Repolishing old lenses with fungus, balsam separation

[Jim C.]

Ok that makes sense now. I took out some of my separated lenses and they look like they've been edge-ground after cementing. So optical alignment is purely angular in a concentric jig. I could technically use the lens body as a jig, after coating with mold release, or the lenses could be held with aluminum tape and aligned with a straight edge. The optical alignment in the video did not look like it required a precision of more that 0.001".

I'm not sure if a vacuum jig can be a helpful air for balsam repair, unless I have the complete optical setup in the promo video.
Attachment 249001

With re cementing elements back together you're basically working with two elements that have already been optically aligned
and post finished ( edge grinding, etc. ) you just want a jig to hold them in physical aligment to each other for gluing hopefully
preserving the opical alignment.
I still think a hard durometer moldmaking rubber ( shore 50 or higher ) would be your best option. It'll be forgiving with both the glue and in removal.

[kfed1984]

I'm now thinking of a 3D printed jig.

[Steven Tribe]

Zylene/Zylol is the standard solvent for softening the balsam layer. You just need to cover the lens with liquid. Can take up to two weeks. It is not a heathy solvent - but evaporates less than, for example, acetone. It is quite fun to follow the phase movement (lighter liquid slowly encroaching into the solid, darker, aged balsam.

Canada Balsam used to be a standard eBay item. Clue for an internet search is “ microscope supplies Canada balsam” as microscopists are, apparently, quite active!
The first link I found was: https://www.laballey.com/pages/searc...=canada+balsam

What some people describe as fungus on older lenses is actually “blooming” - the patchy surface that looks like botched coating experiment.
The only lens surfaces I have seen with etching damage have been coated lenses where clever fungii have able to transform the Florine component to something that attacks glass.

[kfed1984]

Zylene/Zylol is the standard solvent for softening the balsam layer. You just need to cover the lens with liquid. Can take up to two weeks. It is not a heathy solvent - but evaporates less than, for example, acetone. It is quite fun to follow the phase movement (lighter liquid slowly encroaching into the solid, darker, aged balsam.

So what is the solvent used in the Canada balsam cement itself, is it Xylene? How long does it take for the solvent to dry fully when a new one is applied? it will probably take a while as the cement in the center of the lens is farthest from the air.

If Xylene is the solvent used in the cement itself, can you buy dry balsam sap and dissolve it in Xylene to make a DIY balsam cement?

[Steven Tribe]

Well, one of sites says “balsam dissolved in zylol” and I use zylol to reduce viscosity when the bottle has been in store for a year or so.
I place the in a standard domestic oven at 60 deg C for a couple of hours. I check that the edges of the 2,3 or 4 contributing lenses are flush after an hour or so - it is still possible to finger adjust any lenses that are out of alignment. Then back into the oven for the last hour.
I am quite sure only the edge few mm are really hardened but it is solid enough to mount the lens when it cools.

[kfed1984]

Well, one of sites says “balsam dissolved in zylol” and I use zylol to reduce viscosity when the bottle has been in store for a year or so.
I place the in a standard domestic oven at 60 deg C for a couple of hours. I check that the edges of the 2,3 or 4 contributing lenses are flush after an hour or so - it is still possible to finger adjust any lenses that are out of alignment. Then back into the oven for the last hour.
I am quite sure only the edge few mm are really hardened but it is solid enough to mount the lens when it cools.

Can you share more details/pics of your process? Sounds like you do it a lot. Any experience with UV resin?
Could you use 3d printing SLA clear resin?

[Steven Tribe]

I have used UV glue once - horrible to handle, Need to get a perfect match first try in alignment, expensive, even in freezer shelf life is short (months). I like to keep the lens as original as possible and balsam was the original glue! Most of my lenses are 19th century.
I describe the process in
https://www.largeformatphotography.i...-for-a-sick-RR
I have done around 40 swaps of balsam. Mostly achromats (landscape meniscus types) and Petzvals. The life time of balsam in a achromat is about 50 years, so I have seen everthing from edge discolouration and crystal growth to brown opaqueness.

[kfed1984]

I describe the process in
https://www.largeformatphotography.i...-for-a-sick-RR

Thanks for your balsam thread. I will read through it before trying on my own.

I wonder why it is only the Canadian balsam tree that has these qualities, and why it has to be a special UV lens resin and not any other UV resin.

[Steven Tribe]

Refractive index perhaps?

[Jim C.]

I'm now thinking of a 3D printed jig.

A 3D printed jig would work, you could design in glue channels lessen the chance
you'll glue it to the jig even with mold releaase, a continous band will glue really well
as opposed to one tha has empty spaces. Check the material you're going to
print the jig with the glue you're going to use, for release characteristics.