Repolishing old lenses with fungus, balsam separation

[kfed1984]

I want to share a technique for re-polishing lens surfaces etched by fungus residues. In the best case, fungus can be removed with acetone, but usually fungus permanently etches the glass. If the damage is severe, it may be worth re-polishing the glass using same technique as was used in making the lens originally. That is, if the lens is no longer usable and you are willing to experiment.

The old school technique of polishing optical glass is by molding warm polishing pitch against the lens and using it as polishing pad with compounds like cerium oxide or optical rouge. The method is described in books like "Amateur Telescope Making" and the second "Advanced" edition of this book. Also, "How to Make a Telescope" by Jean Texereau. Old school optics were made by hand and can be made to exceed factory made optics in accuracy. So this is not something I just made up.

To polish old lenses:

Polishing pitch is melted and poured into a circular shape, then allowed to cool until it is no longer tacky. The lens is coated in polishing slurry (to prevent sticking) and pressed/molded against the still-soft pitch. When fully hardened, square channels are cut into the pitch to allow better recirculation of the polishing slurry.

Even at room temperature, the pitch behaves like a very slow flowing liquid, so it gradually conforms to the lens shape precisely. Slurry is applied on the pitch and used to polish and refigure the lens surface if required. Particles of polish become embedded in the pitch and work like micro abrasives, where the scratches produced are much smaller than a wavelength of light.

The polishing technique requires the lens and pitch tool to be rotated periodically in opposite directions to maintain a spherical surface. This will work on most old lenses, since majority of them are spherical. Modified shapes of the pitch pattern and special strokes can be used to refigure the lens into geometries other than a sphere, such as parabolas and hyperbolas.

The only issue I encountered is the hard anti-reflection coating that could not be polished off with cerium oxide initially (too soft), so I first had to use a one-micron alumina to polish through the coating. Alumina is harder than cerium oxide apparently. The lens coating will be stripped off with this process as a side effect. But what good is an anti-reflection coating on a fungus-etched lens that cannot be used any more?

The process requires a lot of patience, especially with the pitch wanting to stick to the glass when soft and chipping off when cold. There are many nuances to the process, such as that placing the lens on top of the tool will polish center of the lens more than edges, and vice versa if flipped. Too much pressure and uneven polishing can refigure the spherical surface into something like a parabola or an ellipse. Polishing too dry and with too much pressure will soften the pitch, etc. etc. So you have to decide if the lens if no longer usable due to damage and should be replaced or repaired with this method. Depending on the degree of fungus damage, type of coating, what the lens is worth, etc.

With all this in mind, I was able to polish out most of the fungus damage on an Apo-Ronar 360mm. Only a few small pits were left that went too deep into the glass, but they were not significant enough to affect image quality. The lens looked essentially like new at the end of the process.

[kfed1984]

Pouring the pitch into a mold formed with aluminum tape, and pressing the lens into a softened pitch. The square channels are visible. The white compound is the polishing slurry.
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Pics showing a clean pitch tool, polishing in action.
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[kfed1984]

I completed the polishing today and found the rear element of my 360mm apo-ronar also had a fungus smudge. So I took out that element and polished it out as well, which took about 30 min for one face. The first lens took several days, but it was mostly due to the learning curve, molding the pitch, learning all the tricks, etc. Here are some pics of before and after.

- First pic is the original lens, both sides with fungus etching.
- Second is with front side polished out, the remaining fungus spot is on the opposite side, for which I had to remold the pitch for a different radius of curvature.
- Third is both sides polished out with a bit of the fungus etch still remaining as it goes a little deep into the glass. But the lens was more than satisfactory for me at that point.
- Fourth is the assembled lens. You can see a white reflection from the front lens (coating is gone) and a purple reflection from the second element behind the first.

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Overall, the process may not be worth it for just one lens, as the time and effort spent is more than what the lens is worth. But my personal satisfaction level and morale are elevated significantly. Also I have a bunch of enlarger lenses damaged by fungus and balsam separation that I will work on and provide some updates later.

[kfed1984]

Here is a link to the book, "Amateur Telescope Making, Advanced" which can be downloaded as a pdf. I bought the original paper version for a few bucks.

https://archive.org/details/in.ernet.dli.2015.233318


The book goes a little deep and you may get lost, but it may be searched for relevant material only, ignoring the rest. The lens making procedure described in the book uses a hand-made machine, unlike my manual process.
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[kfed1984]

Later on I want to add information on balsam separation repair, which will involve creating an alignment fixture, separating the lenses with heat, cleaning old balsam, applying new lens cement, and realigning/cementing back to original position.

[mhayashi]

Thanks for sharing your knowledge and experience!

[kfed1984]

Thanks for sharing your knowledge and experience!

Appreciated, thank you for reading. I figured that fungus damage is quite common on vintage lenses and many good ones are scrapped, unfortunately. And no information is available other than cleaning, which does not do much.

[darr]

Thanks for this!

[Tin Can]

Yes

Thank you

And archive.org

[Alwelch1]

Followed your project over from our comment thread on my YouTube channel (Adam Welch). This is quite interesting and as others have said thanks a lot for taking the time to share the process. Glad to see it worked out well.

[kfed1984]

Followed your project over from our comment thread on my YouTube channel (Adam Welch). This is quite interesting and as others have said thanks a lot for taking the time to share the process. Glad to see it worked out well.

The most interesting part will be balsam separation fix, I already have a plan, but that will be later. I will share the before/after photos of the lens fungus etch when I get back to polishing the other side of the lens.

www.largeformatphotography.info is a really good blog, for anything large format related, technical stuff. The other good one is photrio

[ic-racer]

Nice, thanks for sharing.

This may not apply to still photography, but I understand some Kern Yvar Cine lenses were only coated on front and rear surfaces. Therefore, the internal surfaces could be polished without worrying about the lens coating. I have not tried it. For some [good] reason all my old Cine lenses have clear elements.

