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Coating removal
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PostPosted: Wed Jun 26, 2013 4:50 pm    Post subject: Coating removal Reply with quote
I had posted a while back about using hydrofluoric acid for the subject purpose. It's a really dangerous acid.

I recently hauled out my 610mm Aero Ektar (there is a separate thread on this) with repair in mind. Attached to the lens was a really old note I had written to myself, I think back in the 1990's. The note (in essence) mentions use of 10% ammonium bifluoride and water to remove lens coatings. Ammonium bifluoride is less dangerous to work with than is hydrofluoric acid.

Here are a few references:

http://www.ehow.com/way_5635130_home-remedy-scratched-prescription-glasses.html

http://flowcyt.salk.edu/abf.html

http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=1228904


PostPosted: Wed Jun 26, 2013 6:52 pm    Post subject: Reply with quote
Interesting articles!
But the ammonium bifluoride seems still much to dangerous for me :-/
The dangers sounds nearly as horrible like those from hydrofluoric acid.

Take care in case you really try this!


PostPosted: Wed Jun 26, 2013 11:29 pm    Post subject: Reply with quote
I would guess that less much dangerous acids like hydrochloric acid or lyes like sodium hydroxide (often available as 90%+ NaOH (mixed with some tensides and aluminium which don't disturb) as drain cleaner in supermarkets and drug stores) would also work when hot. And they are not highly toxic.
Also HF might not only dissolve the coating - it might also etch the glas and ruin sharpness. Hydrochloric doesn't etch glas but should remove most coatings.

If HF is really necessary (I doubt that) I would buy less dangerous sodium fluoride and normal hydrochloric acid which produce HF in situ when mixed in thinned solutions (use only in thinned solutions - adding conc. HCl to pure NaF directly might be deadly as gasous HF can ne evolved!). And adding a mild lye like chalk or lime could be use to neutralise HF and HCl excess afterwards.


PostPosted: Wed Jun 26, 2013 11:46 pm    Post subject: Reply with quote
I've read distilled vinegar will remove coatings. Might take a bit of patience but the coating will rub off without any risk to the glass.


PostPosted: Thu Jun 27, 2013 1:40 am    Post subject: Reply with quote
themoleman342 wrote:
I've read distilled vinegar will remove coatings. Might take a bit of patience but the coating will rub off without any risk to the glass.


Boy, that sounds a lot safer than the chemicals I wrote about earlier . . . . . . . if it works.


PostPosted: Thu Jun 27, 2013 3:59 am    Post subject: Reply with quote
themoleman342 wrote:
I've read distilled vinegar will remove coatings. Might take a bit of patience but the coating will rub off without any risk to the glass.


Nope, it doesn't work, even the strongest one.


PostPosted: Thu Jun 27, 2013 12:07 pm    Post subject: Reply with quote
so what is the best way to remove coating?
and: would removing damaged coating improve the performance, resp. in which cases, I am thinking of level of coating damage or kind of lens, would removing coating improve lens performance?

thank you for any input, andreas


PostPosted: Thu Jun 27, 2013 1:04 pm    Post subject: Reply with quote
kuuan wrote:
so what is the best way to remove coating?
and: would removing damaged coating improve the performance, resp. in which cases, I am thinking of level of coating damage or kind of lens, would removing coating improve lens performance?

thank you for any input, andreas


Well, it's difficult for me to recall what I was thinking circa fifteen years ago. At that time, with a severely impaired Aero Ektar in hand, I was viewing a catastrophe with no clue whatsoever how to deal with it. My note to myself lists two telephone numbers, both to gentlemen at Kodak (Kodak is the manufacturer of the Aero Ektar lens). One fellow I have listed as being with Kodak's Optical Products Division. The other chap, on my note, shows as being Engineering Manager of Traditional Optics.

Then, at the bottom of my note it says "10% ammonium bifluoride and water". Did one of those fellows tell me that's how Kodak strips coatings, or that's a good way to strip coatings? I cannot recall. I just know the ammonium bifluoride reference appears adjacent to the two names.

From the references I listed in the OP, though, it does seem as if ammonium bifluoride is available and will strip coatings. Whether that's how the professionals do their stripping, I do not know.

