How I did it: new balsam for a sick RR

[Steven Tribe]

THIS THREAD IS CURRENTLY WITHOUT THE PHOTOS I UPLOADED AT THE TIME DUE TO AN OUTAGE SOME TIME AGO. I NO LONGER HAVE THE IMAGES FROM 2011. SO UNLESS THERE IS A MIRACLE AT LFPF, IT WILL REMAIN PRETTY MUCH WITHOUT VALUE! SORRY! April 2017.




The job has actually been completed and I will post photos, explain what went well - and badly - along with text and diagrams over the next week or so. I have done this before - mostly with modern UV glues - but this time I have experimented a little to find out "the most foolproof method". Please don't let this be one way communication -and I don't claim to have discovered "the method". But perhaps if you restricted contributions to the step I am describing it would make the whole more logical for those who join later.

The candidate for rejuvenation is a Suter Aplanat A No. 4. A nice big extra rapid RR.
This first picture is what the front cell looked like after removal from the mount. This is cheating a bit - but the yellowing is more obvious out of the mount. Apart from the discoloured ring, there were much larger areas of colourless separation well towards the centre. The rear cell was in perfect condition.

[Scott Davis]

Steven-

I would be very happy to hear full details (or even better, see a YouTube video??? hint hint!) of this process, as I have a lens or two that could benefit as well.

[Steven Tribe]

This first balsam replacement (there are two others underway) is probably the easiest as Suter used a system of mounting the cemented lens which was designed for servicing. The classic book on objective construction shows a slightly less sensible system (photo 1). I think the thickness of the brass edge to be "turned down" over the lens has been exagerated for the purpose of illustration - I would guess that something made like this would be capable of being "rolled back" with hand tools.
The Suter mount is photo 2. This looked quite formidable as the ring was obviously a separate piece of brass. But it could have had a rim at 90 degrees which acted as a spacer between the side of the lens and the interior wall of the cell mount.
Lifting the paperthin edge of the mount, which had only a 1mm overlap, showed the construction to be fortunately as figure 3 (no artist here!). This done using a hobby knife blade to get it started and then a suitable piece of hardwood (boxwood is tradition but I had to settle for walnut). Note that Suter's made a sloping edge on the loose rim to make things easy.
If the cementing edge had been at the side, rather than the top, the lens pair might have been difficult to remove, as I suspect some makers used excess balsam to "fix" their lenses in position. Or seeping balsam has run out and hardened.

[Steven Tribe]

I think the glueing might be a suitable candidate for YouTube - but I'll try and illustrate that process as well as I can here (if you stand my sketches). There is a very real problem of sticky fingers which is unavoidable in the process and seconds count.

[BrianShaw]

I think the thickness of the brass edge to be "turned down" over the lens has been exagerated for the purpose of illustration - I would guess that something made like this would be capable of being "rolled back" with hand tools.

Maybe worth a try. But my experience in clock/watch and other types of repair is that more often than not the brass has been sufficiently "work hardened" by the original peening/turning that it won't roll back without a fight or breaking. My personal experience includes trying this and ending up with stab wounds in my hand or damaged (insert name of thing being fixed). Maybe if it were chucked in a lathe it could be coaxed to roll back just one more time... I don't know, but suspect if that were the case the "classic texts" would have mentioned that option. Cutting the rolled-over part might be more expedient. All that being said, I've never tried this kind of work on an antique lens.

This is very interesting; please keep documenting your progress!

[Steven Tribe]

In all the period instructions it says "mount the brass lens in a boxwood chuck on a lathe and turn back the brass rim". The first sketch from Orford's "Lens work for Amateurs" shows an edge that I would remove rather than try and lift back. The third objective I am working on, does require a cut out and a new "fixing" system. The turnback edge of the Suter was almost foil thickness.

[Steven Tribe]

I have done a quick check of my old brassies with non-screw lens mounts.

The "Suter" system seems quite popular - a bendable brass foil edge turned down about a 1mm. The lens edge has, in some instances, been ground down to form a curve to accept the retaining brass. The only "solid" turned over edge is on the "Anon." large rectilinear I have up for repair. There is just a chance that this is really a flat rim with an edge at 90 degrees which was been pressed into the barrel. Whatever the structure - it will still have to be cut out.

There is another type I haven't noticed before. Here the lens appears to have a pressure brass ring forced into the barrel opening. It looks rather like the system used to make the light-tight reduction flanges in objectives with Waterhouse slots. Next posting will be about Canada Balsam and Zylene/Zylol.

