Differentiating between silver and aluminium mirroring

[Norlesh]

How can I test if my primary mirror is coated with silver or aluminium? Currently the surface has the appearance of having been sitting under a chalk board collecting dust (this is after cleaning with isopropyl and distilled water).

I found a page on the net describing a technique for rejuvenating silver surfaces using a galvanic cell reaction (about 1/3 page down on this link (http://telescopeandbinoculars.com/oldscope.htm) but would like to be certain first that my mirror is silver instead of aluminium since there is no indication what the process would do to the latter.

[billdan]

Hi Shane

I don't know how you would test for the difference except that silver has a weaker reflectivity in the faster wavelengths ( shorter than 450nm).

My guess is - if its a commercial mirror most likely aluminium. Silver degrades too quickly 3 - 12 months depending on humidity and air pollution. Even those using a protective coating are only extending the life by 6 months.

Protected aluminium will last 10 years or more.

Regards
Bill

[Don Pensack]

If it's a commercially-made mirror, it's coated with aluminum and overcoated with either SiO or SiO2.
The coating can be removed with Ferric Chloride in solution, but it sounds like you need a re-coat. Time to investigate who does coatings.

[astro_nutt]

Hi Shane.
I had a similar problem after cleaning the primary mirror of my 10" dob. The surface looked like it swirly/dusty appearance after it dried. I found out it was a fine layer of oils on the mirrors surface.
I cleaned the mirror again but this time I gradually warmed the mirror up by adding more and more hot water to the tub every 5 minutes until the water temperature reached about 45C. I then added a few drops of detergent diluted in a cup of water and allowed the mirror to soak for 10mins before cleaning.
I rinsed in warm water, gradually getting cooler until room temperature, then final rinse with distilled water. It came up near new with a few spots remaining.
I hope this helps.
Cheers!

[dannat]

post a pic, silver is quite old & while people may called it silvering I think youll find silver hasn't been used for quite a while

[Merlin66]

If is was a silver coating - you would have realised as soon as you tried to wash it!!
Silver can't be washed like aluminium coatings it just "slides" of the mirror when immersed!!!
(Loooong story, but when I was very young and naïve, I was asked to clean a sad dirty brown 6" mirror for a guy - at the time I was the local expert (yeah!) I had experience washing my own aluminised mirrors so just launched into it. You can imagine my horror, disbelieve and total embarrassment when the "coating" just floated away!!!!
We ended up successfully re-silvering it, but to my eternal shame....)

[AstroJunk]

Kind of re-iterating what has been said - Give it a wash in soapy water and if:

It comes up shiny - its a silicon coated aluminised mirror. Feel smug and go make a cup of tea.

It is dull and possibly scratched - uncoated aluminium. That's ancient but probably a reasonable mirror. Send it to the coaters to have the job done properly.

It's tarnished and bits have flaked off, there is coating down the sides too and possibly on the back - bad news, that's been silvered. The mirror is probably home made and may or may not be worth re-coating...