Silver Coated Mirrors vs. Aluminum Mirrors

A. Well let me first say the cost is cheaper because the silver coated mirror lasts longer. Aluminum is a more reactive metal, therefore will corrode much more easily.

Reactivity series IN ORDER OF MOST REACTIVE

potassium
sodium
calcium
magnesium
aluminum
zinc
iron
tin
lead
hydrogen copper
mercury silver
gold.

Please respond if you think this is an appropriate answer.

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Hi, Michael.

Thanks for your take on this. Although your response certainly tells us some things about aluminum and silver, I personally don't think it quite hits the nail on the head as a comparison of silver and aluminum coated mirrors.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
 

A. Hi, Matthew.

Most metal mirrors are polished stainless steel rather than polished aluminum, and it's probably a better choice overall; but I suppose that aluminum pocket mirrors would be basically functional if you can truly keep such a soft metal from scratching.

Regards,

A. Hi, Archie. I presumed that Matthew was inquiring about inexpensive pocket mirrors made by polishing a sheet of aluminum, but I could be wrong.

Probably the guessing should stop and someone knowledgeable step up to the plate. But in the meanwhile, I'll hazard that due to the propensity to rapidly tarnish, silver is only acceptable on rear surface mirrors where the glass in front and a coating in back keeps air away from the silver. In that application it is rugged, durable, highly reflective, and ubiquitous. And you can wash dust and fingerprints off the mirror with no problem.

But my understanding is astronomical mirrors are always front-surface, so that there will be no double reflection (one off the front of the glass, one off the back), so they use aluminum. Although aluminum is active and corrodes easily, the corrosion product -- aluminum oxide -- can be quite transparent if things are right. And nobody will be getting thumbprints on astronomical mirrors.

A. Silver mirrors reflect more red and yellow tones than their aluminum counterparts which reflect bluer tones. Silver mirrors are preferred in cosmetic applications, such as makeup mirrors, because they give a warmer image than the slightly harsher one from an aluminum mirror.

Aluminum oxide forms rapidly on the surface of the aluminum metal, and, as Ted pointed out, this extremely thin layer is clear. However, the oxide layer acts as protection against further oxidation. It is extremely hard to separate the aluminum oxide from the metal, and if the mirror is scratched, a new protective layer of oxide will reform immediately.

Silver does have a higher optical reflectivity than aluminum, but it also reflects UV light poorly. I'm guessing here (sorry, Ted!), but telescopes may use aluminum mirrors not only to lower the price and minimize upkeep, but for the aluminum mirror's ability to reflect the UV and far IR regions better than any other metal. By using aluminum mirrors, telescopes can have the best reflectance in the UV and far IR regions and nearly the best in the visible and near IR regions.

A. I had a number of sextant mirrors recoated last year. They are rear surface mirrors originally silver (some are 60 years old)
These were resurfaced in Aluminium and I can't tell the difference with a silver mirror when taking star sights.

I was worried that dimmer stars would not reflect as well as a silver mirror but this is not the case.
(These sextants are English and German) I have a Russian sextant with front surface mirrors but I don't know what metal and there is no perceived difference
I understand front surface mirrors gain about 4% due to no loss through the glass

A. For reflective property specifications of an aluminum vs. silver coating, I suggest you read this chapter from the CVI Melles Griot product catalog:

www.mellesgriot.com/pdf/CatalogX/X_05_26-34.pdf

Thanks, J. That is indeed great coverage of the issues. I'd like to thank you and everyone here for building an interesting, civil, and informative thread here!

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
July 2008

A. Hi, Zahid. Aluminum is deposited on mirrors by a PVD vacuum metallizing process; it's not a matter of liquids like for silvering of mirrors.

Regards,

A. We are professional mirror manufacturer in Qingdao, China

1. There are aluminum float mirror, aluminum sheet mirror, silver float mirror, silver sheet mirror, aluminum mirror does not mean sheet mirror, silver mirror does not mean float mirror, this is important.

2. Price of Aluminum Mirror is cheaper than silver mirror, the processing of aluminum mirror is a kind of physical process, silver mirror is a kind of chemical process.

3. Due to the material and processing differences, Aluminum Mirror is very good for clean cut mirror and simple deep-processed mirror, let's say cut aluminum mirror can be used for cosmetic mirror, framed mirror, mirror closed door, safety back mirror with Cat II& Cat; I;, normally the thickness is 1.1-4.0 mm. But for bevel mirror, the thickness is 4-6 mm, silver mirror is better.

4. There is no copper for aluminum mirror, there is copper in silver mirror, so for the environment, aluminum mirror is more ecological, so now there is copper free silver mirror, but the price is very very expensive, we have it.

Wish this information is helpful.

A. The history of "looking glass" is quite interesting and continues to develop as new technologies become economically viable.

Before glass there were small polished bronze and steel plates which were generally reserved for rich folks to see themselves when applying cosmetics. Long, full length mirrors for seeing the whole body were very rare. With improvements in heavy machinery at the time of the Industrial Revolution, tin amalgam mirrors became the standard. Made by dissolving metallic tin in mercury and squeezing the sticky "amalgam" onto the glass surface with heavy weights over a period of weeks, these were the mirrors that made the Hall of Mirrors in the Palace of Versailles famous. If you have one of these types of antique mirror, it probably has white stains of tin oxide in it and a pretty pool of mercury at the bottom as the amalgam disintegrates. Most folks try to avoid pools of mercury these days!

