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Silver Coated Mirrors vs. Aluminum Mirrors

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Q. We are an OEM glass company and we currently supply silver coating mirror only. One of our clients suggests us to supply aluminum mirror as the cost of such kind is much lower then the silver one. Here are the questions: What are the differences between Silver and Aluminum mirror? Which kind has a better quality?

Thanks and warmest regards,

Vincent Lay
- Taichung, Taiwan. ROC


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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.

Michael McDermott
- Edinbourgh, Indiana

AMBIGUITY ALERT:

To most of the contributors in this thread, 'aluminum mirror' means a glass mirror with an aluminum reflective film deposited on it in lieu of the usual silvering.

But to a few readers 'aluminum mirror' apparently meant a polished sheet of aluminum metal.


 

thumbsup2Hi, 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,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


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Q. Is aluminum mirror okay to use for a product that is strictly used for appearance purposes only and will not come in contact with any abrasive materials?

Matthew Wells
- Great Missenden, Bucks, UK


 

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,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

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Q. If aluminum is not as durable, then why do most astronomical telescopes (including observatory based ones) use aluminum coating on their primary mirrors?

I was told by a mirror manufacturer that although aluminum is second to silver in reflectiveness, aluminum is cheaper and easier to maintain. Supposedly silver tarnishes quicker. That seems to be the opposite of what's said here. Anyone care to shed some light on this please?

Archie Chen
- San Francisco, California


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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.

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

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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.

Erin Brocker
- Santa Barbara, California


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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

Paul Owen
navigation - Stockport Cheshire UK


May 12, 2008

We produce the silver mirror, I don't know the exact difference between the silver mirror and aluminium mirror, but I know the aluminium mirror is usually made by the sheet glass, the silver mirror are made by the float glass. In fact, the float glass's quality is higher than the sheet glass.

Kathy Zhao
mirror mfgr. - ZIBO, SHANDONG, CHINA


July 8, 2008

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

J Scott
- Wakefield, Massachusetts


July 2008

thumbsup2Thanks, 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,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


October 24, 2008

Q. HOW TO MAKE ALUMINIUM MIRROR WHAT'S THE CHEMICAL USED IN IT

ZAHID NAGDAWALA
- MUMBAI, INDIA


October 26, 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,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

March 6, 2009

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.

Nancy Liu
glass factory - Qingdao, Shandong, China


September 13, 2009

Q. Greetings All,
Sorry if I am jumping off topic, but I have a few questions that I have not been able to find the answers to. From reading this string it appears that this would be the best place to find the answers that I am looking for.

When did silver start being used as the reflective material for mirrors?

Other than the price how can someone tell the difference between a silver mirror and an aluminum mirror?

How much silver is used in the making of a mirror (lets say per sq foot or sq meter or how ever the industry measures it) and how pure is the silver that is used?

Thanks for helping and satisfying my curiosity.

Regards,

James Ike
Property Management - Fairfax, Virginia


February 27, 2011

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 50-100 nm 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 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?

Mike King
- Oak Park, Illinois, USA


June 14, 2011

Q. I want to thank you on the best answer I have found online about how to tell silver from aluminum. My only question would be is there a less caustic Soda than Drano, like Baking soda or some other, that will do the fizz test?

John Howie
- Citrus Heights, California, USA

April 25, 2011

Q. Sir, I am Bhavani, we make silver coated mirrors. We are interested to click on to aluminium coating due to high silver costs. I will be thankful to you if you provide me the process of aluminium mirrors.

leela bhavani
product designer - Vijayawada, Andhra Pradesh, India

April 25, 2011

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,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

November 8, 2011

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.

Steven Mickenberg
- Davie, Florida, USA


November 23, 2011

Q. Hi,
I am facing problem related to the aluminum mirror coating on glass. When I coat the aluminum by PVD process on glass substrate, in most of the time the coated surface doesn't look very shiny and reflective. Can you tell me any possible reason for that. Is this due to substrate temperature or some thing else which is responsible for this.

M. Shoaib
- Pakistan

November 23, 2011

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!

H.R. Prabhakara
- Bangalore, Karnataka, India


January 26, 2012

Q. We are using polycarbonate .08 to .125 thick mirrors for our aircraft interiors. Unfortunately they are easily scratchable and visual quality is not consistent so I am thinking for alternate stainless steel or probably aluminum sheet mirrors.
Do you have any suggestions?

Thank you,
G. G.

Georgi Georgiev
Aircraft Interiors - Montreal, Quebec, Canada



February 28, 2012

Q. Hi,

I need to know which type of mirrors can be used to minimise ghost images and aberrations due to reflections.

My application requires a total internal reflection from 2-3 mirrors.

regards
Harry

Harry Joseph
- Tamil Nadu, India


February 28, 2012

A. Hi Harry.

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

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

March 13, 2012

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.

Bill Haden
PVD coating - Houston, Texas


June 29, 2012

A. Aluminum is a better reflector for certain wavelengths of light than silver. Aluminum will reflect much better from 1 nm wavelength to around 500 nm wavelength and has only marginally less reflectivity from 500 nm 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.

Don Gross
- Macedon, New York



April 12, 2012

Q. Does anyone know at which temperature aluminum mirrors start to be oxidized under standard environmental conditions?

Manfred Feustel
- Madison, Wisconsin, USA



April 25, 2012

Q. How can we increase the paint film protection to the silver film in the mirror making process?

Samir Mehta
- Jodhpur, Rajasthan, India


April 25, 2012

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,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


October 22, 2012

Q. Dear All,

I am looking for a optimum material for Solar Radiance Concentration dish hence I request all you experts to please let me know whether it would be good option to use Aluminium sheet reflector dish for the said purpose, and if yes what care be taken for its long life.

Secondly, please let us know if there is any manufacturer of such thing.

Regards,

Chetan Dabhade
Maharashtra, India
outdated RFQ


October 22, 2012

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,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

June 14, 2014


Good

Rajesh Verma
- vadodara, Gujarat, India



December 14, 2014

Question on reflectivity. I would like to custom build a large, 4 foot parabolic dish and I need a coating with a mirror finish for a primary mirror for a telescope. As a glass mirror would cost close to $25,000, that is a little out of my price range. Any suggestions?

alan blackburn
- st louis, Missouri ,usa

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