[Steven Tribe]

Canada balsam is a natural product product like maple syrup and latex. Unlike wine, there doesn't appear to many differences between the various vintages of Canada Balsam. The basic material is quite uniform. The problem starts with the purpose to which the balsam is put. Some applications require a thin material whilst others, including lens cementing, require the natural state balsam. The standard thinning solvent used, and which is also used for removing excess, is Zylol/Zylene. This is a very nasty solvent ,healthwise - and at minumum will give you, or some other member of your household, a headache within half-an-hour.

Zylene/Zylol has the additional role of gradually dissolving the aged balsam in the join.
Canada Balsam is used as it has approximately the same refractive index as the commonly used "old" type glass - 1.54. The reasons it is still used are:

1. It is comparitively cheap compared with modern (UV hardening) optical glues.

2. The big problem with UV cured synthetics is their very short shelf lives. Although they contain as much as the 15ml Balsam (1 oz) there is no way that you use more than just a fraction unless you are starting a recementing business. Expiry date is 9 months after manufacturing date. I do not have personal experience of the real shelf life of canada balsam, but would suggest it is indefinite if stored in a cool and dark place. Certainly the lenses I have separated - over a hundred years old - still have large areas which appear to be identical in all features (including a distinct smell/aroma) to the new balsam I have been working with.

Zylene/Zylol is a standard product available locally. Canada Balsam is more problematic because the "mix" of the product appears to vary according to the end use. There has been an e**y provider from Quebec (sounds perfect!) but he/she has gone "belly-up". There is another source in the UK at the moment. The product we are after is the same as that used for geological slides in microscopy - this market is many times bigger than ours.

I would be grateful for extra/contrary comments from those who have had long term experience with Canada Balsam. Additional sources/shelf life/quality issues?

How to work with balsam and general properties of balsam will come under the recementing section.

Next time - Separating the lenses.

[Steven Tribe]

No. This is more about the alternative UV glue. I did a successful recementing with the UV glue a couple of years ago. There may be some of you who don't like the idea of using a very sticky and smelly material which balsam definitely is. I have just retrieved the old instructions and 98% full bottle from 2 years ago. I would say that the glue is still useable - it has the same consistency as 2 years ago. I also note that Norland say " has a minimum shelf life of six months. I thought the technical data might be useful as it contains more than on their website.
I have often seen that this a more "permanent" cementing than balsam. But they can be split quite easily with methylene chloride (carefull - a wicked solvent).

[Steven Tribe]

Separation

First of all, temperatures noted in these next sections all Centigrade/Celsius and all work must be done with forced ventillation or outside!

The first point is about heating the cemented combination as a method of separation. I have read that "leaving the objective in the sun" is a cause of the delamination and that the "balsam can be softened by heating and the two lenses can be separated". Whilst the first may be true to some extent but I doubt whether the second is. When the lens has been rebalsamed, the balsam is "baked" for a few hours at 100 degrees. It may be that, at higher temperatures, there is some degree of loosening - but that would involve difficult handling of the lens surfaces and increased risk of glass heat damage.

So the method I found most successful and safe after trying out various methods was a simple long term soak in xylene. Total time was about a week on average.
This aplanat delamination was a combination of the serious edge degradation and a central "hole" with a channel to the edge.

The first photo shows the layout of the soak. Nothing high technological here. It is not necessary with much solvent as the capillary forces at work are much more important than a "header" of solvent. A small as possible hard thermoplastic, sealable, container. Small size reduces the use of solvent and reduces the amount of released vapours when you make your inspection of progress! A white container has advantages.

Photo 2 shows the various stages in the soak. The first figure show the starting point with a central clear delamination and the typical edge breakdown. The colour of this approaches brown and usual has a blotchy/mottled appearance.

Not much seems to happen the first few days. But you may note that the colour of the edge become more uniform and the boundary between the delamination and the uneffected balsam becomes more distinct. The central delamination was uneffected at this stage.
After a few more days, the solvent will be noticably colored and the edge lamination will be much paler as well. Don't be alarmed at the apparently slow inward march of the phase line showing the progress of the dissolving process. The speed starts at about 1 mm per 24 hours once the solvent has reached the edge of the delamination. At this stage I have changed the solvent in order to be able to monitor progress without taking the lens up (works only if you have a white container!). Photo 3 shows this stage - note the brownish clump which suddenly appear in the solvent.

In the final phase (stage IV), there is not longer any difference between the edge and central clear delamination. The gradual loss of colour makes it difficult to spot the edge. A separation should be possible at this stage.
It is easy to forget that the insert glass has very thin edges so I have always attempted to turn the glass, rather than push to one side. Be prepared for a very sticky inside surface and a quite delicate lens. The strange thing is the smell - which is exactly the same as the fresh canada balsam you already know. A short section on cleaning/preparation will follow very soon.