[kfed1984]

Nice, thanks for sharing.

This may not apply to still photography, but I understand some Kern Yvar Cine lenses were only coated on front and rear surfaces. Therefore, the internal surfaces could be polished without worrying about the lens coating. I have not tried it. For some [good] reason all my old Cine lenses have clear elements.

Usually it is one or two surfaces that are damaged by fungus, and in a typical 4 element lens there are 8 surfaces. So we will loose about 1/4 of the total anti-reflection quality if the coatings are polished off.

[Mark J]

This is all good and makes sense. I am familiar with the process, I got into optics by grinding and polishing telescope mirrors. the pitch needs to be optical quality.
Yes, modern AR coatings are dense and very hard. they will take ages to polish off.

[kfed1984]

This is all good and makes sense. I am familiar with the process, I got into optics by grinding and polishing telescope mirrors. the pitch needs to be optical quality.
Yes, modern AR coatings are dense and very hard. they will take ages to polish off.

Thank you for the feedback. I came across a passage in "How to Make a Telescope" that surface accuracy on lenses needs to be 1/4 of what is required in mirrors. Not sure if this makes any potential refiguring/despherizing done by my polishing technique less damaging than it would be on mirrors, as I am not doing any Foucault testing on the lenses. Just trusting that my polishing process did not alter the sphere too much.

Do you have any experience with cementing of lenses and balsam separation or are you only into mirror making?

I bought the pitch and cerium oxide from here: https://firsthanddiscovery.com/gugolz-73-pitch.html

[kfed1984]

"How to Make a Telescope" by Jean Texereau. This book has good illustrations and is considered a classic for this topic. Archive does not have a downloadable version, but you can borrow the book virtually for one hour.
https://archive.org/details/howtomaketelesco0000texe/mode/2up

I have the paper version.


Amateur Telescope Making, book 3. Lots of good stuff on lens making: https://archive.org/details/amateurtelescope0000na/page/162/mode/2up?view=theater

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[Mark J]

The surface accuracy on photo lenses is typically less stringent than on telescope optics - eg. 1 to 3 fringes, rather than less than 0.5 fringes on a telescope.

I work in an optical company. Balsam went out decades ago, for us. We use either UV-curing cements or specific optical epoxies.

[kfed1984]

The surface accuracy on photo lenses is typically less stringent than on telescope optics - eg. 1 to 3 fringes, rather than less than 0.5 fringes on a telescope.
I work in an optical company. Balsam went out decades ago, for us. We use either UV-curing cements or specific optical epoxies.

Concerning realignment of cemented optics, I am thinking of using a cast-plaster jig, molded around the lenses before separating them, for radial realignment. Prior to that, coating them with Vaseline for easier release and with modeling clay applied in strategic places to keep the plaster out. And, marking the lenses on the edges for angular re-alignment of lenses to each other and to the plaster jig.

Then using heat to separate lenses, solvent cleaning, and recementing with UV-curable resin in the plaster jig. Balsam I heard takes a long time to set, so UV-resin it is.

[Steven Tribe]

Later on I want to add information on balsam separation repair, which will involve creating an alignment fixture, separating the lenses with heat, cleaning old balsam, applying new lens cement, and realigning/cementing back to original position.

There are plenty of threads here which cover balsam renewal.
“….separating the lens with heat….” Doesn’t sound like a suitable method with irreplaceable Antique lenses!
Originally, pitch was only used to fix the glass to the “lathe” - a more stable material was used to make the optical surface polishing tool.

Amateur cementing is best done with Canada balsam.
Because:
- easy to redo. Yes, it may be necessary!
- much cheaper.
- synthetics have short shelf lives. Viscosity of Balsam is easily adjusted.
- balsam is non-toxic.

[Mark J]

Originally, pitch was only used to fix the glass to the “lathe” - a more stable material was used to make the optical surface polishing tool.
.

Well, the polishing tool has a former made out of metal, but the polishing surface was a layer of pitch that conforms to the lenses and holds the rouge or cerium. Polishing with pitch was industry-standard for most (quality) optics until about 25 years ago, from where it has gradually been phased out by polyurethane and then other polymers used in the high-speed automated machines. However we still retain pitch polishing in out factory for small batches and certain special materials.

[pgk]

Amateur cementing is best done with Canada balsam.

I'm struggling to find any suppliers of CB in the UK. Any suggestions?

[kfed1984]

I completed the polishing today and found the rear element of my 360mm apo-ronar also had a fungus smudge. So I took out that element and polished it out as well, which took about 30 min for one face. The first lens took several days, but it was mostly due to the learning curve, molding the pitch, learning all the tricks, etc. Here are some pics of before and after.

- First pic is the original lens, both sides with fungus etching.
- Second is with front side polished out, the remaining fungus spot is on the opposite side, for which I had to remold the pitch due to different radius of curvature.
- Third is both sides polished out with a bit of the fungus etch still remaining as it goes a little deep into the glass. But the lens was more than satisfactory for me at that point.
- Fourth is the assembled lens. You can see a white reflection from the front lens (coating is gone) and a purple reflection from the second element behind the first.

248981 248982 248983 248984

Overall, the process may not be worth it for just one lens, as the time and effort spent is more than what the lens is worth. But my personal satisfaction level and morale are elevated significantly. Also I have a bunch of enlarger lenses damaged by fungus and balsam separation that I will work on and provide some updates later.

[kfed1984]

There are plenty of threads here which cover balsam renewal.
“….separating the lens with heat….” Doesn’t sound like a suitable method with irreplaceable Antique lenses!

Any suggestions for separating the lenses other than heat? The only effective method I've seen is gradual/uniform heating. Also solvent was mentioned.

[Jim C.]

