.
  Steve Grimes also has a little on lens re-cementing on his web site
http://www.skgrimes.com
  He uses prisms to clamp the lens edges. I've found that even large
machine nuts seem to be suitabley square. A sheet of thick glass is used as
the reference surface. I've used an ordinary gas oven for curing although a
temperature controlled electric oven would be ideal. 
  I have also recemented using Canada Balsam, but it is actually more
difficult and fussy to use and the results are not as good. 
  Lenses cemented with Canada Balsam can be gotten appart by gentle
heating. The text books say to use a frying pan but I've also had good luck
placing the elements in water and heating it until the fall apart. 

- ----
Richard Knoppow
Los Angeles, CA, USA
dickburk  

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