OT: How to remove silvering from a Mirror?

[Doozer]

I have an old parabolic mirror that I want to use as a clear lens. It has most of the silvering off from being so old, but how do I get the rest of it off. And what is silvering originally?
-Doozer

[lynnl]

..... how do I get the rest of it off. ...
-Doozer

Apparently with great difficulty. This question has come up here before, and I don't think any surefire solution has been offered yet.
You can search for "silvering". Here's the most recent offering:

http://bbs.homeshopmachinist.net/sho...ight=silvering

[Just Bob Again]

Depends on whether it's actual silver or aluminum and what the backside coating is. Silver comes off with nitric acid. Doesn't need much and doesn't need to be strong. If you don't have jeweler's pickle or something with nitric in it, you can make a little impure dilute acid by mixing a little potassium nitrate (root destroyer in the garden department) with sulfuric acid. Drain cleaner or battery acid. If it's aluminum, lye takes it off. Whatever coating on the backside can be hard to remove. Old coatings may just be shellac. Soak in alcohol for a while. New coatings can be a bear. Try multiple solvents. Protected silver or aluminum with an antioxidant coating are harder to remove but should still come off after a while.

[Greg Parent]

Aluminium comes off with sodium hydroxide.

Silver with nitric acid.

[Evan]

Aluminum may come off with lye. In this case it is probably silver. If any of it has turned black then it is silver. Many aluminum scope mirrors are overcoated with silicon dioxide (SiO2). It is impervious to all acids except hydroflouric acid.

[darryl]

My question would be how to put the reflective coating back on- with minimum hassle-

I'm wondering about the use of the parabolic mirror as a lens- it might not work as a lens at all. Is it built like a lens, or more like a clear cooking pot lid-

[Paul Alciatore]

You say parabolic mirror and that suggests a telescope mirror. If that is what it is, then the silver or aluminum will be on the front or parabolic surface. The problem with using such a mirror as a lens is only one side needs to be optically perfect for it to be a mirror. Most telescope mirror makers will only put a coarsely ground finish on the other, back side. This will probably be flat in the machine shop sense, but not optically flat. There may also be some difference in thickness around the OD so even if you finish grinding and polishing the back side, it may exhibit characteristics of a wedge combined with a negative lens. This will add additional abberations to the lens.

If you need a lens, you are probably better off looking for a surplus lens of the approximate diameter and focal length. There are surplus lens suppliers on the web.

[Astronowanabe]

If it is really old you may be able to take it off with with vinagar
Not kidding we did this a couple of months ago.

More typically you can use what is called "Green River"
hydrochloric acid and copper sulfate. I stripped two mirrors last summer
first was very easy the second had stubborn spots. I have since learned one trick is to put cellophane tape over the tough spots and yank it off. The stress puts fractures in the protective surface coating that lets the acid go to work.

I am going to strip one more this winter but as I am refiguring it anyway
I'll just rub it off.

[Doozer]

Thanks for all the replies. Ok, the truth of my weird idea. I am looking for a headlight for this motorcycle I am building. It has a really wide front end, and I want a beefy looking headlight.

So after many ideas I settled on a 12" lens traffic light. Yes, really. Before that, I bought an old search light off ebay. Paid to much for it, and it was old and rusty. Turns out the search light was from the 1920s, maybe older. It was made be George McBeth in Alleganey county, PA. Apparently McBath was a popular glass maker in that area years ago. Anyhow, I decided this spotlight was too much of an antique to put on the front of a motorcycle, as I might crash into a tree (on/off road bike) and the sweet old lens would be broken for ever. The spotlight does have a curved glass reflector mirror behind the bulb that most of the silvering has come off of. It was also exactly 12". So my traffic signal light has a green arrow lens that I gotta replace with something clear, and I was thinking of using this reflector mirror from this old spotlight. Probably should have it re-silvered and put back to original and put the restored spotlight in my living room. So I really just need a 12" piece of round glass for converting this traffic light to a headlight for my bike. I will put a 12volt halogen bulb in the housing, and I could just use a piece of flat round glass or Lexan for the lens, but a convex lens like original would probably be better. I was thinking of going to WalMart and looking for 12" clear glass dinner plates. Or, where could I get a 12" convex lens, glass or plastic? Curve I guess does not matter, and I would not know how to calculate a focal distance anyways. It mounts in a weatherstrip like seal of sorts, and retained in the housing by four metal tabs. Any help on finding a convex lens that is 12"?

[Evan]

We need to get some terminology straight. The front glass can be flat in which case it is a cover glass. It can be curved with an equal thickness at all points in which case it is not a lens but a meniscus and has zero magnifying or diverging properties. Such a "lens" is called plano-plano (pronounced plan-oh) meaning it has front and back surfaces that are parallel even though they are curved. Then you have various lenses that may have one or both sides curved. A front outward curve with a flat back is convex-plano. Both sides curved out is convex-convex and one side curved out and the other in is convex-concave.

I am presuming that what you need is a plano-plano cover with no magnification. Is that correct?