Concerning realignment of cemented optics, I am thinking of using a cast-plaster jig, molded around the lenses before separating them, for radial realignment. Prior to that, coating them with Vaseline for easier release and with modeling clay applied in strategic places to keep the plaster out. And, marking the lenses on the edges for angular re-alignment of lenses to each other and to the plaster jig.

Then using heat to separate lenses, solvent cleaning, and recementing with UV-curable resin in the plaster jig. Balsam I heard takes a long time to set, so UV-resin it is.

Plain old plaster of paris to use as a jig for lens alignment is such a bad idea, all gypsum products expand as they set ( from exotherm ) your jig will be loose fitting
and if your plaster mix isn't proper you'll have plaster bits falling onto your glue surfaces. There are some foundry plasters that have very low expansion rates like
US Gypsums Ultra Cal 30. But you're back at endlessly blowing bits off your glue surfaces.
I've never re cemented any lenses, I do have a few brass oldies that could use re cementing, but off the top of my head would be to machine a jig to hold
the lens elements since I have a machine shop. An alternative is to look for a thick wall HDPE or nylon tube that might have the same inner diameter
as your lens elements that you could build a jig to hold the lens elements in alignment for cementing.

[kfed1984]

Plain old plaster of paris to use as a jig for lens alignment is such a bad idea, all gypsum products expand as they set ( from exotherm ) your jig will be loose fitting
and if your plaster mix isn't proper you'll have plaster bits falling onto your glue surfaces.

I've considered the plaster expansion as a good thing actually. The expansions should not be too great and should facilitate easy removal from the plaster jig.

I thought about making a metal jig, but the whole point of plaster is that it will remember the radial positions of the lenses, since they are not perfectly aligned mechanically but optically. Watching videos of lens cementing and optical alignment, you can see the lens being pushed by hand until optical axii align together, which means that mechanically/radially they will be offset somewhat. And the idea is that plaster will locate the two lenses back to their original optical axii. That will work only if both lenses are aligned angularly back the same way with respect to the jig. So that's my justification for plaster.

In the video below at 3:20 you can see the lenses wobbling a bit relative to each other after optical alignment. Meaning that mechanically they are not aligned in the radial position. But maybe that's just my perception. Share some thoughts.
https://www.youtube.com/watch?v=kIa42sOWG0I

[Alwelch1]

Yes I hold a membership at Photrio.

[kfed1984]

I'm struggling to find any suppliers of CB in the UK. Any suggestions?

Most of them will be purchased online anyways, so does it really matter if in UK or not? I've seen 100ml bottles online for 60 euro. I wonder if all it is, is tree sap dissolved in some solvent.
Also, what is so difficult about UV resins? Difficult to separate afterwards?

[Jim C.]

I've considered the plaster expansion as a good thing actually. The expansions should not be too great and should facilitate easy removal from the plaster jig.

I thought about making a metal jig, but the whole point of plaster is that it will remember the radial positions of the lenses, since they are not perfectly aligned mechanically but optically. Watching videos of lens cementing and optical alignment, you can see the lens being pushed by hand until optical axii align together, which means that mechanically/radially they will be offset somewhat. And the idea is that plaster will locate the two lenses back to their original optical axii. That will work only if both lenses are aligned angularly back the same way with respect to the jig. So that's my justification for plaster.

In the video below at 3:20 you can see the lenses wobbling a bit relative to each other after optical alignment. Meaning that mechanically they are not aligned in the radial position. But maybe that's just my perception. Share some thoughts.
https://www.youtube.com/watch?v=kIa42sOWG0I

There is a slight eccentricity of the lens elements, but I don't think that doublet is final, it may be destined for
more grinding if it were to be a final lens or not it was a demo more of the machine than lens making.

I think you have the right idea but bad choice of material for the jig, the lens you're taking apart is already optically
aligned and all you want is to hold it in the same position, I would suggest a hard durometer moldmaking rubber
like a shore 50 either polyurethane rubber or silicone, either rubber would need some mold release for the lens
and possibly for the UV glue. You would have to test the glue against whichever rubber you choose.
The rubber would be a lot more forgiving than plaster when time to remove the lens comes.

The video was interesting in particular how the alignment machine works. You could make a crude approximation
of that machine with PVC pipe, a few pieces of wood for a base ( even lower tech, a old drawer ) and a vacuum cleaner.
You're not going to get micron accuracy.

For couple of reasons PVC pipe would work, it's cheap and readily available, it's easily worked, and it's soft so the risk of scratching is less.
The alignment machine in the video uses a light vacuum to hold the element in place so that it can tapped into optical alignment, then adjusted to hold,
a house hold vacuum cleaner should work, even a hand vacuum. You would have to port the the pipe jig so that it's not a full vacuum to adjust
then more vacuum to hold, you probably wouldn't need the full vacuum suction a vacuum cleaner would supply.

[kfed1984]

There is a slight eccentricity of the lens elements, but I don't think that doublet is final, it may be destined for
more grinding if it were to be a final lens or not it was a demo more of the machine than lens making.

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.
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[kfed1984]

I'm now thinking if both lenses are same diameter (like the one on above-left) I could rest them on a surface plate against a v-block. That should be more accurate than any other method. For the other lens above-right, I could set two v-blocks on top one another for two lens diameters. Maybe not actual v-blocks but something rigged up to function like v-blocks, clamped with thumb screws. Then it can be used for all kinds of lenses like an adjustable jig.
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[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.
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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/search-results-page?q=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.info/forum/showthread.php?58763-How-I-did-it-new-balsam-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.info/forum/showthread.php?58763-How-I-did-it-new-balsam-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.

