Separating Lens Elements - Problem

[Paul Metcalf]

Howdy-
I have an old lens (Scientific Lens Co brass Portrait 5x7) which was laying around not getting much/any use due to rear elements having signficant separation. (It is a cemented pair of aplanatic meniscus lenses as it turns out.) So, being bored this weekend, I experimented with separating the elements using heat per what I've read about separating lens elements cemented with Canada balsam. Didn't work (up to 300 deg F), and the thinner meniscus element ended up with a gritty surface, almost crystallin in nature (under a microscope it shows lots of small hairline cracks, but only on the outer surface. I subsequently was able to separate the two elements chemically, and the other three surfaces (inside surface of damaged meniscus and both inner and outer surfaces of the thicker, non-damaged meniscus) are pristine. As best I can tell, both meniscus are glass, and I can't imagine anything else for the vintage of this lens (unless it's not that old). Any ideas what happened? And more important, any ideas as to how to correct the surface that's gone bad? Maybe a replacement meniscus? Thanks. Paul

[Jim Galli]

Bummer. Sounds like another lens ready to depart for the big roundup in the sky. Glad you posted because, oddly, I've got the identical lens with the identical problem. Guess I'll put the groups in a jar of acetone and forget about them for a week or 2 instead of applying heat.

At this point you've got nothing to lose. May as well finish the clean up and re-cement. After it's all cured, what if you coated the crazed surface with a clear polymer to make it smooth again. After that focus some light, make an image, and claim you've discovered the panacea with your one of a kind impossible to duplicate optic. ;)

[Kevin Crisp]

I agree with Jim. I had many conversations with Steve Grimes on this subject and he said sometimes old glass just does that and it cannot be avoided. He was going to recement a rapid rectilinear for me and he called to say the patient did not survive.

I have separated many protar elements by very slowly and carefully raising the heat in a convection oven. I start at 100 degrees F and once an hour bump the temp 25 degrees. I put the glass on crumpled aluminum foil so that it is not in contact with anything much. At the final stage I push the lenses apart (if they have not come apart on their own) then I put it back in the oven and turn the oven off and let them gradually cool down with the door closed. This is a very conservative way to do it compared with what is out there on the web. It takes a lot of patience to do it this way, and patience isn't my strong suit. Maybe I've just been lucky, or maybe protars aren't inclined to crack up.

I know a long soak in Acetone (it can take weeks) will separate more modern UV cure cements sometimes, I have not tried this on balsam but I suppose it should work at least as well.

[Paul Metcalf]

Jim-
The acetone disassembled the elements in less than a day, wish I'd gone that route to begin with. Lesson learned. Hey, here's an idea. There are two cemented groups in this lens (and a corrector between them, behind the apeture), and best I can tell they're the same. So, assuming you have one "good" group left, and you send it to me for a ridiculous miniscule sum, and I mount it in place of my "bummer" group, take some pictures that eventually sell for $120k, and I pass along 25%, we'd both be way ahead. Good idea?

BTW - the lens covers 8x10 at infinity with just the front group and the middle corrector lens, but for portraits it now needs a lot of extension (but workable).

[Harold_4074]

If memory serves, the solvent of choice for Canada balsam is xylene, and not acetone, although if acetone works well there seems to be no downside to using it. (Xylene certainly has a more interesting smell, though...)

Anyone?

[Paul Metcalf]

I actually used goopy paint stripper (that has some amount of acetone in it). And it worked quickly, with no apparent damage. Better than heat!

[Kevin Crisp]

Acetone is very effective at taking off the residual balsam stuck to both surfaces after separating, so I assume that with sufficient time you could just soak a lens and get there.

[Paul Metcalf]

Upon inspection of the "adhesive stripper" (not paint stripper as I previously posted)(http://www.kleanstrip.com/adhesiveremover.htm) that I used, it contains methanol (and methylene chloride). It worked within day. The acetone was used to clean up.

[Harold_4074]

On reflection, it occurs to me that there is a potential downside to using one or another agent for separating lens elements: some solvents cause swelling of particular plastics, without necessarily dissolving them. In an extreme case, it seems that this could build up enough stress to fracture the glass. Canada balsam is probably pretty tolerant, since it is a thermoplastic, but UV-cured cements might be another matter.

It would be nice to hear from someone in the industry; the methylene chloride in paint stripper may work on balsam, but it will definitely swell epoxy without dissolving it and I for one don't know if there are optical cements with the same property.

[Kevin Crisp]

I am not in the industry but I have had several goofs (excuse me -- learning experiences) while using two of the available Saunders Optical UV cure modern cements. (One that cures with a sunlamp, one that cures with a UV light.) A long soaking (a week or more) would separate the lenses I needed to redo, I used MEK for this and it did the trick. This worked on protars from around 1910 and small Wolly's from the 1940's. Saunders makes some more aggressive stuff you buy in a metal can that has to be heated on a hotplate for doing this; that is what they recommend using for separating more modern cements. I have learned through trial and error that it is best to give the cement just enough light to harden it up. You can then inspect the elements to make sure you are happy with the job you did. If it isn't rock hard, the MEK soaking will take it apart a lot sooner than fully cured cements.

What Steve Grimes told me when my lens died in his hands was that really old glass gets brittle and sometime the heat causes it to craze.

[Harold_4074]

Kevin--

Thanks. The fact that your procedure has worked more than once is reassuring, even if we don't always know just what the cement is.

I once worked with an cement formulated specifically for bonding glass to metal (not necessarily for optical purposes) that involved a UV initial set, followed by several hours at something like 200 C (yep, a really "hot" kitchen oven) for final cure. It would be interesting to know what it would take to get that stuff off!

[John Schneider]

I would stay away from anything containing methanol or methylene chloride. Both pose far greater health risks than the alternatives. Acetone, despite its pungent smell, is not that unhealthful (the TLV is 5000 ppm, as opposed to methanol at 200 ppm). You should always avoid chlorinated solvents if possible (methylene chloride is a probable carcinogen), as well as aromatics like xylene (suspected to be carcinogenic as well -- see http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=3053447&dopt=Abstract).