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UltravioletPhotography

Oxidation of ionic glass


rfcurry

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Andrea and I were discussing the issues associated with ionic glass filters and oxidation. Some of the Schott glass can, according to Schott, oxidize in just a few months.

 

In filter glass types UG5, UG11, BG39, S8612, S8022 and S8023 a change in the glass surface is possible within a few months during normal storage. For this reason, we recommend a protective coating or lamination to a durable filter glass from Group 1 (SCHOTT can do both).
[from the Schott glass catalogue]

 

Of course, many UV photographers use uncoated glass filters. These filters are exposed to many sources of contaminants, the acid from hands not the least; but oxidation is a certainty with some filters. Apart from coating the filters with an AR coating, or cladding them with quartz, which is what I do, the question is "How can we store our filters to protect them from oxidation?"

 

When I used to restore cane fly rods as a hobby, I had a dip-tube about four feet tall, filled to ten inches from the top with expensive varnish. I kept the varnish in the tube year-round but never had a problem with oxidation ....because oxygen couldn't contact the varnish surface. After dipping a rod I would spray some Bloxygen -- pure argon gas -- into the tube before capping it. The argon gas is heavier than air and prevented any air from reaching the varnish.

 

We could do the same with our filters when putting them away for the winter. The Bloxygen is not expensive per shot and should be good protection for our UG11, BG's, and other ionic glass.

 

See http://www.bloxygen.com/

 

Cheers,

Reed

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Some of the filters may sit in packages on store shelves for years before even being purchased. I doubt they are packed in an inert atmosphere. How much of a problem is this in real life if filters are stored in a dry container? I have seen pictures of filters that showed severe, obvious surface damage; but is a filter compromised long before it reaches this stage? The typical filter case is ill-suited for injection of a protective gas due to its geometry. I have not found a lot of practical guidance on this issue.
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Clark, you can look at your 13.5mm filter, both surfaces are in that list Reed posted, and you can judge for your self how those surfaces are fairing in your climate, and climate does seem to have something to do with it.

It can fist show up as a faint cloudy appearance (especially on black glass).

Some absorptive filter glass is not too prone to oxidation, but for those that are I recommend polishing them with cerium oxide slurry. This is the best way to keep oxidation from ruining your filter surface.

If you do this on a regular basis, it will keep oxidation from becoming too deep too remove.

Basically anything that will not clean off with hydrogen peroxide should be polished with cerium oxide.

If you wait until you see oxidation, it may be too late to remove with cerium oxide, and can cost as much to re-lap (re-surfaced) as buying a new filter, and can reduce thickness some.

Some people recommend 'toothpaste', but I don't. I have compared toothpaste to cerium oxide, and it works, but not as well, and frankly, you don't know what all is in toothpaste.

So get some cerium oxide for you uncoated filters.

Don't polish coated filters with cerium oxide.

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I have had ionic glass oxidize while still in a roll separated by lens tissue from its fellows.

 

When well oxidized, it is still possible to restore some glass. I use a single edge razor blade, as sold by hardware stores in boxes of 100. Going around the disc in a circle, use short cutting motions, holding the blade at a 30 degree angle, with the end of the blade not past the center of the disc. The blade must be sharp, so only one blade per filter. You will be amazed at how easily an otherwise useless filter can be restored to a smooth, unblemished appearance.

 

JME, YMMV

 

P.S. - A note on Ceria, a.k.a., cerium oxide. The fine particles are apt to work their way into the pores of the glass. Ceria is highly UV absorbent.

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Perhaps the key thing with filters -- and lenses -- is that we inspect the glass on some regular basis so that we can tend to any oxidation or fungus or other problems in a timely fashion. This is something I try to do but sometimes fail to do. :) I've lost a B+W BG filter more than once to deterioration especially in earlier days when I was less knowledgeable about how they deteriorate and how that can be slowed by proper maintenance.