[kfed1984]

Makes it seems like the excess balsam squeeze out is a big issue. Does it stick to everything that hard? Is it possible to cure the cement without heating the lenses, just by waiting for the balsam to evaporate?
If so, then I can wait a week for the balsam to set hard enough and then soften the 3D printed jig in an oven and peel it off in case it sticks. If I print with PLA, it becomes quite soft by about 80C. Also, PVB 3D printing filament can be dissolved or softened in isopropyl.

[PRJ]

V blocks are the way to go.


If there is any balsam around the glass after you bake it, the balsam is easy to remove.

[kfed1984]

V blocks are the way to go.


If there is any balsam around the glass after you bake it, the balsam is easy to remove.

the v blocks are the way to go for concentric lenses for sure, but the harder one is a combination of two lenses with different diameters.
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[Mark J]

I'm racking my brains trying to think how this sort of thing was done in the days before air-bearings and optical alignment telescopes. Have you got access to any of the old textbooks on optical production ?
We have some at work, might have a quick look if I get time.

[kfed1984]

I do have some books on telescope making mentioned at the beginning of the thread. If you find some good books, let me know the titles if you don't mind.

[Tin Can]

Since it is etched into the spheres

Grinding will change the lens from optimal

I have one etched lens, I use the good 1/2

buy another lens

life is short

[kfed1984]

Well, we're not really grinding too deep but polishing, and even if we were to grind then the spherical shape would be retained mostly.
No, that's not how I like to do things, lens polishing makes me more connected to my equipment. Also balsam separation repair is a must, as I have some good lenses with separation.
Also I have some other lenses with micro scratches all over, from bad cleaning. Now that I know how to do this, it should not take too long.

This whole exercise is more of as Zen thing, not about being an efficient photographer. Otherwise I would just get a new lens, yes.

[Mark J]

I do have some books on telescope making mentioned at the beginning of the thread. If you find some good books, let me know the titles if you don't mind.

Yes, however the telescope making books won't deal with the kit required to cement two dissimilar-diameter lenses.
I think we still have a copy of 'Optical production technology' by Horne , at work, i think that's the title.

The principal of optical centring of a doublet is that you have one lens set on a circular 'knife edge' on a precision spindle, then you need to send light down at the lens and view its reflection coming back. You adjust the first lens till its top surface is running true ( the reflection is stable, not moving ) then add the cement and the second lens ; then observe the reflection from the top surface of the second lens and adjust till this is also running true.
I just need to see how this might be done with relatively simple equipment.

[kfed1984]

well if we use the jig to get everything aligned shouldn't that work? after grinding the mechanical and optical axes should coincide. If we re-align mechanically to about 0.001", should that be accurate enough for the optical axis?

[Mark J]

I thought you were hoping to re-cement the 300 Symmar-S above, where the lenses are not the same diameter , or easy shapes ?
What kind of jig did you mean, maybe I need to re-read.

For same-size lenses, 0.001" is pretty good, but only if each lens is also optically true to about 1' to 2' to its ground edge - which is not a given.

[kfed1984]

Yes the Symmar. I was thinking a 3D printed jig with two pockets for the two lenses with different diameters, we covered it a bit earlier. For the lenses of same diameter I could rest the lenses on the side in a v-block.

[Mark J]

Well, OK, but typical 3D-printed plastic will not be accurate to 0.001" . Our expensive new machine at work for product mock-ups, might be approaching that level.

[kfed1984]

The accuracy of internal diameter I can verify with an inside micrometer. If printed with let's say 0.1mm layer thickness, and adjusting the extruder settings, I think it's possible to make a good jig.

For balsam gluing, will it cure by itself if left at room temperature? How long could it take before the lenses can be removed?

[Steven Tribe]

For balsam gluing, will it cure by itself if left at room temperature? How long could it take before the lenses can be removed?

Yes it will harden at room temperature. Length of time depends on the amount of solvent (zylene) added to the raw Canada balsam. Days rather than hours or weeks. I use stretched rubber bands ( 4, at least) to maintain the position of the 2 or more single lenses - the V block set-up is hardly mobile!

[Mark J]

Helps if you can iterate the dimensions yes.
I would take off the tops of the surface texture with fine emery cloth too.

How are you going to hold the lenses perpendicular to the jig ?
I can imagine you might put the negative element down on its concave side on a piece of glass on a bench, then bring in the jig from the side ?

Are you aiming to use the 300 for contact prints or enlargements ?

[kfed1984]

I plan to rest the lenses flat on a surface plate, the bottom of one lens is concave, so it will sit flat on its perimeter. Then I will position them against the edge of a v-block, lying on its side on the surface plate. This applies to the two lenses of same diameter on the left in the pic below. The other larger lens is from a symmar, for which I want to make a jig. I will use the symmar for contact prints mainly, but would be good if it can be used for enlargements. Again I don't think the lens will be compromised by a bit, if its off by a thou.
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again I could just machine a jig, I just don't want to take it that far

[kfed1984]

Does anyone have anything against this balsam or it needs to be some optical grade Canada balsam?
https://arttreehouse.com/artstore/product/canada-balsam/?attribute_canada-balsam-sizes=8+oz.+size

[Mark J]

Again I don't think the lens will be compromised by a bit, if its off by a thou.

again I could just machine a jig, I just don't want to take it that far

I think you have a few more sources of error than you think, but I wish you good luck.
One point is that the cap ( flat ) around the cave side of the symmar doublet is not used for mounting in the original lens, so it will not have been made particularly tight to the optical axis. It should be good to under 3' though. So your first two optical surfaces may not be fully 'true' when you bring in the positive.

However I think you've got a good chance of at least getting a lens good for contact prints.

[Steven Tribe]

Does anyone have anything against this balsam or it needs to be some optical grade Canada balsam?
https://arttreehouse.com/artstore/product/canada-balsam/?attribute_canada-balsam-sizes=8+oz.+size

I Think there is only grade of Canada balsam! A Quick check shows that fancy labeling and special use (microscope slides) pushes the price up quite a lot!.
It is fairly important that the screw top ( with or without built-in dispensing bulb) is cleaned up after use or you will have to use xylene to break the seal for subsequent use.
Single air bubbles in the deposited balsam layer can “rubbed out” to the lens edge, but these are best avoided by having super clean lens surfaces and contact transfer from a glass rod or similar.

