Glass oxidation vs. tarnish

[rfcurry]

I just thought that we may be hasty in cleaning our ionic glass filters that are showing signs of oxidation. As we recall, Lord Rayleigh (John William Strutt, 3rd Baron Rayleigh of Terling Place) discovered the first type of antireflection coating known, in 1886. He found that old, slightly tarnished pieces of glass transmitted more light than new, clean pieces due to this oxidation coating. Of course, we cannot know, without spectrophotometer testing, whether the increased transmission, if any, benefits our particular wavelengths of interest.  Still, the possibility that the oxidation is advantageous should give us pause.

Another reason for delaying cleaning, is the possible deleterious effects of cleaning. Some have suggested the polishing of the glass with fine ceria (cerium oxide) particles. Ceria is a wonderful substance, and is of especial note as a nanoparticle. I received a patent for the use of such nanoparticles in camouflage in the ultraviolet. In the image below, the the subject is wearing a U.S. Army ACU (FRACU) with the right side of the top and trousers - and the Multicam boonie hat - treated with ceria nanoparticles The top image in visible light and the middle image in NIR light reflect the light as expected. However, the bottom image, in ultraviolet light, shows the wearer's left side as a bright image, while the treated right side blends with the background. (I am simply presenting this as an illustration of the UV absorption of cerium oxide nanoparticles. )

Ceria nanoparticles are inexpensive to produce and readily available down to a size of 10nm. In comparison, human hair is 30,000nm - 90,000nm in diameter. So it is quite easy for ceria to lodge in the invisible pits and crevices of a filter glass. While the ceria would not be visible, it would be absorbing UV. Would it be sufficient to alter the images shot through the filter? I don't know, but caution would suggest avoiding the possibility.

Just some thoughts. ymmv.

 

 

 

[colinbm]

Interesting Reed, thanks for sharing.

 

[Andy Perrin]

Interesting, but any cerium in the cracks would be a very thin coat and would have correspondingly small absorption. The main problem from the oxidation, on the other hand, is that it’s irregular and patchy and scatters light, which destroys image quality. We can deal with small transmission losses by changing exposure, but a blurry picture is not acceptable. So I will continue to polish my filters— although the preferred method (I thought) is hydrogen peroxide in any case, not cerium. 

[rfcurry]

My apologies, Andy. I now realize that my title was somewhat didactic, that was not my intent. 

 

I haven't enjoyed success with hydrogen peroxide. It may be due to the particular metals in the glass which are oxidizing. I'll try again, putting it in the ultrasonic cleaner next time. Maybe I need to pull a vacuum on it as well. I have some boxes of 3-year old ionic glass, still wrapped as it came from the foundry, that is oxidized to the point I can't use it in a customer's filter. Naturally, I am always looking for a non-abrasive technique for restoring that glass.

 

Again, sorry.

regards,

Reed

[Andy Perrin]

Reed, no need to be sorry? I weighed in with a different take, but you didn't say anything bad, I just didn't necessarily agree. There can be many points of view when a subject isn't settled yet!

[ulf]

Cerium oxide is used for polishing fine optical surfaces:

https://iopscience.iop.org/article/10.1088/1757-899X/28/1/012007

Open the pdf and read more.

 

[Andy Perrin]

Ulf, it's known that cerium oxide is used on visible light transmitting glasses, but Reed raised the issue of using it on UV-transmitting ionic glasses, which are a different kind of glass than was tested in that PDF, and more delicate. I thought the question of absorption was reasonable (even though I suspect it's not a serious problem due to how thin any layers of particles would be, and the fact that we can adjust for it even if it does decrease transmission). Cadmium has long recommended against Cerium oxide as a first choice -- he says peroxide is a better method to clean small amounts of oxidation, and that's the advice I was repeating above.

[Cadmium]

Well, rfcurry... I think it was you who first recommended hydrogen peroxide. I think it is great stuff.

My recommendation is to soak the filter in HP first, days, week, depending. Then use more to manually 'scrub' it off with HP and a PEC pad (or such).

That works really well for me. For me that includes removing the filter glass from the ring.