 

Filters do not last forever. This can be upsetting because of the cost of replacing them. I suppose the replacement cost has to be factored into the general expenses of our UV photography. Don't start this as a hobby or as a professional pursuit unless you realize this. :D

 

If I've used a particular UV-pass filter several times a week during a spring and summer season of shooting outdoors "in the field", then that filter unavoidably gathers some (or all of) pollen, dust, moisture, oxidation, minute scratches and sometimes dents in the edges of its metal mount no matter how hard I try to prevent this and how many times I clean the filter after use. But that's OK. These filters are meant to be used and therefore -- eventually -- used up.

 

Anyway, I'll use Eclipse, hydrogen peroxide, cerium oxide or other filter cleaners to keep a filter going a bit longer.

 

I'll add a razor blade to this list of cleaning techniques. I think that you could just as easily embed small metal particles into the glass when using a razor blade. But I wouldn't mind a few particles of this, that, or the other if it kept the filter going for a while longer.

 

It might be interesting to put on my official UVP hat and ask Schott what they recommend for filter cleaning.

 

Maybe if you used a small ziplock bag, you could seal it except for a small opening for a "straw" and then spray argon into it and then quickly finish sealing it?

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The ziplock bag shouldn't be necessary. A simple old coffee can should be fine, as argon is heavier than air and completely inert. As long as there are no leaks at the bottom of the container, you shouldn't need a good seal on the top of the container.

 

I never thought about the steel particles from the blade. Good call. I do wipe off all ionic glass with acetone after cleaning it and have never picked up visible steel; however, microparticles of steel might still be in the pores and not be visible.

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The 13.5mm filter I have is mounted BG side out, I believe. I do not see any obvious surface defects on that face. Nor do I see any such damage to my three KolariVision deconverters, so far. My two 403 filters seem also to be fairly mirror-like on the surfaces--I have had them for 8 years and I have not made any special attempt to protect them from oxygen. However, that kind of glass is opaque enough that anything going on just beneath the surface would not be visible to the eye. Perhaps a pinhole test of the filter would reveal any hidden degradation. I did such a test in my post earlier on pinhole tests, but lacking a standard for comparison, I had no way to interpret the result.

 

As to climate, I live in the southern Appalachian rain forest and it is very humid for 8 months of the year. I have not used my 403 filters a great deal since getting the Baader, but the air in which they are stored is definitely the air from around here. I do, however, generally store them in dark places. Perhaps light exposure is also a factor.

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Chapter five of the Schott Filter Catalogue is an excellent, imo, precis on the subject of glass care.

The B&W 403 is a Schott UG1, not the UG11 listed in my first post. The UG1 is a 1.0 for acid resistance; the UG11 is a 3.0.

 

"5.4.2 Acid Resistance (ISO 8424: 1987), Classification into Acid Resistance Classes SR 1 – 4, 5, 51 – 53 Acid resistance classifies the behavior of glass surfaces that come in contact with large quantities of acidic solutions (from a practical standpoint for example, perspiration, laminating substances, carbonated water, etc.). Acid resistance is denoted by a two or a three digit number. The first or the first two digits indicate the acid resistance class SR. The last digit (separated by decimal point) tells the change in the surface visible to the unaided eye that occurs through exposure (see 5.4.4). " from Schott Filter Catalogue

 

It would require more than 100 hours for the UG1 to exhibit the same corrosion as the UG11 displays in 1-10 hours. Add to this the UG1 is a 1.0 for alkali resistance while the UG11 is a 2.0...

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Clark, The 403 is UG1 if I am not mistaken. UG1 and U-360 don't tend to oxidize like UG11 and U-340 might.

Your little 13.5 stack is UG11 and S8612, it was the only stack thickness I could fit in there.

So keep an eye on your 13.5 filter, and clean it with hydrogen peroxide first, if that doesn't get it clean then use cerium oxide.

It is kind of like alkaline batteries... you never know when they might start leaking all over the inside of something.

In other words, just because you have a filter for a year, with no problems, it can still happen and probably will at some point.

Preventative cleaning/polishing of filters with cerium oxide will maintain them indefinitely. But if you leave the batteries in the remote... then they will eventually destroy the remote.

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