[Kevin Crisp]

I won't claim I'm an expert, but I have separated and reglued at least 20 lenses. They all worked after I was done and I didn't break any of them. I'll just share my thoughts and conclusions based on that experience in hopes that somebody doing this finds it useful. I have no doubt that lots of people will quibble with my suggestions and my only defense is that it has worked for me and it is cheap and when I get done the lenses worked. Others, knowing the forum, will have ideas on how to select the very best trees for obtaining balsam sap and others will suggest mixing it with Pyro. Doubtless a professional with more equipment and experience can do this for you at considerable expense. And if you DIY you will understand why they charge so much for this.

You are responsible for looking into health aspects of any chemicals involved.

This will be -- sort of -- chronological.

1. First, let's be more precise. Many people are talking about "balsam" when they should be talking about more modern UV cure cement. I can't nail it down precisely, but as a general rule of thumb lenses manufactured after the late 1930s weren't cemented with balsam. The classic uncoated 159mm Wollensak I have, which dates to 1939, isn't cemented with balsam. My B&L protars, some of which date to much earlier, are all cemented with balsam. I have seen aluminum housed very late B&L protars that were not. Ebay listings are often wrong on this, with sellers boasting about there being "no balsam separation" on lenses that don't have any balsam in them and never did. (Reminds me of all the snack foods made of corn that boast "Gluten free!" Corn? Of course.)

2. Separation can happen with balsam and UV-cure cements. First question to ask yourself is whether it matters. This may be the most important question to ask yourself. If it is just on the edge (let's say a few mm's of a large surface) it is probably stable and it isn't going to affect your negatives at all given the apertures most commonly used in LF. So leave it alone. I've seen several 1950s Symmars that had very small silvery snowflake/star looking separations which are usually not on the edge. The lenses all worked great and you should leave ones like that alone. Actually separation of much more than a few mm will likely cause no issue at typical working apertures. I'll take it a step further and suggest you not start down this DIY path unless you've actually used the lens and find the results unacceptable.

3. If you need to fully separate a lens, the manner depends on what it was glued with. For balsam, some on here have said boiling water works great. You position the lens at an angle, slowly heat the water, and the different pieces of glass will slide apart. I haven't tried this, but it has some appeal for how slowly and evenly the glass will heat, minimizing the chances of breakage. What I have done is crumple up some aluminum foil so that I can position the lens at an angle so that gravity will let the elements slowly slide apart. I put that in an oven. With the glass already in the oven on a pan, held by the foil, I start out at 100F. NO PREHEATING. Slowly the glass gets warm, then I bump up the temperature 25F at a time, slowly, to avoid shocking the glass and breaking it. Eventually -- and this whole process is not for the impatient -- the balsam will melt and the lens will slide apart. This usually happens around 290 - 325F, and the smell in your kitchen is a dead give away that you're there. Depending on how they slide apart, you can open the door briefly and push them further apart with a tooth pick. Don't touch the glass with anything metal other than the foil it sits on. Once the stack has mostly come apart, turn the oven off, keep the door closed and leave the glass in there until it is barely warm. With the oven technique they don't come apart at 212F or close to it, which is why I question whether boiling will work. I'm still going to try it.

Modern cements are much more of a problem. Many times starting out in photo things I heard warnings about how careful I should be cleaning a lens to keep the cleaner out of the edge where it will damage the lens cement. If only it worked that way. Don't try heating them apart. There are chemicals for this. Summers Optical sells some very flammable stuff you use on a chemist's hotplate. (Flash point 167F) I bought a can years ago but after reading all the warnings never used it. What worked for me was "MEK" solvent. I put a folded paper towel in the bottom of a jelly jar, put the lens in there, and added the MEK, put the lid on and let it sit submerged. It will SLOWLY do the job. It will wick into the edge that is separated and SLOWLY work its way across, turning the cement white in the process. It can take weeks and weeks but it always worked for me. If your lens group has surfaces that are not separated, and some that are partially separated, it is going to take a long time. It is a lesson in just how effective modern cements are.

4. Before recementing, you must get the lens surfaces cleaner than you've ever gotten anything in your life. Acetone will help take off the remaining cement, but be careful and don't let anything contaminate your can of acetone. I used two cans, one only for the final cleaning. Lint-free "cheese cloth" with clean acetone makes a good final wipe. This step is critical, and remember how much time a mistake will cost you if you're gluing together something like a single Protar cell. That's 4 pieces of glass. If you cement the first two joints and screw up the final one, you have to start all over again. Don't even think about trying to recement more than one joint at a time.

5. I never tried using balsam for a repair. I used only modern UV-cure cements. I first used Summers' UV-69 ($64 for a big jar) because I had an old sunlamp. It worked. Then I called Summers and told them what I was doing and they suggested Type J-91 ($52.90) which can cure with an ordinary UV lamp. The "use by" dates on the cements are unfortunately not all that far in the future. For a time people on Ebay were selling smaller quantities for less. If you're really on a budget, a windshield damage repair kit might work, but you're back to the sunlamp.

6. The hardest part of this (for me) to get the hang of was how much cement to put on the concave surface before putting the two pieces together. As you clean off the goo resulting from a misjudgment, you'll develop a feel for it. Too little cement can't be undone; you have to pull them apart and hit the acetone again and get them CLEAN. Too much is less of a problem, the extra will run out the edges, which is fine up to a point. Look carefully at the puddle of cement before you push the two lenses together. Are there bubbles? If so, touch them with the plastic tip of the cement bottle and they'll pop. This should be done in an area as dust free as possible. Any dust in the puddle of cement? Start over. I used a little 12V computer fan with a filter on it to blow across the bottom lens. Gently press the top cell into the bottom one. If you're just short of cement on a edge you can gently rotate them a little. Press them firmly together, then keep your lens sandwich level so they don't slide apart.