I have no coatings to strip at this point; not personally. But if I did I would try the 10% ammonium bifluoride and water, taking care to wear eye and skin protection, and to work in a very well ventilated area. However, regardless protection or ventilation, I would be very, very hesitant to use hydrofluoric acid for anything whatsoever. It frightens the crap out of me.

Also, for the record and to clarify as best I'm able, I think fifteen years ago (or whatever) I had the notion stripping its coatings would help restore my Aero Ektar lens. I no longer hold that view.


PostPosted: Thu Jun 27, 2013 1:17 pm    Post subject: Reply with quote
thank you very much for your reply Guardian.
just now reading up on ammonium bifluoride, sounds like the one to try
( http://flowcyt.salk.edu/abf.html http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=1228904 )


PostPosted: Thu Dec 19, 2013 6:28 pm    Post subject: Reply with quote
Do not use HF, it will dissolve the glass and is one way to commercially etch things into glass. Also the metal content of some glass can be seriously harmed using other acids, even the acetic acid. I'll have to read up on the ammonium bifluoride stuff and see what I think. I have a lens that I really want to treat because it has "cleaning" scratches all over the front element and flares very badly when light falls on that surface. It is also something that needs to be done before anyone can even think of recoating a lens (not that I can find a place to recoat lenses in small batches).


PostPosted: Thu Dec 19, 2013 7:32 pm    Post subject: Reply with quote
After reading this PDF, I have to say that ABF is just as bad for lenses as it is for your health. ABF is also apparently used for etching glass which is why I say probably not so good for removing the coating from lenses. But I bet it works well getting the metal coating off, not worth the risks though.

http://komfg.com/files/2012/02/ABF_HF1.pdf

Warm (hot?) vinegar and time still seem like a better option, even if it takes a lot of time. Dilute HF or ABF might work and be safe for the lens, but it might also not be safe and damage the glass.

Also this PDF (among others):
http://nj.gov/health/eoh/rtkweb/documents/fs/0089.pdf


PostPosted: Tue Dec 31, 2013 3:00 pm    Post subject: Reply with quote
This is a very interesting topic and has lots of interest for photographers, not just for lenses, but also for sensors.

A visible scratch on a sensor could pretty much be a scratch in coating. I wonder if we could use such a method to remove the coating off a sensor's hot mirror (removing the coating to make an IR sensitive camera too!)

This guy uses aluminium oxide found in a paint renovator cream.
http://www.instructables.com/id/IR-digital-camera-mod-keeps-autofocus-intact/step2/Remove-the-IR-reflective-coating/

I feel like doing this if I have to. I've been trying to find a cheap glass replacement for my hotmirror with no luck.

Any ideas?


PostPosted: Tue Dec 31, 2013 6:46 pm    Post subject: Reply with quote
Not every camera has only coated IR cut filters. Many seem to have colored glass - there coating removal will not help (much).


PostPosted: Fri Jan 03, 2014 9:31 pm    Post subject: Reply with quote
Just a little Question from a newbie like me.
What are the benefits of removing the coatings?

TIA!


PostPosted: Sun Jan 05, 2014 12:44 pm    Post subject: Reply with quote
Old lenses have no coating, and so coating removal is a way to get a kind of vintage image quality - with modern DSLR cameras.
One could use those old uncoated lenses instead to get the real thing - but these are most likely longer focal lengths (e.g. 150mm) and higher f-stop numbers (~f/5.6 for example).
For a cheap DSLR I love for example a Helios 44 58mm/2.0.

Another reason for the coating removal is scratch repair (with risks!), or to get rid of coating defects. But I think even a bad coating is in most times better than no coating (IQ wise).


PostPosted: Sun Jan 05, 2014 3:16 pm    Post subject: Reply with quote
Greg_E wrote:
Do not use HF, it will dissolve the glass and is one way to commercially etch things into glass. Also the metal content of some glass can be seriously harmed using other acids, even the acetic acid. I'll have to read up on the ammonium bifluoride stuff and see what I think. I have a lens that I really want to treat because it has "cleaning" scratches all over the front element and flares very badly when light falls on that surface. It is also something that needs to be done before anyone can even think of recoating a lens (not that I can find a place to recoat lenses in small batches).


It will etch glass, we used to use it in the dental trade to etch ceramic, its about the only thing that will etch ceramic. AVOID HF


PostPosted: Mon Jan 06, 2014 8:28 pm    Post subject: Reply with quote
Oh ok. Thanks, ZoneV.
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