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careysub
Pooh-Bah

Reged: 02/18/11

Loc: Rancho Cucamonga, CA

Does Anyone Silver Their Mirrors?
#5220832 - 05/13/12 07:49 PM

I picked up a copy of Sidgwick's "Amateur Astronomer's Handbook" 3rd edition*, that is currently in print by Dover, and he has a section on the procedures for silvering mirrors (must been the only option for ATM early in the 20th Century, no?).

I can find articles about this on-line, but it seems more like demonstrations and experiments, rather than a practice being followed.

The disadvantages are obvious: it is essentially temporary - duration at acceptable reflectivity probably no more than some months**, there is the cost of materials, and you have to master the process) but it would seem neat after making a mirror to be able to test it under full field conditions yourself and a fresh silver coating could qualify has "enhanced".

(I know you can field test an untreated mirror, it just performs in light gathering as a mirror about 1/5 its diameter or so).

*Curiously this was not Sidgwick's last edition, there was a 4th. I wonder why Dover does not reprint that one.

**Sidgwick says several months to several years, but that is using 70% reflectivity as an acceptable cut-off. Finicky amateurs might not stand for it

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Gene7
scholastic sledgehammer

Reged: 06/10/10

Loc: Mid Ohio, USA

Re: Does Anyone Silver Their Mirrors? new

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[Re: careysub]
#5220969 - 05/13/12 09:50 PM

Yes, a very interesting subject Carey. I have done some thinking about it and made some steps toward a different application, but not completed it.

I used to do back silvering of sheets of glass, perhaps 2 X 3 feet, at PPG for test purposes. Results were about perfect. I do remember we used sugar as a reducer, which leads me to believe it was Brashears method. I have pulled out some files where the book pages go from 120 to 131, but do not know what book.

My recent interest was in making a narrow beam spot LED spotlight by silvering the back of a double convex lens. Thus would would have a curved reflector plus the double lens effect in front. I unexpectedly had good results from a couple of lenses and did not complete the project. A high powered, small source LED was used, with heat sink.

Some thoughts on 1st surface silvering.

There is a silver polish that claims to convert silver tarnish back to silver. Although a person would not want to polish a first surface, perhaps some ingredient or procedure could be applied to keep a mirror un-tarnished. Do not recall the name, but it is sold in a unique black plastic bottle.

One method to keep sterling un-tarnished is to store it with moth balls to keep out the sulfur compounds in the air.

What is a good scavenger for sulfur compounds? Well, polished copper is one. Copper foil is available. Perhaps a sheet laid over a mirror would help keep the sulfur away.

Since a mirror is perhaps used only one or two percent of the time life would be greatly extended by sparging the air away and keeping an atmosphere of nitrogen around it with a cover of some kind.

Silver nitrate is available on e-bay in a moderate amount for a decent price. People want to rob you for common chemicals, so go to www.caroline.com for some good prices.

I will be following your post. Gene

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PhilHerring
member

Reged: 12/06/09

Loc: -33.87800,151.19630

Re: Does Anyone Silver Their Mirrors? new

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[Re: Gene7]
#5221061 - 05/13/12 10:41 PM

These days we have anti-tarnish strips that could probably be used in this situation.

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Crayfordjon
Vendor - Zerochromat

Reged: 06/17/09

Loc: UK

Re: Does Anyone Silver Their Mirrors? new

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[Re: PhilHerring]
#5221252 - 05/14/12 01:55 AM

I have silvered mirrors, its just a matter of learning a technique. The criterion is cleanliness, you musr make sure that the surface of the glass is absolutely clean! In fact I found it is easier to silver plastic, polytghene takes a coating just like that, and perspex is easier than glass. Try ATM books one or two, does anybody read these books anymore. You will find a more practical approach than Sidgwick, it is a myth that sivering is only for the experienced chemist.

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FlorinAndrei
scholastic sledgehammer

Reged: 09/28/10

Loc: California

Re: Does Anyone Silver Their Mirrors? new

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[Re: Crayfordjon]
#5221288 - 05/14/12 02:35 AM

These days some labs offer protected silver coatings - that is, silver covered with SiO. That should be very durable.

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sopticals
sage

Reged: 03/28/10

Loc: New Zealand

Re: Does Anyone Silver Their Mirrors? new

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[Re: FlorinAndrei]
#5221331 - 05/14/12 06:07 AM

Watching this post with interest.Looking to soon get a 22" mirror silvered. 50 years back I had mirrors silvered. As I remember got 6 months plus before the tarnish took over.

Stephen.

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DAVIDG
Post Laureate

Reged: 12/02/04

Loc: Hockessin, De

Re: Does Anyone Silver Their Mirrors? new

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[Re: sopticals]
#5221510 - 05/14/12 09:18 AM

Yes, I have silvered my own mirrors many times and being chemist also helps. I also did the field mirror on my Schupmann refractor. It's covered by a little door when not being used and I lined the inside of the door with 3M anti-tranish paper. The silver coating stayed bright for over two years until I forgot to close the door after an observation session in the Summer and didn't discover it for about a month.
One also needs to be able to dispose of the spent silver solutions correctly since silver metal is considered a toxic waste. Also the solution must never be allowed to dry out since it then can form silver fulminate which is a stock sensitive explosive.