How to center them during this process? If the lens elements are the same diameter, machinists' V blocks work fine -- so long as you keep the bottom glass level. These are the easy ones. But what if the elements have different diameters? You can very patiently eyeball it and get a perfectly usable lens. F:16 and f:22 are your friends.

Or you can use a precise custom-manufactured jig to get them precisely centered. Q: So where do you get one of those and how much does it cost? A: You already have one and it costs you nothing. Your lens mount probably holds the glass darned tight and precisely centered. Put the uncured glass into the mount and gently screw it together if necessary. You are screwing it together to get it centered, not to smash it down. It will center the glass. Check to make sure the cement is still uniform across the surface. Keep the concave surface down.

7. One everything is perfect or as close you you can get, you pre-cure the lens. Use the UV source (I use a Feit compact florescent UV bulb. NEVER use a UV "C" bulb!!!!) at close range for 5 to 10 seconds. (Summers suggests 3 to 5 seconds) It varies with your UV source and distance. You can get some idea by just putting a drop of cement on a piece of scrap glass and hitting it with the light. You want it tacky, but not rock hard. This will get you in the ballpark, though in actual practice with the UV shining through the glass it will be a little different. You can inspect your work, remove the glass from the mount, and make sure everything is exactly how you want it before you take the big final step and really cure it. (If just pre-cured, the soaking to separate doesn't take that long.)

8. You can use a damp (but not soaked) rag with acetone to clean off your mount, but it may take off the black paint so you'll need a Sharpie. You can use a just damp rag to clean off excess glue on the edges or on surfaces where you don't want it.

9. For full curing after you've done everything right, I give it a good hour, flipping the part over half way through. Too much won't hurt anything, but the bulb does put off some heat so don't get it too close. (If you have an radioactive-browned lenses, that same bulb with a week of exposure will largely clear them up.)

OR...you can do this the way you think best. I'm OK with that.

[kfed1984]

Damn that was a good post! I will read again several times. MEK is Methyl Ethyl Ketone? If so, I just happen to have one gallon of the stuff for a different purpose, that didn't materialize. Do you have anything against real balsam? Auto glass repair resin? Never heard of it. I now like the balsam because it keeps well. Also I considered 3D printing clear uv resin. But I think its a thermoset polymer what cannot be reheated and melted again.

[Tin Can]

but what about the FUNGUS?

[Mark J]

Yes, great info from Kevin.
However, here's a question, going back many steps, has anyone just tried oil ( preferably microscope immersion oil ) applied to the gap where the cement has separated, to see if it will wick into the gap and cure the problem optically ?

[Kevin Crisp]

I haven't tried oil, though I suppose it might help in some situations. But, again, if the issue is some relatively minor separation on the edge, the lens is probably going to work well anyway without it.

I never considered or looked into using balsam, figuring there's a modern product for this that is stable and long-lasting so I'll use that. I like the fact that I can check my work with the UV-cure cements before hitting them with a fully curing dose of light. So I just don't know how to work with it. I have nothing against it.

Lenses can have a lot wrong with them and still work acceptably. I bought a compromised 360mm Wollensak protar-copy. (Coated, in my experience the best Protar version anybody ever made. I think they called them Series Ia) The front element didn't look right. I took it out of the mount and found that one of the formerly UV-glued surfaces had come apart, been cleaned off, then the lens reassembled. Whoever did this wanted the now-separated glass surfaces to be as close together as possible in the mount, but not quite touching. So at 90 degree intervals he/she put little pieces of mylar tape on the edge as extremely thin spacers. I tried it out before I fixed it, and at f:16 or lower it wasn't bad at all.

[Steven Tribe]

This method of tiny spacers at the edge was used by early telescope objective makers. I have discovered them when renovating some about 10 years ago. They were silver coloured metal - tin, I think. I have also seen oil used as an optical contact in early telescopes.

[Mark J]

I haven't tried oil, though I suppose it might help in some situations. But, again, if the issue is some relatively minor separation on the edge, the lens is probably going to work well anyway without it.


Well, not necessarily, it depends on the lens design. The failure mode would be that for high field angles, the light doesn't get out of Lens 1 into lens 2 , it just TIR's ( Total Internal reflect ) on the rear of lens 1 ( the positive).

[Kevin Crisp]

Well, "I suppose it might help in some situations" and "the lens is probably going to work well anyway" do not, in my opinion, suggest that it would "necessarily" have no impact. I was assuming relatively minor separation and use at smaller apertures.

[Mark J]

I'm not trying to get at you, Kevin. I'm just coming at this from a different angle, being a designer.
Re. separation, I was thinking in the context of the OP's 300mm Symmar picture.

As it is, i went away and checked a 210 Apo-Symmar model, and the full field rays for f/5.6 or f/22 don't get that close to the edge of lens 2 , so it's quiet tolerant ( as would a Symmar-S be, most likely ) of a fair bit of edge separation. A typically tele would be less so.

[Bayliss]

I'm planning on repairing some balsam separation, but can't decide on the best adhesive. Is the Canada Balsam you can get from art supply sites (e.g. https://www.cornelissen.com/oils/solvents-and-balsams/roberson-canada-balsam.html) the same as the Canada Balsam sold for microscopy? I haven't been able to find a source for the latter (many sites appear to stock it but will only sell to businesses).

Also has anyone here tried Norland Optical Adhesive 60 (https://www.norlandprod.com/adhesives/noa%2060.html)? It was designed as a cement for lenses and looks pretty good, I'm hesitant to use a UV glue though because they seem so much harder than Canada Balsam to reverse. Thanks!

