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Silver coating life span and protecting methods?

Astrometry ATM Reflector Mirror Making Optics Beginner DIY Dob
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#26 Tom Stock

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Posted 20 September 2021 - 07:40 PM

I have used the chalk method on the mirrors and never had any issues (on high precision optics). The Oregon found that cleaning with chalk and distilled water improved and solved the issues they were having with the coatings. Cleaning the glass is the most important issue with spray silvering, edge cleaning is key. Get your chalk from a good supplier.

 

Starry Nightswaytogo.gif

Hi thanks for the info, which cleaning methods had been tried before the chalk?

 

Thanks


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#27 Oregon-raybender

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Posted 20 September 2021 - 09:58 PM

Chalk is the main cleaner. Polishing compounds with cotton, however there is a danger of polishing in

roughness or surface changes. Chalk is used because it removes stains and other materials that will cause the silver not to take (spots, runs and such) without having a effect on the surface. It is used by major observatories and the spray silver companies for decades. This is the material I was taught to use and had used very successfully. Check out video on cleaning the 200 inch mirror. Cleaning the edge is key as I noted.

 

Starry Nightswaytogo.gif


Edited by Oregon-raybender, 20 September 2021 - 10:13 PM.

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#28 Celestial825

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Posted 21 September 2021 - 06:36 AM

I have sprayed silver hundreds of mirrors (4" to >36") over my years at Tinsley and other companies. I have given to the the Oregon group helpful insights and tech help with their process. They have reviewed the latest process and has developed an improved method both in using the Angel product and the protection of the silver. They detailed the process and have answered many of the concerns listed here. There is no hard overcoat (like used on AL) for sprayed silver. The overcoat is just to allow the coating to last a few months longer. In most cases I have suggested to silver the mirror before the observing time period ( Spring ) and recoat again in 8 to 10 months. The silver coating is what we call a soft coating, with limited methods of cleaning (no touch) and keeping the mirror away from moisture, bugs and dust by covering. I have test mirror in my office which was coated by one of the Oregon member with the Midas and bare coating (50/50). This was over 2 years ago, it looks fine, some edge browning which is normal, stored in Zipper plastic bag, wrapped in lens tissue and paper towel. I would suggest review, read they efforts it would answer many of your questions and concerns. Just be aware, the spray silver is short term soft coating and most likely never be a total replacement of AL with overcoat, they different processes. I understand the why going in this direction because of costs, and shipping issues. I have used the chalk method on the mirrors and never had any issues (on high precision optics). The Oregon group found that cleaning with chalk and distilled water improved and solved the issues they were having with the coatings. Cleaning the glass is the most important issue with spray silvering, edge cleaning is key. Get your chalk from a good supplier.

 

Starry Nightswaytogo.gif

Thank you for all these applicationla and useful information. Every point was what I really needed to know.



#29 Celestial825

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Posted 21 September 2021 - 06:41 AM

You need to read about the process in the old S.A.  ATM books.  It was std practice then, no need to reinvent the wheel

That's good news. Thanks a lot like-button.jpg



#30 Celestial825

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Posted 21 September 2021 - 06:43 AM

No. Midas or lacquer is a separate step, after silvering

This is so promissing to me. Thanks waytogo.gif



#31 Tom Stock

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Posted 21 September 2021 - 09:11 AM

The entire process including the Midas is shown here (not my video):

 

https://m.youtube.co...GNoE31tmPGUebYg


Edited by Tom Stock, 21 September 2021 - 01:38 PM.

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#32 Oregon-raybender

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Posted 21 September 2021 - 03:25 PM

The only issues I see in the video is: Do the side first, then up and over the edge. The issues the Oregon group was having was oils and polishing compounds on the side tend to bleed up and over to the surface.

They were not using chalk, just soap and water. 

 

Use a larger pad or cloth. The starting in the center is correct.

The jury is still out on the long term using Midas product (IMHO),  but worth trying.

 

I wished I had video equipment back when I was doing silvering on the large mirrors (36")  It would

have been amazing to watch.

 

Starry Nightswaytogo.gif


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#33 hbanich

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Posted 21 September 2021 - 05:50 PM

For my first few attempts at spray silvering my 28-inch mirror I used the glass cleaning detergent supplied with the Angel Gilding spray silver kit, but I wasn't careful enough throughout the process to get the optical surface as clean as needed - which needs to be perfectly clean. I didn't understand about cleaning the side of the mirror either. Bob ("Oregon-raybender") explained how important that is, and that using chalk was the best substance to use for cleaning. So I used both the AG cleaner and chalk for my next, and first fully successful coating, but it took hours to clean a mirror that big. This past May I only used chalk for cleaning and it works wonderfully well - plus it's much quicker using only one cleaning material.

 

As Bob also pointed out, a silver coating is soft and fragile. Midas really slows down the tarnishing process but it doesn't completely stop it, so even if you manage to keep bugs and dust off your mirror, it will eventually tarnish. I use my previous coating for 22 months. at which point it measured as 95% reflective in Red, 88% in Green and 78% in Blue - which shows it had started to tarnish. Eyepiece views were still outstanding though. I stripped this coating in May and applied a new spray silver coating then.

 

Also, I recently modified my scope to add Ed Allen's Forced Ventilation System. That was an extensive but fun upgrade, and I hope it will help extend the life of the current silver coating - it will certainly help keep the coating cleaner so I'm optimistic. 

 

Aluminum coatings are much more durable than silver of course, and spray silvering makes the most economic sense for larger mirrors. But saving money isn't the only reason to use a silver coating - it's a fun process that results in a precision optical coating that will out-perform all but the most expensive enhanced aluminum mirrors. Plus, if a silver coating gets too dirty to use, you don't wash it - you simply strip it and spray on a new one.


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#34 Celestial825

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Posted 22 September 2021 - 01:28 AM

 

I wished I had video equipment back when I was doing silvering on the large mirrors (36")  It would

have been amazing to watch.

 

Starry Nightswaytogo.gif

 

 

So I used both the AG cleaner and chalk for my next, and first fully successful coating, but it took hours to clean a mirror that big. This past May I only used chalk for cleaning and it works wonderfully well - plus it's much quicker using only one cleaning material.

 

Thank you for these important hints. But what is chalk? The white substance that we used to write on old blackboards?



#35 hbanich

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Posted 22 September 2021 - 03:03 AM

Chalk = lab grade calcium carbonate. This has been processed so there are no impurities and so no danger to scratch the optical surface.
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#36 Celestial825

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Posted 22 September 2021 - 03:26 AM

Chalk = lab grade calcium carbonate. This has been processed so there are no impurities and so no danger to scratch the optical surface.

Thank you waytogo.gif



#37 Oregon-raybender

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Posted 22 September 2021 - 02:15 PM

You can order it from the companies listed below. As always, I would test on sample optic first, batch testing.

I am sure there are other lab supply companies you can order from. Always check the SDS sheets,

Read, Understand and Follow the guidelines for any materials regardless of any assumed low risk.

I have no associations with any of the companies, reference only. 

 

I hope this helps.

 

Laboratory grade calcium carbonate is precipitated calcium carbonate with a purity of 98–99%.

 

Starry Nightswaytogo.gif

 

SDS for chalk

 

https://www.flinnsci...bonate/sds_195/

 

https://www.carolina...AiABEgLlLfD_BwE

 

https://www.flinnsci...de-100-g/c0347/


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