- Dave

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FeynmanFan
sage

Reged: 02/18/11

Loc: N Colo front range

Re: Does Anyone Silver Their Mirrors? new

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[Re: sopticals]
#5221516 - 05/14/12 09:21 AM

From what I remember of Texereau's discussion of this, it seems like chemically deposited silver coatings leave much to be desired. The deposition is not uniform due to thermals in the solution, and micro-scale changes in the concentration of reactants, leading to a surface that does not accurately reproduce the figure of the glass. First you polish and figure your mirror to 1/16th lambda, then you have to figure your coating? As Florin says above, there are labs that offer silver coatings, which I'll wager are vacuum deposited.

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FeynmanFan
sage

Reged: 02/18/11

Loc: N Colo front range

Re: Does Anyone Silver Their Mirrors? new

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[Re: DAVIDG]
#5221572 - 05/14/12 10:03 AM

Dave, I read your post with great interest. I had considered silvering my first mirror, since the chemicals were easily obtainable (back then a 17-yr old kid could walk into the local chem supply house with a laundry list of stuff, and walk out no questions asked. Try that now). Apparently you were satisfied with the results. How much work was involved in getting the silver polished? Did the final surface compare well to an aluminized surface? It seems like a lot of hassle for a few percent better reflectance.

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DAVIDG
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Reged: 12/02/04

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Re: Does Anyone Silver Their Mirrors? new

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[Re: FeynmanFan]
#5221610 - 05/14/12 10:28 AM

Chris,
If you do the procedure correctly you don't need to polish the silver surface. What is critical is the temperature that you run the reaction at. Since I have all the chemicals in my lab, analytic balance to weight the reactants out, plus a fume hood, all the safety equipment and 30 years of experince, it was straight forward for me to do. I also have access to a vacuum coater were I've done both aluminium and silver. The vacuum method is better.
There are kits available to silver glass to restore antique mirrors. I was one of the technical consultants for the movie "Undaunted" that was just released about the life of John Brashear. http://undauntedthemovie.com/for-students/who-was-john-brashear There is a part in the movie were Brashear silvers a 12" mirror. They used one of these kits to reproduce the silvering.

- Dave

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Starman1
Vendor (EyepiecesEtc.com)

Reged: 06/24/03

Loc: Los Angeles

Re: Does Anyone Silver Their Mirrors? new

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[Re: DAVIDG]
#5221678 - 05/14/12 11:08 AM

http://www.opticalsputtering.com/Characterization%20of%20silver%20and%20aluminum%20coatings.pdf

How cleaning makes all the difference in the world for longevity:
http://www.gemini.edu/documentation/webdocs/rpt/rpt-te-g0073.pdf

http://www.page.sannet.ne.jp/mazmoto/emsultima.htm

http://www.page.sannet.ne.jp/mazmoto/silvertest.htm

If you use silver:
1) have it applied by vacuum deposition, not a chemical bath.
2) have it overcoated with SiO2 or similar.
3) expect to not have it last as long as an aluminum coating
4) expect to have to clean it often as the air we breathe has more agents in it to corrode silver than used to be the case.

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Steven Aggas
scholastic sledgehammer

Reged: 04/15/08

Loc: Arizona

Re: Does Anyone Silver Their Mirrors? new

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[Re: Starman1]
#5221996 - 05/14/12 02:09 PM

I silvered my 8"f6 mirror when the aluminum was starting to become see-through. Using Texereau's book, and my basement chem lab, it was messy, fun, and well worth the full magnitude fainter objects.

I had built a cold-box to store the scope so dew never formed on the mirror. The scope was sold within a year of the silvering and still looked bright (and the new owner new it was unprotected silver). I wish I remembered the guys name (if you're the one that bought it at Astrofest in 1992, let me know).

Steven

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BigC
Carpal Tunnel

Reged: 09/29/10

Loc: SE Indiana

Re: Does Anyone Silver Their Mirrors? new

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[Re: Steven Aggas]
#5233824 - 05/22/12 10:20 AM

A cold box so dew never forms? Would you explain that?My experience is dews forms on the optics once they do cool.

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JohnH
Carpal Tunnel

Reged: 10/04/05

Loc: Squamish BC Moved!!!!!

Re: Does Anyone Silver Their Mirrors? new

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[Re: careysub]
#5235488 - 05/23/12 10:44 AM

Quote:

I picked up a copy of Sidgwick's "Amateur Astronomer's Handbook" 3rd edition*, that is currently in print by Dover, and he has a section on the procedures for silvering mirrors (must been the only option for ATM early in the 20th Century, no?).

I can find articles about this on-line, but it seems more like demonstrations and experiments, rather than a practice being followed.