[Mark J]

Norland cements like 60 ( and 61 ) are widely used in industry, but maybe aren't the best choice for home use. Their expiry date is usually short and you need UV lamps.

[Jody_S]

Just read through the whole thread, matches my personal experience. Comments:

1. Easiest way of separating any lens group, whether Canada Balsam or any lens cement I've ever encountered, is old furniture stripper (MEK-based). Hard to find, I scour flea markets for old cans.

2. Centering most lens groups is easy with a flat surface and a pair of machining V-blocks.

3. If you can't find UV-cure optical cement (I ordered mine off AliExpress), clear-coat UV-cured nail polish is a good substitute, available at any nail salon.

4. The only reason to polish off surface etching from fungus is if you plan to sell the lens, IMHO, and if it's a coated lens and you polish off the coating to make it look pretty, you might get a better price for your lens but it's a little unethical IMHO. I expect all those 'mint' Leica lenses for sale on FleaBay have had some treatment of this sort. If the etching is bad enough to affect a LF lens, then your grinding might manage to keep the exact curvature of the lens, if you follow the proper pitch lap technique, but changing the thickness of the lens by polishing off a huge amount of glass might affect the lens' performance anyway. Even more so if it's a modern coated lens. I don't mind doing this to polish off a huge number of cleaning marks.

[kfed1984]

The only reason to polish off surface etching from fungus is if you plan to sell the lens, IMHO, and if it's a coated lens and you polish off the coating to make it look pretty, you might get a better price for your lens but it's a little unethical IMHO. I expect all those 'mint' Leica lenses for sale on FleaBay have had some treatment of this sort. If the etching is bad enough to affect a LF lens, then your grinding might manage to keep the exact curvature of the lens, if you follow the proper pitch lap technique, but changing the thickness of the lens by polishing off a huge amount of glass might affect the lens' performance anyway. Even more so if it's a modern coated lens. I don't mind doing this to polish off a huge number of cleaning marks.

The polishing process will not remove more that a micron off the lens surface, which will not affect the optical performance. No I am not polishing to make it look "pretty", but to make the image sharp again. Fungus and microscratches make a diffused image. I wrote at the beginning; the decision to polish or not depends on the degree of damage and what the lens is worth to you. I had this damaged lens (the Apo-Ronar), shown in the example pictures, and it was worth for me to repair. Next example will be another damaged Apo-Gerogon lens, which I will show later, with lots of cleaning marks. But maybe polishing my own lenses will be unethical, I don't know.

The idea of using pitch is to preserve the curvature of original glass as done in the factory, not to make it "pretty". I don't think the "mint" lenses on eBay you mentioned had this procedure.

I don't think re-boring and re-honing the cylinders on your worn-out engine is unethical. Yeah, you may sell it as a low mileage engine, which may be unethical. But re-boring a car engine on its own is a good thing without doubt, and always good to learn how to do it for those who want to.

[Jody_S]

The polishing process will not remove more that a micron off the lens surface

You can polish off as much as you want, just by using a variety of grits and spending as much time as you want. You most definitely can change the curvature doing so, particularly if you use a heavier grit to polish out deep scratches and don't follow the proper pattern of movement. The amateur telescope-making book mentioned in the thread gives a variety of movement patterns to use, IIRC, as well as instructions for cutting and polishing a glass blank into a concave mirror.

I am not accusing you of doing anything unethical, but I have had many hundreds of lenses pass through my hands and I have spotted a number that have been imperfectly polished and re-sold. I very nearly bought a collapsible Summicron in LTM a few years back, from what I thought was a reputable photo dealer here in Montreal, only to notice on close inspection that an inner element had been mangled this way. So there are unethical people out there, and if you buy old and valuable lenses, this is something you should look for. I am skeptical of all those Universal Heliars and the like on FleaBay with 'perfect' glass. I am more inclined to take my chances on a lens with a few visible blemishes. Same reason I won't buy a Turner-Reich Triple Convertible that doesn't have separation.

[kfed1984]

You can polish off as much as you want, just by using a variety of grits and spending as much time as you want. .

Well I tried to do this as much as possible in agreement with the telescope making book. Also we should keep in mind that the accuracy required for making mirrors is about 4x more stringent than lenses, due to the nature of refraction vs. reflection. So should be ok. The special polishing strokes that are more concentrated on the mirror center and specially formed pitch laps designed to work more on the center or perimeter are designed for generating parabolas and hyperbolas, etc. In our case, most of the lenses are spherical, and to guarantee the spherical shape the tool and the lens need to be rotated periodically. The repolishing does not take too much time, about 30 min, again depending on the damage, so it should not dig too deep into the glass. For the lens shown in my photo it took about 30 min per side. The grit is also quite small so we are not grinding anything, but polishing. Although polishing is basically grinding on a sub-micron scale.

I think those lenses you are taking about from ebay have been buffed on an industrial buffing wheel. I doubt somebody will go through the trouble of forming the lap, etc.

The whole exercise here is to try to use the same manufacturing techniques as used to make the lens. If you look at the pictures of industrial polishing setups from 1960's, they look quite rudimentary. Just need to follow some rules to ensure the law of averages of the polishing strokes generates a spherical surface than something else.

For the next lens I will try to do an image quality analysis at the center and corners before and after polishing.

[pgk]

The whole exercise here is to try to use the same manufacturing techniques as used to make the lens.

FWIW. It may have been Twyman's (Hilger & Watts) book in which the author states that 200 grinding rotations with the 'finest grit' would remove about 1/4 wavelength of depth from a lens surface . As neither glass type nor specific wavelength was mentioned I think it was simply an illustrative suggestion as to how much polishing is required to remove a small amount of a hard material like glass.

