Repolishing old lenses with fungus, balsam separation
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.
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Re: Repolishing old lenses with fungus, balsam separation
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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Re: Repolishing old lenses with fungus, balsam separation
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.
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Re: Repolishing old lenses with fungus, balsam separation
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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Re: Repolishing old lenses with fungus, balsam separation
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.
Re: Repolishing old lenses with fungus, balsam separation
Thanks for sharing your knowledge and experience!
Re: Repolishing old lenses with fungus, balsam separation
Quote:
Originally Posted by
mhayashi
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.
Re: Repolishing old lenses with fungus, balsam separation
Re: Repolishing old lenses with fungus, balsam separation
Yes
Thank you
And archive.org
Re: Repolishing old lenses with fungus, balsam separation
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.