The disadvantages are obvious: it is essentially temporary - duration at acceptable reflectivity probably no more than some months**, there is the cost of materials, and you have to master the process) but it would seem neat after making a mirror to be able to test it under full field conditions yourself and a fresh silver coating could qualify has "enhanced".

(I know you can field test an untreated mirror, it just performs in light gathering as a mirror about 1/5 its diameter or so).

*Curiously this was not Sidgwick's last edition, there was a 4th. I wonder why Dover does not reprint that one.

**Sidgwick says several months to several years, but that is using 70% reflectivity as an acceptable cut-off. Finicky amateurs might not stand for it

Sidgwick was where *I* got the recipe for the Brashear recipe for silvering (4th edition). The mirror took three tries before I got it right with regards to cleanliness. The coating lasted 5 1/2 years as I remember and was noticeably brighter than a commercial scope with a plain aluminum coating in the first year. Strange sepia tone to brighter objects until you got used to it. I built a cover for the mirror in the bottom of the tube to keep as small a volume of air around it for when it wasn't in use. I probably took years off the coating by cleaning it far too often.

My house at the time was just 8 miles from a pulp and paper mill that released copious amounts of sulfur dioxides before its refit in the late 80's and also was just a half mile from the ocean, both sources of silver coating degradation according to Segdwick

The mirror I am making (100% polished, ~80% parabolized) is a 12 1/2" so it should reflect the same amount of light to the eyepiece as a 4 1/2" mirror, so star testing on Vega should be instructive.

Aluminum has one other large advantage: it is a much smoother coating, so the mirror surface you worked so hard to keep smoother doesn't have a bumpy layer added to it to change the figure.

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ccaissie
sage

Reged: 09/13/10

Loc: Whitefield, Maine

Re: Does Anyone Silver Their Mirrors? new

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[Re: JohnH]
#5239181 - 05/25/12 02:54 PM

ATM (book 1?) had an article on using amyl acetate in solution, poured on the mirror and spun until dry. Reported to protect the silver.

I like the tight fitting cover with the anti-tarnish strips idea myself, though recently I'm an OWL customer for Aluminum.

C

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EyeSage
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Reged: 06/08/12

Loc: Canukistan

Re: Does Anyone Silver Their Mirrors? new

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[Re: ccaissie]
#5322071 - 07/17/12 02:29 PM

Quote:

ATM (book 1?) had an article on using amyl acetate in solution, poured on the mirror and spun until dry. Reported to protect the silver...

C

I've been looking in to the same thing. ATM1 mentions it and ATM3 (c.1.4)goes in to more detail. The idea is to get a thin lacquer coating on the mirror, and amyl acetate is used to drastically thin the lacquer so it will form a very thin protective film on the surface.

I've been considering this myself as silvering mirrors myself would help bring bigger apertures closer to my low budget range and if the life of a coating could be significantly extended there's one more practical disadvantage out of the way. Amyl acetate is just one more chemical to source though, so I'm wondering if it's even necessary as it was apparently used as a thinner. I can get a standard lacquer thinner at a local hardware store but don't know if there is some reason to stick with amyl acetate. Does anyone have any insight on this?

- Joe

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JohnH
Carpal Tunnel

Reged: 10/04/05

Loc: Squamish BC Moved!!!!!

Re: Does Anyone Silver Their Mirrors? new

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[Re: EyeSage]
#5322173 - 07/17/12 03:44 PM

The recipe I mentioned two posts back works quite well as long as you are careful to clean things and use distilled water.

It uses dextrose as the sugar to use as it is commonly available.

Edited by JohnH (07/17/12 03:46 PM)

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EyeSage
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Reged: 06/08/12

Loc: Canukistan

Re: Does Anyone Silver Their Mirrors? new

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[Re: JohnH]
#5322289 - 07/17/12 04:56 PM

Was it difficult to source all the chemicals in Canada? I have only done a bit of poking around so far but even silver nitrate may be difficult as the US suppliers I have found so far will not ship it to Canada.

My other thought has been to get an all in one kit such as the one's offered here:

http://angelgilding.com/mirror-kits.html

I just don't know if the finish would be as acceptable for astronomical purposes as the Brashear or other methods commonly used for telescope optics.

- Joe

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JohnH
Carpal Tunnel

Reged: 10/04/05

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Re: Does Anyone Silver Their Mirrors? [Re: EyeSage]
#5322317 - 07/17/12 05:23 PM

The nitric acid I got from a source at school other things are harder to find as they need to be quite pure

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careysub
Pooh-Bah

Reged: 02/18/11

Loc: Rancho Cucamonga, CA

Re: Does Anyone Silver Their Mirrors? [Re: JohnH]
#5322396 - 07/17/12 05:58 PM

A good source for chemicals at reasonable prices (almost the only source these days...) is Elemental Scientific: http://www.elementalscientific.net/

They have amyl acetate, silver nitrate, nitric acid, reagent acetone and isopropyl alcohol, etc.

They have the U.S. shipping regulations down pat, and know what quantities that can be shipped without incurring exorbitant shipping charges.

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