[kfed1984]

FWIW. It may have been Twyman's (Hilger & Watts) book in which the author states that 200 grinding rotations with the 'finest grit' would remove about 1/4 wavelength of depth from a lens surface . As neither glass type nor specific wavelength was mentioned I think it was simply an illustrative suggestion as to how much polishing is required to remove a small amount of a hard material like glass.

I think this is referring to the grinding operation with the smallest emery grit, which is different from polishing. In grinding telescope mirrors for example, the grit is rolled back and forward between two glass surfaces in contact; the glass tool and the mirror. In the case of polishing, the maximum pressure each particle can exert is limited by the pitch plasticity. Exerting more pressure on the tool, forces the grit into the pitch, which places a limit on the stress each particle can exert on the lens surface. Whereas in grinding, the particles are crushed between two pieces of glass, creating large stresses that breaks down the glass.

There is no way that in polishing a telescope mirror you will remove 1/4 wavelength in 200 strokes, which will be couple minutes of work. Correcting a 1/10 wavelength error will probably take an hr of work or more.

[kfed1984]

FWIW. It may have been Twyman's (Hilger & Watts) book in which the author states that 200 grinding rotations with the 'finest grit' would remove about 1/4 wavelength of depth from a lens surface . As neither glass type nor specific wavelength was mentioned I think it was simply an illustrative suggestion as to how much polishing is required to remove a small amount of a hard material like glass.

Found a link to your book, downloadable:
https://archive.org/details/20210902171836226

[kfed1984]

and I just found this statement in Chapter III. I will probably order this book, this is just what I was looking for. Thanks for sharing.
249818

[Mark J]

There is no way that in polishing a telescope mirror you will remove 1/4 wavelength in 200 strokes, which will be couple minutes of work.

Well, I disagree, that sounds plausible for polishing with pitch ( grinding with fine grit would remove more ) if the glass is soda-lime eg. float glass.
ps. I've made about 6 telescope mirrors.
It may be that the figure would not change much in 200 strokes ( if you were just using a well-running lap and normal strokes ) but that doesn't mean that material isn't being removed.
You can change the figure by 1/10 wave easily in a few dozen strokes if you apply a more extreme stroke, I've seen this several times.

If you are polishing Pyrex/Duran, then the removal would be lower, maybe only half this.

[kfed1984]

Well, I disagree, that sounds plausible for polishing with pitch ( grinding with fine grit would remove more ) if the glass is soda-lime eg. float glass.
ps. I've made about 6 telescope mirrors..
Is there something like a Foucault test for lenses? How do you test and refigure a lens?

[Mark J]

Unfortunately no, not that I'm aware of. That's why optical shops use either a test plate ( sphere ) matched to the design radius, or nowadays, interferometers stationed in the shop.
You will just have to rely on getting the lap running smoothly with the right kind of steady drag , plus a moderate ( 1/3 diameter) stroke in various directions. This should get you a surface that's within a couple of fringes, which is all that's required for a photo lens - less stringent than a telescope mirror.

[Nodda Duma]

Stumbled across this thread. Interesting reading. Some good info, some myths, some not so good info.

Mark J: Your quetion about how lenses wre aligned long ago.

1) Centering bells and edge grinding for the crown and flints, ground to very closely matched diameter. Glue applied, and the lenses installed in the tight-fitting barrel while cement is still wet.
2) Centering bells and edge grinding close to final diameter. Flint set on a precisely leveled flat surface. Crown floated on flint with cement. Gravity centers the lenses. After set, edge grind both to final diameter.
3). You slap them in like Turner Reich did, and damn the wedge error.

(I am a lens designer as well.. have been for a few decades and did a lot of gands-on optical shop work. I now am engineering manager at LaCroix Precision Optics. I also do other things like repair delamination at home for fellow photographers).

[Randy]

Once upon a time I acquired an old 12" rapid rectilinear - cost me nothing. It had some bad balsam separation. I removed the separating cell, let it soak in a dish of acetone, checking the progress of the dissolving balsam every day, and about a week later I could very easily separate the elements. After cleaning the residue off and washing both elements well, I put one drop of canola oil on one element, sandwiched them together, gently rocking to work out any bubbles, wiped the edges with a paper towel, reinstalled the cell into the barrel, made some water-house stops, and used the lens like that for a year or so, then sold it (fully disclosing my very inexpensive repair to the buyer).

[reddesert]

Internet references say vegetable oil has an index of refraction n about 1.47, so that would be a pretty good match to common glasses and a decent choice for a layer between lens elements from an optics point of view. I have never been in a position to consider such a repair; if I did, I might consider using mineral oil (the stuff you can buy to use on cutting boards and wood utensils), as vegetable oil might eventually go rancid.

[Randy]

I might consider using mineral oil (the stuff you can buy to use on cutting boards and wood utensils), as vegetable oil might eventually go rancid.
Yes, vegetable oil will likely turn bad. If I had kept the lens my plan was to remove the cell every so often, clean and replace the drop of canola oil. The last thing I want is for my lenses to start stinking.

[Mark J]

There are oils used for immersion microscope objectives, which would be ideal for this job if you can find some cheaply. The index is not too critical, as long as it's over approx 1.45. The layer between the lenses is so thin that the refractive effect of the oil is tiny once it's in place.

[Tin Can]

Does anybody polish UN coated lenses

with food grad activated charcoal

?

[cowanw]

I have a Rodenstock 105mm f/5.6 whose front element had separated about half way across. I probably should have left it but I soaked the front assembly and dissolved the cement, cleaned it and popped it back in waiting for the day I access some Canada Balsam. I can't tell the difference from my other enlarging lenses either when it was half separated or now with an air space.

[Myriophyllum]

Does anybody polish UN coated lenses

with food grad activated charcoal

?

Hi,

my guess: Too soft to be abrasive at all.

Jens