The chemist Liebig in the mid-19th century is credited with developing a method for depositing silver on glass and that method with numerous enhancements has remained the traditional method until relatively recently when vacuum deposition of aluminum vapor has become the preferred industrial method for "regular" mirrors. As pointed out, their reflectivity is somewhat different from silver both in intensity and wavelength but for most applications they are just fine.

Optical applications generally use front-surface mirroring to avoid refraction through the glass and silver doesn't handle this too well, being prone to tarnishing, although a thin clearcoat on the silver can protect it for long enough to be useful without getting too much optical distortion. Vacuum deposition is expensive and not generally accessible to the average do-it-yourselfer while electroless silver mirroring is simple and - added bonus - can be modified as it deposits to make some very interesting decorative effects.

A typical silver mirror has a layer of 24 carat silver about 50-100nm thick. While this makes a very reflective mirror, it is so thin that the mirror is blue (complementary color to yellow) when held up to the light and the backing paint also therefor serves to make the mirror opaque.

As for resistance to wear and contamination, silver is pretty much inert when properly backed with paint - copper is indeed sometimes used to absorb airborne contaminants that would tarnish the silver but good mirror backing paints generally make this unnecessary. Aluminum on the other hand is corroded by both acidic and alkaline environments, both of which dissolve the thin but tenacious aluminum oxide patina. A simple test for a sample of mirror is to drip caustic soda [affil links] on it (Drano drain cleaner for example). Aluminum will fizz and dissolve while silver is totally uneffected.

Does this answer any of the outstanding questions?

A. Hi, Bhavani. I doubt that anyone will write out the entire process of making aluminum mirrors, but they may point you to a reference book on the subject. But meanwhile, there are many snippets of the process disclosed on this page. Maybe you could summarize the steps and information already listed, and ask specific questions where the steps seem unclear to you?

Regards,

A. My company has been importing and selling aluminum mirrors in the US for over 10 years. We compete with manufacturers of "silver mirrors". The mirrors are both made with high quality float glass. The aluminum mirror is made in a vacuum sealed chamber in which aluminum coils are placed behind the glass in the chamber. The air in the chamber is evacuated and the chamber is heated to a temperature where a fusion takes place between the glass and the aluminum to create a reflective surface on the rear of the glass. The glass is then coated with protective paint the same way the processed silver mirror would be. The aluminum mirrors have been tested to the same ASTM standards as the silver mirror with results that are equal.

A. Aluminum coated glass is a front coated mirror. The reflection is specular i.e. it reflects light in a specific direction. In other directions the mirror looks almost black. Normal shiny surfaces reflect light in all directions due to surface irregularities. You would realise it if you try to use the Al coated glass as mirror - either look at your own image or reflect a beam of light on to a wall. I assume the coating is good!

A. Hi Harry.

I'm no optics expert, but it certainly sounds like front surface mirrors are your only choice.

Regards,

A. There are many topcoat types for Aluminum and Silver top coated mirrors. Al can reflect about 90% max of visible light and silver can be brought up to over 95% with the right coating. Many optical coaters can provide you with their version. Most are multi-layer and depend on the desired use - optical quality reflectivity, illumination reflectivity, IR, UV, etc. The Al or silver layer can be applied via PVD or ion beam assisted PVD (or cathodic arc, ALD, etc.) and the topcoat is put on in the same process as a second or final layer. The coated surface (glass) must be extremely clean and flat. The thin coating follows the contour of the surface and every imperfection shows. Some coaters fill in the surface with a leveling substance to remove surface imperfections first. Look for 'optical reflective coating providers' or similar and several companies which do this type of coating should be shown in the results.

A. Aluminum is a better reflector for certain wavelengths of light than silver. Aluminum will reflect much better from 1nm wavelength to around 500nm wavelength and has only marginally less reflectivity from 500nm through the rest of the visible spectrum. There are some great web pages out there that can give you the formulas and all of the scientific stuff, but in a nutshell it depends what you want to reflect if you should use aluminum. The problem is aluminum oxidizes very quickly.

Hi Samir.

The possibilities strike me as utterly limitless. I think it would be more efficient and productive for you to tell us what you do and in what ways it is less than satisfactory. Good luck.

Regards,

A. Hi, Chetan. Aluminum sheets are bright dipped and anodized for use as reflectors all the time, for outdoor lighting, dentist & medical lights, infrared heat reflectors, etc. I think it would be fine.

Regards,

Good

A. Hi Jean-Philippe. Mirror polishing is not as good as chemical bright dipping, which in turn is not as good as electropolishing.

I don't know enough about the SiO2 coating to answer your question, but the traditional coating is a thin layer (0.00008" or so) of anodizing. The ASM Metals Handbook, Vol. 5, has a good introductory chapter on "Cleaning and Finishing of Aluminum and Aluminum Alloys".

"The Surface Treatment of Aluminum and Its Alloys" has more data than you'll probably ever have time to read: super purity aluminum with 2% magnesium added can give excellent reflectivity (about 85%) even with a durable 0.0002" - 0.0004" anodizing layer, and even more if bare or with a thin anodized layer. Good luck.

Regards,

Hi Jean-Philippe,

Ted's advice is fine if you are only interested in the visible part of the spectrum. But if you also want to reflect infrared there are two extra points

1) Chemical/electro polishing offers little extra reflectivity at IR.

and, more importantly

2) Anodized coatings absorb energy at IR wavelengths. Perhaps this is why SiO2 coatings are used to give corrosion resistance?

A. I don't know if anyone brought it up but I believe cosmetic or everyday mirrors are "silvered" on the back, whereas some optical mirrors are silvered on the front so you would want a less tarnishing metal.