Cerium  Oxide works good too, but I use the HP method first.

I have found nothing wrong with either method.

The thing about cerium getting into small places, I think you just need to wash the filter well after cleaning.

 

For any of you, if you have filters of a certain type especially, don't be afraid to clean your filters with HP or even Cerium Oxide.

These will keep your filters clean and those should be cleaned sooner than later.

Here is a short list, and there may be more.

UG11 (U-340), UG5 (U-330), UG1 (U-360)

S8612

Etc.

Let me know if you have questions. I don't know everything, but fear if not on the list.

 

 

 

[Andrea B.]

I just thought that we may be hasty in cleaning our ionic glass filters that are showing signs of oxidation. As we recall, Lord Rayleigh (John William Strutt, 3rd Baron Rayleigh of Terling Place) discovered the first type of antireflection coating known, in 1886. He found that old, slightly tarnished pieces of glass transmitted more light than new, clean pieces due to this oxidation coating. Of course, we cannot know, without spectrophotometer testing, whether the increased transmission, if any, benefits our particular wavelengths of interest.  Still, the possibility that the oxidation is advantageous should give us pause.
 

Editor's Note:

A reminder that this old glass tarnish is probably not may not be is not the same kind of oxidation seen on uncoated, oxidation prone BG glass of some types. BG glass "oxidizes" by leaching out the ions of ionic glass when in contact with humidity. Hence, very messy stuff -- which should be cleaned immediately upon discovery. 

 

Another little reminder:

Any coated filter should be cleaned ONLY with a good filter cleaner. Do not use cerium oxide or hydrogen peroxide to clean any coated filter. 

 

Added Later:  See next post for description of lead sulfide tarnish on flint glass.

 

[Andrea B.]

Reference:

On the Manufacture of Glass for Optical Purposes

by Michael Faraday, 1830

 

Summary:

 

Flint glass tarnish (lead sulfide) is caused by interaction of atmospheric hydrogen sulfide (or other kinds of sulphur vapors) with the lead oxide in the glass. The H₂S gets into the atmosphere from breakdown of organic waste material or from chemical fumes from oil refining, paper mills, tanning mills, coke ovens and or from similar industrial effluvia. 

 

The kind of ionic leaching oxidation we find on our BG filters is of the 2nd kind - "minute vegetations or crystallizations", as Faraday put it. 

 

It is entirely possible that we are seeing more than one kind of oxidation. I wouldn't really know for sure. But it is definitely the ionic leaching which has ruined a couple of my filters over time. I keep up with it fairly well, but recently had to give up on an old Baader blue-pass filter. It just will not come clean any more.

 

It was really cool to find this old reference from 1830. My search term as you can see was "flint glass tarnish".

 

---

 

 

Screen shot from Faraday's book:

 

[dabateman]

You can read philosophical transactions all the way back to 1669:

https://royalsocietypublishing.org/toc/rstl/1669/4/54

 

https://royalsocietypublishing.org/toc/rstl/1691/17/194

[Andrea B.]

Those old transactions can be fascinating stuff !! 

 

Anyway -- thank you to Reed for this very interesting comment about Lord Rayleigh and his observation about glass tarnish. It led me to that delightful discovery of M. Faraday's book where I learned something new.

And I also learned about ceria particles. Fascinating photos of the UV camouflage. I wonder if UV camouflage would work for bird watching? 

[Andy Perrin]

It's interesting that even Faraday saw the two different kinds of oxidation damage.

[Andrea B.]

It is also interesting that Faraday had such wide ranging interests. 

 

Glass has been made for thousands of years, so I'm thinking that there have been such observations about glass tarnish and glass deterioration for a while? Oxidation-reduction was also known very early on. By the time Faraday was active, things had advanced enough that they knew the tarnish compound, lead sulfide, and its hydrogen sulfide inducer. In Faraday's lecture about glass manufacture, he called it "sulphuret of lead" and "sulphuretted hydrogen".

 

 

Perhaps we all should remember not to lick tarnished or corroded glass. Especially if made with lead or arsenic.

 

.....j/k.........