Seperating lens elements - Problem

[middlecalf]

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

 

[nicolai]

Sorry this isn't more helpful; other than maybe getting it re-ground/polished, I have no idea. But if it's screwed up the image and you want to get rid of it, I'd be interested in buying it. (I like funky, "defective" lenses.)

 

[AgX]

My first thought was a general crystallisation of the glass (trickled by some corrosion?). But the structure you described, and most probably the temperature you applied were not convincing.
But that corrosion idea brought me to the `glass disease´, a growth, due to drying, of hydrates which had been formed before in the glass surface due to be prone to moisture/humidity.
Look at that photo at the very wikipedia article and most probably you will recognize your meniscus…

 

[middlecalf]

Look at that photo at the very wikipedia article and most probably you will recognize your meniscus…

AgX - thanks, I did look at the photo (attached), and it's somewhat similar, although I think my menicus has many more cracks, and they are not discolored (except for effect of light defraction). But your post made me do a little more searching, and I found this:

"In all glass, the sodium and potassium oxides are hygroscopic; therefore, the surface of the glass absorbs moisture from the air. The absorbed moisture and exposure to carbon dioxide causes the NaO2 or NaOH and KO2 or KOH to convert to sodium or potassium carbonate. Both NaCO2 and KCO2 are extremely hygroscopic. At a relative humidity (RH) of 40 percent and above (and in some cases as low as 20 percent RH), drops of moisture appear on the glass surface. In water, especially salt water, the Na and K carbonates in unstable glass may leach out, leaving only a fragile, porous hydrated silica (SiO2) network. This causes the glass to craze, crack, flake, and pit, and gives the surface of the glass a frosty appearance. In some cases, there is an actual separation of layers of glass from the body. Fortunately, these problems are not commonly encountered in glass manufactured in the 18th century and later. Pearson (1987b, 1987d) discusses glass deterioration and reviews the various glass conservation procedures."

I'm assuming this glass is newer than 18th century, so not sure if this is what is happening. The initiator of the problem though was applied heat, so I'm still researching that (this is now an academic exercise).

And, I posted this on the large format forum and Jim Galli suggested (somewhat seriously, I think) I use a clear polymer resin. Apparently, this is a method for fixing diseased glass:

"Glass can be repaired and reconstructed with the same glues as described for pottery. Optically clear epoxy resins are generally preferred as they adhere to the smooth, non-porous glass more readily. They also dry clearer and shrink less than the solvent resins. The resulting bonds, therefore, are less noticeable and stronger than with other glues. The epoxy resins are, however, usually irreversible. Hysol Epoxy 2038 with Hardener 3416 and Araldite are the two brands most commonly used in glass repair. The new 'super glues,' which are made of cyanoacrylate, are used quite often to piece the glass together quickly. After using the cyanoacrylate, epoxy is flowed into the cracks with an artist's brush to permanently bond the pieces. It is exceptionally difficult and time consuming to gap-fill glass. It requires considerable work and experience. The problem of matching transparent glass colors is equally difficult. All of these problems are adequately discussed in greater detail in Newton and Davison (1989)."

Curious as to what happens to the index of refraction using a resin. I might just have to try this and see what happens.

For future reference, I'm going to avoid heat as a means to separate lens elements

 

[AgX]

As far as I remember this or a similar `disease´ was also found in stacked plain glass planes due to capillary effects.

Due to my own experience with epoxy and cyanacrylate glues I would be reluctant. Not all epoxy glues stay colourless and those I know are even heated quite viscous (for such cracks), and cyanacrylates tend to be a mess (or it’s me…). I would consider UV-curing glues offered by those companies who offer lens elements. Spin coating them?

 

[middlecalf]

I would consider UV-curing glues offered by those companies who offer lens elements. Spin coating them?

Hopefully will be giving this a try http://www.epoxies.com/tech/60-7108UV.pdf. Don't know about spin coating, though.

 

[AgX]

I was referring to a technique where a rather flat object to be coated is laid horizontally on a sort of turntable. The coating agent is then poured onto to form a drop, the object is rotated to make centrifugal forces the agent spread evenly.
In your case it would be more apt to spread the glue over the meniscus to let soak into those fractures (if there are any recesses at all) and only then start rotation.
Where are at an academic/guessing stage...

 

[Murray@uptowngallery]

I have had very poor luck with viscous materials and attempting to coat evenly by rolling/spinning. It was not an optical application, but what happened was the material did not move as quickly or smoothly as I hoped, so I got spiral runs and alot of panic.

In the end, I got complete coating, but with conspicuous visible thickness variation. In hindsight, the unevenness I anticipated a brush would cause would have been better.

I think a method posted on rec.photo.equipment.large-format by Richard Knoppow involved a slow-curing optical adhesive applied to one surface and even spread accomplished by squeezing the material outward from the center by gently squeezing the two elements together. He thought out a method of assuring centration to his satisfaction prior to applying any adhesive.

Murray

 

[AgX]

Thank you for awaking us from our academic dreams...

(But where those fractures filled? Did at least this work?)